The Immune System 4th Edition By Parham – Test Bank

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The Immune System 4th Edition By Parham – Test Bank

THE IMMUNE SYSTEM, FOURTH EDITION

CHAPTER 6: THE DEVELOPMENT OF B LYMPHOCYTES

© 2015 Garland Science

 

 

 

6–1      Describe the six functionally distinct phases of B-cell development.

 

6–2      Which of the following cell-surface markers differentiates hematopoietic stem cells from other cell constituents in the bone marrow?

  1. pre-B-cell receptor
  2. BAFF receptor
  3. CD34
  4. CD4
  5. membrane-bound stem-cell factor (SCF).

 

6–3      You are going to use flow cytometry to determine the proportion of developing B cells in the bone marrow that are immature, anergic, or mature. You have three monoclonal antibodies specific for three different B-cell surface proteins. The first has specificity for the cell-surface protein CD19, which is expressed by all developing and mature B cells; the second is specific for the Fc region of IgD; and the third is specific for the Fc region of IgM. The antibodies are conjugated to three different fluorescent tags that can be detected and distinguished by the flow cytometer.

  1. Use histograms to show your analysis of CD19-positive cells and indicate which part of your histogram you would gate to analyze IgM and IgD expression. Indicate the gated population with an arrow.
  2. Using a two-dimensional dot plot, compare the expression of IgD and IgM of these gated cells, and say which of these populations represents (i) immature B cells, (ii) mature B cells, and (iii) anergic B cells.

 

6–4      Which of the following is characteristic of a large pre-B cell?

  1. VDJ is successfully rearranged and μ heavy chain is made.
  2. V–J is rearranging at the light-chain locus.
  3. μ heavy chain and λ or κ light chain is made.
  4. V is rearranging to DJ at the heavy-chain locus.
  5. D–J is rearranging at the heavy-chain locus.

 

6–5      Which of the following statements is correct?

  1. The κ light-chain genes rearrange before the heavy-chain genes.
  2. The κ light-chain genes rearrange before the λ light-chain genes.
  3. The λ light-chain genes rearrange before the heavy-chain genes.
  4. The λ light-chain genes rearrange before the κ light-chain genes.
  5. The μ heavy-chain genes rearrange first and then the λ light-chain genes rearrange.

 

6–6      Immature B cells develop into B cells in the

  1. subendosteum
  2. bone marrow
  3. thymus
  4. blood
  5. secondary lymphoid organs.

 

6–7      Place the following phases of a B cell’s life history in the correct chronological order.

  1. negative selection
  2. attacking infection
  3. finding infection
  4. searching for infection
  5. repertoire assembly
  6. positive selection.

 

6–8      Place the following stages of B-cell development in the correct chronological order.

  1. early pro-B cell
  2. large pre-B cell
  3. immature B cell
  4. stem cell
  5. late pre-B cell
  6. small pre-B cell.

 

6–9

  1. Discuss the importance of the bone marrow stroma for B-cell development.
  2. What would be the effect of anti-IL-7 antibodies on the development of B cells in the bone marrow, and at which stage would development be impaired? Explain your answer.

 

6–10

  1. What are the two main checkpoints of B-cell development in the bone marrow?
  2. What is the fate of developing B cells that produce (i) functional or (ii) nonfunctional heavy and light chains?
  3. Explain how these two checkpoints correlate with the process of allelic exclusion that ensures that only one heavy-chain locus and one light-chain locus produce functional gene products.

 

6–11    Large pre-B cells are characterized by which of the following?

  1. They do not express CD19 at the cell surface.
  2. Rearrangement of light-chain genes commences.
  3. Nonproductive rearrangement of both heavy-chain loci has already occurred.
  4. Allelic exclusion of the immunoglobulin light-chain loci has already occurred.
  5. μ is assembled with VpreBλ5.

 

6–12    All hematopoietic stem cells express

  1. CD34
  2. CD127
  3. CD19
  4. VpreBλ5
  5. Pax-5

 

6–13    Which of the following do not associate with one another during B-cell development?

  1. IL-7: IL-7 receptor of late pro-B cells
  2. Pax-5: CD19 gene
  3. surrogate light chain: δ heavy chain
  4. VpreB: λ5
  5. SCF: Kit
  6. pre-B-cell receptor: Igα and Igβ

 

6–14    The latest stages of late pro-B-cell development are recognized by the association of a surrogate light chain with a μ chain. The surrogate light chain is composed of

  1. E2A and EFB
  2. Igα and Igβ
  3. VpreB and λ5
  4. RAG-1 and RAG-2
  5. Pax-5 and CD19.

 

6–15    A genetic defect in the λ5 gene would cause which of the following consequences? (Select all that apply.)

  1. inability to produce functional μ chains
  2. inability to produce a pre-B-cell receptor
  3. inability to produce functional κ or λ chains
  4. production of different light chains owing to defects in allelic exclusion
  5. B-cell immunodeficiency
  6. chronic bacterial infections
  7. requirement for prophylactic injections of antibodies from healthy donors.

 

6–16    What would be the consequence if terminal deoxynucleotidyl transferase (TdT) were expressed throughout the whole of small pre-B-cell development?

 

6–17    Which of the following is not paired with its correct complement?

  1. N nucleotides: more abundant in rearranged heavy-chain genes than in rearranged light-chain genes
  2. second checkpoint in B-cell development: assembly of a functional B-cell receptor
  3. receptor editing: exchange of light chain for one that is not self-reactive
  4. first checkpoint in B-cell development: selection by the pre-B-cell receptor
  5. large pre-B-cell stage: constitutive expression of RAG-1 and RAG-2 proteins.

 

6–18    Which of the following would occur after the production of a functional μ chain as a pre-B-cell receptor?

  1. RAG proteins are degraded.
  2. The chromatin structure of the heavy-chain locus is reorganized to prevent gene rearrangement.
  3. Transcription of the RAG1 and RAG2 genes ceases.
  4. There is allelic exclusion of a second μ chain.
  5. All of the above would occur.

 

6–19    An important advantage of having two gene loci (κ and λ) for the light chain is

  1. that the likelihood of a successful rearrangement of light-chain genes increases.
  2. that immunoglobulins are homogeneous and not heterogeneous in mature B cells.
  3. that different effector functions are conferred by the two different light-chain loci.
  4. that surrogate light-chain transcription cannot compete with κ and λ transcription and enables B-cell development.
  5. all of the above.

 

6–20    Which of the following is correctly matched? (Select all that apply.)

  1. early pro-B cell: VDJ rearranged
  2. pre-B-cell receptor: VpreBλ5/μu heavy chain
  3. mature B cell: IgM plus IgD
  4. small pre-B cell: VJ rearranged
  5. immature B cell: μ heavy chain plus λ or κ light chain on surface.

 

6–21    Large pre-B cells undergo clonal expansion before the rearrangement of light-chain loci. Which of the following are beneficial consequences of clonal expansion? (Select all that apply.)

  1. Autoreactive B cells are eliminated before the expenditure of energy needed to rearrange a functional light-chain gene.
  2. The energy used to make a functional heavy chain is not wasted as a result of the inability to produce a functional light chain.
  3. RAG gene expression is decreased, which in turn signals light-chain rearrangement.
  4. A diverse population of immature B cells is generated that express the same μ chain but a distinct light chain.
  5. Approximately 85% of small pre-B cells will progress to the immature B-cell stage.

 

6–22    When expression of _______ is turned off in small pre-B cells, the result is the presence of P nucleotides but an absence of N nucleotides in around 50% of light-chain genes.

  1. Kit
  2. CD19
  3. TdT
  4. Pax-5
  5. RAG-1 and RAG-2.

 

6–23    A defect in which of the following proteins blocks B-cell development at the pre-B-cell stage, resulting in almost no circulating antibodies in individuals with this defect?

  1. IL-7 receptor
  2. terminal deoxynucleotidyltransferase (TdT)
  3. Pax-5
  4. Bruton’s tyrosine kinase (Btk)
  5. CD19.

 

6–24    The consequence of allelic exclusion at the immunoglobulin loci ensures that _____. (Select all that apply.)

  1. B-cell receptors have a low-avidity binding
  2. B cells express antigen receptors of a single specificity
  3. hybrid immunoglobulins are formed
  4. all functional copies of a gene are expressed
  5. homogeneous B-cell receptors bind more effectively to antigen.

 

6–25    A developing B cell unable to generate a productive rearrangement on any of the four light-chain loci will undergo

  1. self-renewal
  2. apoptosis
  3. allelic exclusion
  4. malignant transformation
  5. differentiation into a B-1 cell.

 

6–26    All of the following participate in signal transduction in developing B cells except

  1. terminal deoxynucleotidyl transferase (TdT)
  2. FLT3
  3. CD19
  4. Igα and Igβ
  5. Bruton’s tyrosine kinase (Btk)
  6. CD45

 

6–27    Negative selection of developing B cells ensures that

  1. there is not an overabundance of circulating B cells that would compete with other important cell types in the circulation
  2. only antigen-activated B cells leave the bone marrow
  3. clonal expansion of B cells does not occur in the absence of infection
  4. B-cell receptors that bind to normal constituents of the body do not emerge
  5. B cells do not leave secondary lymphoid tissues.

 

6–28    Receptor editing occurs _____. (Select all that apply.)

  1. in the bone marrow
  2. after encounter with foreign antigen in secondary lymphoid organs
  3. in mature B cells
  4. to establish self-tolerance of the B-cell repertoire
  5. to express an excess of IgM over IgD on the surface of mature B cells.

 

6–29    Which of the following statements about the IgD made by B cells of upper respiratory mucosa is not true?

  1. These antibodies bind to airborne bacteria such as Haemophilus influenzae.
  2. λ light chains are used almost exclusively by these IgD antibodies.
  3. Two-thirds of these IgD antibodies possess κ light chains.
  4. These IgD antibodies recruit basophils and induce the secretion of antibacterial peptides.

 

6–30    Individuals who fail to express functional Bruton’s tyrosine kinase exhibit all of the following characteristics except

  1. B-cell development is blocked at the immature B-cell stage.
  2. They are usually male because the Btk gene is on the X chromosome.
  3. They suffer from an immune deficiency known as X-linked agammaglobulinemia (XLA).
  4. Recurrent infections with extracellular bacteria are common.
  5. They benefit from treatment with immunoglobulin infusions.

 

6–31    All of the following are associated with the development of Burkitt’s lymphoma except

  1. The expression of Myc protein is perturbed.
  2. A chromosomal translocation involving a proto-oncogene and an immunoglobulin gene occurs.
  3. Overproduction of the Bcl-2 protein prolongs the lifetime of B-lineage cells.
  4. Cell division restraints on mutated B cells are lifted.
  5. In addition to a chromosomal translocation event, mutations elsewhere in the genome are usually involved.

 

6–32    Which of the following is a characteristic of B-2 cells?

  1. They are sometimes referred to as CD5 B cells.
  2. They comprise only 5% of the B-cell repertoire.
  3. In adults, they are renewed by cell division in the peripheral circulation.
  4. They are located primarily in secondary lymphoid organs.
  5. They are not dependent on T helper cells for activation.

 

6–33    Identify the mismatched pair of chemokine and the cells that secrete it.

  1. CCL19: lymph-node dendritic cells
  2. CXCL13: follicular dendritic cells
  3. CCL21: stromal cells of secondary lymphoid tissues
  4. All of the above are correctly matched.

 

6–34    Plasma cells have all of the properties listed except

  1. they rapidly proliferate in secondary lymphoid follicles
  2. they secrete antibody
  3. they are terminally differentiated B cells
  4. they no longer express MHC class II molecules
  5. they cease expressing membrane-bound immunoglobulin
  6. they differentiate into plasma cells after migration from germinal centers to other sites in lymphoid tissue and bone marrow.

 

6–35    All of the following events occur within germinal centers except

  1. centrocytes mature from centroblasts
  2. isotype switching
  3. centroblasts arise from activated B ells
  4. B cells are activated by CD4 helper T cells
  5. affinity maturation
  6. somatic hypermutation
  7. production of memory B cells.

 

6–36    In which location would plasma cells not be present?

  1. bone marrow
  2. afferent lymphatic vessels
  3. medullary cords of lymph nodes
  4. lamina propria of gut-associated lymphoid tissues
  5. red pulp of spleen
  6. efferent lymphatic vessels.

 

6–37    Match the name of the B-cell tumor in Column A with its correct description in Column B.

Column A Column B
___a.   multiple myeloma 1.         most cases caused by B-1 cells
___b.   chronic lymphocytic leukemia (CLL) 2.         derived from lymphoid progenitor in bone marrow and rearrangement of immunoglobulin loci has not occurred
___c.   acute lymphoblastic leukemia (ALL) 3.         expresses VpreBλ5
___d.   Burkitt’s lymphoma 4.         derived from plasma cells in the bone marrow
___e.   pre-B-cell leukemia 5.         associated with chromosomal translocations involving the proto-oncogene MYC

 

 

6–38    Which of the following is true of centrocytes? (Select all that apply.)

  1. Somatic hypermutation has occurred.
  2. They are large proliferating cells.
  3. Isotype switching is complete.
  4. They produce secreted forms of immunoglobulins.
  5. They lack MHC class II molecules on the cell surface.

 

 

6–39    Immunological tolerance in the B-cell repertoire is called _______ tolerance when it develops in primary lymphoid organs, and _______ tolerance when it is induced outside the bone marrow.

  1. primary; secondary
  2. apoptotic; anergic
  3. stromal; follicular
  4. receptor-mediated; systemic
  5. central; peripheral.

 

6–40    What is the role of primary lymphoid follicles in eliminating B cells that have antigen receptors specific for soluble self antigen?

 

6–41    A plasma cell is characterized by which of the following features? (Select all that apply.)

  1. It differentiates in the medulla of lymph nodes and the bone marrow.
  2. It dedicates 10–20% of total protein synthesis to antibody production.
  3. Levels of MHC class II molecules are elevated.
  4. It undergoes extensive proliferation in germinal centers.
  5. It produces secreted immunoglobulin instead of the membrane-bound form.

 

6–42

  1. Explain why immunological memory is important in acquired immunity.
  2. Describe how immunoglobulin expressed during a primary immune response differs qualitatively and quantitatively from the immunoglobulin expressed during a secondary immune response.

 

6–43    When producing monoclonal antibodies, why is it important to use as a fusion partner a myeloma cell that is unable to produce its own immunoglobulin?

  1. to ensure that allelic exclusion of μ chain occurs normally
  2. to ensure that the antibodies are homogeneous and able to make strong bivalent attachments to multivalent antigens
  3. to ensure that the monoclonal antibodies are not autoreactive
  4. to provide a greater opportunity for making a successful rearrangement at the light-chain locus
  5. to ensure that antibodies are secreted and not membrane-bound.

 

6–44    The proto-oncogene _______ is associated with the development of Burkitt’s lymphoma.

  1. BCL-2
  2. Myc
  3. CD5
  4. CD19
  5. BTK.

 

6–45    Which of the following characterizes the B-1 cells that develop prenatally?

  1. They lack N nucleotides.
  2. They possess polyspecificity for bacterial polysaccharide antigens.
  3. They arise early in embryonic development preceding the development of the majority subset of B cells.
  4. They have little or no IgD on the cell surface.
  5. All of the above.

 

6–46    Explain how B cells undergo the process of negative selection and indicate at which stage of development and at which location these events occur.

 

6–47    What is the fate of an immature B cell that encounters and has specificity for self antigen?

  1. If further heavy-chain and light-chain gene rearrangements are possible, it undergoes apoptosis.
  2. Somatic hypermutation.
  3. Decrease in production of IgD.
  4. Continued rearrangement of heavy-chain genes.
  5. Continued rearrangement of light-chain genes.

 

6–48    Which of the following pertains to the fate of immature B cells that have specificity for univalent self antigens? (Select all that apply.)

  1. The cells acquire a state of unresponsiveness called anergy.
  2. IgD is retained in the cytosol.
  3. IgD on the cell surface fails to activate the B cell when bound to self antigen.
  4. The cells have a much longer life-span than mature B cells.
  5. The cells die by apoptosis.

 

6–49    The circulatory route through a lymphoid tissue for both immature B cells and mature B cells that do not encounter specific antigen is:

  1. bloodstream →HEV of lymphoid cortex → primary lymphoid follicle → efferent lymphatic vessel
  2. afferent lymphatic vessel → primary lymphoid follicle →HEV of lymphoid cortex →efferent lymphatic vessel
  3. afferent lymphatic vessel →medullary cords → primary lymphoid follicle →efferent lymphatic vessel
  4. primary lymphoid follicle →HEV of lymphoid cortex →afferent lymphatic vessel →efferent lymphatic vessel
  5. bloodstream →afferent lymphatic vessel → HEV of lymphoid cortex →efferent lymphatic vessel.

 

6–50

  1. Give three properties that distinguish B-1 cells from B-2 cells.
  2. Do you think that B-1 cells should be categorized as participants in innate immune responses or in acquired immune responses? Explain your rationale.

 

6–51

  1. Identify properties that are shared by anergic B cells and plasma cells.
  2. What key property is different?

 

6–52    Indicate which of the following statements concerning memory B cells are true (T) and which are false (F):

___ a.  Memory B cells are derived from germinal center B cells as immune responses subside.

___ b.  Memory B cells have long life spans.

___ c.  Memory B cells possess high-affinity antigen receptors as a consequence of affinity maturation.

___ d.  Memory B cells have more stringent requirements for activation than naive B cells do.

___ e.  Memory B cells express only IgM and retain the capacity to switch to the most beneficial isotype during secondary responses.

 

6–53    Hinda Mundy, 26 years old, grew concerned when a lump appeared in her lower neck and she had pain in her chest and a dry cough. She also told her physician that she had experienced fatigue, night sweats, unintentional weight loss, pruritis (dry, itchy skin), and intermittent fevers over the past few months. Immunohistological staining of a biopsy of the enlarged lymph node revealed the presence of large multinucleated Reed–Sternberg cells. Polymerase chain reaction (PCR) tests confirmed immunoglobulin gene rearrangements; however, B-cell antigen expression was absent. Hinda entered complete remission after treatment with four cycles of chemotherapy combined with radiotherapy. These symptoms and treatment are most consistent with a diagnosis of

  1. Hodgkin’s lymphoma
  2. multiple myeloma
  3. acute lymphoblastic leukemia (ALL)
  4. Waldenström’s magroglobulinemia
  5. chronic lymphocytic leukemia (CLL).

 

6–54    Multiple myeloma involves the unregulated proliferation of an antibody-producing plasma cell (myeloma cell) independently of antigen stimulation or T-cell help. Myeloma cells populate multiple sites in the bone marrow, where they produce immense quantities of monoclonal immunoglobulin as well as suppressing normal marrow function. Myeloma cells also synthesize and secrete excessive amounts of free light chains (known as Bence-Jones protein), which, because of their low molecular weight (~25 kDa) are excreted as free light chains in the urine.

In a given patient the free light chains are both monoclonal and all are of either the κ or the λ type.

  1. Explain both of these observations.
  2. Why do you think patients with multiple myeloma are more susceptible than normal to pyogenic infections, such as pneumonia caused by Streptococcus pneumoniae or Haemophilus influenzae?

 

ANSWERS

 

6–1

(1) Repertoire assembly: Bone marrow expression of diverse B-cell receptors. (2) Negative selection: Modification, elimination or inactivation of autoreactive B cells. (3) Positive selection: Selection of a small subset of immature B cells to become mature B cells in secondary lymphoid organs. (4) Searching for infection: Patrolling for infectious material by recirculating continuously between lymph, blood and secondary lymphoid organ compartments. (5) Finding infection: B cells become activated by antigen in secondary lymphoid tissues and then undergo clonal expansion. (6) Attacking infection: B cells differentiate into plasma cells and memory cells in secondary lymphoid tissues.

 

6–2      c

 

6–3

  1. Histogram. Your histogram should look like the one in the left panel of Figure A6.3, with axes labeled ‘cell number’ (y) and ‘relative fluorescence intensity’ (x) depicting total bone marrow cells stained with anti-CD19. Two peaks will be observed. One is the CD19-negative population and the other is the CD19-positive population. Gate the CD19-positive population (developing and mature B cells) for two-dimensional dot-plot analysis.
  2. Two-dimensional dot plot. Your dot plot should look like the one in the right panel of Figure A6–46, with relative fluorescence intensity of anti-IgD versus anti-IgM. Three populations of B cells will be distinguished: (1) IgM-high, IgD-low, which represent immature B cells; (2) IgM-low, IgD-high, which represent mature B cells; and (3) IgM-very low (retained in the cell), IgD-high, which represent anergic B cells.

 

 

<<insert Figure A6–46>>

Figure A6–46 A histogram analysis (left) and two-dimensional dot-plot analysis (right).

 

6–4      a

 

6–5      b

 

6–6      e

 

6–7      e →a →f →d →c →b

 

6–8      d →a →e →b →f →c

 

6–9

  1. Bone marrow stromal cells provide the necessary environment for B-cell development by expressing secreted products and membrane-bound adhesion molecules. For example, VCAM-1 adhesion molecule binds to the integrin VLA-4 on early B-cell progenitors. Cytokines such as IL-7 have an important role in later stages of B-cell development, serving to stimulate the growth and cell division of late pro-B and pre-B cells.
  2. If anti-IL-7 antibody were introduced into this environment, developing B cells would be arrested at the late pro-B-cell or pre-B-cell stage and would not be able to progress normally to the immature B-cell stage. Interestingly, in transgenic mice overexpressing IL-7, significant increases in pre-B cells are observed in the bone marrow and secondary lymphoid organs, whereas in IL-7 knockout mice (in which the gene locus encoding IL-7 is interrupted and no IL-7 is produced) early B-cell expansion is significantly impaired. These experiments in mice clearly demonstrate the importance of IL-7 in B-cell maturation.

 

6–10

  1. Checkpoint 1 is marked by the formation of a complex of a μ heavy chain complexed with the surrogate light chain VpreBλ, Igα, and Igβ. Checkpoint 2 is when a complete B-cell receptor, comprising μ heavy chains, κ or λ light chains, and Igα and Igβ chains, is expressed on the B-cell surface.
  2. At checkpoint 1, if the V(D)J rearrangement gives rise to a functional pre-B-cell receptor the late pro-B cell will be permitted to survive and undergo clonal proliferation. If V(D)J rearrangement produces a nonfunctional heavy chain and no pre-B-cell receptor is assembled, the pro-B cell undergoes apoptosis and dies. Similarly, at checkpoint 2, production of a functional light chain results in the assembly of a functional surface immunoglobulin and the survival and maturation of the B cell. Nonproduction of a light chain results eventually in apoptosis.
  3. Checkpoint 1 delivers an important signal to the cell, verifying that a functional heavy chain has been made. This triggers the cessation of heavy-chain gene rearrangement followed by the inactivation of surrogate light-chain synthesis. Thus, only one heavy-chain locus ends up producing a product. As surrogate light chain becomes unavailable, μ accumulates and is retained in the endoplasmic reticulum, ready to bind to functional light chain when that is synthesized after successful light-chain gene rearrangement. Checkpoint 2 signals the cessation of light-chain rearrangement. This ensures that only one light-chain locus out of the possible four produces a functional product.

 

6–11    e

 

6–12    a

 

6–13    c

 

6–14    c

 

6–15    b, e, f, g

 

6–16    N nucleotides would be added at the VJ joints of all rearranged light-chain genes during gene rearrangement (instead of about half), resulting in an increase in immunoglobulin diversity. It is interesting to note that because TdT is not expressed until after birth, B-1 cells that are generated prenatally lack N nucleotides in the VD and DJ junctions of their rearranged heavy-chain genes as well as in the VJ junctions of all light-chain genes.

 

6–17    e

 

6–18    e

 

6–19    a

 

6–20    b, c, e

 

6–21    b, d, e

 

6–22    c

 

6–23    d

 

6–24    b, e

 

6–25    b

 

6–26    a

 

6–27    d

 

6–28    a, d

 

6–29    c

 

6–30    a

 

6–31    c

 

6–32    d

 

6–33    d

 

6–34    a

 

6–35    d

 

6–36    b

 

6–37    a—4; b—1; c—2; d—5; e—3

 

6–38    a, c

 

6–39    e

 

6–40    To survive, circulating B cells must enter primary follicles where survival signals are delivered by cells in the follicles, including follicular dendritic cells (which are the stromal cells of primary lymphoid follicles). Circulating B cells that fail to enter follicles in secondary lymphoid tissues will die in the peripheral circulation with a half-life of about 3 days. B cells with antigen receptors specific for soluble self antigen are generally rendered anergic in the bone marrow or the circulation. Anergic B cells that enter secondary lymphoid organs are held in the T-cell areas adjacent to primary follicles and are not permitted to penetrate the follicle. As a result, they do not receive the necessary stimulatory signal for survival. Instead, anergic B cells will undergo apoptosis in the T-cell zone. This is an efficient cleansing mechanism and serves to delete potentially autoreactive B cells from the circulation.

 

6–41    a, b, e

 

6–42

  1. Memory enables faster, more efficient recall responses when antigen is encountered subsequently. This enables the body to get rid of a pathogen before it has time to cause disease.
  2. Immunoglobulin produced during a primary immune response is mainly IgM, in low concentration (titer) and of low affinity for the antigen. Immunoglobulin expressed during a secondary immune response has undergone isotype switching and is often of the IgG isotype. It also has a higher titer and, through the process of somatic hypermutation, will have a higher affinity for its corresponding antigen.

 

6–43    b

 

6–44    b

 

6–45    e

 

6–46    Immature B cells that express receptors specific for common multivalent self antigens undergo apoptosis unless receptor editing can produce a receptor that does not have specificity for self antigen. This process of clonal deletion begins in the bone marrow, and apoptotic cells are phagocytosed by macrophages. Immature B cells bearing receptors for monovalent self antigen are instead rendered anergic. These anergic B cells are produced in the bone marrow and when exported to the periphery survive for only 1–5 days. Immature B cells reactive to self antigen in the periphery cannot carry out receptor editing. They either undergo apoptosis or become anergic.

 

6–47    e

 

6–48    a, c

 

6–49    a

 

6–50

  1. Unlike conventional B-2 cells, B-1 cells express the cell-surface protein CD5, possess few N nucleotides at VDJ junctions, and have a restricted range of antigen specificities. They produce IgM antibodies of low affinity and respond mainly to carbohydrate, rather than protein, epitopes. Individual B-1 cells are polyspecific for antigen; that is, their immunoglobulins bind several different antigens.
  2. B-1 cells are probably best associated with innate immune responses because of their rapid response to antigen, their limited diversity, and their polyspecificity.

 

6–51

  1. They both have limited life-spans, express decreased levels of IgM on the cell surface, and are nonresponsive to antigen.
  2. Anergic B cells do not secrete antibody. Plasma cells, in contrast, secrete very large amounts of antibody.

 

6–52    a—T; b—T; c—T; d—F; e—F

 

6–53    Rationale: The correct answer is a. Two clues are crucial to this diagnosis: the first is the presence of Reed–Sternberg cells, a hallmark of Hodgkin’s lymphoma; the second is the existence of rearranged immunoglobulin loci in these cells but their inability to express mature B-cell receptors. Multiple myeloma, Waldenström’s magroglobulinemia, and CLL, in contrast, are B-cell tumors that do make mature B-cell receptors. ALL derives from a lymphoid progenitor, and the immunoglobulin loci are unrearranged.

 

6–54

  1. In a normal response to infection, a diverse array of plasma cells will produce different light chains against many different antigens. In multiple myeloma, the tumor originates from a single plasma cell expressing heavy and light chains with specificity for a single antigen (clonotypic immunoglobulin). Because B cells express only κ or λ light chains, the tumor will also express only κ or λ, but not both. Therefore, Bence-Jones protein for a given patient will be of one type or another, but not both.
  2. Although patients will have elevated immunoglobulin levels (usually IgG or IgA), most of the immunoglobulin will be produced by the myeloma cells and will be monospecific. Hence, normal concentrations of polyclonal immunoglobulin will be severely compromised. Pyogenic infections caused by encapsulated bacteria are cleared by humoral immune responses that use antibody-mediated complement activation and antibody-enhanced phagocytosis. An insufficiency of pathogen-specific polyclonal immunoglobulins puts these patients at greater risk of these infections.

 

THE IMMUNE SYSTEM, FOURTH EDITION

CHAPTER 7: THE DEVELOPMENT OF T LYMPHOCYTES

© 2015 Garland Science

 

 

7–1      In which of the following ways does the developmental pathway of α:β T cells differ from that of B cells? (Select all that apply.)

  1. Their antigen receptors are derived from gene rearrangement processes.
  2. When the first chain of the antigen receptor is produced it combines with a surrogate chain.
  3. Cells bearing self-reactive antigen receptors undergo apoptosis.
  4. MHC molecules are required to facilitate progression through the developmental pathway.
  5. T cells do not rearrange their antigen-receptor genes in the bone marrow.

 

7–2      Which of the following cell-surface glycoproteins is characteristic of stem cells, but stops being expressed when a cell has committed to the T-cell developmental pathway?

  1. CD2
  2. CD3
  3. CD25
  4. CD34
  5. MHC class II.

 

7–3      Which of the following processes is not dependent on an interaction involving MHC class I or class II molecules? (Select all that apply.)

  1. positive selection of α:β T cells
  2. intracellular signaling by pre-T-cell receptors
  3. negative selection of αβ T cells
  4. peripheral activation of mature naive T cells
  5. positive selection of γ:δ T cells.

 

7–4      If a double-negative thymocyte has just completed a productive β-chain gene rearrangement, which of the following describes the immediate next step in the development of this thymocyte?

  1. A pre-T-cell receptor is assembled as a superdimer.
  2. Rearrangement of γ- and δ-chain genes commences.
  3. Expression levels of RAG-1 and RAG-2 are elevated.
  4. The linked δ-chain genes are eliminated.
  5. This cell will inevitably differentiate into a committed γ:δ T cell.

 

7–5      All of the following cell-surface glycoproteins are expressed by double-negative thymocytes undergoing maturation in the thymus except _____. (Select all that apply.)

  1. CD2
  2. CD5
  3. CD127 (IL-7 receptor)
  4. CD34
  5. CD1A
  6. CD4.

 

7–6      _____ is a T-cell-specific adhesion molecule expressed before the expression of a functional T-cell receptor while the thymocytes are still in their double-negative stage of development.

  1. CD4
  2. CD8
  3. CD25
  4. CD2
  5. CD3.

 

7–7      Which of the following is mismatched:

  1. double-negative CD3– thymocytes: cortico-medullary junction
  2. double-negative CD3– thymocytes: subcapsular zone
  3. double-positive CD3+ thymocytes: cortico-medullary junction
  4. cortical epithelial cells: subcapsular regions
  5. dendritic cells: cortico-medullary junction.

 

7–8      After interaction with thymic stromal cells, _____, a glycoprotein not expressed by the uncommitted progenitor cell is activated in developing thymocytes. (Select all that apply.)

  1. CD2
  2. CD34
  3. CD5
  4. CD127 (IL-7 receptor)
  5. CD44.

 

7–9      Which of the following statements about Notch 1 is correct? (Select all that apply.)

  1. Notch 1 is expressed on thymic epithelial cells.
  2. In the absence of Notch 1 expression, T cells can complete their differentiation.
  3. Notch 1 is to T-cell development as Pax-5 is to B-cell development.
  4. Notch 1 contains two distinct domains, one of which is proteolytically cleaved and becomes a transcription factor in the nucleus.
  5. The extracellular domain of Notch 1 must interact with a ligand on thymic epithelium to initiate cleavage and separation of the Notch 1 extracellular and intracellular domains.

 

7–10    Which of the following is the first stage of T-cell receptor gene rearrangement in α:β T cells?

  1. Vα→Dα
  2. Dα →Jα
  3. Vβ→ Dβ
  4. Dβ→Jβ
  5. Vα→Jα.

 

7–11    Which of the following is the first T-cell receptor complex containing the β chain to reach the cell surface during the development of T lymphocytes?

  1. γ:β:CD3
  2. β:CD3
  3. α:β:CD3
  4. β:CD44
  5. pTα:β:CD3.

 

7–12    The T-cell receptor β-chain locus can undergo successive gene rearrangements to rescue unproductive V(D)J rearrangements.

  1. What aspects of gene segment rearrangement at the TCRβ locus make this possible?
  2. Can the immunoglobulin heavy-chain locus, which is also composed of V, D, and J segments, undergo successive rearrangements? If not, give the reasons for the difference.

 

7–13    Indicate which of the following statements is true (T) or false (F).

  1. __ Immature T cells failing to successfully recombine a β-chain locus die by apoptosis.
  2. __ Apoptotic T cells are ingested by medullary epithelial cells.
  3. __ Allelic exclusion of the T-cell receptor α and β chains is effective; therefore, all T cells express only one T-cell receptor on the cell surface.
  4. __ T-cell receptor rearrangements have many features in common with immunoglobulin rearrangement, including the use of the RAG-1 and RAG-2 genes.
  5. __ The expression of the pre-T-cell receptor is required in order to halt β-, γ-, and δ-chain rearrangements.

 

7–14    Genetic deficiencies in all of the following would impair the development of a fully functional T-cell repertoire except

  1. RAG-1 or RAG-2
  2. Notch1
  3. Pax-5
  4. IL-7 receptor (CD127)
  5. TAP-1 or TAP-2.

 

7–15

  1. What is Notch1?
  2. Which cells express the ligand of Notch1?
  3. How does the interaction between Notch1 and its ligand mediate T-cell development?

 

7–16    There are many parallels between the development of B cells and T cells. Identify the incorrectly matched counterpart in B cells (left) versus T cells (right).

  1. VpreBλ5: pTα
  2. Igα/Igβ:CD3
  3. Pax-5: FoxP3
  4. multiple κ and λ light-chain gene rearrangements: multiple α-chain gene rearrangements.

 

7–17    _______ of thymocytes is necessary to produce a T-cell repertoire capable of interacting with self-MHC molecules.

  1. positive selection
  2. negative selection
  3. apoptosis
  4. receptor editing
  5. isotype switching.

 

7–18    Which of the following statements are true of a T cell that expresses two α chains (and thus two different T-cell receptors) as a result of ineffective allelic exclusion of the α chain during rearrangement? (Select all that apply.)

  1. Engaging either of the T-cell receptors on MHC molecules of the thymic epithelium will result in positive selection.
  2. One of the T-cell receptors will be functional while the other will most probably be non-functional.
  3. If either T-cell receptor binds strongly to self-peptides presented by self-MHC molecules, the thymocyte will be negatively selected.
  4. One of the T-cell receptors may be autoreactive but escape negative selection because its peptide antigen is present in tissues other than the thymus.
  5. Subsequent gene rearrangements may give rise to a γ:δ T-cell receptor.

 

7–19    Once a thymocyte has productively rearranged a β-chain gene, which of these event(s) can occur subsequently? (Select all that apply.)

  1. β binds to pTα and is expressed on the cell surface with the CD3 complex and ζ chain.
  2. Rearrangement of β-, γ-, and δ-chain genes ceases as a result of the suppression of expression of RAG-1 and RAG-2.
  3. The pre-T cell proliferates and produces a clone of cells all expressing an identical β chain.
  4. Expression of CD34 and CD2 gives rise to double-positive thymocytes.
  5. α-, γ-, and δ-chain loci rearrange simultaneously.

 

7–20    Which of the following statements regarding positive selection is correct?

  1. All subsets of developing T cells undergo positive selection before export to the peripheral circulation.
  2. T-cell receptor editing is linked to the process of positive selection.
  3. Positive selection results in the production of T cells bearing T-cell receptors that have the capacity to interact with all allotypes of MHC class I and class II molecules, and not just those of the individual.
  4. Positive selection ensures that autoreactive T cells are rendered non-responsive.
  5. If there is a genetic defect in AIRE, then T-cell development is arrested as positive selection commences.

 

7–21    Thymocytes that are not positively selected

  1. undergo genetic reprogramming and differentiate into a different cell type
  2. are exported to the periphery, where they are phagocytosed by macrophages
  3. make up about 98% of developing thymocytes and die by apoptosis in the thymic cortex
  4. are eliminated because of their reactivity with self antigens
  5. try out different β chains to acquire reactivity with self-MHC molecules.

 

7–22    If the process of positive selection did not occur, then

  1. a condition resembling immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) would develop
  2. a condition resembling autoimmune polyendocrinopathy–candidiasis–ectodermal dystrophy (APECED) would develop
  3. naive T cells would be unable to undergo differentiation in secondary lymphoid tissues
  4. malignant transformation would be more likely because of the accumulation of multiple mutations
  5. only a very small percentage of circulating T lymphocytes would be able to become activated.

 

7–23    Immediately after positive selection

  1. the thymocyte reaches maturity and is exported to the periphery
  2. RAG proteins are degraded and are no longer synthesized
  3. receptor editing commences to eliminate reactivity against self antigens
  4. the developing thymocyte acquires a double-negative phenotype
  5. expression of pTα is repressed.

 

7–24    Allelic exclusion occurs for all of the following except

  1. T-cell receptor α genes
  2. T-cell receptor β genes
  3. B-cell receptor heavy-chain genes
  4. B-cell receptor κ-chain genes
  5. B-cell receptor λ-chain genes.

 

7–25

  1. Explain two ways in which the expression and processing of self antigens in thymic epithelium differs from the expression and processing of self antigens outside the thymus.
  2. In what way is the thymic situation advantageous for the purposes of negative selection?

 

7–26    Autoimmune polyendocrinopathy–candidiasis–ectodermal dystrophy (APECED) is caused by a defect in

  1. cathepsin L
  2. a transcription factor that regulates tissue-specific gene expression in the thymus
  3. the production of regulatory CD4 T cells
  4. FoxP3
  5. T-cell receptor gene rearrangement.

 

7–27    Identify which of the following describes how antigen processing and presentation of self antigens by thymic epithelial cells differs from that of antigen-presenting cells in peripheral tissues. (Select all that apply.)

  1. Thymic epithelium expresses MHC class I molecules but not MHC class II molecules.
  2. Thymic epithelium uses cathepsin L for proteolytic degradation of self proteins.
  3. Thymic epithelium expresses MHC class II molecules but not MHC class I molecules.
  4. Thymic epithelium uses the transcription factor AIRE to activate thymic expression of tissue-specific genes.
  5. Thymic epithelium expresses transcription repressor protein FoxP3.

 

7–28    Match the immunodeficiency in Column A with its corresponding cause or consequence in Column B.

 

Column A Column B
___a.   IL-7 receptor deficiency 1.         absence of functional AIRE
___b.   DiGeorge syndrome

 

2.         absence of functional MHC class I or MHC class II molecules
___c.   IPEX 3.         absence of T cells because of signaling defects by thymic stromal ells
___d.   Bare lymphocyte syndrome 4.         absence of functional FoxP3
___e.   APECED

 

5.         absence of T cells due to absence of thymus

 

 

7–29    All of the following types of protein are processed and presented by macrophages in the thymus except _____ proteins.

  1. tissue-specific
  2. soluble proteins from extracellular fluids
  3. ubiquitous proteins
  4. proteins made by macrophages
  5. proteins derived from other cells that macrophages phagocytose.

 

7–30    Healthy individuals have approximately ____ of CD4 T cells compared with CD8 T cells.

  1. one quarter the number
  2. half the number
  3. equal numbers
  4. twice the number
  5. four times the number

 

7–31    The surrogate light chain operating during pre-B-cell development is made up of VpreB:λ. Its expression with μ on the pre-B-cell surface is an important checkpoint in B-cell maturation. Name the T-cell analog of VpreB:λ5 and discuss how it is functionally similar.

 

7–32    Double-negative thymocytes initiate rearrangement at the _____ locus (loci) before all other T-cell receptor genes.

  1. γ and δ
  2. β
  3. α and β
  4. α, γ, and δ
  5. β, γ, and δ.

 

7–33    The function of negative selection of thymocytes in the thymus is to eliminate

  1. single-positive thymocytes
  2. double-positive thymocytes
  3. alloreactive thymocytes
  4. autoreactive thymocytes
  5. apoptotic thymocytes.

 

7–34    In T cells, allelic exclusion of the α-chain locus is relatively ineffective, resulting in the production of some T cells with two T-cell receptors of differing antigen specificity on their cell surface.

  1. Will both these receptors have to pass positive selection for the cell to survive? Explain your answer.
  2. Will both receptors have to pass negative selection for the cell to survive? Explain your answer.
  3. Is there a potential problem in having T cells with dual specificity surviving these selection processes and being exported to the periphery?

 

7–35    Mature B cells undergo somatic hypermutation after activation, which, after affinity maturation, results in the production of antibody with a higher affinity for antigen than in the primary antibody response. Suggest some reasons why T cells have not evolved the same capacity.

 

7–36    MHC class II deficiency is inherited as an autosomal recessive trait and involves a defect in the coordination of transcription factors involved in regulating the expression of all MHC class II genes (HLA-DP, HLA-DQ, and HLA-DR).

  1. What is the effect of MHC class II deficiency?
  2. Explain why hypogammaglobulinemia is associated with this deficiency.

 

7–37    As we age, our thymus shrinks, or atrophies, by a process called involution, yet T-cell immunity is still functional in old age.

  1. Explain how T-cell numbers in the periphery remain constant in the absence of continual replenishment from the thymus.
  2. How does this differ from the maintenance of the B-cell repertoire?

 

7–38

  1. What is the role of regulatory CD4 T cells (Treg)?
  2. How can Treg be distinguished from other non-regulatory CD4 T cells?

 

7–39    Which of the following statements is correct?

  1. In adults the mature T-cell repertoire is self-renewing and long-lived and does not require a thymus for the provision of new T cells.
  2. T cells and B cells are both short-lived cells and require continual replenishment from primary lymphoid organs.
  3. The human thymus is not fully functional until age 30, at which time it begins to shrink and atrophy.
  4. In DiGeorge syndrome the bone marrow takes over the function of the thymus and produces mature peripheral T cells.
  5. None of the above statements is correct.

 

7–40    Individuals with a defective autoimmune regulator gene (AIRE) exhibit

  1. DiGeorge syndrome
  2. autoimmune polyendocrinopathy–candidiasis–ectodermal dystrophy (APECED)
  3. severe combined immunodeficiency (SCID)
  4. MHC class I deficiency
  5. MHC class II deficiency.

 

7–41    Giulia McGettigan was born full term with a malformed jaw, cleft palate, a ventricular septal defect, and hypocalcemia. Within 48 hours of birth she developed muscle tetany, convulsions, tachypnea, and a systolic murmur. A chest X-ray showed an enlarged heart and the absence of a thymic shadow. Blood tests showed severely depleted levels of CD4 and CD8 T cells; B-cell numbers were low but within normal range. Parathyroid hormone was undetectable. Fluorescence in situ hybridization of the buccal mucosa revealed a small deletion in the long arm of chromosome 22. Giulia failed to thrive and battled chronic diarrhea and opportunistic infections, including oral candidiasis and Pneumocystis jirovecii, the latter infection causing her death. Giulia most probably had which of the following immunodeficiency diseases?

  1. AIDS
  2. DiGeorge syndrome
  3. bare lymphocyte syndrome
  4. chronic granulomatous disease
  5. hyper IgM syndrome.

 

7–42    The human thymus begins to degenerate as early as one year after birth. This process is called ______ and is marked by the accumulation of ___ once occupied by thymocytes.

  1. thymectomy; dendritic cells
  2. involution; fat
  3. differentiation; γ:δ T cells
  4. negative selection; γ:δ T cells
  5. involution; thymic stroma.

 

7–43

  1. What is immunological tolerance?
  2. What is the general name for the antigens against which the immune system is normally tolerant?

 

 

ANSWERS

7–1      d, e

 

7–2      d

 

7–3      b, e

 

7–4      a

 

7–5      d, f

 

7–6      d

 

7–7      a

 

7–8      a, c, d

 

7–9      c, d, e

 

7–10    d

 

7–11    e

 

7–12

  1. Successive gene rearrangement is possible at a TCRβ locus because there are two sets of D, J, and C gene segments downstream of the cluster of V gene segments: (Vβ)n…Dβ1…(Jβ1)n…Cβ1…Dβ2…(Jβ2)n…Cβ2. If a first rearrangement involving Dβ1 and a Jβ1 segment is unproductive, an upstream V gene segment can rearrange to the second D gene segment and an associated J segment. If this is unproductive, no more rearrangements can be made.
  2. The answer is no. The heavy-chain locus has the following configuration: (V)n-heptamer…23 spacer…nonamer…nonamer…12 spacer… heptamer-(D)n-heptamer…12 spacer…nonamer…nonamer… 23 spacer…heptamer (J)n…Cμ. After the first DJ rearrangement, the intervening D segments between the chosen D and J will be deleted. After VDJ rearrangement, the D segments that lie between the chosen V and DJ will be deleted. Therefore, no unrearranged D segments remain after these two rearrangement events. V and J cannot rearrange directly because the recombination signal sequences are not paired appropriately and do not follow the 23/12 rule; rather, they both contain 23-bp spacers in their recombination signal sequences. Successive gene rearrangement is thus not possible at a heavy-chain locus.

 

7–13    a—F; b—F; c—F; d—T; e—T

 

7–14    c

 

7–15

  1. Notch1 is a membrane-bound receptor found on thymocytes that participates in the regulation of early T-cell development.
  2. Its ligand (Notch ligand) is a membrane-bound protein on the surface of thymic epithelial cells.
  3. After binding of the extracellular domain of Notch1 to the extracellular portion of Notch ligand, the intracellular domain of Notch1 is released by proteolysis and subsequently translocates to the nucleus. In the thymocyte nucleus, this domain forms a transcription factor complex that displaces repressor proteins from genes involved in T-cell development and initiates transcription of these genes by recruiting transcription activator proteins.

 

7–16    c

 

7–17    a

 

7–18    a, b, c, d

 

7–19    a, b, c, e

 

7–20    b

 

7–21    c

 

7–22    e

 

7–23    b

 

7–24    a

 

7–25

  1. (i) As well as expressing their own thymus-specific self antigens, medullary epithelial cells in the thymus produce a transcription factor called autoimmune regulator (AIRE), which causes several hundred genes normally expressed in other tissues to be expressed in these cells. The proteins can then be processed to form self peptides that will be presented by MHC class I molecules. (ii) The thymic epithelium uses different proteases for self-protein degradation; cathepsin L is used for peptide production instead of cathepsin S, which is used by other cell types.
  2. Generating a more comprehensive repertoire of self peptides in the thymus increases the types of potentially autoreactive T cell that are removed from the peripheral T-cell repertoire during negative selection.

 

7–26    b

 

7–27    b, d

 

7–28    a—3; b—5; c—4; d—2; e—1

 

7–29    a

 

7–30    d

 

7–31    The analog of VpreB:λ5 in developing T cells is the protein preTα(pTα), which combines with the T-cell receptor β chain, the first of the two T-cell receptor chains to be expressed, to form the pre-T-cell receptor. The β chain, like the immunoglobulin heavy chain, contains V, D, and J segments. pTα also binds CD3 and ζ components to this complex, and the assembly of the complete complex induces T-cell proliferation and the cessation of rearrangement at the TCRβ loci (leading to allelic exclusion). Formation of the analogous pre-B-cell receptor complex of VpreB:λ5 and heavy chain with Igα and Igβ in B cells similarly prevents further rearrangement of the heavy-chain loci.

 

7–32    e

 

7–33    d

 

7–34

  1. Only one of the receptors will have to be positively selected for the cell to get the survival signals necessary for it to pass on to the next stage. Even if the other receptor does not react with self MHC this will have no effect on the cell.
  2. In contrast, both receptors will have to pass the negative selection test for the T cell to survive, because if only one of them fails it, the cell will die.
  3. Yes. Imagine this situation. The T cell with dual specificity could be activated appropriately during a genuine infection by a professional antigen-presenting cell plus foreign antigen 1 using T-cell receptor 1. But that same T cell, because it is now an activated effector T cell, would also be able to respond to a second peptide, which might be a self peptide, using T-cell receptor 2, without requiring the co-stimulatory signals that only professional antigen-presenting cells deliver. Thus it could cause a reaction against a self tissue, either directly, if it is a CD8 cytotoxic T cell, or indirectly, if it is a CD4 T cell, by activating potentially autoreactive B cells.

Furthermore, interferon-γ produced in the response against foreign antigen 1 could activate nonprofessional antigen-presenting cells nearby, inducing the expression of MHC class II with presentation of the self peptide above. Effector T cells with T-cell receptor 2 could make an autoimmune response against it.

 

7–35    Because T cells drive almost all immune responses, once they have been activated their receptors must continue to recognize the exact complex of foreign antigen and MHC molecule (which does not itself change) that activated them. Because of this requirement for dual recognition (MHC restriction), somatic hypermutation would be more likely than not to change the T-cell receptor to make it unable to recognize either the peptide or the MHC molecule, or the combination of both, thus rendering it unable to give help to B cells or to attack infected cells. This would destroy both the primary immune response and the development of immunity. Even changes that simply increased the affinity of the T cell for its antigen would have no real advantage, because it would not make the immune response any stronger or improve immunological memory in the same way that affinity maturation of B cells does. Also, if somatic hypermutation changed the specificity of the T-cell receptor so that it now recognized a self peptide, this could result in an autoimmune reaction. These considerations do not apply to B cells, because they require T-cell help to produce antibody and will only receive it if their B-cell receptor still recognizes the original antigen.

 

7–36

  1. MHC class II deficiency affects the development of CD4 T cells in the thymus. If the thymic epithelium lacks MHC class II, positive selection of CD4 T cells will not take place. CD8 T cells are not affected because MHC class I expression is unaffected by this defect.
  2. To produce antibody, B cells require T-cell help in the form of cytokines produced by CD4 TH2 cells. Low immunoglobulin levels (hypogammaglobulinemia) in MHC class II deficiency are attributed to the inability of B cells to proliferate and differentiate into plasma cells in the absence of TH2 cytokines.

 

7–37

  1. After thymic atrophy or thymectomy, T cells in the periphery self-renew by cell division and are long lived.
  2. B cells are short lived and replenish from immature precursors derived from the bone marrow.

 

7–38

  1. Treg suppress the proliferation of naive autoreactive CD4 T cells by secreting inhibitory cytokines. This inhibitory action requires that both the Treg and the other CD4 T cell are interacting with the same antigen-presenting cell.
  2. Unlike non-regulatory CD4 T cells, Treg express CD25 on the cell surface and the FoxP3 transcriptional repressor protein.

 

7–39    a

 

7–40    b

 

7–41    Rationale: The correct answer is b. Depletion of T cells, but not of B cells, and the absence of a thymic shadow on the X-ray are critical clues. Development of both CD8 and CD4 T cells is affected in this patient because the thymus is the primary lymphoid organ required for T-cell development. Bare lymphocyte syndrome affects either CD8 (type I) or CD4 (type II), but not both. Although patients without a thymus succumb to infections also common in individuals with AIDS, this option can be ruled out because Giulia lacks CD8 T cells, a condition not seen in AIDS patients. Chronic granulomatous disease is a defect of neutrophil, not T-cell, function.

 

7–42    b

 

7–43

  1. Immunological tolerance is the mechanism that operates to ensure that lymphocytes do not contain antigen receptors specific for host components. This is achieved through a process involving the removal of self-reactive T and B cells called negative selection. Both T and B cells are removed by apoptosis after the engagement of T-cell receptors and immunoglobulins, respectively, if the interaction with their ligands is especially strong. The consequence is the removal of autoreactive lymphocytes that could cause damage to healthy, uninfected cells and tissues.
  2. Self antigens.