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Basic Immunology Functions And Disorders of the Immune System 4th Edition by Abul K. Abbas – Test Bank
Abbas: Basic Immunology, 4th Edition
Chapter 03: Antigen Capture and Presentation to Lymphocytes
- Antigen-presenting cells (APCs) perform which of the following functions in adaptive immune responses?
- Display major histocompatibility complex (MHC)-associated peptides on their cell surfaces for surveillance by B lymphocytes
- Initiate T cell responses by specifically recognizing and responding to foreign protein antigens
- Display MHC-associated peptides on their cell surfaces for surveillance by T lymphocytes
- Display polysaccharide antigens on their cell surfaces for surveillance by B lymphocytes
- Secrete peptides derived from protein antigens for binding to T cell antigen receptors
Antigen-presenting cells (APCs) degrade proteins derived from either the extracellular environment or the cytoplasm. They form complexes of peptide fragments of these proteins with major histocompatability complex (MHC) molecules and display these complexes on their cell surfaces, where T cells can “see” them. Neither processing nor MHC association of protein or polysaccharide antigens by B cells is required for recognition. APCs do not distinguish between self and foreign proteins and will display peptides derived from a sampling of all cytoplasmic and extracellular proteins. APCs do not secrete peptide antigens, and T cell antigen receptors do not bind free peptides.
- A child who suffers from a persistent viral infection is found to have a deficiency in lymphocyte production and very few T and B cells. Other bone marrow–derived cells are produced in normal numbers, and MHC molecule expression on cells appears normal. Transfusion of mature T cells from an unrelated donor who had recovered from a previous infection by the same virus would not be expected to help the child clear his infection. Which one of the following is a reasonable explanation for why this therapeutic approach would fail?
- Viral infections are cleared by antibodies, not T cells.
- The patient’s own immune system would destroy the transfused T cells before they could respond to the viral infection.
- T cells recognize peptides, not viral particles.
- Donor T cell viral antigen recognition is restricted by MHC molecules not expressed in the patient.
- In responding to the previous infection, the donor would have used up all his T cells specific for that virus.
T cells are “self MHC restricted,” meaning they specifically recognize infected cells that display microbial peptides displayed by self MHC molecules. There may be no MHC molecules shared by donor and patient, and therefore the transfused T cells would not recognize virus-infected cells in the patient. Because the patient has very few B cells and T cells, his immune system is unlikely to be able to recognize and destroy (i.e., “reject”) the transfused T cells. T cells do not recognize structures on intact viral particles but rather peptides derived from viral proteins bound to MHC molecules. Prior viral infection in the donor would be expected to generate memory T cells specific for the virus.
- Many vaccines now in development will include highly purified, recombinant, or synthetic peptide antigens. These vaccine antigens are expected to stimulate highly specific immune responses, but they are less immunogenic than vaccines containing intact killed or live microbes. Adjuvants are substances added to such vaccines to enhance their ability to elicit T cell immune responses. Which of the following statements about adjuvants is NOT correct?
- Adjuvants induce local inflammation, thereby increasing the number of antigen-presenting cells (APCs) at the site of immunization.
- Adjuvants stimulate the expression of costimulators on local APCs.
- Adjuvants enhance local production of cytokines that promote T cell activation.
- Adjuvants prolong the expression of peptide-MHC complexes on the surface of APCs.
- Adjuvants bind to T cell antigen receptors and promote their proliferation.
Adjuvants are not necessarily T cell antigens. Some adjuvants may be T cell antigens, but their adjuvant activity is unrelated to their ability to be recognized, in peptide form, by T cells. Adjuvants are surrogates of the innate immune response to a microbe, required along with antigen component of a vaccine for naive T cell activation. Adjuvants stimulate local inflammation, influx of antigen-presenting cells (APCs), and activation of APCs to secrete cytokines and express costimulatory molecules, and they prolong peptide-MHC expression on the APC membrane.
- A helper T cell response to a protein antigen requires the participation of antigen-presenting cells that express which of the following types of molecules?
- Class II MHC and costimulators
- Class I MHC and CD4
- Class II MHC and CD8
- CD4 and costimulators
- Class II MHC and CD4
Helper T cells are almost always CD4+. The activation of naive CD4+ T cells requires T cell receptor recognition of class II MHC-peptide complexes and the binding of costimulators, both on the antigen-presenting cell (APC) surface. CD4+ helper T cells bind to class II MHC molecules on the APC, not to class I MHC molecules. CD4 or CD8 expression on the APC surface is of no known relevance to T cell activation.
- Which type of antigen-presenting cell is most important for activating naive T cells?
- Dendritic cell
- Endothelial cell
- B lymphocyte
- Epithelial cell
Dendritic cells are the key type of antigen-presenting cell (APC) for activation of naive T cells and initiation of T cell immune responses. Macrophages and B lymphocytes function as APCs for already differentiated effector T cells in cell-mediated and humoral immune responses, respectively. Epithelial cells usually do not function as APCs.
- Which of the following statements about the antigen-presenting function of macrophages is NOT correct?
- Macrophages are particularly important at presenting peptides derived from particulate or opsonized antigens that are internalized by phagocytosis.
- Macrophages become activated by the helper T cells to which they present microbial peptides, and as a result of this activation they become efficient at killing the microbes.
- Resting macrophages express low levels of class II MHC molecules, but higher class II MHC expression is induced on activation by the T cells to which they present antigen.
- Macrophages express highly variable, high-affinity receptors for many different antigens, and these receptors facilitate the internalization of the antigens for processing and presentation.
- Macrophages present antigen to T cells in lymphoid organs and many nonlymphoid organs.
The description of high-affinity and highly variable receptors for antigen applies to B cells, which can present antigen to helper T cells, but does not apply to macrophages. Macrophages express receptors for the Fc region of Ig molecules, and these receptors do facilitate internalization of antibody-opsonized antigens. These Fc receptors are not highly variable and do not recognize the antigen. Macrophages are also highly competent at internalizing intact microbes and other large particulate antigens through phagocytosis. Macrophage class II MHC expression and microbicidal activity are enhanced by signals from the T cells to which they present antigen, including cytokines and CD40 ligand. Macrophages are abundant in spleen, lymph nodes, and most nonlymphoid tissues. They may perform antigen-presenting functions in all these locations.
- Which one of the following statements about dendritic cells is true?
- Immature dendritic cells are ubiquitously present in skin and mucosal tissues.
- Dendritic cell maturation occurs after migration to lymph nodes in response to signals derived from activated T cells.
- Class II MHC and T cell costimulators are highly expressed on immature dendritic cells and are down-regulated during maturation.
- Dendritic cells that enter lymph nodes through draining lymphatics migrate to the B cell–rich follicles in response to chemokines.
- The principal function of mature dendritic cells is antigen capture.
Tissues that are barriers between the external environment and the inside of the body, such as skin and mucosa, are rich in resting dendritic cells. In this location, the dendritic cells are well positioned to internalize samples of the environment and respond to innate immune system signals, which will drive their maturation into competent antigen-presenting cells. Dendritic cell maturation occurs during migration from infected tissues via lymphatics to the T cell zones of draining lymph nodes. Maturation must occur before, and is required for, activation of T cells, not vice versa. Class II MHC molecule up-regulation occurs during dendritic cell maturation and is one of the changes that make mature dendritic cells better able to present antigen to CD4+ T.
- Maturing dendritic cells that migrate to a lymph node from peripheral tissues end up mainly in:
- High endothelial venules
- The medullary sinus
- T cell zones
- Efferent arterioles
Migrating dendritic cells express the chemokine receptor CCR7 and move into the T cell zones, where SLC and ELC, the chemokines that bind CCR7, are expressed. In this location, the dendritic cells they are most likely to interact with are naive T cells that also migrate to the same area.
- A young adult is exposed to a virus that infects and replicates in mucosal epithelial cells of the upper respiratory tract. One component of the protective immune response to this viral infection is mediated by CD8+cytolytic T lymphocytes (CTLs), which recognize and kill virus-infected cells. The CTLs can recognize and kill the infected cells because:
- In response to interferon-g secreted during the innate immune response to the virus, the mucosal epithelial cells express class II MHC, with bound viral peptides, on their cell surfaces.
- Mucosal epithelial cells, like all nucleated cells, express class I MHC molecules and are able to process cytoplasmic viral proteins and display complexes of class I MHC and bound viral peptides on their cell surfaces.
- Antibodies specific for viral antigens bind to these antigens on infected cell surfaces and engage Ig Fc receptors on the CTL, thereby targeting the CTL to the infected cells.
- Virus-infected mucosal epithelial cells migrate to draining lymphoid tissues, where they present viral peptide antigens to naive CD8+T cells.
- Viral infection of the mucosal epithelial cells stimulates them to express E-selectin, which promotes CD8+T cell adhesion.
Differentiated CD8+ cytolytic T lymphocytes (CTLs) can recognize class I–associated viral peptides on epithelial cells, as well as most other cell types, and become activated to kill those cells. Interferon-g may be secreted during the innate immune response to a virus, and this cytokine can up-regulate both class I and class II MHC expression of various cell types, but CD8+ T cells do not recognize class II–associated peptides. Antibodies may form a bridge between Fc receptor-bearing natural killer cells and infected cells expressing viral antigens on their surface, but this phenomenon does not apply to CD8+ CTLs. Mucosal epithelial cells do not migrate to draining lymph nodes in response to viral infection, nor do they express E-selectin, which is an endothelium-specific adhesion molecule.
- Naive CD8+T cells require signals in addition to T cell receptor recognition of peptide-MHC to become activated and differentiate into cytolytic T cells. These signals are called costimulatory signals and are provided by professional antigen-presenting cells (APCs), such as dendritic cells. If a virus infects epithelial cells in the respiratory tract but does not infect professional APCs, what process ensures that naive T cells specific for viral antigens will become activated?
- Cross-reactivity, whereby the naive CD8+T cell recognizes a self antigen that is structurally similar to a viral antigen presented by dendritic cells
- Crossover, whereby part of the viral genome is exchanged with part of one chromosome of the host
- Crosstalk, whereby signals generated by the virus binding to class I MHC molecules intersect with T cell receptor signaling pathways
- Cross-presentation, whereby infected epithelial cells are captured by dendritic cells, and the viral proteins originally synthesized in the epithelial cells are processed and presented in association with class I MHC molecules on the dendritic cell
- Cross-dressing, whereby viral infection of the epithelial cell stimulates the expression of surface molecules that are typically found only on dendritic cells
Cross-presentation (or cross-priming) is the phenomenon by which a protein antigen made within one cell is processed and presented by the class I MHC pathway of a separate professional antigen-presenting cell (APC). Cross-presentation requires that the protein antigen from one cell be internalized from the extracellular milieu into the APC to gain access to the cytoplasm of the APC. Crossover and crosstalk are terms referring to genetic and signaling phenomena, which are not accurately described in the question. Cross-dressing is not a term used in immunology.
- Which of the following is the main criterion that determines whether a protein is processed and presented via the class I MHC pathway in an antigen-presenting cell (APC)?
- Encoded by a viral gene
- Present in an acidic vesicular compartment of the APC
- Present in the cytosol of the APC
- Internalized into the cell from the extracellular space
- Small in size
Regardless of the source of the protein, its presence in the cytosol makes it accessible to the tagging and proteolytic processing mechanisms that initiate the class I MHC antigen presentation pathway. Microbial proteins and self proteins have equal access to this pathway if they are present in the cytosol. Presence in acidic vesicles is the comparable major criterion for inclusion in the class II MHC pathway; such proteins are usually, but not always, internalized from the extracellular space. The size of an intact protein is not relevant to which processing and presentation pathway it will enter.
- Which one of the following molecules does NOT play an important role in the class II MHC pathway of antigen presentation?
- Invariant chain
b2-Microglobulin is one of the polypeptide chains of a class I MHC molecule and is required for assembly of the peptide-class I MHC complex. All the other molecules listed are involved in the class II MHC of antigen presentation. Cathepsins are acid proteases that degrade proteins in acidic vesicles in the class II MHC pathway. The invariant chain directs appropriate sorting of new class II MHC molecules from the Golgi to endosomes, and it protects the class II MHC peptide binding groove from occupancy by peptides until the class II MHC molecules are delivered to the endosome. Calnexin is an endoplasmic reticulum chaperone involved in the assembly of both class I and class II molecules.
- In the class I MHC pathway of antigen presentation, peptides generated in the cytosol are translocated into the endoplasmic reticulum in which of the following ways?
- By ATP-dependent transport via the transporter associated with antigen-processing (TAP) 1/2 pump
- By passive diffusion
- By receptor-mediated endocytosis
- Through membrane pores
- Via the proteasome
The TAP1/TAP2 heterodimer is an ATP-dependent pump that delivers peptides generated by the proteasome into the endoplasmic reticulum.
- In the class I MHC pathway of antigen presentation, cytoplasmic proteins are tagged for proteolytic degradation by covalent linkage with which of the following molecules?
- Nuclear factor (NF)-kB
In the class I pathway, proteins are tagged for proteasomal degradation by covalent addition of several copies of the polypeptide ubiquitin. Ubiquitin-dependent proteasomal degradation is also important in many other cellular processes besides antigen presentation. For example, NF-kB is a transcription factor whose activation is dependent on ubiquitination and proteasomal degradation of an inhibitor (called IkB). Calreticulin, tapasin, and calnexin regulate the assembly of class I MHC proteins within the endoplasmic reticulum.
- Which one of the following statements about T cell tolerance to self proteins is accurate?
- Self proteins are not presented by the class I pathway because only microbial proteins, and not self proteins, are ubiquinated in the cytosol.
- Peptides derived from self proteins are not presented by the class I or class II pathways because MHC molecules are expressed only in response to infections.
- Self proteins are not presented by the class II pathway because endosomal acidic proteases digest microbial proteins but not eukaryotic proteins.
- Self peptide/self MHC complexes are formed and displayed by antigen-presenting cells in both class I and class II MHC pathways, but T cells that recognize these complexes usually are not present or are functionally inactive.
- Peptides derived from self proteins are not displayed by MHC molecules because they usually are displaced by the more abundant microbial peptides.
T cell tolerance is a result of deletion or inactivation of self-reactive T cells. The various steps in both the class I and the class II MHC antigen-presenting pathways do not discriminate between self and microbial proteins. Although expression of MHC molecules is up-regulated as a result of the innate immune responses to infections, there is some degree of constitutive expression of class II MHC on professional antigen-presenting cells, and class I MHC is constitutively expressed on most cells in the body. Only a small fraction of the surface MHC molecules of an infected cell will express peptides derived from microbial peptides.
- In a clinical trial of a new antiviral vaccine composed of a recombinant viral peptide and adjuvant, 4% of the healthy recipients did not show evidence of response to the immunization. Further investigation revealed that all the nonresponders expressed the same, single allelic variant of HLA-DR but all the responders were heterozygous for HLA-DR alleles. Which of the following is the most likely explanation for this finding?
- Response to the vaccine requires T cell recognition of complexes of the viral peptide with HLA-DR, but the peptide cannot bind to the allelic variant of HLA-DR found in the nonresponders.
- The nonresponders could not express class II MHC proteins.
- The viral peptide is not an immunodominant epitope.
- The nonresponders underwent determinant selection of another viral epitope.
- Because of technical errors, the nonresponders had not received adequate doses of the vaccine.
The response to a viral protein (or peptide) requires T cell recognition of the peptide bound to an MHC molecule. Although the viral peptide in the vaccine may bind to many different MHC alleles, it likely will not bind to all. The nonresponders express an allelic variant of HLA-DR, which is a class II MHC molecule. Because the peptide evoked a response in 96% of the people in the trial, it can be considered a dominant epitope. Formally, this can be concluded only when the whole protein is the immunogen and the specificities of the responses for different epitopes are compared. Determinant selection is an older term that predates our knowledge of peptide-MHC binding, but it does not mean active selection for one versus another epitope. It is highly unlikely that the only people in the trial who were not adequately immunized for technical reasons happen to be the only ones homozygous for a particular MHC allele.
- The required number of complexes of a microbial peptide and a particular class II MHC allele on the surface of an antigen-presenting cell to initiate a T cell response specific for the viral peptide is:
- At least equal to the number of complexes of self peptides with class II MHC on the cell surface
- Greater than 103
- Less than or equal to 0.1% of the total number of class II MHC molecules on the cell surface
- Greater than 106
As few as 100 complexes of a particular peptide and a particular class II MHC molecule are needed to activate naive T cells specific for that complex and thereby initiate a detectable T cell response. This represents less than 0.1% of the total class II MHC molecules on a typical antigen-presenting cell surface.
Abbas: Basic Immunology, 4th Edition
Chapter 05: T Cell–Mediated Immunity
- All of the following protein-protein interactions are involved in activation of naive helper T cells by antigen-presenting cells (APCs) EXCEPT:
- Binding of peptide-MHC complexes on the APC to the TCR on the T cell
- Binding of CD4 on the T cell to nonpolymorphic regions of class II MHC molecules on the APC
- Binding of integrins on the T cell with adhesion ligands on the APC
- Binding of B7-2 on the APC with CD28 on the T cell
- Binding of CD40L on the T cell with CD40 on the APC
CD40L is not expressed on naive T cells and is only up-regulated subsequent to activation by an antigen-presenting cell (APC). In the naive helper T cell, the TCR binds to the MHC-peptide complex whereas the CD4 coreceptor engages a conserved region on the MHC II molecule. Integrins on the T cell interact with adhesion ligands on the APC. This region of binding between the T cell and the APC is known as the immunologic synapse and also includes costimulatory interactions, such as CD28 on the T cell binding to B7 on the APC.
- Which one of the following statements about MHC-TCR interactions is NOT true?
- Antigen receptors on T cells bind to MHC molecules for only brief periods of time.
- The affinity of most TCRs for peptide-MHC complexes is similar to the affinity of antibodies for their antigens.
- Only 1% or less of the MHC molecules on any antigen-presenting cell (APC) display a peptide recognized by a particular T cell.
- T cells usually require multiple engagements with an APC before a threshold of activation is reached.
- A subthreshold number of MHC-TCR interactions can lead to T cell inactivation.
In general, the TCR binds to peptide-MHC complexes with lower affinity than antigen-antibody interactions. This relatively low-affinity interaction occurs briefly; thus, a T cell may need multiple engagements with the antigen-presenting cell (APC) before a threshold of activation occurs. If this threshold is not reached, the T cell may enter into an inactive state known as anergy. On any given APC, less than 1% of the MHC molecules display the same peptide.
- Which one of the following cell types would be most potent at activating naive T cells?
- Kupffer cells
- B cells
- Follicular dendritic cells
- Langerhans cells
Antigen-presenting cells (APCs) are responsible for presenting peptide-MHC complexes and costimulatory molecules to naive T cells; this leads to activation of the T cells. The most potent APCs are the dendritic cells, because they constitutively express high levels of costimulatory molecules. Langerhans cells are dendritic cells found in epidermis. Other APCs include macrophages and B cells. Kupffer cells are a type of macrophage found in the liver. Neither neutrophils nor follicular dendritic cells (FDCs) are involved in antigen presentation to T cells. FDCs are unrelated to dendritic cells and are found within the germinal centers of lymph nodes.
- Which one of the following descriptions of cytokine interleukin-2 is NOT true?
- Expression of its gene requires multiple transcription factors, such as Fos, Jun, and NFAT.
- It acts as an autocrine growth factor for T cells.
- It binds to CD25 on the cell membrane of T cells.
- It is only involved in the proliferation of helper T cells and not CTLs.
- It promotes susceptibility of T cells to apoptosis.
IL-2 is involved in the proliferation of both CD4+ and CD8+ T cells. Activation of the naive T cell results in signals transduced via the TCR (signal 1) and CD28 (signal 2). This signaling results in the activation of transcription factors, such as Fos, Jun, and NFAT, which increase transcription of the IL-2 gene. IL-2 is then secreted and acts as both a paracrine and autocrine growth factor for T cells by binding to the IL-2 receptor (one component of which is CD25). In addition to its growth factor activity, IL-2 also “primes” T cells for apoptotic death, and this role for IL-2 is important in homeostasis of the immune system.
- Which one of the following statements about T cells involved in an immune response is NOT true?
- Activated T cells receive survival signals from antigen during an infection.
- Activated T cells contribute to the activation of antigen-presenting cells via CD40 ligand.
- Memory T cells generated during a primary immune response express high levels of interleukin-2 receptors and actively proliferate long after the primary response is completed.
- The major effector function of helper T cells is to activate macrophages and other cells by releasing cytokines.
- When an infection is eliminated, activated T cells die by apoptosis.
Memory T cells are not actively proliferating and do not express high levels of IL-2 receptors. Instead, these cells are functionally quiescent and are not performing effector functions after a primary immune response. Effector T cells continue to survive in the periphery via proliferative signals from MHC-antigen binding to the TCR. Effector helper T cells can then activate macrophages and other lymphocytes via release of cytokines such as IFN-g, as well as through CD40 ligand on the cell surface. On elimination of the infection, the effector T cells die by apoptosis.
- Which one of the following statements about the molecules B7-1 and B7-2 is NOT true?
- B7-1 and B7-2 expression on antigen-presenting cells (APCs) is upregulated by the presence of “danger” signals, such as lipopolysaccharide, as well as cytokines, such as interferon (IFN)-g.
- B7-1 and B7-2 are expressed at low levels on some resting APCs.
- Induction of B7-1 usually occurs before the induction of B7-2 in an immune response.
- B7-1 and B7-2 bind to CD28 on T cells and provide “second signals” for naive T cell activation.
- Activated helper T cells can induce expression of B7-1 and B7-2 on APCs via CD40L binding to CD40.
The temporal patterns of B7-1 and B7-2 expression differ. B7-2 is expressed constitutively at low levels and induced early after activation of antigen-presenting cells (APCs), whereas B7-1 is not expressed constitutively and is induced hours or days later. The expression of B7-1 and B7-2 on APCs is induced by “danger signals” of infection. These signals are mediated by binding of lipopolysaccharide (LPS), unmethylated CpG DNA, and other ligands of Toll-like receptors. Signals mediated through cytokines, such as interferon (IFN)-g, as well as through CD40 ligand, can also up-regulate B7-1 and B7-2 expression on APCs. Both B7-1 and B7-2 bind to CD28 on naive T cells, thus providing the second signal needed for activation of T cells.
- In patients with hyper IgM syndrome, there is a genetically based deficiency in expression of CD40 ligand. In addition to defects in antibody isotype switching, these patients have defects in T cell–mediated immune responses and become infected with intracellular parasites. Which one of the following normal functions of CD40 ligand is important in T cell–mediated immunity?
- CD40-dependent isotype switching is required to produce antibody isotypes that activate T cells.
- CD40 ligand is required for CTL killing of CD40-expressing infected cells.
- CD40 ligand is required for maturation of CD4+T cells in the thymus.
- CD40 ligand on activated T cells binds to CD40 on antigen-presenting cells (APCs), and this enhances the expression of B7-1, B7-2, and cytokines by the APCs.
- CD40 ligand on T cells binds to B7-1 and B7-2 on APCs, and this enhances the function of the APCs.
CD40 ligand, a membrane-bound protein in the tumor necrosis factor (TNF) family of proteins, is expressed after T cell activation. When it binds to its receptor CD40, a TNF-receptor family member expressed on macrophages, and other antigen-presenting cells, signals are transmitted that enhance costimulator and cytokine expression (as well as other functions of macrophages). This serves to amplify the T cell response and enhance the killing of microbes ingested by macrophages. Antibodies are not required to activate T cells. CD40 does not transduce pro-apoptotic signals. CD40 ligand is not involved in T cell maturation and does not bind to B7-1 or B7-2.
- All of the following molecules act as transcription factors in T cell activation signaling EXCEPT:
All of these proteins are involved in the activation of T cells. However, Ras is not a transcription factor but a guanosine triphosphate (GTP)-binding protein present in the cytosol and in association with the plasma membrane. On exchange of guanosine diphosphate (GDP) for GTP, the Ras protein becomes functional and acts as an allosteric activator of MAP kinases, which leads the transcription of Fos.
- Which one of the following statements about the molecule Lck is NOT true?
- It is a member of the Src family of kinases.
- It binds to the cytoplasmic tails of T cell coreceptors CD4 or CD8.
- It phosphorylates ITAM motifs on the CD3 complex.
- It phosphorylates tyrosine residues on Zap-70 and activates it.
- It phosphorylates PIP2 to PIP3 and leads to the activation of Itk.
PI-3 kinase is responsible for the phosphorylation of PIP2 to PIP3, leading to the activation of Itk in T cells and Btk in B cells.
- A 7-month-old boy, the only child of second-degree cousins, saw a pediatrician for immunologic evaluation after developing Pneumocystis carinii pneumonia. Serum IgG, IgM, and IgA levels were normal. Blood cell count showed 10,600 leukocytes/mm3and 80% lymphocytes; 90% of the lymphocytes were TCR ab+ CD4+. In vitro lymphocyte-proliferative responses to PHA and anti-CD3 were absent, and the pattern of tyrosine-phosphorylated cytoplasmic proteins after anti-CD3 treatment of the T cells was distinctly abnormal. This boy most likely carries homozygous mutations in the gene encoding which one of the following proteins?
The patient shows signaling defects in TCR-mediated T cell activation, as well as defects in CD8+ T cell maturation. Zap-70 is a tyrosine kinase required for TCR-mediated T cell activation. Mutations in Zap-70 result in impaired TCR signaling, with abnormal tyrosine phosphorylation of downstream signaling molecules, and also a defect in CD8+ T cell maturation. It is not known why CD8+ maturation is selectively impaired in Zap-70 deficiency. VDJ recombination, and therefore RAG-1 function, must still be intact because TCR-expressing CD4+ T cells do mature. CD3, pre-Ta, and TCRa are also required for maturation of CD4+ T cells, and therefore these molecules must all be expressed by this patient.
- Which one of the following signaling molecules, if mutated, would affect B cell maturation and function primarily without affecting T cell function?
- PI-3 kinase
Btk is a Tec family protein tyrosine kinase that is particularly important in pre-B cell receptor complex signaling, and therefore in B cell maturation and activation. Mutations in Btk are responsible for X-linked agammaglobulinemia. Itk and Tec are other members of the Tec family that are important in T cells. PI-3 kinase is a phospholipid kinase involved in signaling in many cell types, including B and T cells, and Zap-70 is a protein tyrosine kinase particularly important in TCR signaling in T cells.
- All of the following are early T cell events that occur after antigen recognition by the TCR EXCEPT:
- Formation of the immunologic synapse
- Recruitment of signaling molecules, such as LAT, to glycolipid-enriched domains known as lipid rafts
- Enhanced adhesion between T cells and antigen-presenting cells (APCs) via T cell integrin LFA-1 and its ligand on the APC, ICAM-1, at the central zone of the immunologic synapse
- Clustering of the TCR and coreceptors leading to phosphorylation of ITAMs on CD3 by Lck
- Binding of CD28 with costimulators on APCs in the cSMAC, resulting in signal transduction activation
On binding of the TCR complex with MHC-associated peptides on an antigen-presenting cell (APC), several T cell surface proteins and intracellular signaling molecules are rapidly mobilized to the site of contact, known as the immunologic synapse. Molecules that are recruited to the central supramolecular activation cluster, or center of the synapse, include the TCR complex (TCR, CD3, and z chains), CD4 or CD8 coreceptors, and costimulatory molecules (CD28). The clustering of signaling molecules results in the phosphorylation of ITAMs on CD3 by CD4- or CD8-associated Lck. Integrins remain at the peripheral zone of the synapse and stabilize the binding of the T cell to the APC. LAT is a transmembrane adaptor molecule recruited to the synapse whose cytoplasmic tail forms part of a scaffold of signaling molecules.
- An experiment is performed in which a point mutation is introduced randomly into the Zap-70 gene for a particular strain of mice. The mutant mice display a defect in T cell development. However, precursor T cells isolated from the thymus of these mice show normal expression levels of Zap-70 of the correct molecular weight. On further in vitro analysis, the mutant Zap-70 is found to bind to ITAM motifs in the cytoplasmic tail of the z chain, but only when the z chain is phosphorylated. No phosphorylated LAT is detected, however. Given these data, in which of the following protein domains is the mutation most likely to be present?
- Pleckstrin homology (PH) domain
- Proline-rich (PR) domain
- SH1 domain
- SH2 domain
- SH3 domain
In this experiment, the mutant Zap-70 can still bind to the phosphorylated z chains but cannot phosphorylate LAT. This suggests that the SH2 domains are normal but that the SH1 kinase domain has been mutated. Zap-70 does not contain a PH, PR, or SH3 domain. The PH domain allows proteins to localize to the membrane by binding to PIP3. PR domains mediate protein-protein interactions via binding to SH3 domains.
- Which one of the following accurately depicts the correct order of events in a TCR signal transduction pathway?
- TCR → Lck → Zap-70 → LAT → Grb-2 → SOS → Ras → Erk → Fos
- TCR → Lck → Zap-70 → LAT → SOS → Grb-2 → Ras → Erk → Fos
- TCR → Lck → ITK → LAT → Grb-2 → SOS → Ras → Erk → Fos
- TCR → Lck → Zap-70 → LAT → SOS → Grb-2 → Ras → Erk → Jun
- TCR → Lck → Zap-70 → LAT → PLCg → DAG → calcium release
Activated Zap-70 phosphorylates the transmembrane adapter protein LAT at tyrosine residues, which serve as docking sites for SH2 domains of other proteins. In one pathway, the SH2 domain of Grb-2 binds to phosphorylated LAT. Grb-2 is then able to recruit Sos to the membrane. Sos catalyzes GDP/GTP exchange on Ras, a G protein that is active when guanosine triphosphate (GTP) is bound and inactive when guanosine diphosphate (GDP) is bound. Active Ras functions as an allosteric activator of mitogen-activated protein kinases (MAPK), leading to downstream activation of Erk through phosphorylation. Activated Erk stimulates the transcription of Fos (through intermediate activation of a protein called Elk). In a second pathway, PLCg binds directly to activated LAT and is then phosphorylated by Zap-70. Activated PLCg leads to the cleavage of PIP2 to IP3 and DAG. Although DAG activates protein kinase C (PKC), it is IP3 that causes a release of calcium into the cytosol.
- Damage to neurons in patients with multiple sclerosis (MS) may be caused by autoreactive T cells that recognize peptides derived from myelin proteins presented by self MHC molecules. These autoreactive T cells secrete interferon (IFN)-g and promote inflammation, which damages the myelin sheath surrounding neurons. The exact immunodominant epitopes recognized by autoreactive T cells in MS patents have been identified. One potential method of therapy for patients with MS is to administer therapeutic peptides that differ from the immunodominant epitopes by one or two amino acids. Which one of the following statements best describes the basis for this therapeutic approach?
- The therapeutic peptides, called “altered peptide ligands,” could inactivate T cells specific for myelin proteins, or drive them to differentiate into T cells that do not produce IFN-g.
- The therapeutic peptides, called “altered peptide ligands,” could interfere with processing of the natural myelin proteins by the patient’s antigen-presenting cells.
- The therapeutic peptides could bind to the TCRs of myelin-specific T cells but not to the self MHC molecules, thereby blocking T cell activation.
- The therapeutic peptides could down-regulate MHC expression.
- The therapeutic peptides could replace the damaged myelin and restore neuronal function.
Altered peptide ligands are synthetic peptides in which the TCR contact residues have been changed, so that the peptide induces only partial responses by the responding T cell. These peptides still bind to the same MHC molecules as the original peptides, but they can cause T cell inactivation (anergy) or change in the cytokines the T cell produces. Altered peptide ligands do not interfere with processing of the natural proteins nor can they bind to TCRs without being presented by MHC molecules. There is no basis to say that peptides can down-regulate major MHC expression or replace damaged proteins in the myelin sheath.