Immunology Chapter 1-4
How might receipt of a cytokine signal result in the alteration of the location of a lymphocyte?
A cytokine may induce the expression on the cell surface of new chemokine receptors and/or new adhesion molecules that would cause the cell to move to a new location and, once present, to be retained there.
Describe how dimerization and phosphorylation of intracellular signaling molecules contribute to activation of cells by Type 1 cytokines.
Activation of Type I and Type II cytokine receptors results in their dimerization and subsequent phosphorylation by Janus Activated Kinases (JAKs). Th ese JAKs then phosphorylate inactive, cytoplasmic transcription factors, called Signal Transducers and Activators of Transcription, or STATs. Once phosphorylated, the STATs dimerize, with SH2 domains in each STAT binding to a phosphorylated tyrosine residue in its partner. Th is phosphorylation and dimerization allow the STATs to assume a conformation that allows them to dissociate from the cytokine receptor complex, enter the nucleus, and stimulate transcription from selected genes.
Distinguish between a hormone, a cytokine, a chemokine, and a growth factor. What functional attributes do they share, and what properties can be used to discriminate among them?
All four are chemical signaling molecules, which are oft en, but not always, proteins. Th ey deliver a message to a second cell, causing physiological changes in the second, target cell, which must bear receptors that are specifi c to the signaling molecule. Growth factors are oft en secreted constitutively, whereas cytokines and hormones are secreted in response to particular stimuli and their secretion is short lived. Most hormones are produced by specialized glands, and their action aff ects a small number of target cells. In contrast, cytokines are oft en produced by, and bind to, a variety of cells. Chemokines are specialized cytokines that provide chemotactic signals which induce target cells to move in a particular direction toward the cell secreting the chemokine
What is an ITAM, and what proteins modify the ITAMs in Ig(alpha) and Ig(beta)?
An ITAM is an Immunoreceptor Tyrosine-based Activation Motif. It is a motif with a particular sequence containing phosphorylatable tyrosines in the cytoplasmic regions of several immunologically important proteins. Ig and Ig are part of the signaling complex in B cells and are phosphorylated by the Src-family kinase Lyn, when the BCR moves into the lipid raft regions of the membrane upon activation
Defi ne an adapter protein. Describe how an interaction between proteins bearing SH2 and phosphorylated tyrosine (pY) groups helps to transduce a signal from the T-cell receptor to the ZAP-70 protein kinase.
An adapter protein bears more than one binding site for other proteins and serves to bring other proteins into contact with one another without, itself, having any enzymatic activity. Activation of the TCR results in activation of the Src-family kinase, Lck. Lck phosphorylates the tyrosine residues on the ITAMs of the CD3 complex associated with the TCR. ZAP-70 then binds to the pY residues via its SH2 regions, and is then itself phosphorylated and activated.
Th e B- and T-cell receptor proteins have remarkably short intracytoplasmic regions of just a few amino acids. How can you reconcile this structural feature with the need to signal the presence of bound antigen to the interior of the cell?
Both the BCR and the TCR are noncovalently associated on their respective cell membranes with signal transduction complexes that function to transduce the signal initiated by antigen binding to the receptor into the interior of the cell. In the case of the B-cell receptor, the signal transduction complex is composed of IgIg. ITAMs on Ig/Ig are phosphorylated by tyrosine kinases that are brought into close proximity with the B-cell receptor complex upon the oligomerization of the receptor and its movement into the lipid raft regions of the membrane, which occur upon antigen binding. Th e phosphorylated tyrosines of Ig/Ig then serve as docking points for downstream components of the signal transduction pathway. In the case of the TCR, the signal transduction complex is the CD3 set of proteins, which contains six chains and serves a similar function to Ig/Ig. ΙΤΜs on CD3 are phosphorylated by Src family kinases, particularly Lck, and then serve as docking points for downstream components of the TCR signaling pathway. Lck in turn is associated with the TCR co-receptors CD8 and CD4.
Describe how the principle of herd immunity works to protect unvaccinated individuals. What characteristics of the pathogen or of the host do you think would most impact the degree to which this principle begins to take hold?
Herd immunity is what occurs when enough members of a population have protective immunity to a pathogen, either through vaccination or prior infection, that they act as a buff er to spread and help protect those without immunity. Th e effi cacy of herd immunity depends on pathogen characteristics such as how the pathogen is transmitted (airborne, fecal/oral, etc.) and whether it can survive for long outside the host. Herd characteristics include how we interact with one another as a "herd" (e.g., crowding indoors versus sparse populations and outdoor encounters) and the primary target population (e.g., young children versus adults). (Other answers are also acceptable.)
IgM has ten antigen-binding sites per molecule, whereas IgG only has two. Would you expect IgM to be able to bind fi ve times as many antigenic sites on a multivalent antigen as IgG? Why/why not?
I would expect IgM to be able to bind more molecules of antigen than IgG, but perhaps not fi ve times more. Steric hindrance/conformational constraints may prevent all 10 antigen-binding sites of IgM from being able to simultaneously bind to 10 antigenic sites.
Explain the diff erence between a monocyte and a macrophage.
Monocytes are the blood-borne precursors of macrophages. Monocytes have a characteristic kidneybean-shaped nucleus and limited phagocytic and microbial killing capacity compared to macrophages. Macrophages are much larger than monocytes and undergo changes in phenotype to increase phagocytosis, antimicrobial mechanisms (oxygen dependent and oxygen independent), and secretion of cytokines and other immune system modulators. Tissue-specifi c functions are also found in tissue macrophages.
Describe two examples of ways in which vertebrates tune down the intensity of their own cytokine signaling.
8. See Figure 4-14.
Describe one way in which the structure of antibodies is superbly adapted to their function.
(Th ere are multiple possibilities for this answer, and we off er a few here). (a) Th e antibody is a Y-shaped molecule with the antigenbinding regions located at the two tips of the Y. At the junction of the three sections is a fl exible hinge region that allows the two tips to move with respect to one another and hence to bind to antigenic determinants arranged at varying distances from one another on a multivalent antigen. (b) Within the variable region domains of the heavy and light chains, a common -pleated sheet scaff old includes multiple anti-parallel strands in a conserved conformation. However, at the turns between the strands, there are varying numbers and sequences of amino acids, corresponding to hypervariable regions in the immunoglobulin variable regions sequences. Th ese enable the creation of many diff erent antigen-binding sites. (c) Th e constant regions of antibody molecules form the bridge between the antigen-binding region and receptors on phagocytic cells that will engulf antigen-antibody complex, or components of the complement system that will bind to the Fc regions of the antibody and aid in the disposal of the antigen. Diff erent constant region structures bind to diff erent Fc receptors and complement components.
Explain why each of the following statements is false. a. Th ere are no mature T cells in the bone marrow. b. Th e pluripotent stem cell is one of the most abundant cell types in the bone marrow. c. Th ere are no stem cells in blood. d. Activation of macrophages increases their expression of class I MHC molecules, allowing them to present antigen to TH cells more eff ectively. e. Mature B cells are closely associated with osteoblasts in the bone marrow. f. Lymphoid follicles are present only in the spleen and lymph nodes. g. Th e FRC guides B cells to follicles. h. Infection has no infl uence on the rate of hematopoiesis. i. Follicular dendritic cells can process and present antigen to T lymphocytes. j. Dendritic cells arise only from the myeloid lineage. k. All lymphoid cells have antigen-specifi c receptors on their membrane. l. All vertebrates generate B lymphocytes in bone marrow. m. All vertebrates have a thymus. n. Jawless vertebrates do not have lymphocytes
(a) Although T cells complete their maturation in the thymus, not the bone marrow, mature CD4+ and CD8 T cells will recirculate back to the bone marrow. (b) Pluripotent hematopoietic stem cells are rare, representing less about 0.05% of cells in the bone marrow. (c) Hematopoietic stem cells can be mobilized from the bone marrow and circulate in the blood, which is now used as a source of stem cells for transplantation. (d) Macrophages will increase both class I and class II MHC expression aft er activation; however, TH cells are CD4 and recognize antigenic peptide bound to class II MHC. (e) Immature, not mature B cells, are associated with osteoblasts, which help them to develop. (f) Lymphoid follicles are found in all secondary lymphoid tissues, including that associated with mucosal tissues (MALT). (g) Th e follicular dendritic cell (FDC) network guides B cells within follicles. Th e follicular reticular cell (FRC) system guides T cells within the T cell zone (although in some cases it may also help B cells to get to follicles, so aspects of this statement are true). (h) Infection and associated infl ammation stimulates the release of cytokines and chemokines that enhance blood cell development (particularly to the myeloid lineage). (i) FDC present soluble antigen on their surfaces to B cells, not T cells. (j) Dendritic cells can arise from both myeloid and lymphoid precursors. (k) B and T lymphocytes have antigen specifi c receptors on their surface, but NK cells, which are also lymphocytes do not. (l) B cells are generated outside the marrow in birds and ruminants. (m) Th is was true for all jawed vertebrates, but now may even be true for jawless vertebrates; in other words, this is now true, not false! (n) Recent data suggest that at least one jawless vertebrate, the lamprey, has T- and B-like cells.
Th e NFAT family is a ubiquitous family of transcription factors. a. Under resting conditions, where is NFAT localized in a cell? b. Under activated conditions, where is NFAT localized in a cell? c. How is it released from its resting condition and permitted to relocalize? d. Immunosuppressant drugs such as cyclosporin act via inhibition of the calcineurin phosphatase. If NFAT is ubiquitous, how do you think these drugs might act with so few side eff ects on other signaling processes within the body?
(a) Cytoplasm (b) Nucleus (c) By de-phosphorylation with the enzyme calcineurin phosphatase, activated by binding to the calcium-calmodulin complex. Activation of the cell results in an increase in intracytoplasmic calcium ion concentration. (d) Th ere are many possible answers to this question. For example, there may be multiple forms of NFAT, and so these drugs may just interact with particular forms of the enzyme. Other cells may have alternative pathways that can bypass the need for calcineurin, whereas T cells may not. In fact, cyclosporin binds to the protein immunophilin, which is specifi cally expressed in activated T cells, and it is the complex of cyclosporin and immunophilin that inhibits the activity of calcineurin.
True or false? Explain your answers. Interactions between receptors and ligands at the cell surface: a. are mediated by covalent interactions. b. can result in the creation of new covalent interactions within the cell.
(a) False. Receptors and ligands bind through noncovalent interactions such as hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactions. (b) True. Receptor-ligand binding can activate the receptor and result in phosphorylation and receptor cross-linking.
Indicate to which branch(es) of the immune system the following statements apply, using H for the humoral branch and CM for the cell-mediated branch. Some statements may apply to both branches (B) a. Involves B cells b. Involves T cells c. Responds to extracellular bacterial infection d. Involves secreted antibody e. Kills virus-infected self cells
(a) H. (b) CM. (c) B. (d) H. (e) CM
Compare and contrast innate and adaptive immunity by matching the following characteristics with the correct arm of immunity, using I for innate and A for adaptive: a. Is the fi rst to engage upon initial encounter with antigen b. Is the most pathogen specific c. Employs T and B lymphocytes d. Adapts during the response e. Responds identically during a fi rst and second exposure to the same antigen f. Responds more eff ectively during a subsequent exposure g. Includes a memory component h. Is the target of vaccination i. Can involve the use of PAMP receptors j. Involves antigen-specifi c receptors binding to pathogens k. Can be mediated by antibodies
(a) I. (b) A. (c) A. (d) A. (e) I. (f) A. (g) A. (h) A. (i) I. (j) A. (k) A.
For each of the following sets of cells, state the closest common progenitor cell that gives rise to both cell types. a. Dendritic cells and macrophages b. Monocytes and neutrophils c. TC cells and basophils d. TH and B cells
(a) Myeloid progenitor. (b) Granulocyte-monocyte progenitor. (c) Hematopoietic stem cell. (d) Lymphoid progenitor
Describe how the following experimental manipulations were used to determine antibody structure. a. Reduction and alkylation of the antibody molecule b. Enzymatic digestion of the antibody molecule c. Antibody detection of immunoglobulin fragments
(a) Reduction enabled the breakage of disulfi de bonds between the heavy and light chains and the pairs of heavy chains in the IgG molecule. Alkylation ensured that the bonds between the separated chains would not reform. Scientists could then separate the chains and fi gure out their molecular weight and how many chains belonged to each molecule. (b) Papain digested the molecule in the hinge region, releasing two antigen-binding fragments (Fabs) and one non-antigen-binding fragment, which spontaneously crystallized (Fc). Th is told investigators that there were two antigen-binding sites per molecule and suggested that part of the molecule was not variable in sequence (since it was regular enough in structure to crystallize). Pepsin isolated the divalent antigen-binding part of the molecule from the rest. (c) Antibodies were generated that specifi cally recognized the Fab and Fc fragments. Antibodies to Fab were found to bind to both heavy and light chains, thus indicating that the antigenbinding sites had components of both chains. Antibodies to Fc fragments bound only to the heavy chain
Th e following diagram represents the results of a fl ow cytometry experiment in which mouse spleen cells were stained with antibodies directed against diff erent components of the IL-2R. Th e more antibody that binds to the cells, the further they move along the relevant axis. Th e number of cells stained with fl uorescein-conjugated anti- IL-2R antibodies are shown along the x-axis of the fl ow cytometry plot, and cells that stain with phycoerythrinlabeled antibodies to the subunit of the IL-2 receptor move along the Y-axis. We have drawn for your reference a circle that represents cells that stain with neither antibody. On this plot, draw, as circles, and label where you would expect to fi nd the populations representing unstimulated T cells and T cells aft er antigen activation, aft er treatment with the two fl uorescent labels described above
(a) Th e IL-2R is made up of three components: , , and . Quiescent (unactivated) T cells bear only the dimer, which binds IL-2, but at intermediate affi nity insuffi cient to result in a signal under physiological conditions. Upon antigen stimulation, the T cell synthesizes the subunit, which binds to the dimer and converts it into a highaffi nity receptor capable of mediating the IL-2 signal at physiological cytokine concentrations
Indicate whether each of the following statements about the lymph node and spleen is true or false. If you think a statement is false, explain why. a. Th e lymph node fi lters antigens out of the blood. b. Th e paracortex is rich in T cells, and the periarteriolar lymphoid sheath (PALS) is rich in B cells. c. Only the lymph node contains germinal centers. d. Th e FRCC enhances T cell/APC interactions e. Aff erent lymphatic vessels draining the tissue spaces enter the spleen. f. Lymph node but not spleen function is aff ected by a knockout of the Ikaros gene.
(a) Th e spleen has been classically considered an organ that 'fi lters' antigens from blood. Th e lymph node receives antigens principally from the aff erent lymphatics. (b) Both the paracortex of the lymph node and the periarteriolar lymphoid sheath of the spleen are rich in T cells. B cells are found primarily in the follicles. (c) Germinal centers are found wherever there are follicles,which are present in all secondary lymphoid tissue, including lymph node, spleen, and the wide variety of mucosal associated lymphoid tissues. (d) True. (e) Aff erent lymphatics are associated with lymph nodes, not spleen. (Eff erent lymphatics can be found in both.) (f) Ikaros is required for lymphoid development which occurs primarily in the bone marrow (and thymus). However, lymphocytes populate the spleen and mount an immune response there, so the spleen would clearly be compromised in function in the absence of Ikaros and lymphocytes
For each of the following statements, indicate whether the statement is true or false. If you think the statement is false, explain why. a. Booster shots are required because repeated exposure to an antigen builds a stronger immune response. b. Th e gene for the T cell receptor must be cut and spliced together before it can be expressed. c. Our bodies face the greatest onslaught from foreign invaders through our skin. d. Increased production of antibody in the immune system is driven by the presence of antigen.e. Innate immunity is deployed only during the primary response, and adaptive immunity begins during a secondary response. f. Autoimmunity and immunodefi ciency are two diff erent terms for the same set of general disorders. g. If you receive intravenous immunoglobulin to treat a snakebite, you will be protected from the venom of this snake in the future, but not venom from other types of snakes. h. Innate and adaptive immunity work collaboratively to mount an immune response against pathogens. i. Th e genomic sequences in our circulating T cells for encoding a T-cell receptor are the same as those our parents carry in their T cells. j. Both the innate and adaptive arms of the immune response will be capable of responding more effi ciently during a secondary response.
(a) True. (b) True. (c) False. Most pathogens enter the body through mucous membranes, such as the gut or respiratory tract. (d) True. (e) False. Both are involved in each case. Innate immunity is deployed fi rst during the primary response, and adaptive immunity begins later during that fi rst encounter. During the secondary response, innate and adaptive immunity are again both involved. While innate responses are equally effi cient, the second time around adaptive immunity uses memory cells to pick up where it left off at the end of the primary response and is therefore quicker and more eff ective in pathogen eradication during a secondary response. (f) False. Th ese are two diff erent types of disorder: autoimmunity occurs when the immune system attacks self, and immunodefi ciency is when the immune system fails to attack nonself. Th e one caveat occurs in cases of immune defi ciency involving immune regulatory components. Just like broken brakes, these can result in an overzealous immune attack on self structures that thus presents as autoimmunity. (g) False. Th e intravenous immunoglobulin provides protection for as long as it remains in the body (up to a few weeks), but this individual has not mounted his or her own immune response to the antigen and will therefore not possess any memory cells. (h) True. (i) False. Th e genes for encoding a T-cell receptor are rearranged and edited during T-cell development in the thymus so that each mature T cell carries a diff erent T-cell receptor gene sequence. (j) False. Th e innate immune response does not generate memory cells (as the adaptive immune response does), so it is equally effi cient during each infection.
14.Cells that express antigen-specific receptors Cell Types a. Common myeloid progenitor cells b. Monocytes c. Eosinophils d. Dendritic cells e. Natural killer (NK) cells f. Mast cells g. Neutrophils h. M cells i. Osteoblasts j. Lymphocytes k. NK1-T cell l. Microglial cell m. Myeloid dendritic cell n. Hematopoietic stem cell o. Lymphoid dendritic cell
(j,k)
For each description below (1-14), select the appropriate cell type (a-p). Each cell type may be used once, more than once, or not at all. Descriptions 1. Major cell type presenting antigen to naïve T cells 2. Phagocytic cell of the central nervous system 3. Granulocytic cells important in the body's defense against parasitic organisms 4. Give rise to red blood cells 5. Generally fi rst cells to arrive at site of infl ammation 6. Support maintenance of hematopoietic stem cells 7. Give rise to thymocytes 8. Circulating blood cells that diff erentiate into macrophages in the tissues 9. An antigen-presenting cell that arises from the same precursor as a T cell but not the same as a macrophage 10.Cells that are important in sampling antigens of the intestinal lumen 11.Granulocytic cells that release various pharmacologically active substance. 12. White blood cells that play an important role in the development of allergies. 13. Cells that can use antibodies to recognize their targets
1. (d), 2. (l), 3. (c), 4. (m, n), 5. (g), 6. (i), 7. (n), 8. (b), 9. (o), 10. (h), 11. (c, f, g), 12. (c, f), 13. (c, e, f),
What is an antigen? An antibody? What is their relationship to one another?
An antigen is anything that elicits an adaptive immune response—most commonly a part of some foreign protein or pathogen. An antibody is a soluble antigenspecifi c receptor molecule released by B cells that binds to an antigen and labels it for destruction. One interesting twist: antibodies can be antigens if they come from another species and are recognized as foreign by the host. Th is occurs in some instances of passive immunization, when antibodies from an animal such as a horse are given to humans, who then mount an adaptive immune response against horse-specifi c chemical patterns in the antibodies.
You and another student are studying a cytokine receptor on a B cell that has a Kd of 106 M. You know that the cytokine receptor sites on the cell surface must be at least 50% occupied for the B cell to receive a cytokine signal from a helper T cell. Your lab partner measures the cytokine concentration in the blood of the experimental animal and detects a concentration of 107 M. She tells you that the eff ect you have been measuring could not possibly result from the cytokine you're studying. You disagree. Why?
Because you know that, in at least one case, activation of a cell can result in an alteration in the phenotype of the cytokine receptor, resulting in a dramatic increase in its affi nity for the cytokine. For example, the IL2R exists in two forms: a moderate affi nity form consisting of a dimer and a high-affi nity form, synthesized only following cell activation that consists of an trimer
Measurement of the blood concentration of a particular cytokine reveals that it is rarely present above 1010 M, even under the conditions of an ongoing immune response. However, when you measure the affi nity of the cognate receptor, you discover that its dissociation constant is close to 108 M, implying that the receptor occupancy must rarely exceed 1%. How do you account for this discrepancy?
Cells undergoing an immune response oft en move very close to one another and are held together by adhesive interactions. In this case, if, for example, the cytokinereleasing cell is a dendritic cell and the target cell is a T cell, the two cells will be held in close apposition to one another for several hours. In addition, the secretory apparatus of the dendritic cell will be oriented such that the cytokines will be secreted into the tiny space between the dendritic cell and the T cell. Th e actual concentration of cytokines experienced by the T-cell receptors at the point of contact with the dendritic cell will therefore be orders of magnitude higher than that in the surrounding circulation.
Give examples of mild and severe consequences of immune dysfunction. What is the most common cause of immunodefi ciency throughout the world today?
Consequences of mild forms of immune dysfunction include sneezing, hives, and skin rashes caused by allergies. Asthma and anaphylactic reactions are more severe consequences of allergy and can result in death. Consequences of severe immune dysfunction include susceptibility to infection by a variety of microbial pathogens if the dysfunction involves immunodeficiency, or chronic debilitating diseases, such as rheumatoid arthritis, if the dysfunction involves autoimmunity. The most common cause of immunodefi ciency is infectionwith the retrovirus HIV-1, which leads to AIDS.
In general terms, what role do cytokines play in the development of immunity? How does this compare to chemokines?
Cytokines are soluble molecular messengers that allow immune cells to communicate with one another. Chemokines are a subset of cytokines that are chemotactic, or have the ability to recruit cells to the site of infection
In the early days of experiments designed to detect the T-cell receptor, several diff erent research groups found that antibodies directed against immunoglobulin proteins appeared to bind to the T-cell receptor. Given what you know about the structure of immunoglobulins and the T-cell receptor, why is this not completely surprising?
Immunoglobulin proteins and the T-cell receptor share a common motif: the immunoglobulin fold, which may be the binding target for these antibodies.
At what age does the thymus reach its maximal size? a. During the fi rst year of life b. Teenage years (puberty) c. Between 40 and 50 years of age d. Aft er 70 years of age
In humans, the thymus reaches maximal size during puberty. During the adult years, the thymus gradually atrophies.
Th e cytokine IL-2 is capable of activating all T cells to proliferation and diff erentiation. How does the immune system ensure that only T cells that have been stimulated by antigen are susceptible to IL-2 signaling?
In the case of IL-1, cells secrete a protein called IL-1Ra, a soluble form of the IL-1 receptor, which is capable of binding to IL-1, but not of transducing an IL-1 signal. Th e balance of IL-1Ra, vs the signaling receptor will determine the amount of IL-1 able to productively interact with a cell-surface receptor. IL-18BP serves a similar purpose in the IL-18 system. Sometimes cells will cleave the interleukin-binding portion of the receptor from the surface of the cell. Th is has an eff ect similar to that described above, in that the cleaved receptor can bind to the cytokine, blocking its access to the productive receptor. Th is occurs in the case of the IL-2R, where the IL-2- binding portion of the IL-2R chain is secreted, and is able to bind the soluble IL-2 protein
Why was Jenner's vaccine superior to previous methods for conferring resistance to smallpox?
Jenner's method of using cowpox infection to confer immunity to smallpox was superior to earlier methods because it carried a significantly reduced risk of serious disease. The earlier method of using material from the lesions of smallpox victims conferred immunity but at the risk of acquiring the potentially lethal disease.
What was the signifi cance of the accidental re-inoculation of some chickens that Pasteur had previously exposed to the bacteria that causes cholera? Why do you think these chickens did not die after the first exposure to this bacterium?
Pasteur had inadvertently immunized his chickens during the first inoculation, using an old, attenuated bacterial strain. Th e old strain was no longer virulent enough to be fatal, but it was still able to elicit an adaptive immune response that protected the chickens from subsequent infections with fresh, virulent bacteria of the same type.
How are PRRs diff erent from B- or T-cell receptors? Which is most likely to be involved in innate immunity and which in adaptive immunity?
Pattern recognition receptors (PRRs) are germlineencoded receptors expressed in a variety of immune cells. Th ey are designed by evolution to recognize molecules found on common pathogens, and they initiate the innate immune response when they bind these molecules. In contrast, B- and T-cell receptors are expressed in lymphocytes, and the genes that encode these are produced by DNA rearrangement and editing, so that the receptor locus in each B or T cell's genome has a diff erent sequence and encodes a diff erent receptor. B- and T-cell receptors are diverse and have the ability to recognize a much greater assortment of antigens than PRRs, including those never before encountered by the immune system. B- and T-cell receptors are part of the adaptive immune response.
Defi ne the terms pleiotropy, synergy, redundancy, antagonism, and cascade induction as they apply to cytokine action.
Pleiotropy is the capacity to bring about diff erent end results in diff erent cells. Synergy is the ability of two or more cytokines aff ecting a cell to bring about a response that is greater than the sum of each of the cytokines. Redundancy is the property that describes the fact that more than one cytokine can bring about the same eff ect. Antagonism is the tendency for two cytokines binding to the same cell to bring about opposite eff ects, or to reduce/ eliminate the response to the other. Cascade induction is the ability of a cytokine to bind to one cell and to induce that cell to secrete additional cytokines.
You have generated a T-cell clone in which the Src-family tyrosine kinase Lck is inactive. You stimulate that clone with its cognate antigenic peptide, presented on the appropriate MHC platform and test for interleukin 2 secretion, as a measure of T cell activation. Do you expect to see IL-2 secretion or not? Explain.
Since Lck lies at the beginning of the signaling pathway from the T-cell receptor, a T cell with an inactive form of lck will not be able to undergo activation to secrete IL2.
Cytokines signaling through the Class I cytokine receptors can compete with one another, even though the recognition units of the receptors are different. Explain
Some cytokines in this class have distinct cytokinebinding subunits, but share a signaling subunit. For example, the dimeric receptors for IL-3 and GM-CSF have diff erent binding, but the same signaling, subunits. If the cell is responding to IL-3, and all the signaling subunits are occupied in this response, binding of GM-CSF to the binding subunit alone would fail to transduce a signal. Th us, although the two receptors are not competing for binding to their respective ligands, they do compete for signaling effi cacy
Activation of Src-family kinases is the fi rst step in several diff erent types of signaling pathways. It therefore makes biological sense that the activity of this family of tyrosine kinases is regulated extremely tightly. Describe how phosphorylation of Src-family kinases can deliver both activating and inhibitory signals to Src kinases.
Src-family kinases have two tyrosine sites on which they can be phosphorylated: an inhibitory and an activatory site. In the resting state, the inhibitory site is phosphorylated by the kinase Csk, and the kinase folds up on itself, forming a bond between an internal SH2 group and the inhibitory phosphate, shielding the active site of the enzyme. Cleavage of the phosphate group from the inhibitory tyrosine allows the enzyme to open its structure and reveal the active site. Further activation of the enzyme then occurs when a second tyrosine is phosphorylated and stabilizes the activated state
As a graduate student, your adviser has handed you a T-cell clone that appears to be constitutively (always) activated, although at a low level, even in the absence of antigenic stimulation, and he has asked you to fi gure out why. Your benchmate suggests you start by checking out the sequence of its lck gene, or the status of the Csk activity in the cell. You agree that those are good ideas. What is your reasoning?
Src-family kinases such as Lck are located at the beginning of many signal transduction pathways, and their activities are subjected to rigorous control mechanisms. Lck is maintained in an inactive state by being phosphorylated on an inhibitory tyrosine residue. Th is phosphorylated tyrosine is then bound by an internal SH2 domain that holds the Lck in a closed, inert conformation. If the DNA encoding this tyrosine residue has been mutated or eliminated, such that this phosphorylation cannot occur, then there will be constitutive level of Lck activity. Since the enzyme that phosphorylates this inhibitory tyrosine is Csk, a reduction in Csk activity would have the same eff ect.
What two primary characteristics distinguish hematopoietic stem cells from mature blood cells?
Stem cells are capable of self-renewal and can give rise to more than one cell type, whereas progenitor cells have lost the capacity for self-renewal and are committed to a single cell lineage. Commitment of progenitor cells depends on their acquisition of responsiveness to particular growth factors
Preparations enriched in hematopoietic stem cells are useful for research and clinical practice. What is the role of the SCID mouse in demonstrating the success of HSC enrichment?
Th e SCID mouse has no T or B lymphocytes because they cannot rearrange their antigen specifi c receptors. Injection of normal stem cells will result in restoration of these cell types—an easily measured outcome. As HSCs are successively enriched in a preparation, the total number of cells that must be injected to restore these cell populations decreases.
What effect would removal of the bursa of Fabricius (bursectomy) have on chickens?
Th e bursa of Fabricius in birds is the primary site where B lymphocytes develop. Bursectomy would result in a lack of circulating B cells and humoral immunity, and it would probably be fatal.
What is the diff erence between the discarded instructional theory for lymphocyte specifi city and the selection theory, which is now the accepted explanation?
Th e instructional theory stated that the antibody structure is not determined until it is molded by antigen binding. In contrast, the selection theory states that the antigen receptor is fi rst made (randomly) and then selected by antigen binding from a diverse group of receptors with predetermined structures.
List two primary and two secondary lymphoid organs and summarize their functions in the immune response.
Th e primary lymphoid organs are the bone marrow (bursa of Fabricius in birds) and the thymus. Th ese organs function as sites for B-cell and T-cell maturation, respectively. Th e secondary lymphoid organs are the spleen, lymph nodes, and mucosal-associated lymphoid tissue (MALT) in various locations. MALT includes the tonsils, appendix, and Peyer's patches, as well as loose collections of lymphoid cells associated with the mucous membranes lining the respiratory, digestive, and urogenital tracts. All these organs trap antigen and provide sites where lymphocytes can interact with antigen and subsequently undergo clonal expansion.
Name one protein shown to be defective in many cases of X-linked agammaglobulinemia, and describe how a reduction in the activity of this protein could lead to immunodefi ciency.
Th e signaling kinase Bruton's tyrosine kinase (Btk) is defective in 85% of cases of X-linked agammaglobulinemia. It is encoded on the X chromosome, and hence most cases of this disease occur in boys. Phosphorylation by Btk activates PLC2, which cleaves PIP2 into inositol trisphosphate (IP3) and diacylglycerol (DAG) following activation of pre-B cells through the pre-B-cell receptor, or of B cells through the immunoglobulin receptor. Th is leads eventually to activation of the NFAT and MAP kinase transcription factor pathways, culminating in B-cell diff erentiation and proliferation, which cannot occur in the absence of a functioning Btk protein
Did the treatment for rabies used by Pasteur confer active or passive immunity to the rabies virus? Is there any way to test this?
The Pasteur method for treating rabies consists of a series of inoculations with attenuated rabies virus. This process actively immunizes the recipient, who then mounts an immune response against the virus to stop the progress of infection. A simple test for active immunity would be to look for antibodies specific to the rabies virus in the recipient's blood at a time aft er completion of treatment, when all antibodies from a passive treatment would havecleared from circulation. Alternatively, one could challenge the recipient with attenuated rabies to see whether a secondary response occurred (this treatment may be precluded by ethical ramifications).
Adaptive immunity exhibits several characteristic attributes, which are mediated by lymphocytes. List four attributes of adaptive immunity and briefl y explain how they arise.
The four immunologic attributes are specifi city, diversity, memory, and self/nonself recognition. Specificity refers to the ability of certain membrane-bound molecules on a mature lymphocyte to recognize only a single antigen (or small number of closely related antigens). Rearrangement of the immunoglobulin genes during lymphocyte maturation gives rise to antigenic specificity; it also generates a vast array of different specificities, or diversity, among mature lymphocytes. The ability of the immune system to respond to nonself-molecules, but (generally) not to self-molecules, results from the elimination during lymphocyte maturation of immature cells that recognize self-antigens. Aft er exposure to a particular antigen, mature lymphocytes reactive with that antigen proliferate, differentiate, and adapt, generating a larger and more effective population of memory cells with the same specificity; this expanded population can respond more rapidly and intensely after a subsequent exposure to the same antigen, thus displaying immunologic memory.
Infants immediately aft er birth are oft en at risk for infection with group B Streptococcus. A vaccine is proposed for administration to women of childbearing years. How can immunizing the mothers help the babies?
The immunized mothers would confer passive immunity upon their off spring because the anti-streptococcal antibodies, but not the B cells, cross the placental barrier and are present in the babies at birth. In addition, colostrum and milk from the mother would contain antibodies to protect the nursing infant from infection.
Name three features of a secondary immune response that distinguish it from a primary immune response.
The secondary immune response is faster (because it starts with an expanded population of antigen-specific cells), more effective (because the memory cells have learned and adapted during the primary response), and reaches higher levels of magnitude than the primary response (again, because we begin with many more cells that have already honed their strategy).
What are the two primary roles of the thymus?
The two primary roles of the thymus are the generation and the selection of a repertoire of T cells that will protect the body from infection
What is meant by the term tolerance? How do we become tolerant to the structures in our own bodies?
Tolerance means that our immune system can discern between ourselves and foreign antigens, and does not attack self-antigens. Lymphocytes learn tolerance by being exposed to self-antigens during development, when most potentially self-reactive cells are either destroyed or inhibited from responding.
Briefl y describe the four major categories of pathogen. Which are likely to be the most homogenous in form and which the most diverse? Why
Viruses live inside host cells and require the host cell's machinery to replicate. Fungi are extracellular and are oft en kept in check by the immune system, but they can be a problem for people with immune defi ciencies. Fungi are the most homogenous in form. Parasites are the most varied in form and can range in size from single-celled, intracellular microorganisms to large macroscopic intestinal worms. Some parasites also go through several life-cycle stages in their human host, altering their antigenic structures and location so signifi cantly between stages that they require completely diff erent immune eradication strategies. Bacteria can cause intracellular or extracellular infections, which require diff erent immune targeting and elimination methods. Many bacteria express cell surface molecular markers (PAMPs) that are recognized by receptors that are part of our innate immune system (PRRs).
Describe one mechanism by which Type I interferons "interfere" with the production of new viral particles.
When a Type I interferon binds to its receptor on a virally infected cell, the interferon signal results in the activation of a ribonuclease that breaks down cytoplasmic RNA. It is particularly eff ective against double-stranded RNA
Signaling by tumor necrosis factor can paradoxically lead to cell activation or cell death. Explain how, by drawing diagrams of the relevant signaling pathways
When a Type I interferon binds to its receptor on a virally infected cell, the interferon signal results in the activation of a ribonuclease that breaks down cytoplasmic RNA. It is particularly effective against double-stranded RNA.
