By R. Grompel. New York Law School.
Antigen-induced exclusion from follicles and anergy are separate and complementary processes that influence peripheral B cell fate generic 15mcg mircette amex. Outer periarteriolar lymphoid sheath arrest and subsequent differentiation of both naive and tolerant immunoglobulin transgenic B cells is determined by B cell receptor occupancy buy mircette 15mcg line. Sites of specific B cell activa- tion in primary and secondary responses to T cell-dependent and T cell-independent anti- gens. Anatomy of autoantibody production: domi- nant localization of antibody-producing cells to T cell zones in Fas-deficient mice. Immunoglobulin signal transduction guides the specificity of B cell-T cell interactions and is blocked in tolerant self-reactive B cells. Polyreactive antigen-binding B cells are the predomi- nant cell type in the newborn B cell repertoire. Simultaneous induction of rheumatoid factor- and antigen-specific antibody-secreting cells during the secondary immune response in man. Autoantibody production upon lymphocyte stimulation with autoantigen-xenoantigen conjugates. The subtleties of the process of T-cell antigen recognition and the heterogeneity of kinds of functional responses within the T-cell system are a major focus. Finally, some features of the anatomic compartmentalization of the immune sys- tem and how limited access to tissue compartments skews our thinking about in vivo immunity in humans are explored. Unlike antibodies that can bind with high affinity to multiple kinds of biomolecules, T-cells only recognize peptide epitopes that are embedded into one of two classes of specialized antigen-presenting structures (Fig. Two pathways of antigen presentation correlating with two subsets of responding T cells. Several accessory membrane molecules are therefore required to increase this binding affinity. Individual T-cells in the peripheral pool can undergo mitosis without devel- oping the changes associated with specific memory function (12,13), probably with one daughter cell undergoing apoptosis and the other surviving. Although this strategy appar- ently offers a degree of fine physiologic control (to prevent autoimmunity? In either case, the immune response to such an antigen is unproductive and this phenotype is a heritable genetic trait, an Ir gene. Such a diverse set of restriction elements serves to mitigate the likelihood of a single epidemic pathogen escaping detection by most individuals in a localized population. The complex mechanism by which T-cells recognize antigen, in comparison with B-cell/antibody antigen recognition, has several important implications for responses to infectious agents and especially the development of vaccines. First, since antibodies rec- ognize a broad range of conformationally dependent epitopes, whereas T-cells focus on only a limited set of peptide epitopes, the degree of crossreactive immunity to different quasi-species of the same infectious agent is often greater for T-cell immunity than for antibody responses. The functional responses available, however, are quite extensive and range from pro- grammed cell death to initiation of distinct modalities of immune response. The mechanisms by which the functional repertoire of T-cells is developed are less well understood, but they probably involve a similar strategy to that used during thymic Cellular Immunology Principles 27 selection. This complex structure can deliver multiple levels of signal depending on the relative intensity and stability of the interaction. These multiple signals are most likely integrated at the level of multiple dif- ferent promoter complexes, in which biochemical signals initiated at the cell surface are translated into the production of transcription complex components. In turn, these cel- lular signals can interact with multiple promoter motifs (34), resulting in coordinated patterns of expression of multiple unlinked genes (Fig. The proteins produced by such activated genes are of several classes, including those that initiate entry into the cell cycle; expression of unique cytokine receptors; expression of various effector cytokines; expression of new surface adhesion mole- cules; and new transcription factors. The products of this ensemble of gene activation interact in complex ways to determine not only the fate of that particular T-cell, but also the tempo of immune activation in the immediate microenvironment in which T- cell activation occurs. With a longer kinetic delay, activated T-cells change the pat- tern of cell surface adhesion molecules that alter the subsequent recirculation and tis- sue distribution properties of the cell. It is highly likely that such factors account for the significantly lowered antigen dose threshold required for full stimulation found in previously activated (memory) T-cells (35,36), compared with cells that have not been stimulated recently. These distinct thresholds probably generate the significant clonal heterogeneity charac- teristic of antigen-specific T-cell activation (37 41). This hypothetical scheme of T-cell activation (incorporating the functional subtleties of T-cell antigen recognition) contrasts with the more conventional views of classes of T-cells, based on static conceptions of antigen specificity, T-cell response, and 28 Bucy and Goepfert Fig. Multiple distinct signal pathways converge to produce functional transcription com- plexes to allow coordinate activation of multiple genes. Alterations of the dominant pathway of signaling from successive cycles of antigen stimulation result in alternate pathways of func- tional differentiation. First, antigen specificity is not a clean positive/negative phenomenon, even in response to a particular index peptide structure. Thus, in the presence of high doses of a particular peptide, more T-cells (including those with slightly lower avidity) can become fully activated. In the presence of lower doses of the same peptide, (or in the presence of peptide antagonists), these same T-cells receive suboptimal signal- ing that not only results in failure to reach the threshold stimulus for full activation, but probably results in a different kind of activation. The phenotypic characteristics among the daughter cells of such qualitatively different kinds of antigen activation are probably Cellular Immunology Principles 29 distinctive.
Substitutions at nonanchor residues usually have much smaller eects on binding anity order 15 mcg mircette otc. They found that the peptide residue at position three had its side chain buried in the Db binding cleft and order mircette 15 mcg, apparently, certain substitutions such as VAat this location can disrupt binding in the manner of an anchor position (Puglielli et al. Thenine amino acids of the epitope in positions 3341 of the protein are labeled as P1P9. Three of these substitutions occurred at position 8, the primary anchor site, and one substitution occurred at position 2, the secondary anchor site. Two other substitutions reduced binding by less than two orders of magnitude: a substitutionatposition 1 reduced binding by 67%, and a substitution at position 5 reduced binding by 85%. Hosts A and D progressed slowly to disease, whereas host C progressed at an intermediate rate. The other slow progressor, host D, had all four class I molecules listed for hosts A and C, and presented all ve epitopes. For example, host C viruses were dominated by escape mutants in Env497 504 and Nef165173 but not in the other three epitopes. Ideally, experimental studies of escape would provide information about changed functional character- istics of pathogen proteins and the associated tness consequences. The Tax protein is a trans-acting transcriptional regulator that modulates expression of several viral and cellular genes (Yoshida 2001). Tax appears to aect several aspects of the cell cycle, potentially enhancing cell division and reducing cell death. Three substitutions had lowered ability to activate the viral promoter, and all nine substitutions caused lowered or no activation of two cellular promoters. Amino acid sequences of viral proteins may be shaped by two opposing pressures: contribution to viral function and escape from im- mune recognition. Thus, amino acid substitutions in response to a third force, such as a drug, may be likely to reduce protein performance or enhance recognition by the host immune system. Experimentally applied selective pressures such as drugs may provide information about the functional andimmune selective pressures that shaped the wild-type sequence. However, escape at multiple epitopes may be observed within individual hosts (Evans etal. Escapeatadominant epitope provides ben- et if the aggregate rate of killing via subdominant epitopes allows a higher probability of burst before death. If some infected cells survive to produce new virions, the benet of escape at one epitope depends on the expected increase in cellular longevity during the productive phase of virion release and the probability that released viruses transmit to new host cells. The escape mutant benets only to the extent that fewer recognized peptides occur on the cell surfacelower density may reduce the rate of killing, and that reduction may in turn allow more of the escape variants progeny to be transmitted. Higher dose most likely produces larger population size during the initial viremia, increasing the time and the number of pathogens available to make a particular mutant. Experimental manipulations could test the contributions of dosage, pathogen population size within the host, and time to clearance. Wait- ing time for an escape mutant also depends on the mutation rate, which could perhaps be varied by comparinggenotypes that diered in muta- tion rate. If the infection clears rapidly, then the potential escape variants do not increase suciently within the host to contribute signicantly to transmission to other hosts. The changing frequencies of amino acid substitutions could be tracked under dierent regimes of uctuating selection. The role of timing could be studied in the following experimental evolution design. The relative escape rates in the monoclonal hosts focused on early and late epitopes calibrate es- cape rates in the absence of competition between epitopes. In experimental evolution studies, hosts that can eectively present a broader variety of epitopes should restrict the spread of escape substitutions relative to hosts with narrower presentation. If a host is rst exposed to epitope A, subsequent exposure to epitope B tends to reinforce the response against epitope A. For example, what sort of evolutionary response would occur in a series of hosts each previously exposed to epitope A? Experimental evolutioncreatesadaptations to the particular in vitro or in vivo laboratory conditions. Laboratory studies provide an opportunity to relate biochemical mechanism to kinetics, and kinetics to tness. Mathematical models aid the controlled, experi- mental dissection of these relations (Nowak and May 2000). Controlled analysis must be complemented by study of variation and adaptation in natural isolates.
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