Lymphocyte Dynamics

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Lymphocyte numbers are regulated both by responses to conventional exogenous antigens and endogenous microflora, by stimulation by self-peptide/MHC complexes and by the action of a series of cytokines. This multifaceted regulation permits individuals to maintain a broad repertoire of lymphocytes of distinctive specificities, allowing responses against a vast array of foreign substances and, at the same time, providing a pattern of memory based on the immunization history of the individual. The study of the process of lymphocyte dynamics that underlies this regulation requires a multidisciplinary approach, aimed both at the molecular underpinnings of the processes through which lymphocytes survive and proliferate and a systemics/ computational biology approach to appreciate the overall mechanisms governing total numbers of lymphocytes of distinct phenotype and distinct specificity. Emphasis has been placed on four aspects of this problem: the priming, expansion and differentiated phenotype of naive CD4 T cells in response to antigen challenge, the dynamics of lymphocyte memory and of memory phenotype cells, the mechanisms underlying CD4 T cell depletion in HIV infection, and the process of homeostatic proliferation and death. In an effort to develop strategies to enhance response of nave and memory cells to their cognate antigens, it was observed that the most potent stimulant of such expansion was the cytokine IL-1. When expansion of CD4 TCR transgenic T cells in a syngeneic host in response to antigen was measured, it was found that administering IL-1 over a 3 to 5 day period caused a ten-fold or greater enhancement in the degree of expansion when compared to that seen using conventional adjuvants such as LPS. This was equally true for naive and memory cells and was not mediated by other cytokines. The effect could only be partially explained by enhanced proliferation so that greater survival was also implicated. The use of recipients that were IL-1 receptor knockouts and IL-1 receptor-sufficient donors of TCR transgenic T cells showed that IL-1 could act directly on the responding CD4 or CD8 T cells to mediate expansion. The IL-1 receptor antagonist diminished the adjuvant effect of LPS indicating that a substantial portion of the effect of this conventional adjuvant was due to endogenous production of IL-1. Initial analysis of genes activated and suppressed in cells responding to antigen in the presence of LPS suggest avenues for further analysis that may lead to a mechanistic understanding of the IL-1 effect. The very robust effect of IL-1 suggests it may have a role in certain immunization strategies. IL-1 acts directly on CD4 T cells to enhance their differentiation into IL-17 producing cells. However, although IL-1 acts directly on CD8 cells to mediate their expansion, differentiation of CD8 into efficient cytotoxic cells requires the action of IL-1 on non T cells. IL-1 strikingly enhances the frequency of effector/memory CD8 T cells that migrate into peripheral tissues such as lung and liver. Such migration requires expression of IL-1R1 on a cell other than the responding T cells. Strikingly, the effects of IL-1 during priming are retained at the time of secondary challenge even though IL-1 is not administered again. Thus, the secondary response in mice primed in the presence of IL-1 includes increased in numbers of antigen-specific effector CD8 T cells, a greater presence in the tissues, and the expression of large amounts of granzyme B. Administration of IL-1 with weak vaccines, such as heat killed Listeria monocytogenes, the gD2 protein of H. simplex, heat killed Blastomyces or peptides associated with vaccinia, result strikingly enhances their protective capacity. Memory CD4 T cell proliferation was shown to be quite slow. Specific T cells from mice infected with LCMV divide at a rate of <2% per day. By contrast CD44bright CD25- CD4 T cells divide much more rapidly, at 8 to 10% per day. This rapid steady state proliferation of "memory phenotype" CD4 T cells is similar in conventional and germfree mice. Analysis of the repertoire of memory phenotype undergoing proliferation revealed no difference in receptor complexity from that of non-dividing memory phenotype cells. This implies that division is largely stochastic and probably dominantly driven by cytokines or other endogenous ligands rather than by peptide/ MHC complexes, whether of exogenous or endogenous origin. Memory phenotype and authentic memory cells differ from one another not only in their proliferative rates but, based on an RNA-SEQ analysis, also on the expression of NUR77, the latter being highly expressed on authentic memory cells even 30 days after priming, when they are already quiescent. Interestingly, in neonatal mice, the memory phenotype pool is efficiently colonized and preliminary results suggests that after 2 weeks of age, thymic emigrants enter the pool very inefficiently. We suspect then that the memory phenotype pool represents a set of self-specific cells generated early in the life of the individual and capable of producing Th1, Th2 or Th17 type cytokines early in infections as a results of stimulation of these cells through key cytokines, self-peptide mHC complexes or a combination of the two. In a sense, these cells are adaptive Th cells with a potent innate functionallity Efforts to understand the role of distinct priming regimens to induce particular phenotypic CD4 T cells responses to lung immunization reveal a profound effect of different "adjuvants". Priming in the presence of LPS leads to a TH17 response whereas priming in the presence of polyI:C to an exclusive IFNgamma response. Based on studies with KO mice, the results have been interpreted as indicated that LPS, through its activation of Myd88, results in robust IL-10 production, blocking IL-12 production and Th1 priming. In parallel, though its activation of TRIF/TRAM, LPS activates IL-1 production, markedly enhancing Th17 priming. By contrast, polyI:C, acting through TLR3 and TRIF activates type I interferon and IL-12, enhancing IFNgamma priming and blocking Th17 induction. These results give important insight into the precise regulation of CD4 T cells responses to distinct pathogens in the lung.