The instructive signals received by T cells during the programming stages of activation will determine the fate of effector and memory populations generated during an immune response. Members of the tumor necrosis factor (TNF) superfamily play an essential role in influencing numerous aspects of T cell adaptive immune responses including cell activation, differentiation, proliferation, survival, and apoptosis. My thesis dissertation describes the involvement of two such members of the TNF superfamily, TNF and FasL, and their influence on the fate of T cells early during responses to viral infections and to the induction of transplantation tolerance. TNF is a pleiotropic pro-inflammatory cytokine that has an immunoregulatory role in limiting the magnitude of T cell responses during a viral infection. Our laboratory discovered that one hallmark of naïve T cells in secondary lymphoid organs is their unique ability to rapidly produce TNF after activation and prior to acquiring other effector functions. I hypothesized that T cell-derived TNF will limit the magnitude of T cell responses. The co-adoptive transfer of wild type (WT) P14 and TNF-deficient P14 TCR transgenic CD8+ T cells, that recognize the GP33 peptide of lymphocytic choriomeningitis virus (LCMV), into either WT or TNF-deficient hosts demonstrated that the donor TNF-deficient P14 TCR transgenic CD8+ T cells accumulate to higher frequencies after LCMV infection. Moreover, these co-adoptive transfer experiments suggested that the effect of T cell-derived TNF is localized in the microenvironment, since the TNF produced by WT P14 TCR transgenic CD8+ T cells did not prevent the accumulation of TNF-deficient P14 TCR transgenic CD8+ T cells. To determine if T cell-produced TNF is acting on professional APC to suppress the generation of virus-specific T cell responses, I performed co-adoptive transfer experiments with WT P14 TCR transgenic CD8+ and TNF-deficient P14 TCR transgenic CD8+ T cells into TNFR1/2 (1 and 2) deficient mice. These experiments demonstrated that the absence of TNFR1/2 signaling pathway in the host cells resulted in a greater accumulation of WT P14 TCR transgenic CD8+ T cells, thereby considerably diminishing the differences between donor WT P14 TCR transgenic CD8+ and donor TNF-deficient P14 TCR transgenic CD8+ T cells. The increased frequency and absolute numbers of WT P14 TCR transgenic CD8+ T cells in TNFR1/R2 deficient recipients suggests that one mechanism for the suppressive effect of T cell-derived TNF on antigen-specific T cells occurs as a result of TNFR signaling in the host cells. However, the donor TNF-deficient P14 TCR transgenic CD8+T cells still accumulated to higher frequency and numbers compared to their donor WT transgenic counterparts. Together, these findings indicate that T cell-produced TNF can function both in an autocrine and a paracrine fashion to limit the magnitude of anti-viral T cell responses. Given the immunoregulatory role of TNF and the ability of peripheral naïve T cells to produce this cytokine, I questioned at what stage of development do T cells become licensed to produce this cytokine. The peripheral naïve T cell pool is comprised of a heterogeneous population of cells at various stages of development, a process that begins in the thymus and is completed after a post-thymic maturation phase in the periphery. I hypothesized that naïve T cells emigrating from the thymus will be competent to produce TNF only after undergoing a maturation process in the periphery. To test this hypothesis, I compared cytokine profiles of CD4+ and CD8+single positive (SP) thymocytes, recent thymic emigrants (RTEs) and mature-naïve (MN) T cells during TCR activation. SP thymocytes exhibited a poor ability to produce TNF when compared to splenic T cells despite expressing similar TCR levels and possessing comparable activation kinetics with respect to the upregulation of CD25 and CD69 following stimulation. The reduced ability of SP thymocytes to produce TNF correlated with a decreased level of detectable TNF message following stimulation when compared to splenic counterparts. Stimulation of SP thymocytes in the context of a splenic environment did not fully enable TNF production, suggesting an intrinsic defect in their ability to produce TNF as opposed to a defect in antigen presentation. Using a thymocyte adoptive transfer model, I demonstrate that the ability of T cells to produce TNF increases progressively with time in the periphery as a function of their maturation state. RTEs identified by the expression of green fluorescent protein (GFP) (NG-BAC transgenic mice), showed a significantly enhanced ability to express TNF relative to SP thymocytes, but not to the extent of MN T cells. Together, these findings suggest that TNF expression by naïve T cells is regulated via a gradual licensing process that requires functional maturation in peripheral lymphoid organs. This highlights the functional heterogeneity of the naïve T cell pool (with respect to varying degrees of TNF production) during early T cell activation that can contribute to the many subsequent events that shape the course of an immune response. The productive activation of naïve T cells requires at least initial two signals; the first being through the TCR and the second is the engagement of co-stimulatory molecules on the surface of the T cells. T cells activated in the absence of co-stimulation become anergic or undergo cell death. Agents that block co-stimulation of antigen-specific T cells are emerging as an alternative to immunosuppressive drugs to prolong allograft survival in transplant recipients. Targeted blockade of CD154-CD40 interactions using a αCD154 monoclonal antibody (MR1) with a simultaneous transfusion of allogeneic splenocytes (donor specific transfusion or DST) efficiently induces tolerance to allografts. This co-stimulation blockade-induced tolerance is characterized by the deletion of host alloreactive T cells within 24 hours of treatment. Toll-like receptor (TLR) agonists abrogate tolerance induced by co-stimulation blockade by impairing the deletion of host alloreactive T cells and resulting in allograft rejection. The goal of my study was to determine the underlying molecular mechanisms that protect host alloreactive T cells from early deletion after exposure to TLR agonists. I hypothesized that TLR ligands administered during co-stimulation blockade regimen differentially regulate the expression of pro- and anti-apoptotic molecules in alloreactive T cells, during the initial stages of activation thereby preventing deletion. To test this hypothesis, I used syngeneic bone marrow chimeric mice containing a trace population of alloreactive KB5 TCR transgenic CD8+ T cells (KB5 Tg CD8+ T cells) that recognize H-2Kb as an alloantigen. I show here that KB5-CD8+ T cells downregulate CD127 (IL-7R!) and become apoptotic as early as 12 hrs after co-stimulation blockade. In contrast, KB5 Tg CD8+ T cells from mice treated with bacterial lipopolysaccaride (LPS) during co-stimulation blockade failed to become apoptotic, although CD127 was downregulated. Examination of the mRNA expression profiles of several apoptotic genes in purified KB5 CD8+ T cells from mice treated with DST+anti-CD154 for 12 hrs revealed a significant upregulation of FasL mRNA expression compared to the untreated counterparts. However, in vitro FasL blockade or in vivo cytotoxicity experiments with mice deficient in Fas or FasL indicated that the Fas-FasL pathway might not be crucial for tolerance induction. Another pro-apoptotic molecule BIM was upregulated in alloreactive T cells during co-stimulation blockade. This suggests that both the Fas pathway and BIM may be playing complementary roles in inducing deletional tolerance. Although FasL expression was diminished in alloreactive T cells in the presence of LPS, BIM expression was not diminished, suggesting that alloreactive T cells may still be vulnerable to undergo apoptosis. Concomitantly, I also found that LPS treatment during co-stimulation blockade resulted in non-specific upregulation of Fas expression in alloreactive T cells and non-transgenic T cells (CD4+ and CD8+). I demonstrate here that treatment with Fas agonistic antibody in vitrofor 4 hours can selectively induce apoptosis of alloreactive T cells that were believed to be refractory to apoptosis during LPS treatment. I speculate that under these conditions, deletion may be occurring due to the involvement of both Fas and BIM. Further, the mRNA expression profile revealed interleukin-10 (IL-10) as a molecule induced in alloreactive T cells during LPS treatment. Analysis of serum confirmed the systemic expression of IL-10 protein in mice treated with LPS during co-stimulation blockade. I hypothesized that LPS-induced IL-10 can have an anti-apoptotic role in preventing the deletion of alloreactive T cells and mediating allograft rejection. Contrary to my hypothesis, I found that IL-10 KO mice rejected allogeneic target cells similar to their WT counterparts, suggesting that IL-10 may not be required for LPS-mediated abrogation of tolerance induction. In addition to the systemic induction of IL-10, LPS also induced cytokines such as interleukin-6 (IL-6), TNF and interferon-γ (IFN-γ). These findings suggest that both Fas-FasL and BIM mediated apoptotic pathways may play complementary roles in inducing the early deletion of activated alloreactive T cells during tolerance induction. On the other hand, the mechanism of LPS mediated abrogation of tolerance induction can not be attributed to IL-10 alone as it may be playing a synergistic role along with other proinflammatory cytokines that may in turn result in the prevention of alloreactive T cell death during this process. Most importantly, these findings indicate that despite emerging from a pro-inflammatory cytokine milieu, alloreactive T cells are still susceptible to undergo Fas-mediated apoptosis during the first 24 hours after co-stimulation blockade and LPS treatment. Therefore, targeting the Fas-FasL pathway to induce deletion of alloreactive T cells during the peri-transplant period may still be a potential strategy to improve the efficacy of co-stimulation blockade induced transplantation tolerance during an environmental perturbation such as inflammation or infection.