Effector CD4 cell tolerization is mediated through functional inactivation and involves preferential impairment of TNF-alpha and IFN-gamma expression potentials.

It has recently been shown that effector/memory T cells can undergo peripheral tolerization in response to self-antigen. In the present study, we found that within 24h self-antigen profoundly impairs the ability of CD4 effectors to express TNF-alpha (and to a lesser extent IFN-gamma); however, several days of self-antigen exposure is required to impair non-effector functions such as IL-2 expression and proliferation. Since only half of the initial effector CD4 cell population expresses effector cytokines following brief antigenic stimulation, tolerization might have been mediated either through functional inactivation of effector-competent cells, or alternatively by the selective deletion of competent and expansion of non-competent cells. When briefly stimulated effectors were fractionated based on their expression of IFN-gamma, the IFN-gamma(-) sub-population was able to express IFN-gamma following secondary stimulation, indicating that all effector CD4 cells are functionally competent. Furthermore, both IFN-gamma(+) and IFN-gamma(-) sub-populations underwent tolerization in response to self-HA (although the former was slightly more prone to deletion at later time points). Thus, effector CD4 cell tolerization is mediated primarily through the functional inactivation of effector-competent cells.

CD4+ T cells pass through an effector phase during the process of in vivo tolerance induction.

An important process in the generation of tolerance to peripheral self-Ags is the induction of unresponsiveness in mature specific T cells. Although the end stage of this process, termed anergy, is well defined, the pathway by which naive T cells become anergic remains to be elucidated. Using an in vivo self-tolerance model, we demonstrate that CD4(+) T cells pass through a significant effector stage on their way to an anergic state. This stage is characterized by production of effector cytokines, provision of help for CD8(+) T cells, and induction of in vivo pathology within organs that express cognate Ag. These results suggest that the initial activation stage in T cell tolerance is similar to that seen in memory induction. They also suggest that autoimmune pathology can result during the natural process of tolerance induction rather than requiring that tolerance be broken.

Effector CD4 cells are tolerized upon exposure to parenchymal self-antigen.

It has long been established that exposure of naive T cells to specific Ag in the absence of adjuvant leads to tolerization. Nonetheless, the potential of effector CD4 cells to be tolerized has been less well characterized. To address this issue, we have used an adoptive transfer system in which naive TCR transgenic hemagglutinin (HA)-specific CD4(+) T cells are initially primed to express effector function upon exposure to an immunogenic recombinant vaccinia virus expressing HA, and then exposed to forms of HA that are tolerogenic for naive CD4 cells. HA-specific effector CD4 cells residing in both the spleen as well as in two separate nonlymphoid tissues were tolerized upon exposure to high doses of exogenous soluble HA peptide. Additionally, tolerance could also be induced by bone marrow-derived APCs that cross-present parenchymally derived self-HA. Thus, effector CD4 cells are susceptible to similar tolerogenic stimuli as are naive CD4 cells.

CD4 cell priming and tolerization are differentially programmed by APCs upon initial engagement.

Bone marrow-derived APCs present both parenchymal-self and pathogen-derived Ags in a manner that elicits either T cell tolerization or immunity, respectively. To study the parameters that confer tolerogenic vs immunogenic APC function we used an adoptive transfer system in which naive TCR transgenic hemagglutinin (HA)-specific CD4(+) T cells are either tolerized upon encountering HA expressed constitutively as a parenchymal self-Ag (self-HA) or primed to express effector function upon encountering transiently expressed vaccinia-derived HA (viral-HA). When the duration of viral-HA presentation was extended for the period required to elicit tolerization toward self-HA, CD4 cell tolerization to viral-HA did not occur. Furthermore, CD4 cells exhibited both phenotypic as well as functional differences during early stages of tolerization and priming, suggesting that these divergent differentiation processes are programmed soon after the initial APC-CD4 cell interaction. When mice expressing self-HA were infected with an irrelevant vaccinia, CD4 cell tolerization still occurred, indicating that priming vs tolerization cannot be explained by pathogen-induced third parties (i.e., non-APCs) that act directly on CD4 cells. Taken together, these results suggest that CD4 cell tolerization to parenchymal self-Ags and priming to pathogen-derived Ags are initiated by functionally distinct APCs.