Our experiments looking at the single cell biochemistry of lymphocytes in an immune response in vivo
have shown that endogenous Foxp3+ regulatory T cells receive signals from antigen-stimulated cells early in the response. Surprisingly, IL-2, generally believed to function as the key growth factor for antigen-reactive T cells, was identified as the “messenger” cytokine instructing Foxp3+ T cells to proliferate, increase expression of their essential transcription factor Foxp3, and enhance their suppressive activity. The autocrine action of IL-2 on the antigen-responding T cells, as measured by pSTAT5 induction, is not seen in the early phase of the response, despite high levels of CD25 expression. Since the absence of STAT5 signaling was also observed in cells stimulated in the absence of Tregs and competing endogenous T cell populations (), a cell-intrinsic inhibitory mechanism likely regulates IL-2 signal transduction. SOCS proteins play important roles in innate and adaptive immune responses by regulating cytokine responses (37
). SOCS-1 and SOCS-3 have been shown to inhibit STAT5 signals downstream of IL-2 (38
). SOCS-1-deficient T cells show enhanced responsiveness to γc cytokines and such uncontrolled cytokine responses may result in autoimmunity (37
). Thus the presence of SOCS-1 and SOCS-3 in naive T cells may explain their initial inability to transduce STAT5 signals in response to IL-2. Consequently, it would be predicted that T cells signaling through STAT5, such as Tregs and memory cells, express low levels of these inhibitory molecules. In support of this hypothesis, several recent studies have demonstrated a reduced presence of SOCS proteins in Tregs and activated T cells (34
), resulting in increased responsiveness to IL-2. Our results thus suggest that there are important biochemical differences in how Tregs respond to their environment as compared to the naïve T cells they are meant to regulate. On the other hand, memory cells responded to IL-2 in a similar manner as Tregs, phosphorylating STAT5 early after antigenic stimulation. Although the functional significance of these early STAT5 signals in memory cells is unclear at this time, some intriguing hypotheses can be made. For example, the generation of fast recall responses to antigen by memory cells may be dependent on their competence to receive early IL-2 signals. In addition, the enhanced sensitivity to cytokines of memory cells may render these cells less susceptible to regulation by Tregs, which potentially also contributes to their increased antigen reactivity.
What might be the functional consequences of the IL-2-dependent activation of endogenous regulatory T cells? It is likely that most endogenous Tregs are thymus-derived, “natural” Tregs, presumably specific for self-antigens (5
). It may be that early activation of these Tregs is a mechanism to minimize the risk of autoimmunity during an immune response to foreign antigens. Early activation of Tregs may also provide a constraint, or buffer, on immune responses to foreign antigens, in order to prevent pathologic side-effects of physiologic immunity. Only after this “safety net” of regulatory cells is established would the antigen-specific response develop. Although rather high precursor frequencies of TCR transgenic T cells were necessary to elicit STAT5 signaling in the majority of Tregs residing in the lymphoid organs, our experiments with vaccinia
virus infection demonstrate that a broad activation of Tregs is feasible under physiological circumstances. Also, while systemic Treg activation is unlikely after initiation of an immune response by a limited number of antigen-specific T cells, it is likely that Tregs localized in the vicinity of the responding T cells will receive IL-2 signals and control the environment where the response takes place. The strong activation of the regulatory compartment after the initiation of an immune response also raises the obvious question of how protective immune responses develop in the presence of a large number of activated Tregs. One possibility is that in the event of an infection specific signals, e.g. through the Toll pathway, are overriding the suppressors creating a permissive environment for the response to develop, as has been suggested by Pasare and Medzhitov (41
). In a prolonged or secondary response, effector/memory T cells may directly inhibit Treg generation (42
Thus, IL-2 may be the prototype of a signaling molecule that serves to first establish controls on immune responses and then to promote the development of such responses. Such a scenario also suggests that limiting the transient IL-2-dependent activation of regulatory cells may serve to maximize the effectiveness of vaccines.