Regulatory T cells tonically inhibit spontaneous activation of naturally arising memory-phenotype CD4⁺ T lymphocytes

Foxp3^- CD4⁺ T lymphocytes consist of naïve (CD44^lo), foreign antigen-specific memory, and self antigen-driven memory-phenotype (MP; both CD44^hi) cells at homeostasis. We recently showed that self-reactive MP cells spontaneously proliferate and differentiate into the T-bet⁺ subset. How excess proliferation and Th1 differentiation of MP cells are inhibited in steady state remains unclear. Regulatory T cells (Tregs) can inhibit several immune responses, posing a possibility that the same lymphocytes may suppress spontaneous MP cell activation. Here we show using Foxp3-diphtheria toxin receptor-transgenic mice that both preexisting MP and naïve CD4⁺ T cells spontaneously proliferate and differentiate into Th1 cells upon Treg depletion. MP as compared to naïve CD4⁺ T cells more rapidly proliferate and differentiate three to five days post Treg depletion whereas these responses are mainly attributed to by the latter T cells at a later time point. Moreover, we demonstrate that these early MP proliferation and differentiation are driven in a manner dependent on CD28 and IL-2 signals, respectively. Furthermore, our data suggest that such activation of MP and naïve CD4⁺ T cells contribute to development of multi-organ inflammation at early and later time points, respectively. Together our results argue that Tregs tonically inhibit early, spontaneous proliferation and Th1 differentiation of MP CD4⁺ T lymphocytes, thereby contributing to maintenance of T cell homeostasis.