Differential Effects of TCR Stimulation and IL-2 On Apoptotic Pathways in CD4 Regulatory T Cells Compared to Conventional CD4 T Cells and CD8 T Cells

Abstract 3280

CD4+CD25+Foxp3+ regulatory T cells (Treg) play an important role in the maintenance of self-tolerance and immune homeostasis and Treg deficiency contributes to the development of autoimmune diseases. CD4Treg, conventional CD4 T cells (Tcon) and CD8 T cells are derived from lymphocyte progenitor cells that differentiate into distinct functional subsets in the thymus before export to the peripheral circulation. As T cells differentiate and expand in the periphery, each T cell subset is differentially regulated and subjected to distinct homeostatic signals. For example, interleukin-2 (IL-2) is a critical regulator of Treg development, expansion and survival and lack of IL-2 results in selective Treg deficiency. In regulating Treg homeostasis, IL-2 has multiple and distinct effects on Treg differentiation, proliferation and susceptibility to apoptosis. To determine the mechanism whereby IL-2 affects susceptibility of Treg to apoptosis, we used a new flow cytometry-based assay (BH3 profiling) to measure the mitochondrial membrane depolarization in response to a panel of pro-apoptotic BH3 peptides (BIM, BID, BAD, NOXA, PUMA, BMF, HRK). This assay allowed us to compare “priming” which we define as susceptibility to BH3 peptide-induced mitochondrial membrane depolarization in different T cell subsets, including CD4 Treg, CD4 Tcon and CD8 T cells. We also examined cell surface expression of CD95 death receptor (Fas) and cytoplasmic expression of Bcl-2 and Ki67 as additional measures of susceptibility to apoptosis and proliferation in each subset. In resting blood obtained from healthy donors (n=10), CD4 Treg were more “primed” than either CD4 Tcon or CD8 T cells when exposed to several BH3 peptides (PUMA, BMF and the combination of BAD+NOXA). CD4 Treg were also found to have decreased expression of Bcl-2 and increased expression of CD95 and Ki67 compared to CD4 Tcon or CD8 T cells. Thus, Treg in healthy individuals have higher proliferative activity and are more susceptible to apoptosis than other major T cell subsets through both mitochondrial and death receptor pathways.

To establish the functional effects of TCR stimulation and IL-2, CD4 Treg, CD4 Tcon and CD8 T cells were purified by cell sorting and cultured for 5–6 days with or without TCR stimulation (1μg/ml anti-CD3 + 1μg/ml anti-CD28) and IL-2 (100 IU/ml). Results were compared to cells cultured in media alone. Results are summarized in the table below. CD4 Tcon and CD8 T cells responded in a similar fashion to either TCR stimulation alone or TCR plus IL-2. This response included increased BH3 priming, reduced expression of Bcl-2, increased expression of CD95 and increased proliferation (Ki-67). IL-2 alone had no effect on CD4 Tcon or CD8 T cells. In contrast, TCR stimulation alone had no effect on CD4 Treg but IL-2 alone reduced BH3 priming and increased expression of Bcl-2. Combined TCR stimulation plus IL-2 in Treg increased BH3 priming, reduced expression of Bcl-2, increased expression of CD95 and increased proliferation. Thus, TCR stimulation reversed the anti-apoptotic effects of IL-2 alone and markedly increased susceptibility of Treg to apoptosis. When compared with CD4 Tcon and CD8 T cells, these studies demonstrate distinct effects of TCR stimulation and IL-2 on both mitochondrial and death receptor pathways of apoptosis in CD4 Treg and define mechanisms whereby TCR stimulation and IL-2 interact to regulate Treg homeostasis.