Induction of tolerance and immunity by redirected B cell-specific cytolytic T lymphocytes

Chimeric receptors bearing ligand recognition domains linked to signaling regions from the T-cell receptor can redirect T lymphocytes against non-MHC-restricted targets. Cytolytic T lymphocytes (CTL) expressing these chimeric receptors are being tested in preclinical and clinical trials for activity in cancer, infectious diseases and autoimmunity. The chimeric receptors may incorporate antigenic epitopes previously unrecognized by the immune system. Whether a receptor-specific antibody response develops to these neoantigens and whether such a response inhibits therapeutic cell activity is unknown. We hypothesized that upon engagement of a chimeric receptor-specific B cell, receptor-modified CTL will be activated, lysing the B cell and inducing tolerance to the chimeric receptor rather than immunity. We demonstrate that receptor-modified CTL are indeed stimulated by cognate receptor-specific B cells, proliferate and produce cytokines in response and kill the B cells in vitro and in vivo. However, this is insufficient to induce full B-cell tolerance. Modified CTL induce a chimeric receptor-specific antibody response independent of any other source of antigen. Nevertheless, the CTL retain substantial activity even in the presence of saturating doses of receptor-specific antibody. Thus antichimeric receptor antibody responses need to be considered in the clinical use of chimeric receptor-modified T cells. However, the inhibitory activity of these antibodies may in cases be limited.

Uncoupling of IL-2 Signaling from Cell Cycle Progression in Naive CD4+ T Cells by Regulatory CD4+CD25+ T Lymphocytes

Prior reports have shown that CD4(+)CD25(+) regulatory T cells suppress naive T cell responses by inhibiting IL-2 production. In this report, using an Ag-specific TCR transgenic system, we show that naive T cells stimulated with cognate Ag in the presence of preactivated CD4(+)CD25(+) T cells also become refractory to the mitogenic effects of IL-2. T cells stimulated in the presence of regulatory T cells up-regulated high affinity IL-2R, but failed to produce IL-2, express cyclins or c-Myc, or exit G(0)-G(1). Exogenous IL-2 failed to break the mitotic block, demonstrating that the IL-2 production failure was not wholly responsible for the proliferation defect. This IL-2 unresponsiveness did not require the continuous presence of CD4(+)CD25(+) regulatory T cells. The majority of responder T cells reisolated after coculture with regulatory cells failed to proliferate in response to IL-2, but were not anergic and proliferated in response to Ag. The mitotic block was also dissociated from the antiapoptotic effects of IL-2, because IL-2 still promoted the survival of T cells that had been cocultured with CD4(+)CD25(+) T cells. IL-2-induced STAT5 phosphorylation in the cocultured responder cells was intact, implying that the effects of the regulatory cells were downstream of receptor activation. Our results therefore show that T cell activation in the presence of CD4(+)CD25(+) regulatory T cells can induce an alternative stimulation program characterized by up-regulation of high affinity IL-2R, but a failure to produce IL-2, and uncoupling of the mitogenic and antiapoptotic effects of IL-2.

Antigen Nonspecific Suppression of T Cell Responses by Activated Stimulation-Refractory CD4+ T Cells

Several classes of anergic T cells are capable of suppressing naive T cell proliferation and thereby limiting immune responses. Activated T cells, although not anergic, are transiently refractory to restimulation with Ag. We examine in this study whether activated refractory murine T cells can also suppress naive T cell responses. We find that they can, and that they exhibit many of the suppressive properties of anergic T cells. The activated cells strongly diminish Ag-mediated T cell proliferation, an activity that correlates with their refractory period. Suppression is independent of APC numbers and requires cell contact or proximity. Naive T cells stimulated in the presence of activated refractory cells up-regulate CD25 and CD69, but fail to produce IL-2. The addition of IL-2 to culture medium, however, does not prevent the suppression, which is therefore not solely due to the absence of this growth factor. Persistence of the suppressor cells is also not essential. T cells stimulated in their presence and then isolated from them and cultured do not divide. The suppressive cells, however, do not confer a refractory or anergic state on the target T lymphocytes, which can fully respond to antigenic stimulation if removed from the suppressors. Our results therefore provide evidence that activated T cells act as transient suppressor cells, severely constraining bystander T cell stimulation and thereby restricting their response. These results have potentially broad implications for the development and regulation of immune responses.