MHC class II tolerant T cells undergo apoptosis upon re-exposure to tolerogen in vivo

Tolerance of MHC class II alloantigens can be achieved by intravenous injection of semiallogeneic hematopoietic cells into neonatal mice. Lymphoid cells of tolerant mice fail to proliferate or secrete interleukins IL-2 or IL-4 when stimulated in vitro with tolerogen. Since the lymphoid organs of B10.T(6R) tolerant mice contain normal levels of I-E reactive (Vβ11 +) CD4 + T cells, deletion of alloreactive T cells does not appear to be the mechanism involved in the tolerance induction. To test whether T cells from tolerant animals can become activated under conditions that do not involve alloantigen stimulation, we stimulated these cells with immobilized anti-Vβ11 antibodies. Spleen cells from grafted tolerant and rejector mice proliferated in response to anti-Vβ11 + antibodies, suggesting they were not inert. We then tested whether Vβ11 + T cells from grafted mice can be induced to proliferate following stimulation with alloantigen in vivo. We adoptively transferred T cells from grafted tolerant and rejector mice into irradiated (B10.AQR × B10.T(6R))F1 mice and harvested the lymphoid organs after 65 h. Cells from both grafted tolerant and rejector mice underwent blast transformation, but only cells from rejector mice proliferated when exposed to immobilized anti-Vβ11 antibodies. The failure of Vβ11 + cells from tolerant mice to proliferate after in vivo stimulation may be because they are apoptotic. To test this hypothesis, spleen cells from naive or neonatally tolerized (with (B10.AQR × B10.T(6R))F1 cells) B10.T(6R) mice were adoptively transferred into irradiated (B10.AQR × B10.T(6R))F1 mice and bcl-2 expression was analysed in harvested Vβ11 + cells. Large cells recovered from recipients of naive 6R cells expressed bcl-2 mRNA. By contrast, large cells harvested from recipients of tolerized 6R cells did not express bcl-2 mRNA, suggesting bcl-2 mRNA expression was downregulated in these mice. Moreover, in another experiment, large Vβ11 + cells from grafted tolerant animals recovered after transfer into irradiated (B10.AQR × B10.T(6R))F1 mice did not express the bcl-2 protein as determined by flow cytometry and contained fragmented DNA as assessed by the TUNEL method. Taken together, these data suggest that MHC class II tolerant T cells undergo apoptosis upon re-exposure to tolerogen in vivo.