Positive selection of extrathymically developed T cells by self-antigens

Most T cells develop through the thymus, where they undergo positive and negative selection. Some peripheral T cells are known to develop in the absence of thymus, but there is insufficient information about their selection. To analyze the selection of extrathymically developed T cells, we reconstituted thymectomized male or female recipient mice with bone marrow cells of mice transgenic for male H-Y antigen-specific T cell receptor (TCR). It was revealed that the T cells bearing self-antigen-specific TCR were not deleted in thymectomized male recipients. More importantly, the absence of H- Y antigen-specific T cells in thymectomized female recipients suggests positive selection of extrathymically developed T cells by the self-antigen. The extrathymically developed T cells in male mice expressed interleukin (IL)-2 receptor β chain (IL-2Rβ) and intermediate levels of CD3 (CD3(int)) but were natural killer cell (NK) 1.1^-. They rapidly produced interferon γ but not IL-4 after TCR cross-linking. Furthermore, a similar pattern of cytokine production was observed in CD3(int)IL-2RB⁺NK1.1^- cells in normal mice which have been shown to develop extrathymically. These results suggest that extrathymically developed CD3(int)IL-2Rβ⁺NK1.1^- cells in normal mice are also positively selected by self-antigens.