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^intIL-2Rβ⁺NK1.1⁻ cells in normal mice which have been shown to develop extrathymically. These results suggest that extrathymically developed CD3^intIL-2Rβ⁺NK1.1⁻ cells in normal mice are also positively selected by self-antigens.