Development of CD4+ T cells expressing a nominally MHC class I-restricted T cell receptor by two different mechanisms

Differences in T cell receptor (TCR) signaling initiated by interactions among TCRs, coreceptors, and self-peptide-MHC complexes determine the outcome of CD4 versus CD8 lineage of T cell differentiation. The H-2L(d) and K-bm3 alloreactive 2C TCR is positively selected by MHC class I K-b and a yet-to-be identified nonclassical class I molecule to differentiate into CD8(+) T cells. Here we describe two mechanisms by which CD4(+) 2C T cells can be generated in 2C TCR-transgenic mice. In the RAG(-/-) background, development of CD4+ 2C T cells requires the expression of both I-Ab and the TAP genes, indicating that both MHC class I and 11 molecules are required for positive selection of these T cells. Notably, only some of the 2C(+)RAG(-/-) mice (approximate to 30%) develop CD4+ 2C T cells, with frequencies in individual mice varying from 0.5% to as high as approximate to 50%. In the RAG(+) background, where endogenous TCR alpha genes are rearranged and expressed, CD4+ 2C T cells are generated because these cells express the 2C TCR as well as additional TCRs, consisting of the 2C TCR beta and endogenous TCRa chains. Similarly, T cells expressing the OT-1 TCR, which is nominally MHC class I-restricted, can also develop into CD4+ T cells through the same two mechanisms. Thus, expression of two TCRs by a single thymocyte, TCR recognition of multiple MHC molecules, and heterogeneity of TCR, coreceptors, and peptide-MHC interactions in the thymus all contribute to the outcome of CD4 versus CD8 lineage development.