AbstractAlthough CD1d and MHC class Ia share similar overall structure, they have distinct levels and patterns of surface expression. While the expression of CD1d1 is known to be essential for the development of NKT cells, the contribution of CD1d1 to the development of CD8+ T cells appears to be inconsequential. To investigate whether CD1d tissue distribution and expression levels confer differential capacity in selecting these two T cell subsets, we analyzed CD8 and NKT cell compartments in Kb-CD1d-transgenic mice that lack endogenous MHC class Ia and CD1d, respectively. We found that MHC class Ia-like expression pattern and tissue distribution are not sufficient for CD1d to rescue the development of CD8+ T cells, suggesting that unique structural features of CD1d preclude its active participation in selection of CD8+ T cells. Conversely, cell type-specific CD1d surface density is important for the selection of NKT cells, as the NKT cell compartment was only partially rescued by the Kb-CD1d transgene. We have previously demonstrated that increased CD1d expression on dendritic cells enhanced negative selection of NKT cells. In this study, we show that cell type-specific expression levels of CD1d establish a narrow window between positive and negative selection, suggesting that the distinct CD1d expression pattern may be selected evolutionarily to ensure optimal output of NKT cells.