Background.The invariant chain plays a crucial role in antigen presentation by influencing the expression and peptide loading of major histocompatibility complex (MHC) class II molecules. Therefore, coordinate expression of these molecules is important for antigen presentation.Methods.Immunohistological studies were performed on frozen sections of many rat tissues in order to examine expression of invariant chain and MHC class II antigens.Results.Although coordinately regulated in most tissues, the interstitial dendritic cell (and the renal tubular epithelial cell) was always negative for invariant chain, while strongly positive for MHC class II antigens. However, renal tubular epithelial cells strongly expressed invariant chain during kidney graft rejection.Conclusions.The absence of invariant chain in interstitial dendritic cells is unexpected, in view of their presumed function as sentinel antigen-presenting cells in the connective tissues. This might have important implications for antigen presentation for tolerance and immunity.