Major histocompatibility complex class II invariant chain expression in non-antigen-presenting cells

In contrast to the generally accepted belief, the major histocompatibility complex (MHC) class II invariant chain (Ii) is commonly expressed intracell ularly in cells that do not present exogenous antigens. Such cells include resting peripheral blood T cells and natural killer (NK) cells. In T cells, the Ii is associated with a 77 000 molecular-weight molecule (p77) that ha s yet to be identified. This molecule is co-precipitated with the anti-Ii m onoclonal antibody (mAb) VCD-1, but not with mAb BU-45. This suggests that in the p77-Ii complex, the extracellular epitope of Ii recognized by BU-45 is hidden, whereas the Ii epitope for VCD-I remains exposed. In antigen-pre senting cells (APCs), p77 association with the Ii was minimal, if detectabl e. The p77-Ii association in non-professional APCs suggests that the Ii may have another, more general, function other than the one accepted in antige n presentation.