The origin and fate of beta 2m-free MHC class I molecules induced on activated T cells.

We report here that the expression of major histocompatibility complex (MHC) class I heavy chains not associated with beta 2-microglobulin is induced on resting human T cells by a variety of stimuli. These beta 2m-free class I heavy chains are not transported as such from the endoplasmic reticulum but originate from surface beta 2m-associated MHC class I molecules. beta 2m-free class I heavy chains are spontaneously released from the surface of activated cells. Cross-linking of beta 2m-free class I heavy chains with specific monoclonal antibodies results in the rapid down-regulation and internalization of these molecules. In contrast, beta 2m-associated MHC class I molecules display a different pattern of modulation. Previously, we reported that beta 2m-free class I heavy chains interact with CD8 molecules expressed on the same activated T cells. We propose that interactions between these molecules are involved in a mechanism regulating the function of activated T cells.