Expression of beta 2-microglobulin-free HLA class I alpha-chains on activated T cells requires internalization of HLA class I heterodimers.

HLA class I molecules on activated T cells are expressed as heterodimers associated with beta 2-microglobulin (beta 2-m) and also beta 2-m-free HLA class I alpha-chains. Mechanisms leading to the expression of the activation associated beta 2-m-free HLA class I alpha-chains are poorly defined, however. Upon enzymatical removal of HLA class I alpha-chains on activated T cells, re-expression is observed within minutes upon reculture, reaching half-maximal levels within 1 hr. This process is independent of de novo protein synthesis and of export of newly synthesized proteins. Inhibition of the formation of coated pits by potassium depletion of cells abrogated the re-expression of HLA class I alpha-chains, suggesting that recycling events of HLA class I heterodimers via endosomal compartments are required for the generation of monoclonal antibody LA45-reactive alpha-chains. Furthermore, the rate of alpha-chain generation seems to be governed by the amount of cell surface-expressed HLA class I heterodimers. Taken together these findings suggest that beta 2-m-free HLA class I alpha-chains are generated during the process of class I heterodimer recycling.