EXPRESSION OF LA45 REACTIVE BETA(2)-MICROGLOBULIN FREE HLA CLASS-I ALPHA-CHAINS ON ACTIVATED T-CELLS IS REGULATED BY INTERNALIZATION, CONSTITUTIVE AND PROTEIN-KINASE-C INDUCIBLE RELEASE

HLA Class I molecules on activated T cells are expressed as mAb W6/32 reactive heterodimers associated with beta(2)-microglobulin (beta(2)-m ) and also as mAb LA45 reactive beta(2)-m free HLA Class I alpha-chain s. However, the regulation of free alpha-chain expression remained eni gmatic. Here we show, that the amount of cell surface expressed free h eavy chains is influenced by two distinct mechanisms. Firstly, a propo rtion of expressed molecules are cleaved and give rise to a soluble po ol of HLA Class T molecules. We provide evidence that, besides the pre viously described constitutive release of free alpha chains, a second phorbol ester inducible release mechanism involving activation of prot ein kinase C (PKC) does exist. We demonstrate that both the constituti ve and the enhanced release of LA45 reactive HLA Class I alpha-chains are the consequence of a cell membrane bound proteolytic activity with the characteristics of a 1, 10 phenanthroline sensitive metalloprotea se. Secondly, we report that a distinct fraction of mAb tagged free al pha-chains is internalized via an n-ethylmaleimide Sensitive pathway. Together this data suggests that the expression of free alpha-chains i s regulated by pathways governing release and internalization.