EXPRESSION OF BETA-2M-FREE HLA CLASS-I HEAVY-CHAINS IN NEUROBLASTOMA CELL-LINES

Flow cytometry with the specific monoclonal antibody (MoAb) L31 was us ed to analyse the expression of HLA class I heavy chains not bound wit h beta2-microglobulin (beta2m) by neuroblastoma (NB) cell lines IMR-32 and LA-N-1. The cells, which express barely detectable amounts of bet a2m-free (L31-positive molecules) and beta2m-complexed HLA class I ant igens (W6.32- and BBM.1-reactive molecules), expressed MHC class I mol ecules not bound to light chains upon differentiation with either reti noic acid or serum starvation. The expression was not accompanied by a n increase of surface heterodimers. Conversely, recombinant interferon -gamma (rIFN-gamma) treatment led IMR-32 and LA-N-1 cells to almost ex clusively express beta2-complexed HLA class I heavy chains. Surface be ta2m-free MHC class I molecules displayed a molecular mass of approxim ately 45 kDa and did not bind exogenously added beta2m. No changes in the synthesis of either HLA class I and beta2m mRNAs or of L31 protein s were observed in differentiated NB cells, thus suggesting that the s urface exposure of unusual HLA class I antigens is regulated post-tran slationally. These findings indicate that, in addition to activated ly mphocytes, the surface expression of beta2m-free class I heavy chains is a feature of other cell types, such as NB cells.