GENERAL ROLE OF HLA CLASS-I MOLECULES IN THE PROTECTION OF TARGET-CELLS FROM LYSIS BY NATURAL-KILLER-CELLS - EVIDENCE THAT THE FREE HEAVY-CHAINS OF CLASS-I MOLECULES ARE NOT SUFFICIENT TO MEDIATE THE PROTECTIVE EFFECT

Some HLA-C alleles have been shown to exert a specific protective effe ct preventing target cells from lysis by groups of natural killer (NK) clones displaying a defined specificity. In this study, we analyzed w hether class I-mediated protection is a more general phenomenon involv ing ail NK cells. First, we utilized two anti-class I mAbs (6A4 of IgG 1 isotype and A6-136 of IgM isotype), which had been shown to induce l ysis of protected target cells by group 1 and group 2 NK clones, Addit ion of A6-136 or 6A4 used as F(ab')(2) mAb resulted in lysis of protec ted target cells by all NK clones analyzed. Target cells were represen ted by a panel of HLA homozygous Epstein-Barr virus-transformed a cell lines (B-EBV) while NK clones were representative of clones displayin g different GL183/EB6 surface phenotypes and/or different abilities to lyse allogeneic cells, Unselected NK clones derived from seven differ ent individuals were tested against autologous target cells represente d by phytohemagglutinin-induced blasts or B-EBV transformed cell lines , In both instances, addition of a mixture of 6A4 F(ab')(2) and A6-136 mAbs resulted in lysis of autologous target cells, thus suggesting th at class I molecules prevent lysis of normal cells by self NK cells, W e further investigated whether the class I-mediated protection require s the complexed form of class I molecules (composed of alpha chain, be ta(2)-microglobulin and the antigen peptide) or rather the free alpha chain, Acidic treatment of the C1R (Cw4(+)) target cells or 81.22 (Cw3 (+), Cw4(+)) at pH 2.2 resulted in loss of reactivity with 6A4, A6-136 and W6-32 mAb (known to react with the assembled form of class I mole cules) and in the de novo reactivity with L31 mAb (specific for the HL A-C free chain). While the untreated Cw4(+) C1R cells were resistant t o lysis by the Cw4-specific group 1 NK clones, the pH 2.2-treated cell s became highly susceptible to lysis by the same clones. These data in dicate that, at least for the NK clones analyzed, the protection of ta rget cells requires class I molecules in the complexed form.