Masking of HLA class I molecules expressed on K-562 target cells can restore their susceptibility to NK cell cytolysis

The aim of our study was to follow whether class I HLA antigens play a role in the susceptibility of K-562 target cells to NK cell lysis as well as in the interaction between NK and target cells. After K-562 cells were treated with rIFN-gamma, HLA class I antigens appeared on their surface. Those HLA class I+, in comparison to HLA class I-, K562 target cells became about 40-50% less susceptible to NK cell cytolysis. This influence of HLA class I antigens was abrogated when class I+ K-562 cells were incubated with anti-HLA class I monoclonal antibodies Bra 23/9. The restoration of susceptibility of these targets to NK cell cytolysis was not caused by ADCC mechanism as determined by F(ab')2 fragments or by mAb CD16. Further, the percentage of conjugates between effector and target cells was not significantly altered neither when HLA class I antigens appeared on the surface of K-562 target cells nor when HLA class I+ K-562 cells were incubated with anti-HLA class I monoclonal antibodies. To study this question, we used a different approach because both expression of HLA class I molecules and their masking with appropriate monoclonal antibody were done by means of the same cell line of target cells.