THE INFLUENCE OF TWO RECOMBINANT SOLUBLE HLAMOLECULES ON NK CELL-MEDIATED LYSIS OF CELLS WITH DIFFERENT HLA CLASS I EXPRESSION

Background – The lytic function of natural killer (NK) cells is markedly influenced by recognition of class I major histocompatibility complex molecules (MHC). The soluble form of human MHC molecules (sHLA) has been reported to be secreted by lymphocytes and is presented in the normal human serum. The aim of this study was to evaluate the possible immunomodulatory effect of sHLA on NK cell lytic function. Methods – HLA class I expression on three different tumor cell lines including M4, K562, and HSB-2 were determined using flowcytometry and then the susceptibility of the cells to the killing effect of NK- and CD56-positive lymphokine-activated killer (LAK) cells was measured using 51Cr-release assay. Different concentrations of an anti-HLA class I antibody (W6/32) were used to block class I antigens on the surface of target cells and cytotoxic assay was subsequently carried out. Recombinant sHLA-A2 and sHLA-B7 in monomeric and dimeric forms were used for treatment of NK and LAK cells. The killing activity of these effector cells against K562 and M4 cells was measured. Results – HLA class I molecules were abundantly presented on M4 cells followed by HSB-2. K562 cells expressed extremely low levels of HLA class I on the surface. In cytotoxicity assay, the most and the least susceptible cells to the lytic effect of NK cells were K562 (47.2% killing at 25/1 E/T [effector/target] ratio) and M4 cells (7.3% at 50/1 E/T ratio), respectively. Treatment of these cells with 1 and 10 μg/mL of W6/32 monoclonal antibody increased the susceptibility of M4 cells to lysis but had no effect on K562 cells. Treatment of effector cells with 0.7 to 11.2 μg/mL of sHLA molecules lead to a dose-dependent inhibition of LAK cell lysis in the presence of sHLAB7 molecules. In addition, sHLA-A2 decreased the lytic activity of LAK cells. In contrast to the effect of sHLA-B7 on K562 cells, an increase in NK cell activity on M4 cells was observed. Conclusion – M4 cells expressing high amounts of HLA class I were more resistant than other cells to the lytic effect of NK cells; masking these molecules could change M4 susceptibility. Binding of sHLA molecules to NK/LAK cells can downregulate the killing activity against cells with low HLA expression (K562) and upregulate the activity against the ones with high HLA expression.