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CD45‐mediated regulation of LFA1 function in human natural killer cells. Anti‐CD45 monoclonal antibodies inhibit the calcium mobilization induced via LFA1 molecules

The TA218 and T205 monoclonal antibodies (mAb) were selected on the basis of their ability to inhibit the non‐major histocompatibility complex‐restricted lysis of the murine mastocytoma P815 cell line mediated by CD3−CD16+ natural killer (NK) cells. Both mAb were found to react with CD45 molecules, as demonstrated by immunoprecipitation after surface iodination and western blot analysis. A panel of tumor target cells susceptible to lysis by polyclonal or clonal CD3−CD16+ NK cells was used to study the mAb‐mediated inhibitory effect. The inhibition of cytolysis mediated by TA218 and T205 mAb was found to consistently parralel the inhibition mediated (with the same tumor target cells) by the anti‐LFAlα mAb TS.1.22 or by the anti‐LFA1β mAb TS.1.18. However, different from the anti‐LFAl mAb, T205 or TA218 mAb did not inhibit the binding of activated CD3−CD16+ effector NK cells to the same tumor target cells. This finding supported the concept that the anti‐CD45 mAb‐mediated inhibition could occur at a post‐binding stage. In polyclonal or clonal CD3‐CD16+ NK cellsT205 orTA218 mAb were found to reduce by 50–70 % the intracellular Ca++ ([Ca++]i) mobilization induced by anti‐LFAlα or anti‐LFA1β mAb. On the other hand, TA218 and T205 mAb did not inhibit the Ca++ mobilization induced by anti‐CD 16 mAb or phytohemagglutinin, thus suggesting that, in NK cells, CD45 molecules may exert a selective inhibitory effect on the signal transduction mediated by LFA1 molecules. In line with this hypothesis, the cytolytic activity of human NK clones was triggered in the presence of the hybridoma cells secreting either anti‐CD16 or anti‐LFAla mAb (as “triggering targets”). This effect of anti‐LFAlα, but not of anti‐CD16 hybridoma was susceptible to inhibition by the anti‐CD45 mAb T205 or TA218. Further, experiments on cloned NK cells indicated that T205 or TA218 mAb induced a strong decrease in the constitutive phosphorylation of the LFAlα chain (but not of HLA class I antigens). Taken together, these studies suggest that in human NK lymphocytes, CD45 molecule may regulate both the activation state and the function of the LFA1 molecule.