Different enzyme classes associated with human natural killer cells may mediate disparate functions.

Abstract

Abstract Previous studies have shown that degradation of the acute phase reactant serum amyloid A (SAA) is mediated by enzymes on the plasma membrane of lymphocytes and monocytes. The responsible enzymes had properties of neutral elastases. The present investigations were conducted to explore whether human NK cells enriched by Percoll gradient centrifugation have similar activity and if so, whether the same or different enzyme classes are responsible for proteolysis as well as for tumor cell lysis. Accordingly, human NK cells were enriched on discontinuous Percoll gradients after which the cells were incubated either with SAA or with [3H] proline-labeled melanoma cells at various effector to target cell ratios. When SAA degradation was followed by SDS-polyacrylamide gel electrophoresis, NK fractions proved to be as effective in digesting the protein as unfractionated mononuclear leukocytes. To characterize the enzymes that may be involved in cytotoxicity on the one hand, and SAA degradation on the other, the NK fractions were treated with the following inhibitors: diisopropylfluorophosphate (DFP), soybean trypsin inhibitor, N-p-tosyl-L-lysine chloromethylketone (TLCK), the elastase inhibitors elastatinal, Ac-Ala-Ala-Pro-Val-CH2Cl, Meo-Suc-Ala-Ala-Pro-Val-CH2Cl, and an inhibitor of aryl sulfatase, Na2SO4. Preincubation of the cells with DFP or elastase inhibitors abolished their ability to hydrolyze SAA but did not affect their ability to kill tumor cells. On the other hand TLCK, a potent inhibitor of cytotoxicity, did not bring about any reduction in the proteolysis of SAA. DFP and Na2SO4 diminished cytotoxicity partially. Elimination of NK cells by sorting after incubation of lymphocytes with the monoclonal antisera Leu-7 and Leu-11 abolished cytotoxicity as well as proteolysis. The observations are compatible with the concept that NK cells carry several enzymes with different substrate specificities that may be involved in disparate cellular functions.