Modulation of function-associated molecules on natural killer cells alters recognition and cytotoxicity

Natural killer (NK) cells constitute a minor lymphoid population distinct from antigen-specific T and B cells. NK cells express a non-major histocompatibility complex (MHC)-restricted lytic activity without need for prior sensitization with antigen. The role of NK cells in the immune system is unclear, but NK cells have been implicated in immune surveillance against developing tumors, destruction of established malignancies, as well as the spread of metastatic foci, regulation of B cell proliferation and immunoglobulin (Ig) secretion, resistance to bone marrow transplantation, and protection of the host against microbial and viral infections. 1 , 2 NK cells have been found in numerous vertebrate species including cattle, swine, lower primates, and man. In fact, NK cells have been observed in species in which there is inconclusive evidence for the existence of cytolytic T cells. 3 Based on these observations it has been hypothesized that NK cells appeared early in the course of the evolution of the immune system and possibly function as a "primitive" cytolytic cellular protective mechanism. Despite intensive investigations on NK cells for the past 30 years, there is little evidence regarding the identification of either the target cell antigen(s) recognized by NK cells, or of the antigen receptor(s) used by NK cells for this purpose. It has been postulated that NK cells recognize either the presence or absence of normal antigens; e.g., MHC, Hh, and differentiation antigens. However, to date there is no conclusive proof for these postulations. Recently, NK-like cells (termed NCC for nonspecific cytotoxic cells) have been demonstrated in a lower vertebrate species, the teleost fish. 4 It was observed that these effector cells were readily capable of recognizing and killing a variety of transformed rodent and human cell lines. This observation, along with the fact that NCC were able to lyse the naturally occurring protozoan fish parasite Tetrahymena, led us to speculate that NK cells recognize an evolutionarily conserved antigenic structure and would utilize an evolutionarily conserved receptor to accomplish this function. To examine this possibility, mice were immunized with purified fish NCC and the resulting monoclonal antibody (MoAb)-secreting hybridomas were screened with NCC for binding and inhibition of NCC cytolysis. Four MoAb were identified (named 5C6, 6D32, 6D34, and 2B2) that specifically bound to fish NCC. Two of these MoAb (5C6 and 6D32) were found to inhibit NCC lysis, while the other two MoAb (6D34 and 2B2) bound to but had no effect on NCC killing. Each of the anti-NCC MoAb reacted with an identical cell surface molecule as determined by affinity chromatography and Western blotting. This molecule was observed to be a dimer consisting of 43 and 38 kDa polypeptides. 4 Thus, for the first time, an NK cell-specific molecule had been identified which had the requisite characteristics to qualify it to be a putative NK cell antigen receptor. These anti-NCC MoAb then were analyzed for their effects on NK cells from various species. In this fashion it was possible to identify a novel function-associated molecule (FAM) expressed on NK cells and to examine its role in NK cell function.