Recombinant natural killer enhancing factor augments natural killer cytotoxicity

Natural killer enhancing factor (NKEF) was originally identified and studied because of its ability to enhance NK cytotoxicity in vitro. After cloning the two genes responsible for NKEF proteins, NKEF-A and -B, we found that they belong to a newly described and highly conserved antioxidant gene family. We have now produced recombinant proteins of both genes and used them to test for their ability to promote NK cytotoxicity. Although recombinant NKEF (rNKEF)-A and -B have similar levels of antioxidant function, only the reduced form of rNKEF-A can enhance NK cytotoxicity. These results indicate that both the antioxidant and NK-enhancing functions of rNKEF-A and -B probably involve the cysteine residues of the proteins but are mediated by separate domains of the molecules. We pretreated both effector cells and target cells to investigate which population was influenced by rNKEF-A, and determined that the protein must be present during the cytotoxicity assay to enhance the activity. Despite the similarities between NK cytotoxicity and lymphokine-activated killer (LAK) cytotoxicity, rNKEF-A is not effective in augmenting LAK cytotoxicity. Therefore, rNKEFs can be useful tools in not only protecting cells from oxidative damage, but also in selectively promoting NK cytotoxicity against certain tumor cells.