HUMAN INTERLEUKIN-2-ACTIVATED ADHERENT NATURAL-KILLER-CELLS RECOGNIZEA CONSERVED ANTIGEN FOUND ON TUMOR-CELLS AND PROTOZOAN PARASITES

Plastic adherent interleukin-2-activated human natural killer (NK) cel ls (ALAK) lyse many different histological types of tumor target cells . In order to effect their function as cytotoxic mediators of innate i mmunity, ALAKs may 'recognize' antigen(s) of protozoan parasites, sele ct virus-infected cells and they may release certain cytokines in resp onse to bacterial antigens. In the present study, we demonstrate that CD3-/CD56(dim)/CD16(dim)/monoclonal antibody 5C6(bright) human ALAKs b ind to an antigenic determinant on tumor cells independent of target c ell H-2 allotype expression. The conserved antigen was originally obta ined from the protozoan Tetrahymena pyriformis, however it is also loc ated on the membranes of many ALAK-sensitive tumor cells. The sequence of this protein, i.e. NK target antigen/NKTag, was previously deduced from cDNA. One ALAK cognate determinant of NKTag was identified by in hibition of cytotoxicity using NKTag-derived synthetic peptides. Bioti nylated synthetic peptide [amino acids (aa) 58-74] bound to ALAKs, and synthetic peptides corresponding to this sequence inhibited ALAK lysi s of U937 target cells. Inhibition effects of peptide binding were non reversible. To determine the requirements for recognition by ALAKs of this antigenic determinant, the cognate peptide aa 55-74 was truncated to 17-, 14-, 10-, 7- and 6-mer lengths and tested for inhibition of c ytotoxicity. All inhibited except the 6-mer. A possible mechanism of p eptide inhibition of cytotoxicity following ALAK binding to an antigen ic determinant was a requirement for recognition of one anchor peptide (arginine) and receptor occupancy by a minimum of five to six additio nal amino acids. In antibody-dependent cell-mediated cytotoxicity expe riments, synthetic peptide (aa 68-74) inhibited ALAK killing of anti-H -2(d)-sensitized P815 targets. This same peptide also inhibited conven tional lysis of nonsensitized P815 and IM-9 targets. However, the cogn ate synthetic peptide (aa 58-74) did not inhibit conjugate formation b etween ALAKs and U937 target cells. These data demonstrate that ALAK b inding to a soluble monomeric peptide inhibited cytotoxicity. Peptide binding appeared to negatively regulate cytotoxicity, and the inhibito ry effects following peptide binding were nonreversible. Effector:targ et cell conjugate formation was not affected by peptide binding, howev er, recognition was required because inhibition was specific for the a mino acid sequence of the synthetic peptide.