To study the cellular structures involved in NK and lymphokine-activated killer (LAK) cell function, we have produced a panel of mAbs that modulate the cytolytic function of a population of cells with LAK activity that derive from large granular lymphocyte (LGL)/NK cells (adherent LAK [A-LAK] cells). In this report, we describe an mAb (3.2.3; IgG1k) that recognizes a triggering structure that is expressed on rat LGL/NK cells and A-LAK cells. This epitope is also expressed on polymorphonuclear leukocytes (PMN). The expression of the epitope identified by mAb 3.2.3 increased progressively on A-LAK cells after culture in the presence of rIL-2. mAb 3.2.3 enhanced the cytolytic activity of NK and A-LAK cells against FcR+ target cells, but not FcR- target cells. However, this effect was not induced by F(ab')2 fragments of 3.2.3. This antibody also induced the release of N-alpha-benzyloxycarbonyl-L-lysine thiobenzy esteresterase by A-LAK cells. These data suggest that the epitope identified by mAb 3.2.3 is on a triggering structure expressed on rat NK cells and A-LAK cells. The expression of the epitope recognized by mAb 3.2.3 on LGL/NK cells and PMN suggests that this structure may be analogous to that identified by the anti-CD16 (-FcR) mAbs. However, the molecule immunoprecipitated by mAb 3.2.3 was a 60-kD dimer composed of two 30-kD chains. These data suggest that mAb 3.2.3 recognizes a unique triggering structure. As mAb 3.2.3 is the first antibody recognizing a determinant with functional significance, selectively expressed on both rat NK cells and A-LAK cells, it will be a useful tool for the study of NK cell ontogeny and function, and the development of cells with LAK activity from the NK cell compartment.

Natural killer (NK) cells are a subpopulation of large granular lymphocytes characterized by densely staining azurophilic granules. NK cells are able to recognize and lyse various virally infected or neoplastic target cells without previous sensitization or major histocompatibility complex restriction. A 60-kD disulfide-linked dimer, highly expressed on NK cells, was found capable of mediating transmembrane signaling. The gene encoding this signal transduction molecule was cloned and its nucleotide sequence determined. The encoded protein showed significant homology with a number of lectin-related membrane proteins that share receptor characteristics. This protein may function as a receptor able to selectively trigger NK cell activity.

Recent advances in adoptive immunotherapy have used cytotoxic lymphocytes with broad antitumor cytotoxicity . Studies have shown that the coculture of lymphocytes with rIL-2 induces the generation of lymphokine-activated killer (LAK)' cell activity and this activity has proven useful in the control of metastatic tumors in animal models and in some cancer patients (1-3).Adoptive immunotherapy using cells with LAK activity first requires the removal of large numbers of lymphocytes from the patient followed by a period of in vitro culture in IL-2 (1, 4). In general, an overall expansion of lymphocytes in bulk cultures is seldom achieved (4) and it is difficult to accurately assess the number of cells with LAK activity reinfused into the patient. The development of methods that would enable the rapid expansion of purified populations of cells with LAK activity would provide an acceptable and presumably preferable alternative to conventional culture methods.We have recently reported on the generation of cells with LAK activity in rats (5) and have demonstrated that the majority of LAK progenitors are contained within the large granular lymphocyte (LGL) population, expressing surface markers characteristic of rat NK cells (5-7). In the present studies, we demonstrate that one of the first responses by LGL to rIL-2 is their attachment to plastic surfaces . Exploiting this observation, we now show that rIL-2-activated plastic adherent LGL can expand between 30-and 100-fold in 3-4 d of culture to generate exceedingly high levels of broad antitumor cytotoxicity .

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