SummaryHuman lymphocytes with natural killer (NK) activity, including most activated 3'/8 + T lymphocytes, recognize and lyse tumor target cells without requiring recognition of major histocompatibility complex antigen. However, unlike 3'/8 + T lymphocytes, NK cells do not express CD3/T cell receptor (TCR) molecules, and the receptors involved in ceU-mediated cytotoxicity are unknown. To further delineate circulating NK cells, we developed monoclonal antibodies (mAbs) against the human NK leukemia YT2C2. We report the isolation of a mAb termed BY55, recognizing at the cell surface a novel 80-kD protein with restricted expression, In addition to the immunizing cell line, this mAb binds to circulating NK cells, 3'/8* cells, and a minor subset of ot/B + T lymphocytes. Expression of the BY55 mAb-reactive epitope/ molecule is regulated by activation, as short-term culture of peripheral blood lymphocytes (PBL) with phorbol ester induced its downmodulation. Furthermore, BY55 mAb reactivity was found neither with the NK nor with the TCR a/B + and 3"/8 + clones tested. Biochemical studies as well as phenotypic analysis revealed that this structure is different from all previously identified molecules on the lymphocyte cell surface. Interestingly, we found that BY55 + cells exert most NK activity obtained with fresh circulating lymphocytes. We report that within fresh E rosette-positive PBL only a subset of the CD16 +, CD56 +, and CD57 + cells coexpressed BY55 molecule, indicating that BY55 mAb defines a unique subset mediating NK activity of circulating PBL.N 'K cells are circulating lymphocytes found within the large granular lymphocyte population (1, 2) and constitute, with CTL (3), the major cytotoxic effector lymphoid elements of the immune system. The characteristic surface markers expressed by NK cells are CD56 (4-6), CD57 (4, 5), CD16 (7-10), CD11b (11), and CDllc (11, 12). Most circulating cells with NK activity express the CD2 molecule but are distinct from the T lineage, as they do not express CD3 and do not rearrange any of the TCR genes (13, 14). In addition, NK cells mediate cytotoxicity without requiring recognition of MHC molecules on target cells. However, most activated TCR 3'/8 lymphocytes (15-18), and under certain circumstances some TCR o~/B cloned lymphocytes (19), can also mediate MHC-unrestricted cytotoxicity. The NK cell receptor(s) participating in target recognition remain(s) largely unknown despite extensive studies (20)(21)(22).In an attempt to define NK cell-specific surface structures, we characterized a mAb obtained by repeated immunization with YT2C2, a human cell line with NK functional characteristics. This cell line was selected for intermediate affinity binding of 12sI-IL-2 with the IL-2R p75 subunit (23). In the present study, we report the isolation ofa mAb, termed BY55, reacting at the cell surface with a novel 80-kD protein structure. BY55 mAb reactivity is observed with 15-25% of circulating lymphocytes. Two-color immunofluorescence staining of fresh E rosette-positive (E +) PBL o...

When murine macrophages activated in vivo with bacille Calmette-Guérin were triggered with either acetyl-CoA or propionyl-CoA to form PAF-acether (PAF), similar amounts of platelet-aggregating product were recovered. Liquid chromatographic purification and reversed-phase analysis showed that the composition of PAF molecular species formed in the presence of acetyl-CoA was an equimolar mixture of PAF bearing C16:0 alkyl chain (57% +/- 7, mean +/- SD, n = 3) and PAF C18:1. The PAF-like material obtained from the propionyl-CoA-supplemented macrophages was a mixture of the propionyl analogue of PAF (66% +/- 11, n = 3) and native PAF. The rate of lyso-PAF:acetyl-CoA acetyltransferase (EC 2.3.1.67) reaction in a macrophage lysate was similar for either substrate in the presence of an equimolar mixture of propionyl-CoA and acetyl-CoA. We conclude that the exogenously added propionyl-CoA is transferred to lyso-PAF acceptor to form propionyl-PAF by the PAF-forming acetyltransferase. Propionyl-PAF triggers the formation of native PAF probably from the endogenous acetyl-CoA pool. Two specific PAF antagonists, BN 52021 (60 microM) and WEB 2086 (3 microM), did not influence the rate of PAF synthesis in the presence of either acetyl-CoA or propionyl-CoA and did not prevent native PAF formation when propionyl-CoA was added alone, suggesting that the classical PAF receptors are not involved. This is the first description of a possible mechanism of autocrine amplification of PAF biosynthesis in macrophages.

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