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Studies were carried out using the splenectomized mouse bioassay (SMB) to investigate the nature of embryo-derived platelet-activating factor (EPAF) and its relationship to synthetic platelet activating factor (PAF). While both C16-PAF and embryo conditioned media (ECM) induced a significant platelet decline in the SMB at 15 min postinjection, C18-PAF induced a similar effect at 30 min postinjection. The degree of EPAF activity in ECM was not altered with increasing embryo number from 2 to 40/ml of media. In contrast, PAF (C16/C18 mixture) induced a linear increase in activity with increasing concentration, leading to lethal effects at high concentrations. While EPAF activity was not significantly altered when ECM was diluted 1/1,000, PAF activity was abolished at 1/10 dilution. EPAF in ECM was not inactivated by mouse plasma; however, lipid extracted ECM, like PAF, underwent rapid inactivation in the presence of plasma. Aggregometer studies using horse platelets showed that ECM and lipid-extracted ECM were unable to induce platelet aggregation, while thin-layer chromatography (TLC) purified ECM (Rf 0.23) successfully aggregated horse platelets in vitro. Results suggested that EPAF and PAF are not homologous. EPAF might consist of PAF bound to a regulatory carrier molecule and appears to be associated with EPAF-inhibitor substance(s) in ECM.

Summary A mouse IgGi monoclonal antibody (1C4), which recognizes a cell surface molecule on murine natural cytotoxic{NC) cells was produced. By flowcytometry. IC4preferentially reacted with less than 5% of fresh CBA spleen cells and 20-50% of CBA-interleukin-3 (lL-3) cells, an in vitro derived NC-like cell line. In vitro treatment of spleen cells from a number of inbred mouse strains either with 1C4 alone or 1C4 coupled to dynabeads markedly decreased or abolished NC activity of the cells against ^'Cr-labelled WEHl-164 targets. Splenic NC activity of these same mouse strains was also reduced or abolished by in vivo administration of IC4. The effect was evident within 2 h of treatment and persisted for at least I week. In contrast IC4 had little or no effect on splenic NK activity against "Cr-labeiled YAC-I targets over the same range of experiments in vitro and in vivo. Results of strain surveys for both in vitro and in vivo reduction of splenic NC activity by 1C4 treatment showed that CBA. C57BL/6, BALB/c and NZB mice were positive and CE and DBA/2 mice were negative, indicating that 1C4 recognizes an allo-antigen on mouse NC cells. This allo-antibody has been designated NC-II, and thus IC4 is an anti-NC-11 monoclonal antibody.

SUMMARYWe have previously reported the identification by the murine monoclonal antibody (mAb) 1C4 of the first leucocyte receptor which is involved in natural cytotoxicity (NC ) against WEHI-164; the NC-1.1 receptor. We report herein the identification and characterization of a second leucocyte receptor which is involved in NC, NC-2 (MW 50 000), identified by a rat anti-mouse mAb D9 (immunoglobulin G2a; IgG2a). Flow cytometric analysis showed that NC-2 was expressed on<6% of splenic leucocytes of different inbred mouse strains and 96% of the cells of a mast-cell line which has high NC activity. In vitro treatment of splenic leucocytes with the D9 mAb blocked effector cell-WEHI-164 target cell conjugation and NC by # 50% without affecting natural killing (NK ). Western blot analysis of affinity purified NC-2 and NC-1.1 using the D9 and 1C4 mAbs showed specific reactivity of the proteins with D9 and 1C4, respectively. Pretreatment of splenic leucocytes with both mAbs blocked NC 84%, a result which almost doubled that caused by either mAb alone. Flow cytometric screening of 16 different mouse cell lines showed that 19% of the cell lines expressed both receptors, 6% expressed only NC-2, 44% expressed mainly or only NC-1.1 and the remaining cells expressed neither receptor. These data indicate that D9 identifies a xenoantigen, NC-2, which is expressed on cells mediating NC and not NK, and that it is not the previously described NC-1.1 allo-antigen. We conclude that NC-2 is likely to be one of a number of receptor molecules on cells mediating NC against tumour cells.