Immunohistochemical investigation of the tissue distribution of mannan-binding lectin in non-infected and virus-infected chickens.

We 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 approximately 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 xeno-antigen, 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.