Differential effects of anti-rat CD11b monoclonal antibodies on granulocyte adhesiveness

Four different monoclonal antibodies (mAbs) reactive with rat CD11b (ED7, E D8, OX-42 and 1B6c) have been characterized for their ability to induce hom otypic aggregation of granulocytes or to modify granulocyte adhesiveness tr iggered by phorbol myristate acetate (PMA) or N-formyl-methionyl-leucyl -ph enylalanine (fMLP). Cross-blocking experiments showed that these mAbs recog nize at least three different epitopes on CD11b. OX-42 mAb recognizes an in hibitory epitope since the mAb inhibited homotypic aggregation of granulocy tes and their adherence to plastic in the presence of PMA or fMLP. ED7 and EDs induced homotypic aggregation of granulocytes which was blocked by OX-4 2 and anti-CD18 mAb (WT3) suggesting that CR3 itself is involved in the adh esion process. The aggregation was dependent on active cell metabolism, int act cytoskeleton, divalent cations and activation of tyrosine kinases sensi tive to genistein. Staurosporine, okadaic acid and orthovanadate potentiate d the aggregation. ED7 and ED8 potentiated homotypic aggregation and adhesi on of granulocytes to plastic caused by fMLP, but inhibited granulocyte adh esion to plastic induced by PMA. 1B6c recognizes an epitope that transmits a proaggregatory signal upon binding of the mAb but only if the granulocyte s are in contact with plastic or are activated by fMLP. In contrast, 1B6c i nhibited granulocyte adhesion to plastic triggered by PMA or fMLP. These da ta suggest the existence of functionally different epitopes on rat CD11b an d indicate that some anti-CD11b mAbs are able to functionally activate CR3.