Adhesion mechanisms involved in C5a‐induced eosinophil homotypic aggregation

During the process of migration into tissues, leukocytes interact primarily with vascular endothelial cells but they have also been shown to interact with each other. In this study we investigated the adhesion mechanisms involved in guinea pig eosinophil homotypic aggregation as assessed by changes in light transmission. The anti‐CD18 monoclonal antibody (mAb) 6.5E, at concentrations in excess of those previously shown to abrogate CD18‐dependent eosinophil adherence to serum‐coated plastic, inhibited C5a‐induced eosinophil aggregation to a maximum of 49–68%. In contrast, the anti‐intercellular adhesion molecule‐1 (ICAM‐1) mAb RR1/1, which binds to guinea pig eosinophils and has been shown to block guinea pig ICAM‐1 function, had no effect on C5a‐induced responses. Similarly, two functionally active anti‐very late antigen‐4 (VLA‐4) mAbs had no effect on eosinophil aggregation and did not affect the CD18‐independent component of the aggregation response. The role of L‐selectin in eosinophil aggregation was investigated by using heparin, the selectin‐binding polysaccharide fucoidin, and the anti‐L‐selectin mAb MEL‐14. Heparin concentration‐dependently inhibited C5a‐ and platelet‐ activating factor‐ (PAF) induced aggregation but C5a‐induced responses were inhibited more potently. Fucoidin, but not the carbohydrate dermatan sulphate, effectively inhibited C5a‐induced eosinophil aggregation. PMA‐ and PAF‐induced responses were also inhibited by fucoidin. Moreover, fucoidin and 6.5E were additive in their ability to inhibit C5a‐induced aggregation. Similarly, MEL‐14 effectively inhibited C5a‐induced eosinophil aggregation. In conclusion, we have demonstrated that guinea pig eosinophil homotypic aggregation is mostly dependent on CD11/CD18 and L‐selectin present on the eosinophil surface. In addition, VLA‐4 plays no role in mediating this aggregation response.