'Outside-in' Signalling Mechanisms Underlying CD11b/CD18-mediated NADPH Oxidase Activation in Human Adherent Blood Eosinophils

1 Incubation of human eosinophils in BSA-coated tissue culture plates resulted in time-dependent adhesion and attendant activation of the NADPH oxidase that were both inhibited (by >85%) by blocking antibodies raised against CD11b and CD18. 2 SB 203580, an inhibitor of p38 mitogen-activated protein (MAP) kinase, did not influence adhesion but inhibited superoxide anion generation (pIC50=-6.57). 3 PP1, an inhibitor of the src-family of protein tyrosine kinases, inhibited adhesion and CD11b/CD18-mediated superoxide anion generation with similar potencies (pEC50s=-5.53 and -5.99 respectively) suggesting that inhibition of the NADPH oxidase was a direct consequence of blocking adhesion. 4 The protein kinase C (PKC) inhibitors Ro-31 8220 (broad spectrum inhibitor), GF 109203X (inhibitor of conventional and novel isoforms) and Gö 6976 (inhibitor of conventional isoforms) suppressed adhesion-dependent NADPH oxidase activation (pIC50s=-6.61, -6.05 and -4.89 respectively) without affecting adhesion. Based upon the selectivity of these drugs PKCdelta and PKCepsilon are implicated in the suppression of oxidant production. 5 Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PtdIns 3-kinase), abolished superoxide anion production in adherent eosinophils (pEC50=-9.06). Similarly, CD11b/CD18-dependent adhesion was suppressed with the same potency (pEC50=-9.29) although the maximum effect did not exceed 50% implying that wortmannin also had an affect on those processes that govern adhesion-driven oxidase activation. 6 PD 098059 and piceatannol, inhibitors of MAP kinase kinase-1 and the syk tyrosine kinase respectively, had no effect on CD11b/CD18-mediated adhesion or NADPH oxidase activation. 7 The results of this study demonstrate that human eosinophils adhere to BSA-coated plastic by a CD11b/CD18-dependent mechanism, which is responsible for activation of the NADPH oxidase. Although the signalling pathway(s) utilized by CD11b/CD18 is still to be elucidated, the data presented herein implicate p38 MAP kinase, novel PKCs and PtdIns 3-kinase.