Responses of Neutrophils to Anti-Integrin Antibodies Depends on Costimulation through Low Affinity FcγRs: Full Activation Requires Both Integrin and Nonintegrin Signals

Abstract

The relative contribution of integrin and nonintegrin signals to neutrophil activation is incompletely understood. Immobilized anti-integrin Abs were previously shown to induce robust activation of neutrophils without any additional stimulus, suggesting that cross-linking of integrins is sufficient for full activation of the cells. However, the possible contribution from other receptors has not been tested in this system. In this study, we show that neutrophil responses to anti-integrin Abs requires costimulation through low-affinity FcγRs. Murine neutrophils lacking the FcR γ-chain or FcγRIII failed to respond to immobilized Abs against β1, β2, or β3 integrins and the activation of wild-type cells could be prevented by blocking Abs against FcγRII/III. Plate-bound anti-CD18 Abs initiated a respiratory burst from human neutrophils, but this response was abrogated when the F(ab′)2 of the same Abs were used or the cells were preincubated with FcγRIIA-blocking Abs. Lack of FcγRIII or administration of FcγR-blocking Abs had no effect on responses of TNF-stimulated cells plated on fibrinogen or rICAM-1. TNF restored the respiratory burst of FcγRIII-deficient neutrophils plated on anti-CD18 mAbs. The p38 MAPK inhibitor SB203580 attenuated the responses of neutrophils to anti-CD18 mAbs or TNF stimulation on a fibrinogen surface. Taken together, these results indicate that activation of low-affinity FcγRs is required for neutrophil responses induced by anti-integrin Abs and suggest that a second coactivation signal (e.g., through TNF or FcR ligation) is indispensable for full integrin-mediated activation of neutrophils. These second signals are interchangeable and they may converge on the p38 MAPK.