Antigen-presenting cells containing bacterial peptidoglycan in synovial tissues of rheumatoid arthritis patients coexpress costimulatory molecules and cytokines

Objective, To investigate the role of intercellular adhesion molecule 1 (ICAM-1) and beta 2 integrins in the production of superoxide (O-2(-)) by C1q-stimulated human polymorphonuclear leukocytes (PMN). Methods. PMN were pretreated with F(ab')(2) fragments of monoclonal antibodies (mAb) that blocked or did not block beta 2 integrin-mediated adhesion. The cells were added to wells coated with C1q, and the production of O-2(-) was monitored kinetically as a color change due to reduction of cytochrome c, In some experiments, C1q was co-immobilized with purified ICAM-1, Results. Blocking mAb to the shared beta 2 integrin subunit, CD18, completely inhibited the O-2(-) response triggered by immobilized C1q, while blocking mAb to the alpha subunits of the beta 2 integrins each partially blocked the O-2(-) response. PMN treated with C1q were found to activate the beta 2 integrins lymphocyte function-associated antigen 1 and CR3 for binding to ICAM-1. Co-immobilization of ICAM-1 with C1q cooperatively triggered O-2(-) production by PMN, Conclusion. beta 2 integrin binding to an ICAM provided an essential costimulatory signal for O-2(-) production triggered by C1q in PMN, Our findings suggest a model for PMN activation in which 2 stimuli are required for O-2(-) production: a first signal that also activates PMN beta 2 integrins, followed by a second, beta 2 integrin-mediated signal, which occurs physiologically upon PMN binding to ICAM-1, The requirement for this dual signal for PMN generation of O-2(-) would serve as a regulatory mechanism to limit the production of O-2(-) to a tissue environment where C1q, or some other stimulus, is colocalized with stromal cells bearing upregulated ICAM-1, This mechanism may explain why all tissues can express ICAM-1 and may explain in part why inhibitors of tumor necrosis factor alpha, a major physiologic stimulus of ICAM-1 up-regulation, are potent antiinflammatory agents