Alternative pathway complement activation induces proinflammatory activity in human proximal tubular epithelial cells

Background. Proximal tubular epithelial cells express a surface C3-convertase activity which induces C fixation and insertion of the C5b-9 membrane attack complex (MAC) into the cell plasma membrane. The physiopathological consequences of this phenomenon are unknown. Methods. The effect of C fixation on the production of inflammatory mediators by human proximal tubular epithelial cells in culture was explored. Results. Proximal tubular epithelial cells incubated with a sublytic amount of normal human serum as a source of C, but not with heat-inactivated human serum, showed a time-dependent calcium influx and a concomitant release of C-14-arachidonic acid (C-14-AA). Eicosanoid synthesis following the arachidonic acid mobilization was studied as prostaglandin E(2) release; Mg2+/EGTA, which did not prevent C activation by the C3-convertase, and p-bromodiphenacyl bromide, a phospholipase A(2)-inhibitor, inhibited mobilization of C-14-AA. These results suggest the activation of an extracellular Ca2+-dependent, phospholipase A(2). Complement fixation was associated with the synthesis of proinflammatory cytokines such as IL-6 and TNF-alpha. Experiments with C6-deficient sera indicated that the release of C-14-AA and the production of cytokines were dependent on the insertion of the terminal components of complement in the plasma membrane. Indeed, the reconstitution of normal haemolytic activity of CG-deficient sera with purified C6 restored also the release of C-14-AA and the production of cytokines. Conclusions. In vitro complement activation on the proximal tubular cell surface triggers the generation of proinflammatory mediators, which may potentially contribute to the pathogenesis of tubulointerstitial injury.