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

Background. Proximal tubular epithelial cells expressa surface C3-convertase activity which induces C fixation and insertion ofthe C5b-9 membrane attack complex (MAC) into the cell plasma membrane. Thephysiopathological consequences of this phenomenon are unknown.Methods. The effect of C fixation on the production ofinflammatory mediators by human proximal tubular epithelial cells inculture was explored. Results. Proximal tubularepithelial cells incubated with a sublytic amount of normal human serum asa source of C, but not with heat-inactivated human serum, showed atime-dependent calcium influx and a concomitant release of¹⁴C-arachidonic acid(¹⁴C-AA). Eicosanoid synthesis following thearachidonic acid mobilization was studied as prostaglandin E2 release.Mg²⁺/EGTA, which did not prevent C activation by theC3-convertase, and p-bromodiphenacyl bromide, a phospholipase A2-inhibitor,inhibited mobilization of ¹⁴C-AA. These resultssuggest the activation of an extracellularCa²⁺-dependent, phospholipase A2. Complementfixation was associated with the synthesis of proinflammaotry cytokinessuch as IL-6 and TNF-&agr;. Experiments with C6-deficient seraindicated that the release of ¹⁴C-AA and theproduction of cytokines were dependent on the insertion of the terminalcomponents of complement in the plasma membrane. Indeed, the reconstitutionof normal haemolytic activity of C6-deficient sera with purified C6restored also the release of ¹⁴C-AA and theproduction of cytokines. Conclusion. Invitro complement activation on the proximal tubular cell surfacetriggers the generation of proinflammatory mediators, which may potentiallycontribute to the pathogenesis of tubulointerstitial injury.