Interaction of the Factor H Family Proteins FHR-1 and FHR-5 With DNA and Dead Cells: Implications for the Regulation of Complement Activation and Opsonization

Abstract

Complement plays an essential role in the opsonophagocytic clearance of apoptotic/necrotic cells. Dysregulation of this process may lead to inflammatory and autoimmune diseases. Factor H (FH), a major soluble complement inhibitor, binds to dead cells and inhibits excessive complement activation on their surface, preventing lysis, and the release of intracellular material, including DNA. The FH-related (FHR) proteins share common ligands with FH, due to their homology with this complement regulator, but they lack the domains that mediate the complement inhibitory activity of FH. Because their roles in complement regulation is controversial and incompletely understood, we studied the interaction of FHR-1 and FHR-5 with DNA and dead cells and investigated whether they influence the regulatory role of FH and the complement activation on DNA and dead cells. FH, FHR-1, and FHR-5 bound to both plasmid DNA and human genomic DNA, where both FHR proteins inhibited FH–DNA interaction. The FH cofactor activity was inhibited by FHR-1 and FHR-5 due to the reduced binding of FH to DNA in the presence of the FHRs. Both FHRs caused increased complement activation on DNA. FHR-1 and FHR-5 bound to late apoptotic and necrotic cells and recruited monomeric C-reactive protein and pentraxin 3, and vice versa . Interactions of the FHRs with pentraxins resulted in enhanced activation of both the classical and the alternative complement pathways on dead cells when exposed to human serum. Altogether, our results demonstrate that FHR-1 and FHR-5 are competitive inhibitors of FH on DNA; moreover, FHR–pentraxin interactions promote opsonization of dead cells.

Complement Factor H-Related 3 induces inflammation and complosome activation in human RPE cells

“…A similar interaction was also described for FHR-1 on necrotic cells, including RPE cells, but a cellular incorporation of FHR-1 hadn't been reported (20,23). Cellular FHR-1 interaction increases complement activation as well as deposition of C3 and C4 fragments on cell surfaces (23,24). Further, in ammasome reactivity is triggered on monocytes by FHR-1 binding but not upon FHR-3 interaction (20).…”

“…FH is internalized by stressed and apoptotic RPE cells (18,21), resulting in reduced endogenous C3-cleavage and increased cell survival in stressed cells (18), but contrary increased C3b-fragments are detected in apoptotic RPE cells, facilitating RPE cell opsonization (21). A similar interaction was also described for FHR-1 on necrotic cells, including RPE cells, but a cellular incorporation of FHR-1 hadn't been reported (20,23). Cellular FHR-1 interaction increases complement activation as well as deposition of C3 and C4 fragments on cell surfaces (23,24).…”

“…Martin et al examined a speci c FH uptake by apoptotic rendered Jurkat T-and ARPE-19 cells, whereas neither binding nor uptake could be con rmed by living cells (21). FHR1 and FHR-5 have also been reported to bind to necrotic RPE and endothelial cells, but an uptake was not described (20,23).…”

“…Beyond the classical function of the complement system as part of the humoral immune system, which has been known for more than 100 years, more and more non-canonical functions and the complosome have been assigned in the last decade, including the in uence on cell metabolism, cell development and regeneration (22,55,56). In the past years, rising data indicate that FHR proteins have complement activating properties (20,21,23,24,51,53). While FHR-1, FHR-3, FHR-4 and FHR-5 compete systemically with FH for C3b binding, less is known about their non-canonical functions as cellular modulators (23,51,57,58).…”

“…In the past years, rising data indicate that FHR proteins have complement activating properties (20,21,23,24,51,53). While FHR-1, FHR-3, FHR-4 and FHR-5 compete systemically with FH for C3b binding, less is known about their non-canonical functions as cellular modulators (23,51,57,58). Here, we treated serum-free, in vivo-like cultivated ARPE19 and hpRPE cells apically with FHR-3 to investigate the cell-associated complement and in ammatory response independent of any external complement sources.…”