Complement receptor 3 mediates renal protection in experimental C3 glomerulopathy (1)
C3 glomerulopathy (C3G) is a category of kidney diseases that includes dense-deposit disease, C3 glomerulonephritis, and complement factor H–related protein 5 (CFHR5) nephropathy. Mice with homozygous deficiency of factor H (FH, Cfh–/–) have proved to be an informative experimental model of C3G. Cfh–/– mice spontaneously develop low plasma C3 and C5 levels and linear staining of C3 and C9 along the glomerular basement membrane (GBM). Complement C3 along the GBM in these animals includes iC3b, a cleavage product of C3b. Complement receptor 3 (CR3), also known as Mac-1 (CD11b/CD18, αMβ2), is the main leukocyte receptor for iC3b and a β2–integrin receptor expressed mainly on neutrophils, monocytes and macrophages, and dendritic cells. CR3 can promote TLR-induced inflammation, whereas others have reported negative roles for this integrin in TLR responses.
In a cohort of female Cfh–/–.Itgam–/– (FH and CR3) and Cfh–/– mice housed in non-SPF conditions over an 8-month period, we noted significantly reduced survival in Cfh–/–.Itgam–/– animals. Of the 6 Cfh–/–.Itgam–/– animals culled at the 8-month time point, hematuria and albuminuria were present in 4. Glomerular cellularity was similar between the Cfh–/– and Cfh–/–.Itgam–/– groups, but the number of glomerular macrophages was significantly increased in the Cfh–/–.Itgam–/– mice.
In Itgam–/– (n = 8) and wild-type (n = 10) mice, two days after injection of nephrotoxic serum, hematuria was detectable in 7 of the 8 Itgam–/– mice but absent in all wild-type animals. Ten days after administration of nephrotoxic serum, when all animals were culled, the Itgam–/– mice had significantly greater hematuria, plasma urea levels, and glomerular crescents. Glomerular deposition of sheep and mouse IgG and antigen-specific plasma titers of mouse IgG did not differ between the groups.
Preincubation of iC3b-coated guinea pig red blood cells (gRBCs) resulted in reduced secretion of interleukin-6 (IL-6), but enhanced secretion of IL-10 by murine myeloid cells (both monocytes and macrophages at day 2 and day 7 of differentiation in vitro).
Preincubated with iC3b-coated beads in monocytes and monocyte-derived macrophages at day 2 down modulated the proinflammatory cytokine profile triggered by LPS and promoted an anti-inflammatory response. The cytokine effect of iC3b-coated beads on TLR4-stimulated neutrophils was proinflammatory with significantly increased production of IL-8 and CCL3 (MIP-1α).
CR3-mediated signaling is usually viewed as proinflammatory, and prevention of CR3–iC3b interaction by antibody or genetic deletion has been shown to decrease the severity of inflammatory responses in several animal models including heterologous NTN. However, interaction of iC3b with CR3 on both murine and human monocytes and macrophages is associated with an enhanced anti-inflammatory cytokine profile (reduced IL-6 and increased IL-10 secretion) following LPS stimulation. These data are consistent with earlier studies indicating that engagement of CR3 inhibited secretion of proinflammatory cytokines by TLR4-stimulated monocytes and macrophages. ANTN has been extensively characterized in C57BL/6 mice as involving a T helper 1–predominant, delayed-type hypersensitivity–like nephritogenic immune response. T helper 17–directed cellular immune responses have also been implicated in several murine models of crescentic nephritis, including ANTN. CFA is a recognized TLR agonist, and increased renal injury due to TLR4 ligation specifically during sensitization has been demonstrated in ANTN studies.