Apoptotic Cell-Mediated Immunoregulation of Dendritic Cells Does Not Require iC3b Opsonization

Abstract

A number of recent studies show that activation of CR3 on dendritic cells (DCs) suppresses TLR-induced TNF-α and IL-12 production and inhibits effective Ag presentation. Although the proposed physiologic role for these phenomena is immune suppression due to recognition of iC3b opsonized apoptotic cells by CR3, all of the aforementioned investigations used artificial means of activating CR3. We investigated whether iC3b opsonized apoptotic cells could induce the same changes reported with artificial ligands such as mAbs or iC3b-opsonized RBC. We explored the kinetics of iC3b opsonization in two models of murine cell apoptosis, γ-irradiated thymocytes and cytokine deprivation of the IL-3 dependent cell line BaF3. Using a relatively homogenous population of early apoptotic cells (IL-3 deprived BaF3 cells), we show that iC3b opsonized apoptotic cells engage CR3, but this interaction is dispensable in mediating the anti-inflammatory effects of apoptotic cells. TLR-induced TNF-α and IL-12 production by bone marrow-derived DCs occurs heterogeneously, with apoptotic cells inhibiting only certain populations depending on the TLR agonist. In contrast, although apoptotic cells induced homogeneous IL-10 production by DCs, IL-10 was not necessary for the inhibition of TNF-α and IL-12. Furthermore, because the ability of iC3b opsonization to enhance phagocytosis of apoptotic cells has been controversial, we report that iC3b opsonization does not significantly affect apoptotic cell ingestion by DCs. We conclude that the apoptotic cell receptor system on DCs is sufficiently redundant such that the absence of CR3 engagement does not significantly affect the normal anti-inflammatory processing of apoptotic cells.

IgM Antibodies to Apoptosis-Associated Determinants Recruit C1q and Enhance Dendritic Cell Phagocytosis of Apoptotic Cells

“…5 G ). Notably, our in vitro assay also demonstrated that C3 is not essential for the recognition and engulfment of apoptotic thymocytes by conventional DCs, which is consistent with a recent report (63). …”

C3 opsonization regulates endocytic handling of apoptotic cells resulting in enhanced T-cell responses to cargo-derived antigens

“…As shown in Fig. S1C, the percentage of BM-DCs that had engulfed pHrodolabeled apoptotic cells was similar regardless of the opsonization, which is consistent with previous reports (30). Analysis of coded cytospins confirmed that the number of apoptotic cells engulfed by each BM-DC was equivalent, indicating that C3 −/− BM-DCs were as efficient as the WT BM-DCs in the uptake of the apoptotic cells.…”

“…We first excluded the possibility that the reduced presentation/T-cell activation could be the result of an impaired uptake or differences in DC maturation. Indeed, the role of C3 in the clearance of apoptotic cells by DCs remain unclear, with conflicting reports (29)(30)(31). The discrepancies in the literature are likely to be due to differences in the experimental conditions applied.…”

“…Although the contribution of complement to the engulfment of apoptotic cells by macrophages is well established (10), its role in the uptake by DCs remains controversial, with conflicting results reported in the literature (29)(30)(31). To explore whether the observed differences could be attributed to a reduced ability of the C3 −/− BM-DCs to take up the cells, we labeled the apoptotic LPS B-cell blasts with pHrodo, a dye that becomes fluorescent only in the acidic pH inside endocytic vesicles, and incubated them with day 6 immature C3 −/− and WT BM-DCs.…”

Thers1143679(R77H) lupus associated variant ofITGAM(CD11b) impairs complement receptor 3 mediated functions in human monocytes

“…The ‘waste-disposal’ hypothesis proposes that impaired uptake of apoptotic cells is a key element of SLE pathogenesis, and defective uptake of apoptotic cells by lupus macrophages having been observed directly, but it is debatable whether CR3 plays a key role in this process 33–36. Impaired clearance of iC3b-containing immune complexes is another attractive disease mechanism, because immune complex deposition underlies much of the tissue damage in SLE 35 37…”