Complement Component C1q Programs a Pro-Efferocytic Phenotype while Limiting TNFα Production in Primary Mouse and Human Macrophages.

Emily M Smith,Chunfa Jie,Suzanne S Bohlson,Sean D O’Conner,Holly J Hulsebus

doi:10.3389/fimmu.2016.00230

Abstract

Deficiency in complement component C1q is associated with an inability to clear apoptotic cells (efferocytosis) and aberrant inflammation in lupus, and identification of the pathways involved in these processes should reveal important regulatory mechanisms in lupus and other autoimmune or inflammatory diseases. In this study, C1q-dependent regulation of TNFα/IL-6 expression and efferocytosis was investigated using primary mouse bone marrow-derived macrophages and human monocyte-derived macrophages. C1q downregulated LPS-dependent TNFα production in mouse and human macrophages. While prolonged stimulation with C1q (18 h) was required to elicit a dampening of TNFα production from mouse macrophages, the human macrophages responded to C1q with immediate downregulation of TNFα. IL-6 production was unchanged in mouse and upregulated by human macrophages following prolonged stimulation with C1q. Our previous studies indicated that C1q programmed enhanced efferocytosis in mouse macrophages by enhancing expression of Mer tyrosine kinase and its ligand Gas6, a receptor–ligand pair that also inhibits proinflammatory signaling. Here, we demonstrated that C1q-dependent programming of human macrophage efferocytosis required protein synthesis; however, neither Mer nor the related receptor Axl was upregulated in human cells. In addition, while the C1q-collagen-like tails are sufficient for promoting C1q-dependent phagocytosis of antibody-coated targets, the C1q-tails failed to program enhanced efferocytosis or dampen TNFα production. These data further elucidate the mechanisms by which C1q regulates proinflammatory signaling and efferocytosis in macrophages, functions that are likely to influence the progression of autoimmunity and chronic inflammation.

Highlights

C1q, well known for its function as the recognition component of the classical complement pathway, functions independently of the complement system to facilitate phagocytosis and regulate proinflammatory signaling in myeloid cells [reviewed in Bohlson et al [1]]
This suggests that Bone marrow-derived macrophages (BMDM) require a C1q-dependent programming event in order to respond to C1q with this diminished proinflammatory response
Similar experiments were performed with human monocyte-derived macrophages (HMDM), and unlike BMDM, there was a direct inhibition of proinflammatory signaling in the HMDM; HMDM stimulated with C1q for 30 min and activated with LPS produced significantly less TNFα compared to control cells (Figure 1C, range of inhibition 60.2–87.6%, n = 4)

Summary

Introduction

C1q, well known for its function as the recognition component of the classical complement pathway, functions independently of the complement system to facilitate phagocytosis and regulate proinflammatory signaling in myeloid cells [reviewed in Bohlson et al [1]]. To this end, in vitro studies demonstrate that C1q and other members of the defense collagen family, such as mannose-binding. C1q Programs a Macrophage Phenotype lectin (MBL) and surfactant protein-A (SP-A), are bridging molecules that bind to target particles, such as pathogens or apoptotic cells, and facilitate their rapid clearance [reviewed in Galvan et al [2]]. Physical bridging and programming may occur concomitantly; little is known about the molecular mechanisms leading to C1q-dependent regulation of the macrophage phenotype

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