Inflammatory Responses Induced by Lipopolysaccharide Are Amplified in Primary Human Monocytes but Suppressed in Macrophages by Complement Protein C5a

Abstract

Monocytes and macrophages are important innate immune cells equipped with danger-sensing receptors, including complement and Toll-like receptors. Complement protein C5a, acting via C5aR, is shown in this study to differentially modulate LPS-induced inflammatory responses in primary human monocytes versus macrophages. Whereas C5a enhanced secretion of LPS-induced IL-6 and TNF from primary human monocytes, C5a inhibited these responses while increasing IL-10 secretion in donor-matched human monocyte-derived macrophages differentiated by GM-CSF or M-CSF. Gαi/c-Raf/MEK/ERK signaling induced by C5a was amplified in macrophages but not in monocytes by LPS. Accordingly, the Gαi inhibitor pertussis toxin and MEK inhibitor U0126 blocked C5a inhibition of LPS-induced IL-6 and TNF production from macrophages. This synergy was independent of IL-10, PI3K, p38, JNK, and the differentiating agent. Furthermore, C5a did not inhibit IL-6 production from macrophages induced by other TLR agonists that are selective for Toll/IL-1R domain–containing adapter inducing IFN-β (polyinosinic-polycytidylic acid) or MyD88 (imiquimod), demonstrating selectivity for C5a regulation of LPS responses. Finally, suppression of proinflammatory cytokines IL-6 and TNF in macrophages did not compromise antimicrobial activity; instead, C5a enhanced clearance of the Gram-negative bacterial pathogen Salmonella enterica serovar Typhimurium from macrophages. C5aR is thus a regulatory switch that modulates TLR4 signaling via the Gαi/c-Raf/MEK/ERK signaling axis in human macrophages but not monocytes. The differential effects of C5a are consistent with amplifying monocyte proinflammatory responses to systemic danger signals, but attenuating macrophage cytokine responses (without compromising microbicidal activity), thereby restraining inflammatory responses to localized infections.

Biliverdin modulates the expression of C5aR in response to endotoxin in part via mTOR signaling

“…The C5a-C5aR axis cross-talks with TLR-4 [11] and C5a via C5aR concentration-dependently increases LPS-induced secretion of pro-inflammatory cytokines, including IL-6 and TNF-α in human monocytes [25]. Therefore, the effects of BV on TNF-α and IL-6 were also explored.…”

Complement in disease: a defence system turning offensive

“…This differential modulation by C5a was further explored in a 2013 study which showed that in response to lipopolysaccharide, C5a enhanced the secretion of proinflammatory cytokines by monocytes, but induced an anti-inflammatory response in macrophages, with production of anti-inflammatory IL-10 (REF. 63). Additional insight has been provided by the observation that CR3 positively modulates lipopolysaccharide-induced responses in myeloid dendritic cells, but not macrophages, by promoting endosome-mediated endocytosis of the TLR required for subsequent signalling 64 .…”

Sexual dimorphisms in leukocyte trafficking in a mouse peritonitis model

“…C57BL/6 mice, 8-11 wk old, were used in this study (22)(23)(24)(25)(26)(27)(28)(29)(30) g males, 18-25 g females; Charles River Laboratories, Wilmington, MA, USA) and were housed in individually ventilated cages with a controlled 12 h light/dark cycle (lights on 7:00 AM to 7:00 PM) and constant temperature (21 6 2°C). Mice from the same litter were distributed equally between experimental groups, and males and females were treated at the same time.…”