SR-A1, the first scavenger receptor to be identified, is a type II membrane glycoprotein that forms homotrimers. Multiple observational studies support the potential role of SR-A1 in macrophage-mediated immune response 18,19. The most extensively studied area is atherosclerosis pathogenesis, in which phagocytic receptor SR-A1 is demonstrated to be involved in uptake of oxidized low-density lipoproteins in macrophage 20. SR-A1 also emerges as a recognition receptor of macrophage via discriminating and binding different immune ligands. Additionally, SR-A1 helps to guide cell apoptosis and proliferation by initiating multiple cellular signaling pathways 21,22. Under the influence of local microenvironment, SR-A1 can induce macrophages polarization into different subtypes 23.
As a pivotal member of innate immune system, macrophages discriminate between innocuous antigens and potential pathogens to maintain intestinal homeostasis. They control pathogen invasion by activating an appropriate immune response24,25. Besides traditional macrophages, other macrophages, such as scavenger receptor positive macrophages, also are enriched in peripheral blood as well as in colonic mucosa of IBD patients. CD163 is one of scavenger receptors, and Franzè E et al 26 reported that the fractions of CD163-positive monocytes were increased in IBD patients as compared to controls, and cross-linking of CD163 could enhance tumor necrosis factor-α synthesis. In our study, the result showed that scavenger receptor SR-A1 antibody treatment significantly improved DSS-induced colitis symptom, in addition to its established efficacy in reducing mucosal inflammatory infiltration and crypt structure injury. The mRNA expression of macrophage-related cytokines was detected by qPCR, and results revealed that pro-inflammatory cytokines, such as TNF-α, IL-1β and iNOS, was decreased but anti-inflammatory cytokine IL-10 was increased markedly in both PBMC and LPMC after SR-A1 antibody treatment. Further analysis of flow cytometry showed that SR-A1 antibody treatment had no influence on elevated percentage of F4/80+CD11c+ macrophages induced by DSS colitis, but could increase percentage of F4/80+CD206+ macrophages. These findings suggest that SR-A1 antibody treatment reduces circular and mucosal immune response mainly via inducing anti-inflammatory M2 macrophage activity rather than suppressing pro-inflammatory M1-related immunity, finally avoiding amplification of inflammatory cascade and exerting an anti-inflammatory effect.
In the process of macrophage activity induced by SR-A1 receptor, other PRRs, such as Toll-like receptors, can act as a coactivator, bind to intracellular molecules and activate specific transcription factors. Upon HCMV exposure to THP-1 monocytes, the disruption of SR-A1 signal transduction not only blocks the activation of downstream TLR-9 pathway but also alters the downstream TLR-3 pathway, and upregulation of non-canonical TLR-3 induces the expression of NF-κB-dependent p35 subunit of IL-12 gene transcription27. Other study demonstrated that SR-A1 has a synergistic coordination with TLR4-mediated immune consequences, further inducing the activation of transcription factor NF-κB 28. Consistent with other previous studies, we also observed that blockage of SR-A1 with its antibody could inhibit the expression of TLR4, MyD88 and NF-kB p65 in intestinal mucosa during DSS colitis. Therefore, we hypothesize that SR-A1 may promote the activation of inflammatory macrophages via TLR4-MyD88-NF-kB signaling pathway.
Several studies indicated that the presence of macrophages is essential to intestinal epithelial cell regeneration. Depending on the repair microenvironment, they contribute to wound repair and tissue remodeling by clearing apoptotic neutrophils and helping to reduce autoimmune and chronic inflammatory responses 5,29. Tissue-resident macrophages represent a heterogeneous population of non-migratory cells that respond to injury or infection by sensing related molecule. In a punch-biopsy-induced colonic injury model, the infiltration of TREM2+ macrophages was required for epithelial cell proliferation and colonic mucosa healing 30. Another investigation suggested that regenerative response to intestinal epithelium injury could not be observed in the absence of monocyte recruitment in Csf1op/op mice 31. Our results revealed that treatment of SR-A1 antibody could significantly drop the level of cell apoptosis and elevate the level of cell proliferation in colonic epithelium while converting macrophages to M2 subtype, which indicated SR-A1 antibody treatment is beneficial to intestinal epithelial cell regeneration. However, whether specific macrophage type exerts its function in this process remains to be further studied.
A large number of studies have linked gut dysbiosis to IBD pathogenesis. Accompanied by immune cells activation and gut microbiota, including its derived metabolites, can exert anti-inflammatory or pro-inflammatory effects 32,33. For example, adherent-invasive E. coli (AIEC) has the ability to invade intestinal epithelial cells in IBD pathogenesis, which can be swallowed by macrophages and replicates inside them due to the defect in autophagy 34. Ren J et al 35 reported that acacetin alleviated clinical symptoms of DSS-induced colitis by inhibiting the macrophage inflammatory response in vitro, while improving the imbalance of gut microbiota. In DSS-induced colitis mice, we detected the composition of gut microbiota before and after the treatment of SR-A1 antibody. It was observed the blockage of SR-A1 could further reduce the diversity of microbial population although mucosal inflammatory degree had been improved to some extent. However, the proportions of a novel probiotics, Akkermansia, significantly increased after the treatment of SR-A1 antibody. There has been growing evidence in the association of Akkermansia with health and disease in humans 36,37. Akkermansia has the capacity to stimulate mucin synthesis and immune molecules production. Mice fed a high-fat diet with daily gavage of Akkermansia, showed enhanced production of the antimicrobial peptide Reg3γ in the colon 38. Akkermansia is significantly reduced in patients with IBD and mice with colitis. Hence, in our study, it is tempting to speculate that anti-inflammatory effect of SR-A1 antibody on DSS-induced colitis partially accounts for altering the specific strain rather than the overall diversity of gut microbiota.
There were also some deficiencies in our study. We just discussed the mechanism of SR-A1 antibody treatment by regulating macrophage polarization, and did not investigate whether SR-A1 antibody could play its therapeutic role through the response of other immune cells. In addition, the phenomenon of gut microbiota alteration after SR-A1 antibody treatment was observed in our study, but the interaction mechanism between immune cells and gut microbiota still need to be further considered.