Intrigued by the mechanism of anaphylactic shock described in 1902 by Richet and Portier (1), Jules Bordet devoted several experiments to it, in which he showed that anaphylactic shock could be triggered in guinea pigs by intravenous injection of fresh serum previously put in contact with agar, then eliminated by centrifugation.

Bordet hypothesized that anaphylactogenic absorption complexes are thus produced, referring then to the previous works of Friedberger, who had observed a similar phenomenon by injecting fresh serum treated with immune complexes into the animal (2).

As research on complement progressed, researchers discovered the existence of several small aminoterminal cationic fragments resulting from the proteolysis of each of the complement proteins C3, C4 and C5. The C3a, C4a and C5a - 74 to 77 amino acids peptides that are thus derived from enzymatic cleavage - are called anaphylatoxins due to their effect on mast cell histamine release. The most potent of these, C5a, elicits the broadest responses.

Despite relatedness at the genetic level with C3a and C5a, since no specific human C4a receptor has been identified so far, the fact that C4a is anaphylatoxin is highly questionable (3).

C3a and C5a are recognized by specific transmembrane proteins, respectively one C3a receptor, C3aR1 (4,5), and two C5a receptors, C5aR1 and C5aR2, a.k.a CD88 and C5L2 (6,7). They are all members of the rhodopsin-like G-protein coupled receptor (GPCR) family, transducing signals through heterotrimeric G proteins and binding to β-arrestins for internalization (like chemokine receptors CCR5, CXCR1, CXCR2, CXCR4 and CCR7).

C3aR is highly expressed by granulocytes (eosinophils, basophils) and monocytes (macrophages), CD4+ T lymphocytes, but not on B cells. Many tissues are expressing C3aR (360,000 units per cell in average), and especially spleen, appendix, adipose tissue, spinal cord, lung and placenta (combination of the data from the three transcriptomics datasets HPA, GTEx and FANTOM5 for 55 tissue types and 6 blood cell types).

The human C3a receptor (C3aR) is a 54kDa molecule comprising 482 amino acids. It contains an exceptionally large extracellular loop between the fourth and fifth transmembrane domains, and a large second extracellular loop (e2 loop, ∼172 amino acids). Like for CCR5, the analysis of mutant receptors by direct agonist binding assays suggested that e2 loop is necessary for high affinity C3a binding. Both the N- and C-terminal ends of the e2 loop contain anionic residues that may interact with residues in the cationic C3a protein. The equilibrium dissociation constant (Kd) for C3aR is 3.85 ± 0.15 nM, and the interaction with its ligand generates an intracellular calcium flux.

C5aR1 is expressed by many leukocytes such as neutrophils, eosinophils, basophils, mast cells, macrophages, dendritic cells, and certain subset of lymphocytes. Many tissues are expressing C5aR (50-130.000 units per cell in average), and especially lung, adipose tissues and lymph nodes (combination of the data from the three transcriptomics datasets HPA, GTEx and FANTOM5 for 55 tissue types and 6 blood cell types).

The human C5aR (CD88) is a 42kDa protein comprising 350 amino acids. C5a binding involves two distinct sites at the C5aR essential for receptor activation: the N-terminus of C5aR, and the hydrophobic residues from the transmembrane domain.

Although it can bind to C5a and C5adesArg with high affinity, C5L2 does not trigger the biological actions of C5aR, suggesting that C5L2 may act as a decoy receptor for C5a.

The interaction of anaphylatoxins with their receptors triggers the activation of neutrophils and macrophages, increase the recruitment of dendritic cells and the secretion of cytokines and chemokines. But C3aR and C5aR are also expressed on T cells, modulating the adaptive immune response.

Anaphylatoxins are associated with nearly all types of inflammation. Consequently they’re involved in many autoimmune diseases (rheumatoid arthritis, systemic lupus erythematosus, psoriasis), acute respiratory distress syndrome but also inflammation related to cancer.

Markiewski et al demonstrated that blockade of C5a receptor significantly impaired tumor growth since the generation of complement C5a in the tumor microenvironment enhanced tumor growth by suppressing the anti-tumor CD8+ T cell-mediated response. This suppression is associated with the recruitment of myeloid-derived suppressor cells (MDSCs) into tumors and augmentation of their T cell-directed suppressive capabilities (9).

Wang et al on their side suggested the strong upregulation of C3aR and C5aR1 on CD8+ TILs made them a novel class of immune checkpoints that could be targeted for tumor immunotherapy through the inhibition of IL-10 production (10). Since C3aR/C5aR/IL-10 pathway is independant from PD-1/PD-L1 signaling, a combined antagonist therapy should be developed to treat cancer patients.

In fact, at the preclinical level, the combination of an anti PD-1 with the only anti C3aR1 antagonist currently known, SB-290157, shows better efficacy against tumor progression than treatment with an anti PD1 alone or with a C3aR antagonist alone (11). However, SB-290157 also acts as an agonist on the C5aR, which could compromise its therapeutic use (12).

The same year, Carsten Krieg and its team studied the relationship between C3aR expression and the progression of colorectal cancer. The study of sub-groups of both rectal and colon cancer patients demonstrated that C3aR1 is systematically downregulated compared to controls. Curiously enough the expression level doesn’t seem to be correlated to mutation but rather to epigenetic methylation islands (13).

The year 2022 was dedicated to the study of the relationship between C3aR and NK cells. Indeed, many solid tumors present a low infiltration of NK cells initially in charge of destroying the tumor. NK cells naturally express the C3aR1 receptor and many tumors express or have a high concentration of C3a.

Nandagopal and its team (14) recently showed that stimulation of NK cells by C3a prevented their infiltration into the tumor by disrupting the conformation of LFA1 which allows NK migration. On their side, Soddji et al (15) then demonstrated that the combination of radiotherapy and C3a-C3aR1 axis inhibition enhanced NK function in pancreatic cancer in mice.

REFERENCES

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(10) Wang Y, Sun SN, Liu Q, Yu YY, Guo J, Wang K, et al. Autocrine complement inhibits IL10-dependent T-cell-mediated antitumor immunity to promote tumor progression. Cancer Discov. (2016) 6:1–11. doi: 10.1158/2159-8290.CD-15-1412 


(11) Magrini E, Di Marco S, Mapelli SN, Perucchini C, Pasqualini F, Donato A, Guevara Lopez ML, Carriero R, Ponzetta A, Colombo P, Cananzi F, Supino D, Reis ES, Peano C, Inforzato A, Jaillon S, Doni A, Lambris JD, Mantovani A, Garlanda C. Complement activation promoted by the lectin pathway mediates C3aR-dependent sarcoma progression and immunosuppression. Nat Cancer. 2021 Feb;2(2):218-232. doi: 10.1038/s43018-021-00173-0. Epub 2021 Feb 18. PMID: 34505065; PMCID: PMC8425276.

(12) Li XX, Kumar V, Clark RJ, Lee JD, Woodruff TM. The "C3aR Antagonist" SB290157 is a Partial C5aR2 Agonist. Front Pharmacol. 2021 Jan 21;11:591398. doi: 10.3389/fphar.2020.591398. PMID: 33551801; PMCID: PMC7859635.

(13) Krieg C, Carloni S, Weber L, Fosso B, Hardiman G, Mileti E. Loss of C3aR induces immune infiltration and inflammatory microbiota in a new spontaneous model of colon cancer (2021). doi: https://doi.org/10.1101/2021.01.18.426963

(14) Nandagopal S, Li CG, Xu Y, Sodji QH, Graves EE, Giaccia AJ. C3aR Signaling Inhibits NK-cell Infiltration into the Tumor Microenvironment in Mouse Models. Cancer Immunol Res. 2022 Feb;10(2):245-258. doi: 10.1158/2326-6066.CIR-21-0435. Epub 2021 Nov 24. PMID: 34819308; PMCID: PMC9351714.

(15) Quaovi H. Sodji, Dhanya K. Nambiar, Vignesh Viswanathan, Rie von Eyben, Deana Colburg, Michael S. Binkley, Caiyun G. Li, Monica M. Olcina, Daniel T. Chang, Quynh-Thu Le, Amato J. Giaccia; The Combination of Radiotherapy and Complement C3a Inhibition Potentiates Natural Killer cell Functions Against Pancreatic Cancer. Cancer Research Communications 1 July 2022; 2 (7): 725–738. https://doi.org/10.1158/2767-9764.CRC-22-0069