Introduction to anaphylatoxin receptors
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).
GPCR complement 3a receptor 1 (C3AR1)
C3aR tissue expression and distribution
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).
Structure and ligand interaction of C3aR with C3a
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.
Complement component 5a receptor (C5AR)
C5aR tissue expression and distribution
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).
Structure and ligand interaction of C5aR and C5L2 with C5a
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.
Immunological effects of C3 and C5 anaphylatoxins
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.
Pathological effects of C3 and C5 anaphylatoxins
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.
More recently, 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 (11).
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(2) Friedberger E. Weitere Untersuchungen uber Eiweissanaphylaxie. Z Immunforsch Exp Ther 1910;4:636–89
(3) Lienenklaus S,Ames RS, Tornetta MA, Sarau HM, Foley JJ, Crass T, Sohns B, Raffetseder U, Grove M, Hölzer A, et al. (1998) Human anaphylatoxin C4a is a potent agonist of the guinea pig but not the human C3a receptor. J Immunol 161:2089–2093.
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(7) Cain SA, Monk PN. The orphan receptor C5L2 has high affinity binding sites for complement fragments C5a and C5a des-Arg(74). J Biol Chem 2002;277:7165–9.
(8) Kupp LI, Kosco MH, Schenkein HA, Tew JG. Chemotaxis of germinal center B cells in response to C5a. Eur J Immunol 1991;21:2697–701.
(9) Markiewski MM, DeAngelis RA, Benencia F, et al. Modulation of the antitumor immune response by complement. Nat Immunol. 2008;9(11):1225-1235. doi:10.1038/ni.1655
(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) 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