C5aR1 Activation Drives Early IFN-γ Production to Control Experimental Toxoplasma gondii Infection

Abstract

Toxoplasma gondii (T. gondii) is a parasite infecting animals and humans. In intermediate hosts, such as humans or rodents, rapidly replicating tachyzoites drive vigorous innate and adaptive immune responses resulting in bradyzoites that survive within tissue cysts. Activation of the innate immune system is critical during the early phase of infection to limit pathogen growth and to instruct parasite-specific adaptive immunity. In rodents, dendritic cells (DCs) sense T. gondii through TLR11/12, leading to IL-12 production, which activates NK cells to produce IFN-γ as an essential mechanism for early parasite control. Further, C3 can bind to T. gondii resulting in limited complement activation. Here, we determined the role of C5a/C5aR1 axis activation for the early innate immune response in a mouse model of peritoneal T. gondii infection. We found that C5ar1 −/− animals suffered from significantly higher weight loss, disease severity, mortality, and parasite burden in the brain than wild type control animals. Severe infection in C5ar1 −/− mice was associated with diminished serum concentrations of IL-12, IL-27, and IFNγ. Importantly, the serum levels of pro-inflammatory cytokines, including IL-1α, IL-6, and TNF-α, as well as several CXC and CC chemokines, were decreased in comparison to wt animals, whereas anti-inflammatory IL-10 was elevated. The defect in IFN-γ production was associated with diminished Ifng mRNA expression in the spleen and the brain, reduced frequency of IFN-γ + NK cells in the spleen, and decreased Nos2 expression in the brain of C5ar1 −/− mice. Mechanistically, DCs from the spleen of C5ar1 −/− mice produced significantly less IL-12 in response to soluble tachyzoite antigen (STAg) stimulation in vivo and in vitro. Our findings suggest a model in which the C5a/C5aR1 axis promotes IL-12 induction in splenic DCs that is critical for IFN-γ production from NK cells and subsequent iNOS expression in the brain as a critical mechanism to control acute T. gondii infection.

A Protective and Pathogenic Role for Complement During Acute Toxoplasma gondii Infection

“…While C3aR deficiency in BALB/c was partially protective in DDS-induced colitis, there was no significant effect observed in C57BL/6 mice ( Wende et al, 2013 ), indicating that differences between these two mice strains are important factors in these phenotypes. Given this evidence and the recently demonstrated role of C5aR signaling in systemic T. gondii infection ( Briukhovetska et al, 2020 ), it is highly likely that anaphylatoxins play an important role in promoting inflammation in the gut during T. gondii infection.…”

“…These findings are consistent with an established role for locally produced C3a and C5a in regulating Th1 responses through autocrine and paracrine C3aR/C5aR receptor signaling ( Strainic et al, 2008 ; Liszewski et al, 2013 ). More recently, infection studies in C5ar1 −/− mice showed that these mice also die acutely, but with a much less dramatic phenotype or kinetic than the DKO mice ( Briukhovetska et al, 2020 ). In this study, 50 cysts of the Type II strain Me49 were injected intraperitoneally.…”

“…In this study, 50 cysts of the Type II strain Me49 were injected intraperitoneally. At this dose, both WT and C5ar1 −/− mice were susceptible to intraperitoneal infection, with 60% of WT mice dead within 30 days compared to 80% of C5ar1 −/− infected mice, which exhibited a statistically significant higher parasite load during acute disease ( Briukhovetska et al, 2020 ). The authors argued that C3b and its degradation products likely promoted the release of the anaphylatoxin C5a that bound the C5aR1 expressed on CD8α+ dendritic cells (DCs) which were activated by T. gondii -induced TLR signaling to amplify IL-12 and IFN-γ production ( Briukhovetska et al, 2020 ).…”

“…At this dose, both WT and C5ar1 −/− mice were susceptible to intraperitoneal infection, with 60% of WT mice dead within 30 days compared to 80% of C5ar1 −/− infected mice, which exhibited a statistically significant higher parasite load during acute disease ( Briukhovetska et al, 2020 ). The authors argued that C3b and its degradation products likely promoted the release of the anaphylatoxin C5a that bound the C5aR1 expressed on CD8α+ dendritic cells (DCs) which were activated by T. gondii -induced TLR signaling to amplify IL-12 and IFN-γ production ( Briukhovetska et al, 2020 ). Evidence for decreased levels of serum proinflammatory cytokines and increased IL-10 serum levels in C5ar1 −/− mice, and the reduction of IL-12 secretion from C5ar1 −/− splenic DCs in response to STAg in vitro suggested that complement signaling was important for priming appropriate Th1 responses to control parasite replication.…”

The Defensive Interactions of Prominent Infectious Protozoan Parasites: The Host’s Complement System

“…Toxoplasma parasites can directly upregulate the complement factor B (CFB), complement component factor properdin (CFP), C3, and C5aR1 in glial cells. Since the C5a/C5aR1 axis increases interleukin 12 (IL-12) induction in splenic dendritic cells and subsequent inducible nitric oxide synthase (iNOS) expression in the brain [ 64 ], the continued overexpression of C3aR and C5aR1 in the infected brains could be probably correlated with the defense against Toxoplasma infection-induced damages in the CNS [ 63 ].…”