Complement Protein C1q Interacts with DC-SIGN via Its Globular Domain and Thus May Interfere with HIV-1 Transmission

Abstract

Dendritic cells (DCs) are the most potent antigen-presenting cells capable of priming naïve T-cells. Its C-type lectin receptor, DC-SIGN, regulates a wide range of immune functions. Along with its role in HIV-1 pathogenesis through complement opsonization of the virus, DC-SIGN has recently emerged as an adaptor for complement protein C1q on the surface of immature DCs via a trimeric complex involving gC1qR, a receptor for the globular domain of C1q. Here, we have examined the nature of interaction between C1q and DC-SIGN in terms of domain localization, and implications of C1q–DC-SIGN-gC1qR complex formation on HIV-1 transmission. We first expressed and purified recombinant extracellular domains of DC-SIGN and its homologue DC-SIGNR as tetramers comprising of the entire extra cellular domain including the α-helical neck region and monomers comprising of the carbohydrate recognition domain only. Direct binding studies revealed that both DC-SIGN and DC-SIGNR were able to bind independently to the recombinant globular head modules ghA, ghB, and ghC, with ghB being the preferential binder. C1q appeared to interact with DC-SIGN or DC-SIGNR in a manner similar to IgG. Mutational analysis using single amino acid substitutions within the globular head modules showed that TyrB175 and LysB136 were critical for the C1q–DC-SIGN/DC-SIGNR interaction. Competitive studies revealed that gC1qR and ghB shared overlapping binding sites on DC-SIGN, implying that HIV-1 transmission by DCs could be modulated due to the interplay of gC1qR-C1q with DC-SIGN. Since C1q, gC1qR, and DC-SIGN can individually bind HIV-1, we examined how C1q and gC1qR modulated HIV-1–DC-SIGN interaction in an infection assay. Here, we report, for the first time, that C1q suppressed DC-SIGN-mediated transfer of HIV-1 to activated pooled peripheral blood mononuclear cells, although the globular head modules did not. The protective effect of C1q was negated by the addition of gC1qR. In fact, gC1qR enhanced DC-SIGN-mediated HIV-1 transfer, suggesting its role in HIV-1 pathogenesis. Our results highlight the consequences of multiple innate immune pattern recognition molecules forming a complex that can modify their functions in a way, which may be advantageous for the pathogen.

C1q: A fresh look upon an old molecule

“…Interactions between neutrophils and DCs are mediated through interaction of DC-SIGN with CR3 (Ludwig et al, 2006). DC-SIGN and the related protein DC-SIGNR were reported recently to bind to C1q globular heads (Hosszu et al, 2012; Pednekar et al, 2016). In addition, C1q and gC1qR were shown to associate with DC-SIGN on the surface of immature DCs and to regulate DC differentiation and function.…”

Complement Proteins as Soluble Pattern Recognition Receptors for Pathogenic Viruses

“…C1q can suppress DC-SIGN-mediated transfer of HIV-1 to activated peripheral blood mononuclear cells; however, the recombinant form of globular head A, B, and C modules of C1q do not [ 101 ]. The protective activity of C1q was negated by the addition of gC1qR by enhancing DC-SIGN mediated HIV-1 transfer.…”

Complement Evasion: An Effective Strategy That Parasites Utilize to Survive in the Host

“…The classical pathway is antibody-dependent and initiated by C1 complex, when binding of C1q to antibody IgM or IgG complexed with antigens ( Duncan and Winter, 1988 ; Gaboriaud et al, 2014 ; Pednekar et al, 2016 ). Such binding events cause C1q conformational changes, which lead to the activation of C1r and C1s.…”