title: Complement Protein C1q Interacts with DC-SIGNviaIts 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 DCsviaa 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 TyrB175and LysB136were 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.
authors.affiliation: Biosciences, College of Health and Life Sciences, Brunel University London , Uxbridge , UK.
authors.forename: Lina
authors.initials: L
authors.lastname: Pednekar
authors.affiliation: Department of Innate Immunity, National Institute for Research in Reproductive Health (ICMR) , Mumbai , India.
authors.forename: Hrishikesh
authors.initials: H
authors.lastname: Pandit
authors.affiliation: Biosciences, College of Health and Life Sciences, Brunel University London , Uxbridge , UK.
authors.forename: Basudev
authors.initials: B
authors.lastname: Paudyal
authors.affiliation: Biosciences, College of Health and Life Sciences, Brunel University London , Uxbridge , UK.
authors.forename: Anuvinder
authors.initials: A
authors.lastname: Kaur
authors.affiliation: Department of Infection and Immunity, King Faisal Specialist Hospital and Research Centre , Riyadh , Saudi Arabia.
authors.forename: Maha Ahmed
authors.initials: MA
authors.lastname: Al-Mozaini
authors.affiliation: Biosciences, College of Health and Life Sciences, Brunel University London , Uxbridge , UK.
authors.forename: Lubna
authors.initials: L
authors.lastname: Kouser
authors.affiliation: Department of Medicine, State University of New York , Stony Brook, NY , USA.
authors.forename: Berhane
authors.initials: B
authors.lastname: Ghebrehiwet
authors.affiliation: Clinical Sciences Research Laboratories, University of Warwick , Coventry , UK.
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Mitchell
authors.affiliation: Department of Innate Immunity, National Institute for Research in Reproductive Health (ICMR) , Mumbai , India.
authors.forename: Taruna
authors.initials: T
authors.lastname: Madan
authors.affiliation: Biosciences, College of Health and Life Sciences, Brunel University London , Uxbridge , UK.
authors.forename: Uday
authors.initials: U
authors.lastname: Kishore
keywords: C1q; DC-SIGN; HIV-1; globular head domain; protein–protein interaction