The role of Major Histocompatibility Complex class II (MHC II) molecules in antigen presentation to CD4+ T cells has been amply characterised over decades of research. Here we describe a new function, namely binding activated Complement C3. Activation of C3 is central to its protective function against pathogens. We now show that C3 also regulates the development of the acquired immune system via binding to MHC II.
MHC II expression is tightly controlled genetically and post-translationally. Ubiquitination by Membrane-Associated RING-CH ligase 1 (MARCH1) is a key regulatory step. Mice deficient in MARCH1 show increased MHC II surface expression in professional antigen presenting cells. Here we show that they also contain fewer dendritic cells (DCs) but increased number of DC precursors. To gain insights into the cause of this phenomenon we used plasma membrane proteomics to identify other components enriched on MARCH1-deficient DCs. We found complement C3 and confirmed by flow cytometry and immunoprecipitation its high expression on DCs. Further characterisation showed that C3 binds specifically to the N-linked glycan on the MHC II α-chain. Indeed, C3 linking to MHC II was also observed in normal cells, albeit the accumulation of C3 was limited via MARCH1 regulation of MHC II expression. Furthermore, C3 was also increased on the surface of DCs of mice that expressed MARCH1 but also expressed a mutant form of MHC II (K225R) that cannot be ubiquitinated. DC development was restored in mice deficient in MARCH1, or expressing the K225R mutant form of MHC II, if the mice also lacked C3 expression.
Our results imply that DC development and turnover are controlled by MHC II-C3 complex formation on their surface. They also demonstrate a novel role for MHC II molecules as a C3 receptor. Other potential functions of this phenomenon are currently investigated and will be presented.