The elucidation of protein complexing events and their cellular localisation are foundation to cytokine and signalling research. Traditional methodologies such as immunoprecipitation and immunofluorescence cannot provide data on complexing, localisation and abundance of molecules simultaneously. Here we use dSTORM [1] super-resolution imaging to study the interactions between the powerful anti-inflammatory cytokine IL-37 [2] and the receptor chains IL-18 receptor alpha (IL-18Ra) and SIGIRR. We investigate the formation of this tripartite complex with respect to subcellular localization, complex arrangement and relative molecular numbers of its three proteins in their endogenous, native context, i.e. without exogenous additions, transfections or lysis.
Our results show that IL-37 forms a ligand:receptor complex with SIGIRR and IL-18Ra (sub 50 nm proximity with each other) on the cell surface of primary, untransfected human PBMC. We could observe increased complex formation upon LPS stimulation of inflammatory responses as well as quantify bi- and tripartite complex presence. To our knowledge this is the first demonstration of any super-resolution technique to study three-way protein interactions in primary human samples and our results highlight the potential of super-resolution techniques to investigate formation of individual multi-protein complexes and respective intermediate stages.