Many cytokine receptors are cell surface proteins that promiscuously combine to form active signalling homo- or heterodimers. Thus, receptor chain dimerization can be viewed as a direct measure of a high probability of intracellular signalling by specific cytokines. Proximity ligation assay (PLA) is an antibody-based method for selective and highly sensitive detection of protein interactions by microscopy. As proof of concept, the aim of this study was to combine antibodies towards interleukin 7 receptor alpha (IL-7Rα) and the common gamma chain (γc) with PLA and flow cytometry to enable the detection of IL-7 receptor heterodimers. The presence of IL-7 receptor heterodimers on the surface of the HPB-ALL T cell line was detected by PLA and microscopy with a resolution of one complex per cell. Optimisation of the PLA reaction on cell suspensions identified buffer effects with critical importance for the flow cytometric outcome. In addition, blocking, fixation and incubation conditions were optimised to prevent unspecific antibody binding. PLA combined with flow cytometry very sensitively detected receptor heterodimers on the cell surface. Thus, the method is a powerful tool for the investigation of cytokine receptor dimerization.
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