CCR1 and CCR2 co-expression on monocytes is nonredundant and delineates a distinct monocyte subpopulation. [RNA-Seq]
The interactions between chemokines and their receptors, particularly in the context of inflammation, are complex with individual receptors binding multiple ligands and individual ligands interacting with multiple receptors. In addition, there are numerous reports of simultaneous co-expression of multiple inflammatory chemokine receptors on individual inflammatory leukocyte subtypes. Overall, this has previously been interpreted as redundancy and proposed as a protective mechanism to ensure that the inflammatory response is robust. In contrast we have hypothesised that the system is not redundant but exquisitely subtle. Our interests relate to the receptors CCR1, CCR2, CCR3 and CCR5 which, together, regulate non-neutrophilic myeloid cell recruitment to inflammatory sites. Here we demonstrate that, whilst most murine monocytes exclusively express CCR2, there is a small subpopulation, which is expanded during inflammation, which co-expresses CCR1 and CCR2. Combinations of transcript and functional analysis demonstrate that this is not redundant expression and that co-expression of CCR1 and CCR2 marks a phenotypically distinct population of monocytes characterised by expression of genes otherwise typically associated with neutrophils. Single cell RNA sequencing confirms this as a monodisperse population of atypical monocytes. This monocytic population has been previously described as having immunosuppressive activity. Overall, our data confirm combinatorial chemokine receptor expression by a subpopulation of monocytes but demonstrate that this is not redundant expression and marks a discrete monocytic population. Overall design: Bulk RNA-seq of Monocytes isolated from mouse bone in a resting or inflamed state.