CSF1R ligands promote microglial proliferation but are not the sole regulators of developmental microglial proliferation.

Microglia - the predominant immune cells of the brain and spinal cord - perform essential functions for the development and maintenance of the central nervous system, contingent upon the regulated developmental proliferation of microglia. However, the factor(s) that regulate microglial proliferation remain unclear. Here, we confirmed the timeline of developmental proliferation and used bioinformatics to identify potential signalling onto microglia in mouse from datasets collected at an age of high developmental microglial proliferation. Of the predicted factors, we found that colony stimulating factor 1 receptor (CSF1R) ligands boosted proliferation in vitro and were increasingly expressed in the brain across development with each displaying a distinct regional and temporal expression pattern. However, we did not observe a coincident alteration to CSF1R ligand levels in a model of abnormal developmental proliferation. Together, although CSF1R ligands can promote microglial proliferation in culture, their developmental expression patterns suggest that they function alongside other unknown factors to regulate developmental microglial proliferation.