Systematic comparison of culture media uncovers phenotypic shift of primary human microglia defined by reduced reliance to CSF1R signaling

Efforts to understand microglia function in health and diseases have been hindered by the lack of culture models that recapitulate in situ cellular properties. In recent years, the use of serum-free media with brain-derived growth factors (colony stimulating factor 1 receptor [CSF1R] ligands and TGF-beta 1/2) have been favored for the maintenance of rodent microglia as they promote morphological features observed in situ. Here we study the functional and transcriptomic impacts of such media on human microglia (hMGL). Media formulation had little impact on microglia transcriptome assessed by RNA sequencing which was sufficient to significantly alter microglia capacity to phagocytose myelin debris and to elicit an inflammatory response to lipopolysaccharide. When compared to immediately ex vivo microglia from the same donors, the addition of fetal bovine serum to culture media, but not growth factors, was found to aid in the maintenance of key signature genes including those involved in phagocytic processes. A phenotypic shift characterized by CSF1R downregulation in culture correlated with a lack of reliance on CSF1R signaling for survival. Consequently, no improvement in cell survival was observed following culture supplementation with CSF1R ligands. Our study provides better understanding of hMGL in culture, with observations that diverge from those previously made in rodent microglia.

Automated summary

Efforts to understand the function of microglia in health and disease have been hindered by the lack of appropriate culture models. Recent studies have shown that the use of serum-free media with specific growth factors can better maintain rodent microglia morphology. However, the same media formulation has little impact on human microglia transcriptome but significantly affects their phagocytic capacity and inflammatory response. The addition of fetal bovine serum to the culture media, but not growth factors, helps maintain key genes involved in phagocytosis. Our study provides a better understanding of human microglia in culture and highlights differences from rodent microglia.