Single-Cell Sequencing Reveals Microglia Population Vulnerability Following Maternal Immune Activation in Mice

Background:

Maternal immune activation (MIA) during pregnancy has been linked to increased autism risk in offspring, but the mechanisms that lead to these changes are still largely unknown. Microglia, the resident immune cells in the brain colonize the brain during gestation, are highly sensitive to changes in the brain microenvironment and are altered in the brain’s of some individuals with autism. This suggests that microglia could play a role in the disease manifestation and raises the intriguing possibility that their normal role guiding neural network formation during development could be disturbed.

Objectives:

MIA risk in humans and mice has been linked to the first two trimesters of pregnancy, a period when microglia are populating the brain and initiating their normal developmental programs. In order to understand whether and how MIA affects this developmental trajectory - as a whole or in small subsets of cells - we wanted to profile microglia during gestation and in young mice.

Methods: MIA was induced in pregnant mice using Poly I:C. We performed high throughput single cell sequencing (Drop-seq) of microglia at three developmental time points with and without Poly I:C exposure. We also performed immunohistochemistry on brains from littermates to identify different microglia subpopulations based on the markers identified by Drop-seq.

Results:

Based on the sequencing results of several thousand microglia per condition we found that microglia clustered into distinct subpopulations. These different subpopulations expressed markers of cell division, inflammation/phagocytosis, and cell motility. We found that exposure to Poly I:C changed the size of the phagocytic microglia group at the expense of the other subpopulations. These results persisted into early adolescence. Interestingly, this affected a relatively small subset of cells in specific brain regions.

Conclusions:

Our results are the first to create a detailed single cell sequencing map of microglia development in thousands of cells. This power gave us the resolution to detects small subpopulations of interesting microglia in different brain regions. Interestingly, MIA exposure shifted the number of cells occupying each of these states in a region specific manner, suggesting that certain microglia are more vulnerable than others to immune challenge. Further analysis will be needed to figure out potential mechanisms by which these subpopulations might affect neural network formation and behavior