Depletion of Peripheral Monocytes Alters Long-Term Gene Expression in Microglia in a Murine Model of Traumatic Brain Injury.

Background: Traumatic brain injury (TBI) is a growing and under-recognized public health threat with significant long-term complications suffered by its survivors. Monocytes are amongst the first immune cells to infiltrate the injured brain and have the ability to both foster wound repair and promote inflammation. Microglia, the resident innate immune cells in the brain, drive the long-term direction of the inflammatory response within the injured brain. Previously published data from our laboratory has shown that a brief course of peri-injury monocyte depletion attenuates long-term neurocognitive deficits and preserves white matter connectivity post-injury. Nonetheless, the role that monocytes play in directing the local inflammatory response to injury via crosstalk with microglia remains unknown. To this end, we hypothesized that infiltrating monocytes shape the long-term transcriptional response of microglia to TBI.

Methods: To test this hypothesis, we employed a 2x2 study design consisting of four experimental groups-TBI, sham, TBI with monocyte depletion, and TBI with sham monocyte depletion. Male C57BL/6 mice were randomly assigned to groups. Monocyte depletion and sham depletion were induced via intravenous injection of liposome-encapsulated clodronate versus naked liposomes 24 hour prior to TBI. Depletion was maintained via repeat injections on days 2 and 5. Monocyte depletion was confirmed via flow cytometry. TBI was induced using our established model of controlled cortical impact. Behavioral phenotyping was conducted at 30 days post-injury to assess motor coordination and memory. Mice were euthanized on post-injury days 1, 7, 14, 30, and 60. Brains were harvested and microglia sorted via flow cytometry. The transcriptional response of microglia across groups and timepoints was assess via bulk RNA sequencing.

Results: Monocyte-depleted mice demonstrated improvement in motor coordination, contextual and associative learning, and memory. These neurocognitive differences were associated with distinctly different microglial transcriptional profiles evident within the first 1-2 weeks post injury. In particular, microglia within the monocyte-depleted group showed distinct upregulation of pathways including synaptic signaling, regulation of neuron differentiation, and myeloid leukocyte activation as compared to those from Vehicle TBI groups. By 60 days post injury, microglia from the monocyte-depleted TBI group had upregulation of the heat shock protein transcripts as compared to microglia from the Vehicle TBI group.

Conclusion: These data shows that short-course of peri-injury depletion of peripheral monocytes may have a neuroprotective effect after TBI. While the mechanisms of this protection are multifactorial, alteration of the long-term transcriptional profile of microglia may, in part, be responsible for the observed improvements in motor coordination, learning, and memory. Identified pathways include inflammation, neuroplasticity and regeneration, and a neuroprotective heat shock response. These data warrant further investigation into possible therapeutic benefits of peri-injury immune modulation.