The border-associated macrophage marker MRC1 contributes to an early neuroprotective inflammatory response to traumatic brain injury in mice.

Macrophages are crucial for neuroinflammatory responses following traumatic brain injury (TBI), encompassing various subtypes, such as border-associated macrophages (BAMs) that contribute to both brain damage and repair. However, the pathophysiological relevance of subtype-specific molecular markers is poorly understood. This study investigated the role of the BAM marker mannose receptor C-type 1 (MRC1, also known as CD206) during the early phase of TBI using controlled cortical impact (CCI). MRC1 gene expression was up-regulated, peaking between 3 to 7 days post-injury (dpi), and MRC1 protein expression predominantly localized to BAMs. To assess pathophysiological relevance, MRC1-deficient (MRC1-KO) and wild-type littermates (MRC1-WT) were examined following CCI for early neurological deficits, brain structural damage, intracerebral hematoma, and neuroinflammatory marker expression. At 5 dpi, MRC1-KO mice showed increased brain lesion volume and hippocampal neuron loss, with minor differences in neurological deficits compared to MRC1-WT mice. Intracerebral hematoma size increased in male but remained unchanged in female MRC1-KO mice. Immunostaining revealed no genotype-specific effects on GFAP+ astrocytes, while the number of perilesional CD68+ macrophages/microglia were reduced in MRC1-KO mice. Analysis of neuroinflammatory gene markers revealed an overall reduction in MRC1-KO mice. Sex-specific regulation was observed for the M2-like macrophage/microglia marker Arg1, with decreased expression in male and increased expression in female MRC1-KO compared to MRC1-WT mice. In conclusion, lack of MRC1 exacerbated brain tissue damage following experimental TBI. Reduced CD68+ macrophages/microglia and neuroinflammatory marker expression suggests impaired neuroinflammatory response in MRC1-KO, indicating MRC1 expression on BAMs contributes to beneficial early neuroinflammatory response following TBI.