The CCL2/CCL7/CCL12/CCR2 pathway is substantially and persistently upregulated in mice after traumatic brain injury, and CCL2 modulates the complement system in microglia

Traumatic brain injury (TBI) is the leading cause of death in the global population. Disturbed inflammatory processes after TBI exacerbate secondary brain injury and contribute to unfavorable outcomes. Multiple inflammatory events that accompany brain trauma, such as glial activation, chemokine release, or the initiation of the complement system cascade, have been identified as potential targets for TBI treatment. However, the participation of chemokines in the complement activation remains unknown. Our studies sought to determine the changes in the expression of the molecules involved in the CCL2/CCL7/CCL12/CCR2 pathway in the injured brain and the effect of CCL2, CCL7, and CCL12 (10, 100, and 500 ng/mL) on the classic and lectin complement pathways and inflammatory factors in microglial cell cultures. Brain injury in mice was modeled by controlled cortical impact (CCI). Our findings indicate a time-dependent upregulation of CCL2, CCL7, and CCL12 at the mRNA and protein levels within the cortex, striatum, and/or thalamus beginning 24 h after the trauma. The analysis of the expression of the receptor of the tested chemokines, CCR2, revealed its substantial upregulation within the injured brain areas mainly on the mRNA level. Using primary cortical microglial cell cultures, we observed a substantial increase in the expression of CCL2, CCL7, and CCL12 after 24 h of LPS (100 ng/mL) treatment. CCL2 stimulation of microglia increased the level of IL-1β mRNA but did not influence the expression of IL-18, IL-6, and IL-10. Moreover, CCL2 significantly increased the expression of Iba1, a marker of microglia activation. CCL2 and CCL12 upregulated the expression of C1qa but did not influence the expression of C1ra and C1s1 (classical pathway); moreover, CCL2 increased ficolin A expression and reduced collectin 11 expression (lectin pathway). Additionally, we observed the downregulation of pentraxin 3, a modulator of the complement cascade, after CCL2 and CCL12 treatment. We did not detect the expression of ficolin B, Mbl1, and Mbl2 in microglial cells. Our data identify CCL2 as a modulator of the classical and lectin complement pathways suggesting that CCL2 may be a promising target for pharmacological intervention after brain injury. Moreover, our study provides evidence that CCL2 and two other CCR2 ligands may play a role in the development of changes in TBI.