Activation of Myeloid TLR4 Mediates T Lymphocyte Polarization after Traumatic Brain Injury

Abstract

Traumatic brain injury (TBI) is a major public health issue, producing significant patient mortality and poor long-term outcomes. Increasing evidence suggests an important, yet poorly defined, role for the immune system in the development of secondary neurological injury over the days and weeks following a TBI. Herein, we tested the hypothesis that peripheral macrophage infiltration initiates long-lasting adaptive immune responses after TBI. Using a murine controlled cortical impact model, we used adoptive transfer, transgenic, and bone marrow chimera approaches to show increased infiltration and pro-inflammatory (M1) polarization of macrophages for up to three weeks post-TBI. Monocytes purified from the injured brain stimulated the proliferation of naïve T lymphocytes, enhanced the polarization of T effector cells (Teff: TH1/TH17), and decreased the production of regulatory T cells (TREG) in a mixed lymphocyte reaction. Similarly, elevated Teff polarization within both blood and brain tissue was attenuated by myeloid cell depletion after TBI. Functionally, C3H/HeJ (TLR4 mutant) mice reversed both M1 macrophage and TH1/TH17 polarization after TBI, as compared to C3H/OuJ (wild-type) mice. Moreover, brain monocytes isolated from C3H/HeJ mice were less potent stimulators of T lymphocyte proliferation and TH1/TH17 polarization, as compared to C3H/OuJ monocytes. Taken together, our data implicate TLR4-dependent, M1 macrophage trafficking/polarization into the CNS as a key mechanistic link between acute TBI and long-term, adaptive immune responses.

White matter damage after traumatic brain injury: A role for damage associated molecular patterns

“…Consistent with a role for TLR4 in mediating M1 macrophage polarization [222] and in reprogramming M2 macrophages toward a M1 phenotype [223, 224], we reported that both TLR4 −/− mice and C3H/HeJ mice, which contain a spontaneous inactivating point mutation in the TLR4 signaling domain, exhibited a reduction in M1 polarization over the first 72h after experimental TBI [225]. Of note, a progressive increase in the ratio of M1:M2 macrophages temporally correlated with evolving neurological injury, including white matter loss and reduced brain volume, after TBI [37, 187, 225, 226]. In line with the aforementioned studies, M1 macrophages accumulated within peri-contusional white matter, within grey matter, and within the contralateral cortex by d3 after TBI, preceding oligodendrocyte apoptosis and widespread myelin loss (hallmarks of white matter injury) over the next several weeks [215].…”

“…Toward this end, “classically activated” (M1) macrophages exhibit a pro-inflammatory phenotype whereas “alternatively activated” (M2) macrophages release anti-inflammatory mediators to dampen immune responses [220, 221]. Consistent with a role for TLR4 in mediating M1 macrophage polarization [222] and in reprogramming M2 macrophages toward a M1 phenotype [223, 224], we reported that both TLR4 −/− mice and C3H/HeJ mice, which contain a spontaneous inactivating point mutation in the TLR4 signaling domain, exhibited a reduction in M1 polarization over the first 72h after experimental TBI [225]. Of note, a progressive increase in the ratio of M1:M2 macrophages temporally correlated with evolving neurological injury, including white matter loss and reduced brain volume, after TBI [37, 187, 225, 226].…”

“…We showed that monocytes isolated from the CNS after TBI stimulated T-cell proliferation and increased T H 1 and T H 17 polarization [225]. Moreover, activation of myeloid TLR4 simultaneously increased T H 1/T H 17 polarization and reduced T REG production in both blood and brain for several weeks after TBI in mice [225].…”

“…We showed that monocytes isolated from the CNS after TBI stimulated T-cell proliferation and increased T H 1 and T H 17 polarization [225]. Moreover, activation of myeloid TLR4 simultaneously increased T H 1/T H 17 polarization and reduced T REG production in both blood and brain for several weeks after TBI in mice [225]. While the efficacy of T H 1/T H 17 inhibition remains unexplored after TBI, T H 17 cells drove chronic microglial polarization after EAE and co-culture studies showed myelin-specific T-lymphocytes that secreted IL-17 potently activated inflammatory cytokine expression in microglia [268, 269].…”

The CNS lymphatic system modulates the adaptive neuro-immune response in the perilesional cortex in a mouse model of traumatic brain injury

“…TBI is defined as an alteration in brain function, or other evidence of brain pathology, caused by an external force (4), which results in immediate neuronal cell death, diffuse axonal injury, ischemia, and hemorrhage (5). These primary insults initiate a progressive cascade of secondary injuries, which include macrophage infiltration (6), neuro-inflammation (microglia and astrocyte activation associated with cytokine production), edema formation, oxidative stress, neuronal necrosis and apoptosis, and white matter atrophy (5). Secondary injuries can progress for years in patients and rodent models of TBI, and are the causes of the neurological and psychiatric deficits associated with the pathology (7).…”