Meningeal lymphoid structures are activated under acute and chronic spinal cord pathologies.

Tertiary lymphoid structures (TLS) are organized aggregates of B and T cells formed ectopically during different stages of life in response to inflammation, infection, or cancer. Here, we describe formation of structures reminiscent of TLS in the spinal cord meninges under several central nervous system (CNS) pathologies. After acute spinal cord injury, B and T lymphocytes locally aggregate within the meninges to form TLS-like structures, and continue to accumulate during the late phase of the response to the injury, with a negative impact on subsequent pathological conditions, such as experimental autoimmune encephalomyelitis. Using a chronic model of spinal cord pathology, the mSOD1 mouse model of amyotrophic lateral sclerosis, we further showed by single-cell RNA-sequencing that a meningeal lymphocyte niche forms, with a unique organization and activation state, including accumulation of pre-B cells in the spinal cord meninges. Such a response was not found in the CNS-draining cervical lymph nodes. The present findings suggest that a special immune response develops in the meninges during various neurological pathologies in the CNS, a possible reflection of its immune privileged nature.

M2 microglia and macrophages drive oligodendrocyte differentiation during CNS remyelination.

The lack of therapies for progressive multiple sclerosis highlights the need to understand the regenerative process of remyelination that can follow CNS demyelination. This involves an innate immune response consisting of microglia and macrophages, which can be polarized to distinct functional phenotypes: pro-inflammatory (M1) and anti-inflammatory or immunoregulatory (M2). We found that a switch from an M1- to an M2-dominant response occurred in microglia and peripherally derived macrophages as remyelination started. Oligodendrocyte differentiation was enhanced in vitro with M2 cell conditioned media and impaired in vivo following intra-lesional M2 cell depletion. M2 cell densities were increased in lesions of aged mice in which remyelination was enhanced by parabiotic coupling to a younger mouse and in multiple sclerosis lesions that normally show remyelination. Blocking M2 cell-derived activin-A inhibited oligodendrocyte differentiation during remyelination in cerebellar slice cultures. Thus, our results indicate that M2 cell polarization is essential for efficient remyelination and identify activin-A as a therapeutic target for CNS regeneration.

Recruitment of beneficial M2 macrophages to injured spinal cord is orchestrated by remote brain choroid plexus.

Monocyte-derived macrophages are essential for recovery after spinal cord injury, but their homing mechanism is poorly understood. Here, we show that although of common origin, the homing of proinflammatory (M1) and the "alternatively activated" anti-inflammatory (M2) macrophages to traumatized spinal cord (SC) was distinctly regulated, neither being through breached blood-brain barrier. The M1 macrophages (Ly6c(hi)CX3CR1(lo)) derived from monocytes homed in a CCL2 chemokine-dependent manner through the adjacent SC leptomeninges. The resolving M2 macrophages (Ly6c(lo)CX3CR1(hi)) derived from monocytes trafficked through a remote blood-cerebrospinal-fluid (CSF) barrier, the brain-ventricular choroid plexus (CP), via VCAM-1-VLA-4 adhesion molecules and epithelial CD73 enzyme for extravasation and epithelial transmigration. Blockage of these determinants, or mechanical CSF flow obstruction, inhibited M2 macrophage recruitment and impaired motor-function recovery. The CP, along with the CSF and the central canal, provided an anti-inflammatory supporting milieu, potentially priming the trafficking monocytes. Overall, our finding demonstrates that the route of monocyte entry to central nervous system provides an instructional environment to shape their function.

Rejuvenation of regeneration in the aging central nervous system.

Remyelination is a regenerative process in the central nervous system (CNS) that produces new myelin sheaths from adult stem cells. The decline in remyelination that occurs with advancing age poses a significant barrier to therapy in the CNS, particularly for long-term demyelinating diseases such as multiple sclerosis (MS). Here we show that remyelination of experimentally induced demyelination is enhanced in old mice exposed to a youthful systemic milieu through heterochronic parabiosis. Restored remyelination in old animals involves recruitment to the repairing lesions of blood-derived monocytes from the young parabiotic partner, and preventing this recruitment partially inhibits rejuvenation of remyelination. These data suggest that enhanced remyelinating activity requires both youthful monocytes and other factors, and that remyelination-enhancing therapies targeting endogenous cells can be effective throughout life.