Microglia and infiltrating macrophages are thought to orchestrate the central nervous system (CNS) response to injury; however, the similarities between these cells make it challenging to distinguish their relative contributions. We genetically labeled microglia and CNS-associated macrophages to distinguish them from infiltrating macrophages. Using single-cell RNA sequencing, we describe multiple microglia activation states, one of which was enriched for interferon associated signaling. Although blood-derived macrophages acutely infiltrated the demyelinated lesion, microglia progressively monopolized the lesion environment where they surrounded infiltrating macrophages. In the microglia-devoid sciatic nerve, the infiltrating macrophage response was sustained. In the CNS, the preferential proliferation of microglia and sparse microglia death contributed to microglia dominating the lesion. Microglia ablation reversed the spatial restriction of macrophages with the demyelinated spinal cord, highlighting an unrealized macrophages-microglia interaction. The restriction of peripheral inflammation by microglia may be a previously unidentified mechanism by which the CNS maintains its “immune privileged” status.

Although immune attack against central nervous system (CNS) myelin is a central feature of multiple sclerosis (MS), its root cause is unresolved. In this report, we provide direct evidence that subtle biochemical modifications to brain myelin elicit pathological immune responses with radiological and histological properties similar to MS lesions. A subtle myelinopathy induced by abbreviated cuprizone treatment, coupled with subsequent immune stimulation, resulted in lesions of inflammatory demyelination. The degree of myelin injury dictated the resulting immune response; biochemical damage that was too limited or too extensive failed to trigger overt pathology. An inhibitor of peptidyl arginine deiminases (PADs), enzymes that alter myelin structure and correlate with MS lesion severity, mitigated pathology even when administered only during the myelin-altering phase. Moreover, cultured splenocytes were reactive against donor myelin isolates, a response that was substantially muted when splenocytes were exposed to myelin from donors treated with PAD inhibitors. By showing that a primary biochemical myelinopathy can trigger secondary pathological inflammation, "cuprizone autoimmune encephalitis" potentially reconciles conflicting theories about MS pathogenesis and provides a strong rationale for investigating myelin as a primary target for early, preventative therapy.

AlphaB-crystallin (αBC) is a small heat shock protein that is constitutively expressed by peripheral nervous system (PNS) axons and Schwann cells. To determine what role this crystallin plays after peripheral nerve damage, we found that loss of αBC impaired remyelination, which correlated with a reduced presence of myelinating Schwann cells and increased numbers of nonmyelinating Schwann cells. The heat shock protein also seems to regulate the cross-talk between Schwann cells and axons, because expected changes in neuregulin levels and ErbB2 receptor expression after PNS injury were disrupted in the absence of αBC. Such dysregulations led to defects in conduction velocity and motor and sensory functions that could be rescued with therapeutic application of the heat shock protein in vivo. Altogether, these findings show that αBC plays an important role in regulating Wallerian degeneration and remyelination after PNS injury.T he robust regenerative capacity of the damaged peripheral nervous system (PNS) is partly determined by cellular and molecular events that occur in the nerve segment distal to the injury site (1). For instance, during Wallerian degeneration, influx of calcium into the damaged nerve within 12-24 h of PNS injury activates proteases (2) that result in cytoskeletal breakdown and subsequent disintegration of the axon membrane. This axon degeneration is then followed by breakdown of the myelin sheath within 2 d (3). Schwann cells, the glial cells that characterize the PNS, subsequently undergo a number of reactive physiological changes that benefit the damaged axon. Within 48 h of peripheral nerve damage, myelinating Schwann cells decrease their expression of myelin proteins, such as myelin basic protein (MBP), peripheral myelin protein 22, and protein 0 (P0) (4), and along with their nonmyelinating counterparts, revert to a nonmyelinating phenotype (4). At ∼3-4 d postinjury, the dedifferentiated Schwann cells proliferate (5-7) and align within the basal lamina to form bands of Büngner that provide a structural and trophic supportive substrate for regenerating axons. These Schwann cells secrete neurotrophic factors that provide trophic sustenance to damaged neurons until they reestablish contact with their targets (8) and produce extracellular matrix molecules that encourage and guide outgrowing axons (9), whereas secretion of chemokines is thought to mediate the infiltration of blood-derived macrophages, which along with Schwann cells, phagocytose myelin debris and its associated axon growth inhibitors (9). Finally, based on the level of neuregulin 1 Types I and III on Schwann cells and axons, respectively, and their binding to their cognate receptors ErbB2/ ErbB3 on Schwann cells, these glia will revert to a myelinating or ensheathing phenotype on contact with regrowing axons (10). Altogether, these morphological and physiological changes in Schwann cells create an environment that encourages longdistance axon growth.In humans, however, regrowth of damaged peripheral nerves is often incomplete, wh...

Highlights d Cystatin C (CST3) has a detrimental role in MOG 35-55 -induced EAE but only in female mice d Activation of female but not male antigen-presenting cells is promoted by CST3 d CST3 in antigen-presenting cells is associated with activation of female CD4 T cells d The sex-dependent effect of CST3 in EAE is sensitive to gonadal hormones