Inflammation is a prominent feature of CNS injury that heavily influences neuronal survival, yet the signals that initiate and control it remain poorly understood. Here we identify the nuclear alarmin, interleukin (IL)-33, as an important regulator of the innate immune response after CNS injury. IL-33 is expressed widely throughout the healthy brain and is concentrated in white mater due to predominant expression in post-mitotic oligodendrocytes. IL-33 is released immediately after CNS injury from damaged oligodendrocytes, acting on local astrocytes and microglia to induce chemokines critical for monocyte recruitment. Mice lacking IL-33 have impaired recovery after CNS injury, which is associated with reduced myeloid cell infiltrates and decreased induction of M2 genes at the injury site. These results demonstrate a novel molecular mediator contributing to immune cell recruitment to the injured CNS and may lead to new therapeutic insights in CNS injury and neurodegenerative diseases.

Fighting pathogens and maintaining tissue homeostasis are prerequisites for survival. Both of these functions are upheld by the immune system, though the latter is often overlooked in the context of the CNS. The mere presence of immune cells in the CNS was long considered a hallmark of pathology, but this view has been recently challenged by studies demonstrating that immunological signaling can confer pivotal neuroprotective effects on the injured CNS. In this review we describe the temporal sequence of immunological events that follow CNS injury. Beginning with immediate changes at the injury site including death of neural cells and release of damage-associated molecular patterns (DAMPs), and progressing through innate and adaptive immune responses, we describe the cascade of inflammatory mediators and the implications of their post-injury effects. We conclude by proposing a revised interpretation of immune privilege in the brain, which takes beneficial neuro-immune communications into account.

No abstract

No abstract

Whereas thousands of new neurons are generated daily during adult life, only a fraction of them survive and become part of neural circuits; the rest die, and their corpses are presumably cleared by resident phagocytes. How the dying neurons are removed and how such clearance influences neurogenesis are not well understood. Here, we identify an unexpected phagocytic role for the doublecortin (DCX)-positive neuronal progenitor cells during adult neurogenesis. Our in vivo and ex vivo studies demonstrate that DCX+ cells comprise a significant phagocytic population within the neurogenic zones. Intracellular engulfment protein ELMO1, which promotes Rac activation downstream of phagocytic receptors, was required for phagocytosis by DCX+ cells. Disruption of engulfment in vivo genetically (in Elmo1-null mice) or pharmacologically (in wild-type mice) led to reduced uptake by DCX+ cells, accumulation of apoptotic nuclei in the neurogenic niches and impaired neurogenesis. Collectively, these findings indicate a paradigm wherein DCX+ neuronal precursors also serve as phagocytes, and that their phagocytic activity critically contributes to neurogenesis in the adult brain.

No abstract

Previous research investigating the roles of effector (Teff) and regulatory (Treg) T cells after injury to the central nervous system (CNS) has yielded contradictory conclusions, with both protective and destructive functions being ascribed to each of these T-cell subpopulations. Here we study this dichotomy by examining how regulation of the immune system affects the response to CNS trauma. We show that in response to CNS injury, Teff and Treg subsets in the CNS-draining deep cervical lymph nodes are activated, and surgical resection of these lymph nodes results in impaired neuronal survival. Depletion of Treg, not surprisingly, induces a robust Teff response in the draining lymph nodes and is associated with impaired neuronal survival. Interestingly, however, injection of exogenous Treg cells, which limits the spontaneous beneficial immune response after CNS injury, also impairs neuronal survival. We found that no Treg accumulate at the site of CNS injury, and that changes in Treg numbers do not alter the amount of infiltration by other immune cells into the site of injury. The phenotype of macrophages at the site, however, is affected: both addition and removal of Treg negatively impact the numbers of macrophages with alternatively activated (tissue-building) phenotype. Our data demonstrate that neuronal survival after CNS injury is impaired when Treg cells are either removed or added. With this exacerbation of neurodegeneration seen with both addition or depletion of Treg, we recommend exercising extreme caution when considering the therapeutic targeting Treg cells after CNS injury, and possibly in chronic neurodegenerative conditions.

A r t i c l e A m e n d m e n t s2 5 4 7