The Role Of NgR On Microglial/Macrophage Activity During Experimental Autoimmune Encephalomyelitis

Myelin-associated inhibitory factors within the CNS are considered one of the main obstacles for axonal regeneration following disease or injury. The corpus of works performed in the mammalian CNS illustrates the major role played by NgR in the promotion of axonal damage. Moreover, NgR signalling may promote immune cells activity during the adaptive and innate immune responses that occur during inflammatory challenge to the CNS, as occurs during EAE. Microglia/macrophages are vital immune cells that are regarded as central pathogenic conditions to MS lesions. In our study, we hypothesised that the expression of NgR within microglia/macrophages may further promote myelin and axonal damage during EAE. Thus, targeting this specific mechanism may reduce the clinical and pathological severity of EAE.
   We investigated the expression of NgR within microglia/macrophage populations during the progression of EAE. The relationship of NgR1-dependent microglial/macrophage activity was demonstrated by studying these cell populations in the CNS of ngr1^-/- mice during EAE progression, whereby no receptor-dependent activity could be attributed. Interestingly, we indicated for the first time the inducible role of NgR3 in these cells within enhanced chronic lesions and, importantly, the novel collaborative mechanism elicited between NgR1 and NgR3 during microglial/macrophage activity in EAE. The results identified that the effect of NgR3 was associated with the increased NgR1 levels in these cells during the chronic stage of EAE. We documented the disparity in the microglia/macrophage numbers between WT and ngr1^-/- mice; thus, we studied the presence of myelin degradation proteins within these activated cells during EAEprogression. Remarkably, we showed that, as EAE progressed, the phagocytic activity of microglia/macrophages to clear neural and myelin debris in ngr1^-/- mice was significantly enhanced. These findings have the implication that the mice lacking NgR1 may offer an environmental conducive to regeneration of neural cells by expediting of clearance of inhibitory molecules. Additionally, we demonstrated the possible myelin turnover occurring in NgR1-deficient mice. During EAE progression we showed there exists a sustain switch for M1-pathogenic cells to M2-neurotrophic cells in the ngr1^-/- mice implicating that these cells can exhibit a permanent physiological alteration which may favour neurotrophic support.
   Fundamentally, the current study has set the groundwork for the therapeutic strategy of administering the NgR(310)ecto-Fc fusion protein to clear myelin debris and enhance neural repair during neuroinflammation. We have shown that we can deliver the specific NgR(310)ecto-Fc fusion protein through the transplantation of LV-transduced HSCs that encode the NgR-Fc protein to sites of EAE pathology. We exclusively identified microglial/macrophage cells that were positive for the myc-tag (NgR-Fc- positive) and occupied areas exhibiting inflammatory and demyelinating lesions, signifying the engulfment of NgR-Fc-myelin protein complex by activated microglia/macrophages, which may increase the phagocytic activity of these populations and enhance repair. Here, we present a novel strategy to promote an expedited microglia/macrophage cells clearance of neural inhibitory molecules that are sequestered in the inflammatory lesions milieu that by virtue of their fast neural can promote endogenous repair. The further work in this strategy may indeed be fundamental to identify new neurological strategies for progressive MS.