Highlights d Microglia engulf and eliminate synapses in the visual thalamus of MS patients d MS-relevant animal models show synapse engulfment and loss occur early in disease d Complement C3, but not C1q, localizes to synapses in demyelinating disease d AAV-Crry inhibits C3 and microglia-mediated synapse loss and preserves function

Studies of individual T cells receptors (TCRs) have shed some light on structural features that underlie self-reactivity. However, general rules that predict whether TCRs are self-reactive have not been fully elucidated. Analyses of thymocytes expressing all Vβ family members show that the interfacial hydrophobicity of amino acids at positions 6 and 7 of the CDR3β segment robustly promotes the development of self-reactive TCRs. An index based on these findings distinguishes Vβ2+, Vβ6+ and Vβ8.2+ regulatory T cells from conventional T cells, as well as T cells selected on a major histocompatibility complex (MHC) allele associated with mouse type-1 diabetes from those selected on a non-autoimmune promoting MHC. These results provide a means for distinguishing normal and autoimmune-prone T cell repertoires.

Multiple sclerosis (MS) is a demyelinating, autoimmune disease of the central nervous system. While work has focused on axon loss in MS, far less is known about synaptic changes. Here, in striking similarity to other neurodegenerative diseases, we identify in postmortem human MS tissue and in nonhuman primate and mouse MS models profound synapse loss and microglial synaptic engulfment. These events can occur independently of local demyelination, neuronal degeneration, and peripheral immune cell infiltration, but coincide with gliosis and increased localization of complement component C3, but not C1q, at synapses. Finally, we use AAV9 to overexpress the complement inhibitor Crry at activated C3-bound synapses in mice and demonstrate robust protection of synapses and visual function. These results mechanistically dissect synapse loss as an early pathology in MS. We further provide a novel gene therapy approach to prevent synapse loss by microglia, which may be broadly applicable to other neurodegenerative diseases.1 1 induced in these mice ( Fig. 6C). Mice were then analyzed at the onset of moderate clinical scores (average: AAV-Crry: 1.4±0.3; AAV-EGFP 1.5±0.3), which were typically observed around day 11 post-immunization (AAV-Crry: 11.2±0.6; AAV-EGFP: 11.4±0.4) ( Fig. 7A). Using confocal imaging, we first determined the degree of EGFP colocalization with VGluT2 + -retinogeniculate terminals in the LGN and found ~45% of VGluT2 + -retinogeniculate terminals in the LGN colocalized with EGFP after injection of both AAVs (Fig. 6D). We then immunostained for Crry protein and identified that Crry was highly enriched in presynaptic bouton structures in EGFPlabeled retinogeniculate arbors vs. other fine processes (i.e. axons) within the LGN of AAV-Crry injected mice (Fig. 6E). Previous work has demonstrated that these boutons represent VGluT2 +presynaptic terminals along retinogeniculate arbors (Hong et al., 2014), precisely where we originally identified enrichment of C3 (Fig. 5). These data demonstrate successful CR2-mediated Crry targeting to retinogeniculate synapses tagged by activated C3. In contrast, AAV-EGFP injected mice showed only diffuse Crry immunoreactivity with no concentration on or around retinogeniculate synapses in the LGN following EAE. Regardless of AAV treatment, mice showed comparable development of EAE clinical scores, infiltration of peripheral immune cells, and micro-and astrogliosis (Fig. 7A-D). These data suggest that retinogeniculate overexpression of Crry did not have global, systemic effects, which is important when considering the beneficial and detrimental effects of inflammation in the development of new therapeutic strategies. Crry-mediated inhibition of activated C3 at retinogeniculate synapses blocks microglial synapse engulfment, rescues synapse loss, and restores visual functionFollowing validation that Crry is successfully expressed using our AAV9 strategy and localizes to retinogeniculate presynaptic terminals, we asked if Crry overexpression reduced deposition of C3 following EAE....