Improved recovery in Rag2?/? mice was associated with decreased microglia/macrophage infiltration, which the authors attributed to the absence of T cell-derived cytokines. secondary injury was examined in Ab-deficient Rag1?/? or wild type C57BL/6 mice using Ab reconstitution experiments and neoepitope-targeted therapeutic studies, respectively. Results Administration of B4 or C2 mAb following murine SCI increased lesion size and worsened functional outcome in normally guarded Ab-deficient Rag1?/? mice. Injury correlated with colocalized deposition of IgM and C3d in hurt spinal cords from both mAb reconstituted Rag1?/? mice and untreated wild-type mice. Depletion of peritoneal B1 B cells, a source of natural Abs, reduced circulating levels of IgM with B4 (annexin-IV) and C2 (subset of phospholipids) reactivity, reduced IgM and match deposition in the spinal cord, and guarded against SCI. We therefore investigated whether the B4 neoepitope represents a therapeutic target for match inhibition. B4-Crry, a fusion protein consisting of a single-chain Ab derived Rabbit polyclonal to PAI-3 from B4 mAb, linked to the match inhibitor Crry, significantly protected against SCI. eCF506 B4-Crry exhibited a dual function in that it inhibited both the binding eCF506 of pathogenic IgM and blocked match activation in the spinal cord. Conclusions This study identifies important neoepitopes expressed within the spinal cord after injury. These neoepitopes are recognized by clonally specific natural IgM Abs that activate match and drive pathology. We demonstrate that these neoepitopes represent novel targets for the therapeutic delivery of a match inhibitor, and possibly other payload, to the hurt spinal cord. Electronic supplementary material The online version of this article eCF506 (doi:10.1186/s12974-017-0894-6) contains supplementary material, which is available to authorized users. strong class=”kwd-title” Keywords: Match, Spinal cord injury, Natural antibodies, IgM, Neoepitope, Therapy Background Traumatic spinal cord injury (SCI) results in severe debilitation, often with total paralysis and loss of sensory function below the injury site. Following a main mechanical insult, a secondary wave of cell death ensues, that is accompanied by ischemia, vascular damage, glutamate excitotoxicity, ionic dysregulation, and inflammation. The area of secondary injury emanating from the site of trauma represents a therapeutic target. There is strong evidence that match plays an important role in secondary SCI, and both the classical  and option [1, 2] pathways of match activation have been implicated in propagating injury (examined in ). The alternative pathway functions as an amplification loop for the classical and lectin pathway of activation, although the role of the lectin pathway has not been investigated in SCI. Also, match fragment levels have been found to be elevated in sera of spinal cord injury patients . In an experimental model, it has been shown that match activation and the propagation of SCI can be caused by pathogenic autoantibodies that arise following SCI-mediated activation of B cells [5, 6]. However, the role of naturally occurring pre-existing self-reactive antibodies (nAbs) produced by innate B1 B cells, and how match is usually activated acutely after SCI, has not been investigated. Since SCI shares some pathophysiological characteristics with ischemia reperfusion injury (IRI), and since IRI is usually driven by natural IgM Ab-mediated activation of match in some organs [7C10], we investigated a role for natural IgM Abdominal muscles in propagating SCI. Tissue ischemia exposes neoepitopes known as damage-associated molecular patterns (DAMPs) around the cell surface, and these neoepitopes are recognized by circulating natural IgM Abs that subsequently activate match . Some of these post-ischemic neoepitopes have been recognized in the context of IRI and include non-muscle myosin , annexin IV , and various phospholipids [10, 12]. However, the nature of the neoepitopes uncovered as a result of trauma around the injured spinal cord has not been investigated. Natural IgM Abs differ from adaptive immune Abs in that they are germline encoded, arise without exogenous antigen activation, and are produced by the B1 subset of B cells. B1 cells differ from standard B2 cells by their phenotype, self-renewing capacity, and production of natural Abs. Although most natural IgM Abs are polyreactive, the subsets that initiate IRI are unique in that they identify a specific set of epitopes or patterns and, importantly, are specific for neoepitopes on stressed or hurt cells. Here, we investigate a pathogenic role for natural IgM Abs in promoting spinal cord injury, but it is usually important to note that natural Abs also have important physiological functions, such as protection from pathogens and the removal of cellular and molecular waste [13, 14]. Indeed, a monoclonal IgM Ab corresponding to.