Unveiling Distinct Neuroimmune Responses in Mouse Models of Cervical Spinal Cord Injury: Hemisection versus Hemicontusion

Traumatic cervical spinal cord injury (cSCI) causes severe neurological deficits and long-term disability. Preclinical models such as cervical 2 (C2) hemisection (C2HS), resulting in disrupted communication between the respiratory centers and the phrenic motoneurons (PMN) pool, have been used since decades to study respiratory dysfunction and neuroinflammation after cSCI. Recently, contusive injuries such as C3 hemi-contusion (C3HC) have been increasingly used, as they induce phrenic motoneuron damage and offer a more clinically relevant model of SCI. However, these two different models may engage distinct pathophysiological cascades, raising concerns about the generalizability of findings across injury paradigms. In this study, we compared neuroimmune responses following C2HS or C3HC in mice. Animals underwent C2HS or C3HC, and spinal cord segments (C1-C8) were collected seven days post-injury for immuno-histological analyses around the lesion level and flow cytometry analyses at the lesion level. We observed that C2HS preserved more neurons and exhibited elevated CD86 and F4/80 expression. These markers are typically expressed by activated microglia and are indicative of a response oriented toward phagocytic and reparative functions. This phenotype was associated with limited pro-inflammatory cell infiltration and normalized level of systemic IL-6 in this model. Conversely, C3HC induced more extensive tissue damage, heightened microglial activation, a trend toward increased astrocytic reactivity, and significantly elevated CSPG levels on the contralateral side. Moreover, a persistent NK cell, neutrophil, and CD43⁺ antigen-presenting cells infiltration, along with persistently high circulating IL-6 has been observed following C3HC. These findings demonstrate distinct neuroinflammatory signatures and repairing mechanisms between models, with C2HS promoting a microglia profile toward repair and C3HC leading to a prolonged and potentially harmful immune response. This study underscores, for the first time, how injury type shapes neuroimmune mechanisms, reinforcing the need for lesion-specific therapeutic strategies in cervical spinal cord injury.