The role of complement anaphylatoxin receptors in experimental spinal cord injury

Abstract

Disruption of spinal pathways following traumatic spinal cord injury (SCI) results in significant functional impairments. This is partly due to cellular and molecular ‘secondary injury’ immune responses that compromise the integrity of intact but vulnerable neural tissue surrounding the primary site of damage. The innate immune complement system is a critical modulator of this response. However, the functions of two potent complement anaphylatoxins, C3a and C5a, have remained largely unknown in the context of SCI. The main aim of this PhD thesis was therefore to comprehensively investigate the role of these peptides in SCI, and examine whether their receptors, C3aR and C5aR, can be targeted therapeutically to improve SCI outcomes. The central working hypothesis at the outset was that both receptors would have a pro-inflammatory, detrimental role in secondary injury. A series of behavioural and molecular experiments in wild-type and C3ar-/- mice revealed that C3aR is in fact a key facilitator of neurological recovery by negatively regulating the mobilisation of peripheral granulocytes from their reservoirs in response to SCI; recruitment of mobilised granulocytes to the lesion site exacerbated tissue damage whereas their depletion alleviated the C3ar-/- phenotype. Activation of C3aR in the acute phase of SCI may thus be a novel therapeutic strategy to improve recovery. In contrast, both genetic ablation and pharmacological antagonism of C5aR significantly improved injury outcomes in the (sub-) acute stage of injury. However, C5aR appeared to serve a reparative role in the chronic phase, as absence of C5aR signalling ultimately impaired long-term recovery. Regulation of astrocyte proliferation and glial scar development, which normally protects spared tissue, was identified as a novel mechanism for this chronic phenotype. Translation of putative SCI therapies, including those targeting complement proteins like C3aR and C5aR, would be greatly facilitated by the development of non-invasive, quantitative techniques that can stratify subjects based on injury severity, provide a functional prognosis in association, and visualise therapeutic efficacy. It was determined that diffusion tensor imaging (DTI) can reliably monitor the progression of secondary injury in ventrolateral white matter following a dorsally inflicted contusive SCI, revealing a therapeutic window here for at least three days post-SCI. Post-mortem histological analysis showed that SCI-induced spatial changes in select DTI indices were reflective of demyelination, axonal loss, Wallerian degeneration and gliosis, respectively. Preliminary studies further showed that DTI is also sensitive enough to detect beneficial changes in SCI outcomes following therapeutic intervention. Collectively, the insights gained with regards to involvement of complement peptides in neuroinflammatory responses to SCI, along with characterising advanced and transferable imaging techniques, may facilitate the development and implementation of novel therapeutic approaches for acute SCI into the clinic.