Chemokines and complement: Understanding their roles in neonatal brain injury and brain development.

Abstract

Recent evidence suggests that inflammatory mediators and their receptors are expressed in the central nervous system (CNS) and that their functions extend beyond their roles in inflammation. Clinical data indicate that inflammatory mediators including cytokines, chemokines, and complement activation products may be involved in the pathogenesis of neonatal brain injury. Primarily, this thesis investigated the roles of chemokines and complement activation in acute neonatal brain injury in the rat, using a well-characterized model of hypoxic-ischemic (HI) neonatal brain injury. The chemokine macrophage inflammatory protein-1alpha (MIP-1alpha) acts as a monocyte chemoattractant in several models of CNS inflammation. We used enzyme-linked immunosorbent assays, immunohistochemistry, and immunofluorescence assays, analyzed with confocal microscopy, to demonstrate that HI stimulates the production of MIP-1alpha by both blood-derived monocytes and resident microglia in the acutely injured neonatal brain. To identify potential cellular targets for MIP-1alpha, we performed immunohistochemistry assays for the MIP-1alpha receptors CCR1 and CCR5 and found that both were constitutively expressed by multiple cell types in the neonatal brain and that they were differentially regulated after H1 injury. In additional studies, we documented the coordinated expression of CCR1 and MIP-1alpha in the developing cerebellum, during times of neuronal and astrocytic maturation. Thus, monocytic/microglial production of MIP-1alpha may influence numerous cell types in the injured brain, and pathological elevations in chemokine expression may have the potential to disrupt CNS development. To further identify potential mediators of the HI-induced inflammatory response, we investigated the roles of complement activation in HI-induced injury and chemokine production. We used pharmacological, biochemical, and immunohistochemical approaches to demonstrate that neonatal brain injury activated the complement cascade, that systemic and parenchymal sources of complement contributed to this response, and that complement activation amplified injury-induced chemokine production. Overall, these results highlight the complexity of the injury-induced inflammatory response in the neonatal brain and demonstrate that numerous cell types in the CNS both generate and are targets of inflammatory mediators. Furthermore, the developmental expression of inflammatory mediators and their receptors may render the neonatal brain uniquely vulnerable to inflammatory insults.