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Several extracellular matrix (ECM) molecules have been identified as potent inhibitors of neurite outgrowth in vitro and are believed to limit axonal growth after CNS injury. Recent studies have shown that different members of the chondroitin sulfate proteoglycan (CSPG) class of putatively inhibitory ECM molecules are expressed after a number of CNS injuries. The purpose of this study was to evaluate the relative amounts of individual CSPGs expressed after spinal cord injury (SCI) and identify their cells of origin. Evaluation of total soluble CSPGs 2 weeks after dorsal column lesion in the rat demonstrated that NG2 is highly upregulated and is a major CSPG species. Immunocytochemical analysis further demonstrated that NG2 expression is upregulated within 24 hr of injury, peaks at 1 week, and remains elevated for at least an additional 7 weeks. NG2 expression results from a multicellular response to injury, including both reactive macrophages and oligodendrocyte progenitors; astrocytes were not identified as a major source of NG2. Immunocytochemical analysis of other CSPG family members 7 d after injury showed moderate upregulation of versican, brevican, and neurocan, and downregulation of phosphacan. Axonal tracing experiments demonstrated dense NG2 labeling adjacent to the forward processes of transected corticospinal tract axons in a spatial profile that could restrict axonal growth. Thus, NG2 is a major component of this putatively inhibitory class of ECM molecules expressed at sites of SCI and may restrict axonal regeneration.

Although the CNS is an established immune-privileged site, it is under surveillance by the immune system, particularly under pathological conditions. In the current study we examined the lymphocyte infiltration, a key component of this neuroimmune surveillance, into the axotomized facial motor nucleus and analyzed the changes in proinflammatory cytokines and the blood-brain barrier. Peripheral nerve transection led to a rapid influx of CD3-, CD11a (alphaL, LFA1alpha)- and CD44-immunoreactive T-cells into the axotomized mouse facial motor nucleus, with a first, low-level plateau 2-4 d after injury, and a second, much stronger increase at 14 d. These T-cells frequently formed aggregates and exhibited typical cleaved lymphocyte nuclei at the EM level. Immunohistochemical colocalization with thrombospondin (TSP), a marker for phagocytotic microglia, revealed aggregation of the T-cells around microglia removing neuronal debris. The massive influx of lymphocytes at day 14 was also accompanied by the synthesis of mRNA encoding IL1beta, TNFalpha, and IFN-gamma. There was no infiltration by the neutrophil granulocytes, and the intravenous injection of horseradish peroxidase also showed an intact blood-brain barrier. However, mice with severe combined immunodeficiency (SCID), which lack differentiated T- and B-cells, still exhibited infiltration with CD11a-positive cells. These CD11a-positive cells also aggregated around phagocytotic microglial nodules. In summary, there is a site-selective infiltration of activated T-cells into the mouse CNS during the retrograde reaction to axotomy. The striking aggregation of these lymphocytes around neuronal debris and phagocytotic microglia suggests an important role for the immune surveillance during neuronal cell death in the injured nervous system.