Interaction of NG2⁺ glial progenitors and microglia/macrophages from the injured spinal cord

Spinal cord contusion produces a central lesion surrounded by a peripheral rim of residual white matter. Despite stimulation of NG2⁺ progenitor cell proliferation, the lesion remains devoid of normal glia chronically after spinal cord injury (SCI). To investigate potential cell-cell interactions of the predominant cells in the lesion at 3 days after injury, we used magnetic activated cell sorting to purify NG2⁺ progenitors and OX42 ⁺ microglia/macrophages from contused rat spinal cord. Purified NG2⁺ cells from the injured cord grew into spherical masses when cultured in defined medium with FGF2 plus GGF2. The purified OX42⁺ cells did not form spheroids and significantly reduced sphere growth by NG2 ⁺ cells in co-cultures. Conditioned medium from these OX42 ⁺ cells, unlike that from normal peritoneal macrophages or astrocytes also inhibited growth of NG2⁺ cells, suggesting inhibition by secreted factors. Expression analysis of freshly purified OX42⁺ cells for a panel of six genes for secreted factors showed expression of several that could contribute to inhibition of NG2⁺ cells. Further, the pattern of expression of four of these, TNFα, TSP1, TIMP1, MMP9, in sequential coronal tissue segments from a 2 cm length of cord centered on the injury epicenter correlated with the expression of Iba1, a marker gene for OX42 ⁺ cells, strongly suggesting a potential regional influence by activated microglia/macrophages on NG2⁺ cells in vivo after SCI. Thus, the nonreplacement of lost glial cells in the central lesion zone may involve, at least in part, inhibitory factors produced by microglia/macrophages that are concentrated within the lesion.