Hematogenous macrophages express CD8 and distribute to regions of lesion cavitation after spinal cord injury.

Historically, CD4 and CD8 antigens have been used to designate functionally distinct T-lymphocyte subsets. However, these antigens also have been described on macrophages in the normal and pathologic central nervous system (CNS). Signaling through CD4 or CD8 may impart unique functions in macrophage subsets that express these antigens. In the current study, the distribution and temporal patterns of expression of CD4 and CD8 were evaluated on various cell types within the traumatically injured spinal cord. The data reveal divergent patterns of CD4 and CD8 expression on unique macrophage populations. Specifically, we show sustained elevations of CD4 expression on microglia and macrophages throughout the lesion site and spared white matter. In contrast, CD8 is predominantly associated with hematogenous macrophages that are recruited from the blood during the first week postinjury. The distribution of CD8-positive cells is restricted to areas of necrotic cavitation. Differential signaling of resident and recruited macrophages through CD4 or CD8 may explain the apparent dichotomy of CNS-macrophage-mediated injury and repair.