Human brain parenchymal microglia express CD14 and CD45 and are productively infected by HIV-1 in HIV-1 encephalitis

Microglia are endogenous brain macrophages that show distinct phenotypes such as expression of myeloid antigens, ramified morphology, and presence within the neural parenchyma. They play significant roles in a number of human CNS diseases including AIDS dementia. Together with monocyte-derived (perivascular) macrophages, microglia represent a major target of HIV-1 infection. However, a recent report challenged this notion based on findings in SIV encephalitis. This study concluded that perivascular macrophages can be distinguished from parenchymal microglial cells by their expression of CD14 and CD45, and that macrophages, but not microglia, are productively infected in SIV and HIV encephalitis. To address whether parenchymal microglia are productively infected in HIV encephalitis, we analyzed expression of CD14, CD45 and HIV-1 p24 in human brain. Microglia were identified based on their characteristic ramified morphology and location in the neural parenchyma. We found that parenchymal microglia are CD14+ (activated), CD45+ (resting and activated), and constitute approximately two thirds of the p24+ cells in HIV encephalitis cases. These results demonstrate that microglia are major targets of infection by HIV-1, and delineate possible differences between HIVE and SIVE. Because productively infected tissue macrophages serve as the major viral reservoir, these findings have important implications for AIDS.