Background:   Although the primary CNS cell population infected with HIV-1 is of monocytic lineage, there is evidence that GFAP⁺ astrocytic cells can be infected. Noting that viral entry into astrocytes is inefficient, we hypothesized that astrocyte-precursor, nestin⁺ progenitor cells are susceptible to HIV-1 infection, and that infected progenitors might differentiate into infected astrocytes.  We explored this possibility in HIV⁺ pediatric brain tissue, and in a cell culture model system of human neural progenitor cells isolated from fetal human brain. 

Methods:    Archival brain samples from vertically infected children were tested by in situ hybridization and immunohistochemistry to detect HIV-1 in nestin-rich areas. In addition nestin⁺ progenitor cells, plus astrocytes differentiated from progenitors, were tested for HIV-1 infection following exposure to HIV-1 strains IIIB and NL4-3, or transfection with infectious pNL4-3 DNA.  Supernatant p24 production was measured by ELISA. Cells were stained and examined by confocal microscopy to detect cellular and viral antigens. Infected progenitors were tested for viral reactivation by differentiation toward an astrocytic phenotype, or by stimulation with TNFa. AZT was used to block reactivation.

Results:  We found evidence for HIV-1 mRNA and protein in nestin⁺ periventricular regions of infected pediatric brain. In addition, both neural progenitor cells and progenitor-derived astrocytes in culture were transiently infected by incubation or transfection with HIV-1, consistent with previous studies of primary astrocytes. Confocal microscopy confirmed colocalization of p24 antigen with nestin or GFAP. Progenitor infection was restricted, but was stimulated 5-fold by differentiation toward an astrocytic phenotype. This effect was blocked by AZT, suggesting phenotypic differences in viral replication. In addition, during the nonproductive phase of infection as long as 5 weeks post-transfection, progenitor virus production was increased 2- to 3-fold by TNFa.

Conclusions:  Periventricular colocalization of HIV-1 and nestin in archival pediatric brain, combined with infection of human neural progenitor cells in culture, support the idea that neural progenitor cells may harbor HIV-1. Differentiation into an astrocytic phenotype is associated with higher viral titer, as is stimulation with TNFa. Progenitor cells might be an additional reservoir of HIV-1 in the pediatric brain.

Keywords: Progenitor; Astrocyte; HIV-1