HIV-1 Tat causes enduring microglial activation and transient leukocyte infiltration into hippocampal neuropil.

Both microglia and leukocytes were visualized with CD11b+ and they could be distinguished by their morphology because infiltrating leukocytes had round or ovoid morphology whereas microglia had ramified, often hypertrophied processes. Wild-type mice received either Tat (top panels, n = 3) or saline control vehicle (bottom panels, n = 3) injection into the right hippocampus and were sacrificed 8 hours (Panels A and E), 1 (Panels B and F), 7 (Panels C and G) and 28 (Panels D and H) days later, and hippocampal tissue sections processed for CD11b immunocytochemistry as described in Methods. All microscopic fields depicted here are directly adjacent to the injection site. White arrows point to examples of ramified microglia and white arrowheads point to examples of round or ovoid infiltrating leukocytes. The blue arrowhead in E points to a microglial cell fragment that was frequently seen in both the Tat and saline treatment groups 8 hours after injection. These cellular fragments typically appeared as isolated globules scattered in and around the injection site but were much smaller than leukocytes. Scale bar = 16 µm for all micrographs.