Resolving inflammation: The impact of antiretroviral therapy on macrophage traffic in and out of the CNS

The effects of antiretroviral therapy (ART) and treatment interruption on myeloid cell egress from the central nervous system (CNS) during human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infection remain poorly defined. We hypothesized that CNS macrophages normally traffic out of the CNS, accumulate during viral infection and inflammation, and are released as inflammation resolves. To test this, we administered intracisternal (i.c.) injections of two different colored fluorescent superparamagnetic iron oxide nanoparticles (SPION) to SIV-infected macaques either early in infection (12-14 dpi) or 30 days before necropsy. SPION are preferentially taken up by perivascular, meningeal, and choroid plexus macrophages, enabling us to track macrophage turnover, infection, and migration. In non-infected macaques, SPION+ macrophages trafficked from the CNS to peripheral sites including the deep cervical lymph node (dCLN), lumbar lymph nodes, spleen, and dorsal root ganglia (DRG). With SIV infection, these cells accumulated in the CNS and showed reduced peripheral trafficking. ART decreased the number of SPION+ perivascular macrophages, and to a lesser extent, meningeal or choroid plexus macrophages. After ART interruption, SPION+ perivascular and choroid plexus macrophage numbers remained stable, whereas SPION+ meningeal macrophages increased. ART eliminated SIV-RNA+ perivascular macrophages, leaving few- scattered SIV-RNA+ cells in the meninges and choroid plexus. Following ART interruption, perivascular macrophages remained virus-negative, but scattered viral RNA+ meningeal macrophages persisted. In non-infected macaques, SPION+ macrophages trafficked to the dCLN, spleen, and DRG, but this trafficking diminished with SIV infection and AIDS with SIVE. Importantly, SIV-RNA+ SPION+ macrophages that exited the CNS were cleared by ART and did not reappear after treatment interruption. Using two differently colored SPION to assess establishment of CNS viral reservoirs, we observed higher numbers of early-labeled macrophages within and outside the CNS in animals with AIDS and SIVE, ART treatment, and ART interruption. These findings support a model in which SIV-infected perivascular macrophages seed an early CNS viral reservoir, while the meninges and choroid plexus undergo continual viral seeding during infection. ART reduces trafficking of infected macrophages out of the CNS and clears the perivascular macrophage reservoir, but SIV-RNA+ meningeal macrophages can persist, in low numbers, and can rebound after ART interruption.