The primary cells in the Central Nervous System (CNS) infected with SIV and HIV are macrophages/microglia. It has been hypothesized that these cells serve as a reservoir of SIV and HIV infection within the CNS, but recent data suggest they can leave the CNS and join lymphatics in some case with virus. Understanding the biology of these cells and their traffic out of the CNS is critical to assess this hypothesis.
To asses the fate of brain macrophages we introduced fluorescent Super Paramagnetic Iron Oxide Nanoparticles (SPIONs) into the cisterna magna which are taken up by phagocytic cells in the brain. This was done in six SIV infected and two normal macaques. We also trace labeled the CSF with dextran-red for short periods. We introduced the SPIONs and follow their fate detecting also their point of exit from the CNS.
We found, one hour after SPIONs were instilled at the cisterna magna, they were present throughout the perivascular space mainly as extracellular single beads with some being phagocytized by perivascular cells (PVCs). By 24 hours post inoculation all the SPIONs were intracellular and over the next 28 days the frequency of cells containing SPIONs fell throughout the CNS with the exception of the cranial nerves. There were larger numbers of SPIONS in the CNS of animals with SIV infection than non infected animals. Beginning at 7 days after intracisternal instillation of SPIONs we found SPION containing cells outside of the CNS with the greatest number in the cervical lymph node 7 days after SPION administration. This timing was consistent in both SIV-infected and control animals. We determined that SPIONs are carried out by cells (PVCs) and not in the fluid phase traced with dextran red. In a normal animal the cervical lymph node, with a volume of 0.5 ml, the number of SPIONs carrying cells is a million showing that there is a relative large CNS efflux of cells. We identified several points for emigration out of the CNS as those at the cribriform plate, the cranial nerves, along the neural cord at the ganglia at the brachial plexus and the sacral cord.
The SPION+ cells were consistently CD163+ indicating that they were monocyte/macrophage lineage cells. Interestingly, rare SPION+CD163+ cells were also SIV+. This experiment shows for the first time that brain perivascular cells (macrophage lineage) leave the CNS into the periphery and may carry virus with the possibility of reseeding the body with potentially new viral clones.