Growing evidence implicates the brain as a long-term latent HIV reservoir. The spinal cord, like the brain, contains HIV-infected microglia, but has not been evaluated as a potential HIV reservoir. This SIV/macaque study compared brain versus spinal cord by quantitating SIV DNA after long-term ART suppression, and by measuring SIV RNA tissue levels after release from ART to test if the spinal cord serves as a source of rebound virus.
10 SIV-infected pigtailed macaques were treated with PMPA, FTC, and dolutegravir for 4 months. Plasma and CSF SIV RNA became undetectable within 40 days of starting ART. 4 of the 10 SIV-infected treated animals were euthanized after 3 months of suppression; the other 6 animals were euthanized after stopping ART (post 3 months suppression) when rebound plasma SIV RNA reached set-point levels. SIV DNA and RNA were measured in brain and spinal cord by qPCR. Spinal cord microglia were cultured from the ART release group to evaluate production of SIV by qRTPCR.
In animals euthanized after 3 months of suppressive ART, SIV DNA levels were significantly higher in lumbar spinal cord than basal ganglia (31.2 versus 0.46 median SIV copies/10×5 cells; p = 0.029, Mann-Whitney). After stopping ART and tracking rebound SIV RNA in paired plasma and CSF samples, time to initial detection of SIV RNA in plasma after stopping ART was 4 days (group median) while CSF time to first rebound was 12 days. Animals reached plasma viral load set-point (10×5-10×6 SIV copies/mL) at an average of 20 days after ART withdrawal. CSF viral loads were lower at this time-point (10×4 SIV copies/mL). To establish the source of CSF SIV RNA, qRTPCR was performed on basal ganglia and spinal cord. Whereas SIV RNA was present in spinal cord of 5 of the 6 animals (median = 1,211 SIV copy eq/ug of tissue RNA), low level SIV RNA was only detected in the brain of one animal (10 copy eq/ug tissue RNA) To identify the cellular source of SIV RNA, spinal cord microglia were cultured; culture supernatants from all 6 animals were positive for SIV RNA by qRTPCR.
This study shows the spinal cord serves as a substantial SIV reservoir in the setting of ART. Our findings of higher levels of SIV DNA and RNA in spinal cord than brain suggest rebound SIV RNA in CSF may arise predominantly from SIV-infected spinal cord microglia rather than brain. The spinal cord merits consideration as a HIV reservoir that could challenge HIV cure efforts.