High viral load in the cerebrospinal fluid and brain correlates with severity of simian immunodeficiency virus encephalitis

Abstract

AIDS dementia and encephalitis are complications of AIDS occurring most frequently in patients who are immunosuppressed. The simian immunodeficiency virus (SIV) model used in this study was designed to reproducibly induce AIDS in macaques in order to examine the effects of a neurovirulent virus in this context. Pigtailed macaques (Macaca nemestrina) were coinoculated with an immunosuppressive virus (SIV/DeltaB670) and a neurovirulent molecularly cloned virus (SIV/17E-Fr), and more than 90% of the animals developed moderate to severe encephalitis within 6 months of inoculation. Viral load in plasma and cerebrospinal fluid (CSF) was examined longitudinally to onset of AIDS, and viral load was measured in brain tissue at necropsy to examine the relationship of systemic and central nervous system (CNS) viral replication to the development of encephalitis. In all animals, plasma viral load peaked at 10 to 14 days postinfection and remained high throughout infection with no correlation found between plasma viremia and SIV encephalitis. In contrast, persistent high levels of CSF viral RNA after the acute phase of infection correlated with the development of encephalitis. Although high levels of viral RNA were found in the CSF of all macaques (six of six) during the acute phase, this high level was maintained only in macaques developing SIV encephalitis (five of six). Furthermore, the level of both viral RNA and antigen in the brain correlated with the severity of the CNS lesions. The single animal in this group that did not have CNS lesions had no detectable viral RNA in any of the regions of the brain. The results substantiate the use of CSF viral load measurements in the postacute phase of SIV infection as a marker for encephalitis and CNS viral replication.

FIG. 1

Peripheral blood absolute CD4⁺ lymphocyte counts of six macaques coinoculated with SIV/17E-Fr and SIV/DeltaB670. CD4⁺ lymphocyte counts declined rapidly after inoculation and remained low throughout infection.

FIG. 2

Representative brain section from macaque 18033 at euthanasia. There were numerous perivascular cuffs (lumen of vessel is indicated by an asterisk) consisting of macrophages, multinucleated giant cells, and lymphocytes. Glial foci were also scattered throughout white matter (example indicated by arrowheads). These pathological changes are typical of those seen in the five macaques in this study with SIV encephalitis.

FIG. 3

Plasma viral RNA of six macaques coinoculated with SIV/17E-Fr and SIV/DeltaB670. Plasma viral RNA increased rapidly after inoculation in all six macaques and peaked at 10 days p.i. Thereafter, levels ranged between 10⁷ and 10⁹ copy eq/μl.

FIG. 4

CSF viral RNA of six macaques coinoculated with SIV/17E-Fr and SIV/DeltaB670. CSF viral RNA levels increased rapidly in all six macaques during acute infection, reaching a peak at 10 to 14 days p.i. CSF viral RNA levels in 17834, the macaque that did not develop CNS disease, then declined to very low levels. In contrast, CSF viral RNA in the five macaques that developed CNS disease remained the same or declined only slightly until 28 days p.i., after which they increased to levels that were 4 to 5 logs higher than those of 17834.

FIG. 5

Viral RNA in various regions of the brain. (A) SIV RNA copy equivalents per microgram of total RNA extracted from brain tissue homogenates. No viral RNA was detected in macaque 17834, whereas there were 10⁶ to 10⁸ copy eq/μg of total RNA in the five macaques with CNS lesions. (B) Ratio of SIV RNA copies to copies of PBGD in brain homogenates. (C) Ratio of SIV RNA copies to copies of HPRT in brain homogenates. BG, basal ganglia; Th, thalamus; PC, parietal cortex; Cb, cerebellum.