Virus-specific markers are limited when researching the neurologic complications of HIV infection, especially for participants on antiretroviral therapy. HIV-Tat protein can be released from infected cells despite antiviral therapy and experimental studies show that it can cause neuroglial dysfunction. Hence we investigated its presence in CSF, in an HIV-Tat/ amyloid protein transgenic animal model and in vitro to determine interactions between Tat and amyloid beta (Aβ) peptide.
Tat was measured by ELISA in CSF of patients with HIV infection on antiretroviral therapy. Evaluations included neurocognitive tests, CSF cytokines, Aβ peptide, tau, and neurofilament-light chain by Quanterix SIMOA immunoassay. Tat levels were dichotomized: undetectable or detectable; or <1000 or ≥ 1000 pg/ml and associations with clinical outcomes (GDS, average T score), with neurocognitive impairment (NCI) defined by GDS ≥ 0.5. APP-PS1 transgenic mice were injected with Tat protein or crossed with Tat transgenic mice, and amyloid plaques were examined for Tat by immunohistochemistry. Aβ peptide-Tat complexes were studied by atomic force microscopy, circular dichroism and single fiber imaging by TIRF and molecular modeling.
All patients (n=48; mean age 53.2 yrs; 62.5% AA; 64.6% men) had a plasma viral load <40 c/ml. CSF VL was >40 c/ml in 5 (10.4%). Tat was detectable in CSF in 21 (43.8%) and >1000 pg/ml in 12 (25%). Tat was detectable in 4 (80%) with CSF escape and 17 (39.5%) without escape (p=0.08). Tat levels >1000 pg/ml associated with higher likelihood of NCI (41.7% vs. 11.10%). Detectable CSF Tat was associated with lower Aβ40 (p=0.03), lower Aβ42 (though not significantly at p=0.08), and higher total tau/Aβ42 ratio (p=0.03). Tat localized to the amyloid plaques in the brains of both animal models and in vitro studies showed that in the presence of Tat, uniform amyloid fibrils become double twisted fibrils and formed populations of thick unstructured filaments and aggregates. Tat binding to the exterior surfaces of the Aβ fibrils increased β-sheet formation and lateral aggregation into multifibrillar structures, producing fibers with increased rigidity and mechanical resistance.
The presence of Tat in CSF is an indicator of restricted viral replication and may be associated with cognitive impairment. It also indicates the formation of Tat-amyloid complexes in the brain which may contribute to the pathophysiology of HAND.