Abstract
Despite the success of combination antiretroviral therapy (cART) to suppress HIV replication, HIV persists in a long-lived reservoir that can give rise to rebounding viremia upon cART cessation. The translationally active reservoir consists of HIV-infected cells that continue to produce viral proteins even in the presence of cART. These active reservoir cells are implicated in the resultant viremia upon cART cessation and likely contribute to chronic immune activation in people living with HIV (PLWH) on cART. Methodologies to quantify the active reservoir are needed. Here, an automated immunocytochemistry (ICC) assay coupled with computational image analysis to detect and quantify intracellular Gag capsid protein (CA) is described (CA-ICC). For this purpose, fixed cells were deposited on microscopy slides by the cytospin technique and stained with antibodies against CA by an automated stainer, followed by slide digitization. Nuclear staining was used to count the number of cells in the specimen, and the chromogenic signal was quantified to determine the percentage of CA-positive cells. In comparative analyses, digital ELISA, qPCR, and flow cytometry were used to validate CA-ICC. The specificity and sensitivity of CA-ICC were assessed by staining a cell line that expresses CA (MOLT IIIB) alongside a control cell line (Jurkat) devoid of this marker, as well as peripheral blood mononuclear cells (PBMCs) from HIV seronegative donors before or after ex vivo infection with an HIV laboratory strain. The sensitivity of CA-ICC was further assayed by spiking MOLT IIIB cells into uninfected Jurkat cells in limiting dilutions. In those analyses, CA-ICC could detect down to 10 CA-positive cells per million with a sensitivity superior to flow cytometry. To demonstrate the application of CA-ICC in pre-clinical research, bulk PBMCs obtained from mouse and non-human primate animal models were stained to detect HIV CA and SIV p27, respectively. The level of intracellular CA quantified by CA-ICC in PBMCs obtained from animal models was associated with plasma viral loads and cell-associated CA measured by qPCR and ELISA, respectively. The application of CA-ICC to evaluate the activity of small-molecule targeted activator of cell-kill (TACK) in clinical specimens is presented. Overall, CA-ICC offers a simple imaging method for specific and sensitive detection of CA-positive cells in bulk cell preparations.