HIV-1 latent reservoir: size matters

Abstract

More than 35 million people remain infected with HIV-1. Upon antiretroviral therapy cessation, HIV-1-positive individuals systematically fail to achieve sustained virological remission, revealing the presence of a reservoir. This reservoir takes into account anatomical sanctuaries where HIV-1 continues to replicate, and latently infected cells also known as the latent reservoir (LR). A better understanding of the nature and features of the LR and its quantification are crucial to evaluate the efficiency of therapeutic strategies aiming at purging HIV-1. Culture- and PCR-based assays have already been implemented to measure the LR, and new assays are continuously being developed. In this review, we will discuss these methods highlighting the difficulties to accurately measure the LR, one main obstacle in curing HIV-1.

Keywords: DNA; HIV-1; RNA; assays; latent reservoir; quantification; size.

Figure 1.. Characterization of the latent reservoir.

Two major types of resting CD4⁺ T cells are shown, containing either intact or defective HIV-1. Defective HIV-1 contains large internal deletions, G→A hypermutations or other defects. In both cases, following a single round of T-cell activation, proviruses are either induced to produce virions that can infect other cells (induced, replication-competent HIV-1) or to produce defective virions unable to infect other cells (induced, replication noncompetent HIV-1). Proviruses that are not induced to produce virions following a single round of T-cell activation are termed noninduced HIV-1. The noninduced HIV-1 can still be induced after additional rounds of activation. On subsequent rounds of cellular activation, some noninduced proviruses can produce either competent or noncompetent virions. Culture-based assays detect only induced replication-competent HIV-1, while PCR-based assays detect all types of proviruses. The true latent reservoir includes only induced replication-competent HIV-1 and noninduced intact HIV-1.

Figure 2.. Measurement of the latent reservoir by seven different assays.

From top to bottom: Culture-based assays, PCR-based assays and novel assays. For each assay, the methodology used to measure the latent reservoir is indicated proportionally to the time needed to obtain the results. Culture-based assays are very time-consuming compared with PCR-based assays, but neither offers an accurate estimation of the latent reservoir. Novel assays approach the ‘true’ size and are less time-consuming compared with culture-based assays. PBMC: Peripheral blood mononuclear cell; PHA: Phytohemagglutinin; PMA: Phorbol myristate acetate; qRT: Quantitative real-time reverse transcription PCR.