Measuring HIV reservoirs: Do we need to choose between quantity and quality?

The persistence of latent viral genomes in people receiving antiretroviral therapy (ART) is the main obstacle to a cure for HIV infection. Viral reservoirs can be defined as cells harboring HIV genomes that have the ability to produce infectious virions. Precise quantification of the cellular reservoirs of HIV is challenging because these cells are rare, heterogeneous and outnumbered by a larger number of cells carrying defective genomes. In addition, measuring the inducibility of these proviruses requires functional assays and remains technically difficult. The recent development of single cell and single viral genome approaches revealed additional layers of complexity: the cell subsets that harbor proviruses are heterogeneous and their ability to be induced is variable. A substantial fraction of intact HIV genomes may be permanently silenced after years of ART, revealing the underappreciated importance of induction assays. As such, a simple approach that would assess simultaneously the genetic intactness and the inducibility of the reservoir is still lacking. In here, we review recent advances in the development of methods to quantify and characterize persistently infected cells, and discuss how these findings can inform the design of future assays aimed at measuring the size of the intact and inducible HIV reservoir.