Experimental approaches to the study of HIV-1 latency

HIV-1 can establish a state of latent infection in resting memory CD4+ T cells. These cells carry a stably integrated copy of the viral genome and persist even in patients who have had prolonged suppression of viraemia to undetectable levels on highly active antiretroviral therapy (HAART).Latently infected cells are rare in vivo and are therefore difficult to detect. Approaches to detection rely on three main types of assay: the isolation of pure populations of resting CD4+ T cells; the detection of integrated HIV-1 DNA in these cells; and the recovery of replication-competent virus from these cells by cellular activation.There are many difficulties associated with these assays. Most of the HIV-1 DNA in the resting CD4+ T cells of viraemic patients is unintegrated and labile, so the assays used must be able to distinguish between integrated and unintegrated HIV-1 DNA. Most of the HIV-1 DNA in resting CD4+ T cells is not replication-competent, therefore special assays are also needed to detect the cells that harbour replication-competent viral genomes. Finally, no single assay can simultaneously demonstrate both integration status and replication-competence. Culture assays on purified resting CD4+ T cells from patients on HAART provide the best indication of the true frequency of latently infected cells.Drug-resistant viruses can enter the latent reservoir and persist there. Developing assays to detect archived resistance is an important research goal.Cell-line models for HIV-1 latency have provided much useful information but suffer from the caveat that continuously proliferating, transformed cells might not accurately mimic the profoundly quiescent G0 state of the cells that harbour latent HIV-1 in vivo.Latently infected resting CD4+ T cells have been demonstrated in a SCID/hu mouse model and in the simian immunodeficiency virus (SIV) model. These systems could prove useful in the development of approaches to target the latent reservoir.