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Objective To determine whether HIV-1 can be recovered from blood monocytes as well as resting, memory CD4 T lymphocytes of patients on highly active antiretroviral therapy (HAART) with undetectable plasma viraemia and whether infection is active or latent. Design Five patients with plasma HIV-1-RNA levels of less than 500 copies/ml for at least 3 months and less than 50 copies/ml at the time of sampling were initially selected, followed by an additional five patients with viral loads of less than 50 copies/ml for 3 months or more. Methods Monocytes were isolated from blood by plastic adherence, then further purified by a second adherence step or CD3 depletion before co-culture with CD8-depleted donor peripheral blood mononuclear cells. Virus isolates were examined for mutations conferring resistance to reverse transcriptase or protease inhibitors and for genotype. The highly purified monocytes were also analysed for the presence of proviral and unintegrated viral DNA and multiply spliced (MS) viral mRNA by polymerase chain reaction. Results Virus was recovered from monocytes of five patients. Sequencing of the recovered viruses did not reveal multiple drug resistance, and was consistent with a non-syncytium-inducing/CCR5 phenotype. Proviral DNA was detectable in monocytes from all subjects, and unintegrated HIV-1 DNA and MS RNA was found in four out of five populations examined. Conclusion Recovery of replication-competent virus from some HAART patients indicates that monocytes can also harbour HIV-1. Detection of circular, viral DNA and spliced RNA, albeit at very low levels, in these cells suggests that their infection is recent and transcriptionally active rather than latent.

Markers of HIV infection are used in the clinical setting to predict prognosis in HIV-infected persons, measure disease progression and monitor responses to therapy. In the past, the most commonly used tests were CD4 cell counts and serum p24 antigen levels, although serum beta-2 microglobulin, serum neopterin, serum IgA levels and CD8 cell numbers were used as adjunctive measures by some clinicians. Since the early to mid 1990s new technologies for the detection of HIV RNA in human plasma have allowed relatively accurate and precise measurement of plasma HIV RNA concentrations, which directly relate to the production of HIV-1 in tissues, particularly the lymphoid tissues. The available assays to quantify plasma HIV RNA, in combination with CD4 counts, enable clinicians to monitor responses to antiretroviral therapy and to predict the risk for disease progression.1,2,3,4