In August of 2011, researchers from the laboratory of David Baltimore published a high profile paper in the journal Nature suggesting that cell-to-cell HIV transmission facilitates ongoing viral replication in the face of antiretroviral therapy (ART). The authors proposed that this might represent a mechanism that sustains the reservoir of HIV-infected cells in people on long-term ART.
A new study by Marc Permanyer and colleagues, published online recently by the Journal of Virology, disputes the Baltimore lab’s interpretation of their results. The essence of the disagreement relates to the method used to measure cell-to-cell HIV spread in a laboratory culture system. The original study used a modified HIV that expresses a green fluorescent protein (GFP) tag, and assessed whether the tag became detectable in cells as a surrogate for viral replication. Permanyer’s work shows that transfer of HIV from cell-to-cell, as measured by GFP, does not necessarily equate to continuation of the viral replication cycle. In the presence of ART, the transfer is shown to be abortive because replication is blocked. The researchers conclude: “data on cell-to-cell spread should be taken with caution as it is crucial to correctly distinguish and measure abortive virus transfer or subrogate markers of infection (LTR-driven GFP) from effective viral replication.”
J Virol. 2012 Jun 13. [Epub ahead of print]
Permanyer M, Ballana E, Ruiz A, Badia R, Riveira-Munoz E, Gonzalo E, Clotet B, Esté JA.
IrsiCaixa, Hospital Universitari Germans Trias i Pujol, Universitat Autònoma de Barcelona, Badalona, Spain.
Cell to cell transmission of HIV has been proposed as a mechanism contributing to virus escape to the action of antiretrovirals and a mode of HIV persistence during antiretroviral therapy. Here, cocultures of infected HIV-1 cells with primary CD4+ T cells or lymphoid cell lines were used to evaluate virus transmission and the effect of known antiretrovirals. Transfer of HIV antigen from infected to uninfected cells was resistant to the reverse transcriptase inhibitors (AZT and tenofovir), but was blocked by the attachment inhibitor IgGb12. However, quantitative measurement of viral DNA production demonstrated that all anti-HIV agents blocked virus replication with similar potency compared to cell-free virus infections. Similarly, cell-free and cell-associated infections were equally sensitive to inhibition of viral replication when measuring HIV-1 LTR-driven GFP expression in target cells. However, detection of GFP by flow cytometry may incorrectly estimate the efficacy of antiretrovirals in cell-associated virus transmission, due to replication independent Tat-mediated LTR transactivation consequence of cell-to-cell events that did not occur in short-term (48h) cell-free virus infections. In conclusion, common markers of virus replication may not accurately correlate and measure infectivity or drug efficacy in cell to cell virus transmission. When accurately quantified, active drugs blocked proviral DNA and virus replication in cell to cell transmission, recapitulating the efficacy of antiretrovirals in cell-free virus infections and in vivo.