AIDSWEEKLY Plus, Monday, 9 December 1996
Daniel J. DeNoon, Senior Editor

The bad news is that these infectious virions apparently are not affected by inhibitors of HIV reverse transcriptase.

The good news is that they can be measured by a new technique that can estimate HIV viral load in lymphoid tissues reliably from relatively non-invasive outpatient tonsil biopsies.

"Image analysis of the presymptomatic stage of HIV-1 infection in lymphoid tissue revealed a relatively spatially homogeneous and temporally stable pool of virions associated with follicular dendritic cells that exceeds the viral load in plasma by orders of magnitude," wrote University of Minnesota researcher Ashley T. Haase and colleagues. "The cellular sources of virus production and storage in lymphoid tissues can now be studied with this approach over the course of infection and treatment."

Haase et al. reported their findings in the journal Science ("Quantitative Image Analysis of HIV-1 Infection in Lymphoid Tissue," Science, 1996;274(5289):985-9).

Currently, measures of HIV viral burden are based solely on the amount of virus in the bloodstream. They cannot detect HIV in lymphoid tissues where the virus exists in two cellular compartments: in association with mononuclear cells (MNCs), which are productively infected; and in association with follicular dendritic cells (FDCs), which bind HIV on their surfaces in immune complexes. This may slow the course of HIV disease, but virus in this compartment retains its infectivity.

Using an inventive combination of image analysis and in situ hybridization, Haase et al. quantified the amount of HIV in these two lymphoid compartments. Because the density of HIV was consistent between various lymphoid tissues, they estimated total-body lymphoid viral load by extrapolating from the weight of the relevant lymphoid tissues in the biopsy samples.

They assessed HIV burden in blinded samples from nine people with HIV infection. All were mostly asymptomatic and all but one were receiving antiretroviral therapy with reverse-transcriptase inhibitors.

"We focused in seven of the nine cases on lymphoid tissue obtained by biopsy of tonsillar tissue because tonsil is an accessible source of lymphoid tissue that can be biopsied frequently and repeatedly in an outpatient clinic setting," Haase et al. wrote. "We also analyzed spleen and lymph node tissues from one individual and spleen from another."

The study results showed that in the asymptomatic stage of infection, there is 10 to 40 times more HIV in the FDC than in the MNC compartment. The size of the FDC pool was not related to clinical stage, CD4 count, or treatment.

Ominously, the size of the FDC viral burden dwarfed the plasma viral burden: "The concentration of viral RNA in the FDC pool exceeded concentrations of viral RNA in plasma by factors of 10(2) to >10(4) and, at times when the concentration of viral RNA was below the detection limit of the assay (5x10(3) copies per milliliter), there were >10(6) copies of viral RNA per gram of LT in the FDC pool," wrote the researchers.

Haase et al. obtained sequential tonsillar biopsies from four of their subjects over the course of more than a year. They found that the FDC pool of HIV was relatively stable and rarely exceeded HIV RNA copies of 5x10(8) per gram of tissue.

In a 70 kg individual with HIV infection, Haase et al. estimated that the total-body viral burden of FDC-associated HIV is about 10(11) RNA copies.

"Our estimates are consistent with a large but saturable pool of virus with the potential to perpetuate infection, but also with virus with restricted access to the whole population of susceptible host cells," they suggested.

The antiretroviral agents taken by eight of the nine study subjects had no effect on HIV load in either the FDC or MNC pools in the lymphoid tissues. The patients were receiving zidovudine (AZT) plus lamivudine (3TC), AZT plus delavirdine, AZT alone, AZT plus zalcitabine (ddC), stavudine (D4T) alone, or ddC alone.

"In future studies it will be important to determine the magnitude and dynamics of infection in these cellular compartments throughout the course of infection and during treatment with more potent combinations of antiretroviral drugs," Haase et al. concluded.

"These studies should yield additional insights into pathogenesis and provide a rational basis for optimizing the timing and mix of antiviral treatments that will have the greatest effect on virus production and load in the principal tissue reservoirs."

The corresponding author for this study is Ashley T. Haase, Department of Microbiology, University of Minnesota Medical School, Minneapolis, Minnesota 55455.

Support for this study was provided by the Ramsey Foundation; the Santa Fe Institute, the Joseph P. and Jeanne M. Sullivan Foundation, and the National Institutes of Health (grants AI 28246, AI 27661, and RR 06555).

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