I have always been fascinated by the central question of how lentiviruses elude host defenses, and, now in the case of the human lentivirus, HIV, the mechanisms by which the virus persists despite currently effective anti-retroviral therapy (ART). In our early studies of the prototypic lentivirus and distant relative of HIV, visna virus, we thought that the answer was that visna virus escaped the immune response in infected sheep by “hiding” in cells in a quiescent state, analogous to bacteriophage lysogeny, but on a grand scale, in an animal (1-3). So when Simon Wain Hobson linked HIV to visna and the ungulate lentiviruses (4), we set out to see if HIV persisted in this way.
But, where and how to look? The where seemed obvious-most of the then known CD4+ T cell and macrophage targets would be concentrated in lymphoid tissues-and the how to search for 10 attomoles of proviral DNA in single cells became possible with the development of in situ PCR (5). Armed with the technology and lymphoid tissue samples, I was astonished to see one night how many DNA+ cells there were in our lymphoid tissue samples, and could only find the floor’s janitor to share the discovery. We published the story in Nature in back-to back papers with Tony Fauci’s lab (6, 7), both papers confirming earlier work by Paul Racz that the lymphoid tissues were the principal sites of HIV infection (8). We emphasized covert (latent) infection in our paper, while Pantaleo et al. reached the opposite conclusion. HIV infection was always active. This Janus-faced pairing is actually a recurrent theme in lentivirus infections (9) from Visna to HIV-silent infection to hide and persist, and active infection to maintain and spread infection and indirectly cause disease from immunopathological responses to active infection.
The two papers caused a stir at the time. John Maddox, then Nature’s editor, and Howard Temin and Dani Bolognesi wrote Commentaries (10, 11). All agreed that we must now think of lymphoid tissues as the reservoir where HIV had been hidden and was now fully revealed. Maddox focused on the implications for refuting Peter Duesberg’s “HIV-hypothesis” that HIV was not the cause of AIDS, while the Temin/ Bolognisi prescient piece anchored the findings to the likelihood in any retrovirus infection of defective proviruses accounting for a substantial portion of transcriptionally silent cells.
The advent of effective ART and discovery of HIV dynamics by the Ho and Shaw labs (12, 13) quickly shifted attention to the active nature of HIV infection, only to be soon followed by the diametrically opposed theme of HIV latent infection by Siliciano (14) and others. Fast forward to the present and the quest for curing HIV infections. Lymphoid tissues are the principal reservoirs in HIV and SIV infections (15), but these reservoirs harbor populations of largely defective proviruses (16). So, how are we to count the replication competent proviruses from which infection rebounds when ART is interrupted? And, can we discount immune and drug sanctuaries in lymphoid tissues where active infection continues (17, 18) as a source of virus for rebound and stimulus for immune activation and its associated contribution to morbidity and mortality (19)? Engaging questions, a quarter century after the initial discovery of HIV’s two faces in lymphoid tissues.
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