Anthony Fauci

The concept of HIV viral reservoirs has become increasingly important in the search for optimal antiretroviral therapy, as investigators attempt to stop or reduce therapy in aviremic patients. This subject formed the basis of a full symposium, which began with a comprehensive overview by Anthony Fauci, NIAID director and chief of the Laboratory of Immunoregulation.

Fauci recapped the independent findings of his own and two other teams that despite successful suppression of plasma virus with highly active anti-retroviral therapy (HAART)—in many patients for years—replication-competent virus was still isolated from the resting CD4+ T cells of every patient tested. Rebound viremia, he said, invariably follows therapy discontinuation.

The "sobering realization of the recalcitrant nature of this reservoir," the corollary that currently available therapies will have to be taken lifelong, and the surfacing of troublesome short- and long-term effects of HAART mandate a search for maneuvers to diminish the burden of the HAART regimen, as well as reduce the reservoir, Fauci said.

Several clinical centers have been experimenting with the strategy of interrupting therapy to lessen total drug exposure (and cost), as well as to increase drug adherence. Some experiments have demonstrated that such structured treatment interruption can enhance immunologic responses, leading to control of viremia in acutely infected persons. The situation is different, however, in those chronically infected, who likely have sustained substantial damage to their immune system.

Several interruption protocols—usually with treatment resumption when viral load increases to predetermined levels—have been tried with these patients. The aim, Fauci said, is to "elevate the immunologic setpoint to decrease acute viremia over time—[but] that is not happening."

The NIAID team has taken a somewhat different approach to therapy interruption, evaluating the effect of structured intermittent therapy (SIT) at predetermined times, either two months on and one month off (long cycle) or seven days on and seven days off (short cycle). Fauci described the results of these studies, which were also presented at separate poster presentations.

The Long and Short of SIT

The results of the trial of long-cycle SIT were reported by NIAID’s Mark Dybul and his NIAID and industry colleagues.

In this randomized, controlled study, 24 patients were placed on the intermittent regimen, and 23 were maintained on continuous treatment. All enrolled patients had plasma viral loads below 500 copies/mL for at least three months and below 50 copies/mL at screening; CD4+ T cells were greater than 300 cells/mm3.

Long-cycle SIT resulted in rebound HIV RNA in all 15 patients who’d completed between three and six cycles of eight weeks on and four weeks off therapy; viral load returned to screening levels in many by the end of the next eight-weeks-on cycle—or was no different from viral levels seen in the continuous treatment arm.

In general, then, rebound was followed by resuppression, there was no effect on CD4+ or CD8+ T-cell counts, and individuals on long-cycle SIT were able to reduce their drug exposure by one-third.

However, one of the 15 patients had evidence of genotypic and phenotypic decreased sensitivity to HAART during the third cycle. "That’s a concern," Fauci commented. Although the regimen has not been associated with any risks after one year, repeated increases and decreases in viral concentrations on long-cycle SIT could potentially have negative effects on the immune system, diminish susceptibility to HAART, and increase the risk of transmission during viremia, he suggested.

The same NIAID team reported more encouraging results with short-cycle—seven days on and seven days off—SIT. Twelve patients (with viral load and T-cell counts similar to those of the patients in the long-cycle SIT trial) were enrolled to receive short-cycle HAART for 24 months or until "failure," defined as plasma HIV RNA above 500 copies/mL or a CD4+ T-cell count decline to 25 percent below baseline.

Not one instance of rebound viremia had occurred—up to 44 weeks at the time of the conference—in the 10 protocol-compliant patients, the team reported. Viral load remained below detectable levels. Both noncompliant patients experienced rebound.

There was no increase in proviral DNA or in the frequency of replication-competent HIV in CD4+ T cells; there was no accumulation of HIV RNA in the follicular dendritic cell network of lymph nodes; and there was no evidence of genotypic or phenotypic resistance in these patients.

However, the preliminary data also indicated that there was no evidence of selective amplification of HIV-specific immunity (with either long- or short-cycle SIT) or that the latent HIV reservoir was affected. Although the suppressive efficacy of short-cycle SIT, as well as the absence of untoward side effects and the 50 percent reduction in drug exposure, appears promising, Fauci noted, there is a great need for expanded clinical trials of this strategy.

The IL-2 Boost

The addition of subcutaneous interleukin-2 (IL-2) to antiretroviral therapy (ART) to boost the immune system by expanding the pool of CD4+ T lymphocytes has been the subject of continuing NIH clinical studies conducted by NIAID clinical director Cliff Lane and Joe Kovacs, head of the AIDS section of the Clinical Center’s Critical Care Medical Department, and their teams (see The NIH Catalyst, September-October 1997, "IL-2 Immune Boost in HIV-Infected Patients . . .").

Team members presented updates on these ongoing studies (as did other investigators worldwide, who are also testing this strategy). The long-term feasibility of this approach was evaluated in a longitudinal study of patients who had entered three separate prospective trials of IL-2 plus ART between 1993 and 1997. Findings from 63 of these patients, reported by Doreen Chaitt, Richard T. Davey, Jr., and their colleagues at the CC and NIAID, suggested that substantially elevated CD4+ T-cell counts can be sustained over long periods with a low frequency of intermittent IL-2 cycling and no adverse effects on viral load.

Attempts to blunt the associated inflammatory effects of IL-2 with the addition of prednisone to the regimen, however, reported by NIAID’s Jorge Tavel and colleagues, resulted in a blunting of desirable immunologic effects as well.

In his overview, Fauci noted that IL-2, like the SIT strategy, does not appear to selectively enhance HIV-specific immunity.

Acute HAART and the Long Haul

Another study out of Fauci’s lab, reported by Tae-Wook Chun and colleagues, suggested that early initiation of HAART at HIV infection onset primes CD8+ T cells to take on the task of long-term suppression of viral replication in latent viral reservoirs, independent of cytotoxic T-lymphocyte (CTL) activity. The finding has implications particularly relevant in the context of long-term structured therapy interruptions.

Although CD8+ T cells are known to participate in antiviral activity against HIV, their role in controlling HIV replication in the latent CD4+ T-cell reservoir has been unclear, the investigators noted. They set out to evaluate the ability of autologous CD8+ T cells to suppress viral replication in the resting CD4+ T-cell reservoir.

Coculture experiments assessed cells from nine antiretroviral drug-naïve patients—six chronically infected patients and three long-term nonprogressors (LTNPs)—and eight patients with viral load below detection associated with HAART therapy.

Results showed that HIV was suppressed in the latent CD4+ cell reservoir during coculture with autologous CD8+ cells of LTNPs and with cells from those receiving HAART, but not with cells from some of the chronically infected patients not receiving therapy. The mechanism of viral suppression was independent of CTL. RANTES, MIP-1a, and MIP-1b–HIV-suppressive chemokines produced by CD8+ cells–were found to play a major role in viral suppression in some LTNPs and in patients receiving HAART, but not in untreated chronically infected patients. However, it was an unidentified non-CC-chemokine factor secreted by the CD8+ cells that exhibited the most potent antiviral activity and was found predominantly in patients in whom HAART was initiated shortly after the acute infection.

The presence of an unidentified soluble factor in patients in whom HAART was initiated during the acute phase of disease suggests that early initiation of therapy may play a role in containing viral replication during therapy interruption.

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