By Roberto Fernandez-Larsson, Ph.D.
Anti-HIV Neutralizing Antibodies Delay Viremia Rebound
What will be most important in an effective AIDS vaccine—robust humoral or cellular immunity? The current consensus among HIV researchers is that most likely both will be required, at least for a vaccine that provides sterilizing immunity.
Passive administration of neutralizing antibodies (NAbs) can protect nonhuman primates from SIV and SHIV infection, at least when they are administered at or soon after the time of exposure to virus. Evidence that the NAb response is important in humans remains largely associative, the strongest evidence being the rapid selection of antibody escape mutants in natural HIV infection.
In a recent paper (Nat. Med. 11, 615, 2005) Trkola and colleagues have tried to better define the potential benefit of NAbs in humans in vivo through passive administration of monoclonal NAbs to try to control viremia in HIV-infected patients. Three well-characterized monoclonal NAbs (2G12, 2F5 and 4E10) directed against either gp120 or gp41 were administered to 6 acutely and 8 chronically infected patients who had recently been taken off antiretroviral therapy (scheduled treatment interruption), selected because their autologous viruses were sensitive to those antibodies in vitro. Patients received several doses of the 3 NAbs during an 11-week period, the first dose given immediately before ARV suspension to mimic a successful therapeutic vaccination, and then followed immunologically and virologically until week 24.
Two chronically and 4 acutely infected individuals showed a prolonged delay (>9 weeks) in viral rebound compared to both their own rebound times during treatment interruptions prior to the current trial and rebound times in a control group of HIV-infected patients who were not treated with NAbs, indicating that, in principle, NAbs can control viremia during HIV infection. Acutely infected patients benefited better from the antibody treatment than patients with chronic infection. Overall, NAb-treated patients took about 8 weeks to rebound to levels above 10 HIV RNA copies/ml, whereas untreated patients rebounded in about 3.75 weeks. In all, 12 of the 14 Nab-treated individuals had rebound virus that was resistant to 2G12, indicating that virus had escaped from this antibody and likely accounted for their viral rebound. There was no detectable resistance to either 4E10 or 2F5, suggesting these two antibodies had negligible effect in vivo.
This proof of principle trial demonstrates that NAbs can exert some control on viremia in established HIV infection, despite the fact that the treated and the control groups of patients were neither matched for antibody sensitivity nor randomized, so other factors might have influenced the difference in rebound times between groups. The viremic control was transient and very high doses of Nabs were required, two orders of magnitude above the in vitro inhibitory concentration, and there was evidence of antibody escape in the majority of subject’s virus, so comparable treatment regimens will not be practical in the near future. However, dosage and perhaps even viral escape concerns might be surmountable in a protective setting in uninfected people, before the virus is able to get a foothold.
HIV Transmission During Early-Stage Infection
Heterosexual vaginal transmission accounts for the majority of HIV infections worldwide, yet knowledge of the biology and epidemiology of transmission has remained inadequate. Previous studies have calculated the frequency of transmission to be about 1 per 1000 coital acts, and transmission risk was thought to be greatest during the early post-seroconversion period and during advanced disease, when blood viral loads are highest. Wawer and colleagues (J. Infect. Dis. 191, 1403, 2005) have now examined the rates of HIV transmission per coital act by stage of infection, shedding new light on this important subject.
The researchers studied a cohort of more than 15,000 people in the Rakai district of Uganda, originally designed to determine if intermittent mass therapy against sexually-transmitted diseases could reduce HIV transmission. Although antiretroviral drugs were not available in Uganda at the time, participants were provided with condoms and voluntary counseling and testing. After the end of the trial, they identified 235 discordant couples—where one partner is HIV infected and the other is not—who were monogamous and performed a retrospective transmission investigation based on archival samples and collected data. The monthly frequency of intercourse, as reported by the couples, was used to calculate the number of coital acts through the trial. HIV transmission was corroborated by sequence analysis of donor and recipient viruses.
They found that the overall frequency of HIV transmission in these discordant couples was 1.2 per 1000 coital acts, in good agreement with other studies. However, HIV transmission varied markedly depending on the stage of disease of the infecting partner. During early infection, in about the first 2.5 months after seroconversion, the frequency of HIV transmission jumped to 8.2 per 1000 coital acts. In the period 6 to 15 months after seroconversion, the rate of transmission was 1.5 per 1000 coital acts. The frequency of transmission observed during established infection was 0.7 per 1000 coital acts, a 12-fold lower risk than during acute infection. Later in infection, within the two years before the infecting partner’s death, the frequency increased again to 2.8 per 1000 coital acts. So the rate of HIV transmission follows a U-shaped curve, with the highest values seen in early infection.
Higher viremia has been previously associated with increased rates of HIV transmission per coital act, but this work shows for the first time the variable risk of HIV transmission by stage of infection. Diagnosing this stage of disease requires an understanding of who is at risk, but identifying these individuals allows for counseling to prevent further transmission to others and the potential benefits associated with early therapy. The authors acknowledge the implications for a vaccine that could mitigate the initial infection, when a great number of transmission events take place, and state that an AIDS vaccine that reduces primary viremia may have a greater effect than antiretrovirals in controlling the spread of HIV. They warn, however, that a high early viremia may not be the only factor contributing to the increased risk of transmission seen in early infection. Other factors, such as genital ulcer disease, cannot be disregarded. In fact, they claim that a high rate of genital ulcers was observed in participants to the study during early HIV infection, especially in persons who were HSV-2 seropositive.
SIV Destroys Immune Cells Early in Infection
Both HIV and SIV destroy CD4+ T cells, and the slow and steady decline in the number of these cells correlates with disease progression. But the very early events in acute HIV and SIV infection have been more difficult to study since it is hard to identify people immediately after infection. Loss of CD4+ T cells in that period had been considered moderate and of short duration but two papers published last year confirmed previous data in SIV-infected monkeys: the acute depletion of activated CCR5+ CD4+ memory T cells in the gut mucosa of HIV-infected patients, strongly suggesting that these are HIV-infected cells killed directly by the virus, and debunked once again the idea of early HIV disease being relatively benign.
Two studies of SIV-infected monkeys have now examined more closely the depletion of CD4+ T cells during acute infection and find that SIV immediately wipes out more than half of these cells and provide definitive evidence that they are killed directly by the virus.
In the first study, Roederer and colleagues (Nature 434, 1093, 2005) studied flow cytometry-sorted populations of naïve and memory CD4+ T cells from various tissues of recently-infected rhesus macaques. They found that the acute depletion of CCR5+ CD4+ memory T cells from the gut mucosa can be explained by a severe (30-60%) loss of these cells throughout several body tissues of infected monkeys, first appearing in the blood and lymph nodes and then spreading to the gut. Using a quantitative PCR assay for SIV gag DNA sensitive enough to detect single copy molecules in cells, they found that cells had been infected by day 10 post-inoculation and had disappeared four days later. How the CD4+ memory T cells are killed is not clear but the authors suggest it may be due to direct viral cytopathic effects or CD8+ T cell killing. They found that naïve, resting CD4+ T cells are not infected with SIV, and since these cells are the majority of CD4+ T cells in blood, their numbers mask the massive loss seen in the CD4+ memory fraction.
In a related paper, Haase and colleagues (Nature 434, 1148, 2005) examined SIV replication in ‘resting’ CD4+ memory T cells during acute infection. The consensus opinion has been that resting T cells cannot support SIV (or HIV) replication. CD4+ memory T cells, their resting phenotype and their infection status were identified by combined in situ hybridization for the presence of SIV RNA and immunohistochemical staining of cell markers of gut cells. Interestingly, they found that resting CD4+ memory T cells are capable of supporting virus production. In agreement with the Roederer paper, SIV RNA production peaked at day 10 post-inoculation. In contrast to that study, these authors found that CD4+ T cells may be depleted by Fas-mediated cell apoptosis.
These papers further evaluate SIV pathogenesis in monkeys and show that the acute phase of infection is not only restricted to gut lymphoid tissue as previously thought. Rather, the two studies support the view that the infection is a systemic and huge infection of CD4+ memory T cells. Previously studied depletion of CD4+ cells in the gut reflects the predominance of CD4+ memory T cells over naïve CD4+ T cells in this tissue compartment. These findings stress the point that an AIDS vaccine will have to produce an effective immune response able to arrest this early peak in virus replication in the early days of the infection to thwart the enormous destruction of CD4+ memory T cells that seem to have irreversible consequences.
The significance of this very early massive infection of CD4+ memory T cells on the chronic phase of the infection and disease progression has not been well understood. Evidence pointing to the importance of early events has been accumulating that supports the initiation of antiretroviral treatment in HIV patients as early as possible, regardless of peripheral blood viral counts and CD4+ T cell counts. Two papers reinforce the importance of this very early immunological insult. In a study in 205 HIV-infected infants, Moldanado and colleagues (JAMA 293, 2221, 2005) found that initiating very early treatment of infants (by 2 months after birth as opposed to 3-4 months) with one or two antiretrovirals was associated with decreased and delayed progression to AIDS. And Wong and colleagues (J. Infect. Dis. 191, 1410, 2005) have studied the effect of initiating antiretroviral treatment during either primary infection (before or less than 6 months after seroconversion) or chronic infection on cellular reservoirs of HIV, as determined by decay kinetics of HIV DNA and cell-associated infectivity ex vivo. The data suggest that early treatment initiation controls HIV cellular reservoirs as cell-associated infectivity was not detectable in most early-treated patients after 1 year of treatment whereas all patients that started treatment during chronic infection still had detectable cell-associated activity after 3-6 years of treatment.