January 21, 2014
A treatment based on the combination of an antibody and a bacterial toxin has successfully reduced HIV-infected cells in mice modified to have a human immune system and undergoing antiretroviral therapy (ART). In the future, the technique could be part of a strategy to eliminate reservoirs of latent virus from HIV-infected individuals, according to results published on Jan. 9 in the journal PLOS Pathogens.
ART is an effective way to control HIV infection, however, it requires a constant adherence to the medications to prevent rebounds in viral load and the emergence of drug-resistant mutations. This viral persistence makes ART a lifelong commitment. That is why researchers are looking for HIV eradication therapies called "kick and kill," which induce cellular reservoirs to express latent HIV (the kick) and then kill these cells.
J. Victor Garcia, Ph.D., from the Center of AIDS Research at the University of North Carolina, and his group decided to test the effectiveness of an immunotoxin as a viable candidate for the "kill" step of the strategy. Called 3B3-PE38, this compound combines an antibody fragment (3B3 scFv) that recognizes the gp120 HIV envelope protein and the active part of the Pseudomonas bacteria exotoxin A (PE38). Thus, the immunotoxin can recognize and kill infected cells by identifying the fragments of gp120 they present on their membranes.
Since the effects of ART on viral persistence in tissues other than the blood are poorly understood, there was no baseline with which to characterize the systemic effects of the immunotoxin on HIV persistence during ART. This type of comprehensive multi-tissue analysis cannot be performed in humans, so the researchers employed bone marrow-liver-thymus (BLT) humanized mice, whose modified immunological system reacts to HIV infection as HIV-infected patients do, showing CD4 T-cell depletion and immune activation.
The BLT mice were treated with a triple antiretroviral drug combination that included tenofovir/emtricitabine (Truvada) and raltegravir (Isentress). Garcia and his group found that the drugs were able to penetrate all the tissues analyzed and reach the mice organs, where they reduced the cells producing viral RNA (vRNA). However, vRNA-producing cells were still detectable during therapy in all tissues analyzed, making them potential targets for HIV-specific immunotherapies such as 3B3-PE38.
Beginning on day 28 of ART, when vRNA reduction reached a plateau, the researchers added the immunotoxin by means of seven intraperitoneal injections given on alternate days. Compared to antiretroviral treatment alone, ART plus 3B3-PE38 reduced cell-associated vRNA (those produced in infected cells) by more than a thousand fold (3.2 log, where 1 log is equal to 10) in the bone marrow and from around 3 to 30 fold (0.4 to 1.5 log) in the human thymic organoid, spleen, lymph nodes, liver, lung, intestines and peripheral blood cells.
"When you take all the analyzed samples from all the tissues and combine them, the difference in vRNA levels is almost 10-fold, or 0.8 log exactly," says Garcia.
The study further showed that ART plus the immunotoxin resulted in an approximately 60-fold reduction of vRNA-producing cells throughout the mice's bodies. According to the researchers, the immunotoxin kills these cells and the extreme decrease in their numbers is the explanation for the reduced amount of cell-associated vRNA in the tissues, when compared with the mice that received only ART.
It is important to note that the study did not target the latent HIV reservoir. For the immunotoxin to identify the infected cell, the latter needs to present pieces of HIV proteins in its membrane and this can only happen if the virus is being produced inside it. In the latent reservoirs, HIV is transcriptionally silent, meaning it is not being replicated. Still, the immunotoxin approach is a valid "kill" system for "kick and kill" strategies.
Garcia states the plan now is to test 3B3-PE38 with the mechanisms being studied to function as the "kick." "We just have not settled on which strategy yet," he states, adding, "At this point, there is no date yet for when the immunotoxin would be available for treatment in humans."
Fred Furtado is a science writer based in Rio de Janeiro, Brazil.
Follow Fred on Twitter: @Patchlord.