The increasing research focus on curing HIV infection has led to an admirable uptick in the number of large scale collaborative studies in which multiple scientific groups work together in order to advance the field. Last year saw the publication of one such study that compared approaches to measuring the HIV reservoir, highlighting the strengths and weaknesses of each technique. Now several independent laboratories have collaborated on a comparison of the different laboratory methods for measuring the effectiveness of compounds that aim to awaken latent HIV (believed to be a key step for eliminating the latent HIV reservoir in HIV-positive individuals on ART). These methods are important because they help researchers decide whether a compound is active enough to be studied in people.
The ideal approach would be to test whether the compound reactivates latent HIV in cells isolated from HIV-positive people on ART, but this is extremely difficult because latently infected cells are so rare (~1 per million resting CD4 T cells) that very large numbers of CD4 T cells have to be sampled from each person by the cumbersome process of apheresis. As a workaround, several research groups have developed methods to create latent HIV infection in a lab dish, using either sampled CD4 T cells (these are called primary cell models) or cell lines that are maintained in the laboratory (cell line models).
The new study, published on December 26th in the journal PLoS Pathogens, compares results obtained with a panel of different anti-latency compounds in five primary cell models, four cell line models, and latently infected cells isolated from HIV-positive people on ART. A key finding is that there is significant variability in the performance of the anti-latency approaches from model to model, and no single model accurately predicts activity in latently infected cells from HIV-positive people. The most consistent anti-latency effect is seen with broad stimulators of T cell activation such as PHA; unfortunately these types of approaches are too risky to be used in people (clinical trials of the broad T cell stimulator OKT3 were discontinued due to severe toxicity). Some compounds, such as bryostatin-1 (which was in the news last summer as a potential anti-latency approach), show activity in some models but none in latently infected cells from HIV-positive people. The researchers note that a primary cell model developed by the laboratory of Sharon Lewin at Monash University in Melbourne, Australia, came closest to mirroring latently infected cells isolated from HIV-positive people, but there were still some significant differences.
Overall, the results highlight that the factors that contribute to HIV latency are complex and can vary in different models compared to the human body. Further studies will be needed to better understand the reasons for the variation and the strengths and weaknesses of the current models. One immediate recommendation made by the authors is that potential anti-latency compounds should be evaluated in at least two separate models with complementary properties (specifically mentioned are the Lewin model and another primary CD4 T cell model such as those from the laboratories of Robert Siliciano at Johns Hopkins University or Celsa Spina at the University of California at San Diego). Although the study demonstrates the challenges associated with evaluating anti-latency approaches in the laboratory, it also offers a testament to the willingness of scientists involved in HIV cure research to engage in collaborative problem solving.
PLoS Pathog. 2013 Dec;9(12):e1003834. doi: 10.1371/journal.ppat.1003834. Epub 2013 Dec 26.
Spina CA, Anderson J, Archin NM, Bosque A, Chan J, Famiglietti M, Greene WC, Kashuba A, Lewin SR, Margolis DM, Mau M, Ruelas D, Saleh S, Shirakawa K, Siliciano RF, Singhania A, Soto PC, Terry VH, Verdin E, Woelk C, Wooden S, Xing S, Planelles V.
The possibility of HIV-1 eradication has been limited by the existence of latently infected cellular reservoirs. Studies to examine control of HIV latency and potential reactivation have been hindered by the small numbers of latently infected cells found in vivo. Major conceptual leaps have been facilitated by the use of latently infected T cell lines and primary cells. However, notable differences exist among cell model systems. Furthermore, screening efforts in specific cell models have identified drug candidates for "anti-latency" therapy, which often fail to reactivate HIV uniformly across different models. Therefore, the activity of a given drug candidate, demonstrated in a particular cellular model, cannot reliably predict its activity in other cell model systems or in infected patient cells, tested ex vivo. This situation represents a critical knowledge gap that adversely affects our ability to identify promising treatment compounds and hinders the advancement of drug testing into relevant animal models and clinical trials. To begin to understand the biological characteristics that are inherent to each HIV-1 latency model, we compared the response properties of five primary T cell models, four J-Lat cell models and those obtained with a viral outgrowth assay using patient-derived infected cells. A panel of thirteen stimuli that are known to reactivate HIV by defined mechanisms of action was selected and tested in parallel in all models. Our results indicate that no single in vitro cell model alone is able to capture accurately the ex vivo response characteristics of latently infected T cells from patients. Most cell models demonstrated that sensitivity to HIV reactivation was skewed toward or against specific drug classes. Protein kinase C agonists and PHA reactivated latent HIV uniformly across models, although drugs in most other classes did not.