Evaluation of Different CD4 T-Cell Reservoirs
A new model allowing the evaluation of many primary CD4 T-cell reservoirs is described. It shows potentially important differences within the latently infected cell pools and looks promising for the in vitro testing of different agents aimed at reactivating HIV from latency.
The study of HIV latency has been generally hindered by the lack of a robust and rapidly deployable cell model that involves primary human CD4 T lymphocytes.
Current primary lymphocyte models more closely reflect the in vivo state of HIV latency and may not reflect the heterogeneous nature of the latent reservoir.
Even studying latently infected cells from HIV-infected subjects is challenging. These cells are very rare in the blood, and there are no methods to enrich them.
Immortalized T-cell lines have improved our understanding of HIV latency but they do not recapitulate the non-dividing G0 state of resting CD4 T cells in vivo.
Most primary cell models often takes several weeks or months of continuous culture to get sufficient latently infected non-dividing T cells.
They are, therefore unappropriate for large-scale screening for agents that could reactivate and eliminate latent proviruses.
Furthermore, it is now demonstrated that the latent reservoir in vivo might be more complex than previously thought. It involves both central memory T cells that typically harbor a larger proportion of the latent proviruses, but also transitional memory cells that appear to live longer and to be continually renewed by cytokine-induced homeostatic proliferation.
In this paper (1) Warner Greene laboratory adapated the model previously described by Una O'Doherty laboratory (2) to make it more dynamic and suitable for drug screening. This model is more rapid (viral proteins can be detecetd in latently infected cells within 2 hours after induction) and allows the quantification of the release and accumulation of viral particles after reactivation.
This model may also be useful for further characterizing the subset of latently infected cells that fail to respond to classic reactivation signals to discern the underlying mechanism(s).
The authors tested the effects of Prostratin (NF-kB stimulator), HDAC inhibitors (valproic acid, SAHA, TSA), HMBA (p-TEFB activator) and different combinations of these drugs in their model.
Interestingly, they found that transcriptional activators failed to synergize with chromatin-modifying agents. Unlike prostratin alone, the effects of the synergies were not consistent among different donors. each HDAC inhibitor was screened in combination with HMBA. Again, only modest, transient synergy was observed with any of the combinations.
When they addressed the reactivation of different parts of the reservoir, they found that transitional memory cells may be more susceptible to certain inducers. Transitional memory cells were significantly more susceptible to reactivation with PMA+ ionomycin, anti-CD3+anti-CD28 antibodies, and prostratin, although central memory cells and transitional memory cells exhibited similar levels of reactivation with IL-7.
These data also suggest that the chromatin environment might have a more significant role in establishing latency in proliferating cell lines than in quiescent primary CD4 T cells.
1-Lassen KG, et al. A Flexible Model of HIV-1 Latency Permitting Evaluation of Many Primary CD4 T-Cell Reservoirs. PLoS One. 2012; 7(1): e30176. Epub 2012 Jan 24.
2-Swiggard WJ,et al. Human immunodeficiency virus type 1 can establish latent infection in resting CD4+ T cells in the absence of activating stimuli. J Virol 2005; 79: 14179–14188
Key words: HIV latency model, anti-latency agents, model, models