HIV-1 latency is considered the last hurdle toward viral eradication in the presence of antiretroviral therapy. Studies of viral latency in vivo are complicated by the low frequency of latently infected cells found in HIV-1 patients (about 1 in 106 resting CD4+ T cells). Understanding the molecular mechanisms governing HIV-1 latency in vivo is also complicated by the lack of known phenotypic markers that can distinguish latently infected from uninfected cells.
No single experimental system of HIV-1 latency is thought to represent a complete recapitulation of the biologic state of the latent cell reservoir in vivo. This is due to two reasons:
-first, the latent or persistent HIV reservoir in vivo is likely to reside in multiple cell types;
-second, the molecular mechanisms that lead to the establishment of a latent infection are likely to be multiple as well.
Here is a short list of the current available models to study HIV latency:
1-Chronically infected cell lines:
-The U1 model consists of a chronically infected clone from the parent promonocyte cell line, U937. U1 cells contain two HIV-1 pro-viruses, which are non-replicating. In these cells, HIV latency was found to be associated with a suboptimal level of Tat protein activity;
-The ACH2 model is based in the generation of a chronically infected T-cell clone from the parental cell line and contains one latent provirus. In ACH2 cells, TNF-alpha is able to reactivate HIV-1 from latency;
Early work with the ACH2 and U1 cell lines facilitated studies aimed at understanding how the state of the chromatin impacts HIV-1 transcription.
-JDK is a chronically infected cell line derived from the parental Jurkat cell line. The HIV-1 strain in these cells contains a deletion in the long terminal repeat (LTR) NFkB binding sites. In JDK cells, HIV-1 expression can be restored by incubation with PMA, sodium butyrate, or hexamethylene bisacetamide in an NFkB-independent fashion;
-The J-Lat model where latency was attributed to the integration of HIV-1 in or in close proximity to heterochromatic regions.
The Thy/Liv SCID-hu model uses human fetal thymus and liver tissues implanted into severe-combined immunodeficient mice. This model allows for the study of latency in naïve CD4+ T cells.
3-Primary cell cultures:
In the model developed by Saleh and colleagues (Blood 2007), resting CD4+ T cells can efficiently be infected after incubation with one the CCR7 ligands, CCL19 or CCL21. An important contribution of this ex vivo model is the concept that a viral latent state can be achieved through direct infection of a quiescent, resting memory CD4 T cell.
Another model involves lentiviral transduction of primary CD4+ T cells with a Bcl-2 cDNA to increase cell survival in vitro (Yang HC, J Clin Invest 2009). It is unclear whether ectopic expression of Bcl-2 in this system may introduce artifactual effects in terms of cellular activation, differentiation that, in turn, may affect viral latency.
Vincente Planelles laboratory (Salt Lake City, USA) has recently developed an ex vivo paradigm of viral latency that utilizes primary T cells. The main difference with other primary cell models resides in the isolation of naïve CD4+ T cells to near purity. Naïve cells are then activated and induced to differentiate into a TCM-like phenotype known as non-polarized cells (NP).
This in vivo derived T cell sub-set exhibits an intermediate level of differentiation between naïve and effector cells, devoid of expression of effector cytokines that define Th1 and Th2 polarized cells, but with the potential for differentiation into Th1 or Th2.
Rational design of drugs to target HIV latency is not possible at the moment, because we do not have precise knowledge of all the cellular factors and activation pathways that impact viral transcription. Another obstacle to rational therapy design lies in the notion that while the desired compound should trigger viral reactivation, it should induce minimal to no cellular activation. Improving the availability of new models to study HIV-1 latency should accelerate this process.
Learn more: Bosque A, Planelles V. Studies of HIV-1 latency in an ex vivo model that uses primary central memory T cells. Methods 2010 ; Oct 21. [Epub ahead of print].
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Key words: HIV, HIV latency model, cell lines, in vitro, latency, model