Sunday, December 11, 2011

HIV-1 Integration is Reduced in CD4 T Cells from Elite Controllers

In 2010, 34 million people were living in the world with Human Immunodeficiency Virus (HIV) (1). The current reality, however daunting, is that there are no vaccines available against HIV. Indeed, even modern day treatments are unable to stop progression of HIV to acquired immunodeficiency syndrome, or AIDS. In 2010, 1.8 million people died from AIDS, so clearly this is a high priority area of research (1). Scientists are working all over the globe, continually researching and studying HIV in hopes of developing a vaccine. So far the efforts have been met with continual challenges and not much success. However, there is a group of people referred to as elite controllers, who are able to maintain undetectable levels of HIV-1 replication despite being HIV positive. One could understand why they would be of great interest to scientists, as they provide a model to study for an effective immune response against HIV infection.
A recent article studying elite controllers investigated HIV-1 reverse transcription and integration in directly ex vivo­ isolated CD4 T cells (2). They used CD4 T cells from elite controllers, HIV-1-negative volunteers, and untreated HIV-1 progressors. The quantified CD4 T cell count using negative immunomagnetic selection ex vivo. Next, cells were infected with a yellow fluorescence protein (YFP)-encoding vesicular stomatitis virus G protein (VSV-g) pseudo typed HIV-1 virus. These big words may sound confusing, but just know that this process infects cells, causing only a single round of HIV-1 infection, allowing for detailed assessments of individual early HIV-1 replication steps. Understanding these processes could have very important implications for vaccine development.

Forty-eight hours after infection, cells were collected and subjected to quantification of HIV-1 late reverse transcripts (LRT). They found that the proportions of YFP-positive CD4 T cells were .58% in elite controllers, 1.4% in progressors, and 1.79% in HIV-1-negative persons. The results show that infection of CD4 T cells resulted in significantly lower levels of LRT and HIV-1 DNA in elite controllers than in cells from HIV-1 negative persons or progressors. They also examined the levels of 2-LRT (a different set of transcripts) and found that they were disproportionally elevated in elite controllers. Further, the ratios of integrated DNA to LRT were significantly lower in elite controllers than in HIV-1-negative persons or progressors. So what does all of this mean?
Overall, we now know that early HIV-1 replication is different in elite controllers. The altered pattern of early HIV-1 replication suggests a defect at the level of HIV-1 integration in ex-vivo infected CD4 T cells from elite controllers. Prior studies have found that p21 is highly up regulated in CD4 T cells from elite controllers and can inhibit HIV-1 integration in hematopoietic stem cells (HSC) (3). To see whether p21 is involved in the observed inhibition of HIV-1 integration in ex vivo-infection CD4 T cells from controllers, they performed the first experiment again but in the presence of a small molecule inhibitor of p21. They found that the addition of p21 had no effect on LTR, 2-LTR, and HIV-1 DNA from the HIV-negative persons. However, in cells from elite controllers, LTR and 2-LTR significantly increased, and reached similar levels of the HIV-negative cells. This suggests that p21 inhibition can overcome “intrinsic restriction at the level of HIV-1 reverse transcription in elite controllers” (2). In contrast, HIV-1 DNA only weakly increased after p21 inhibition in CD4 T cells from elite controllers, remaining much lower than the HIV-negative condition. Further, the disproportionate ratios of integrated DNA to LRT described above cannot be corrected by the inhibition of p21. Therefore, the inhibition of HIV-1 integration in ex vivo-infected CD4 T cells from elite controllers cannot be due to p21.
The next step was to investigate whether an inhibition of HIV-1 integration is also detectable in CD4 T cells from elite controllers with prior in vitro activation of CD4 T cells. Therefore, they activated CD4 T cells from elite controllers and HIV-negative people using specific antibodies. The cells were subsequently infected with the same HIV used previously. They found that LRT and integrated DNA levels were much lower in elite controllers than in HIV-negative people, but the ratios were not significantly different between the two. This suggests that reduced levels of integrated HIV-1 DNA in ex-vivo activated CD4 T cells from elite controllers is a result of reduced HIV-1 reverse transcripts and not an independent restriction at the level of HIV-1 integration.
In conclusion, the paper suggests that in these circumstances, HIV-1 integration can be significantly reduced in CD4 T cells from elite controllers. Further studies are necessary to identify molecular mechanisms that contribute to this ability. There is increasing evidence that intrinsic restriction of HIV-1 replication plays an important role in the ability of elite controllers to maintain undetectable viral loads and avoid progressing to AIDS. Future studies should try to identify cell-intrinsic inhibitors of HIV-1 integration in CD4 T cells from elite controllers. Once these mechanisms are understood, perhaps forward progress can be made in vaccine and treatment developments.
2. Buzon, M. J., et al. (2011). Inhibition of HIV-1 Integration in Ex Vivo-Infection CD4 T Cells from Elite Controllers. Journal of Virology. 85: 9646-9650.
3. Zhang, J., et al. (2007). Primitive Hematopoietic Cells Resist HIV-1 infection via p21. J. Clin. Invest. 117:473-481.

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