CORDIS - Forschungsergebnisse der EU

Innate intracellular blocks to HIV-1 in New World monkeys

Final Report Summary - HIVMARMOD (Innate intracellular blocks to HIV-1 in New World monkeys)

The presence of several barriers to HIV-1 replication in cells of many species limits the species tropism of human immunodeficiency virus type 1 (HIV-1) to humans and chimpanzees. This narrow species tropism of HIV-1 is due to two types of host factors: 1) factors required for HIV-1 replication that exhibit species-specific changes that do not allow efficient use by HIV-1; and 2) dominant-acting factors that block replication in many species. The latter, also known as restriction factors (RF), are part of the so-called intrinsic antiviral immunity. Altogether, RFs can act as a powerful barrier to stop viral replication, and consequently, viruses have evolved mechanisms that can antagonize restriction in a species-specific way. Up to date a few of these restriction factors have been identified and include TRIM5alpha, BST2, APOBEC3G (A3G), SAMHD1 and Mx2.

The main goal of this project was to study the host cell RFs that block the replication of HIV-1 in common marmoset cells. Previous studies had shown that a major block to HIV-1 replication in New World monkeys operates at the level of viral entry and that this block can be overcome by adaptation of the virus in tissue-cultured cells. In addition, we have observed that the common marmoset A3G (marA3G) and BST2 (marBST2) proteins, but not TRIM5alpha protein, restrict HIV-1 in cell cultures. Using a direct evolution method, we have been able to derive HIV-1 variants that are able to replicate in cell cultures expressing those RFs. We have found that a few specific changes in the viral proteins Vif and Env are responsible for the escape of the virus from A3G and BST2, respectively. We have shown that the Vif changes reduce the expression levels and the encapsidation of marmoset APOBEC3G, pointing out to a mechanism of escape similar to that previously reported. In contrast, the HIV-1 Env variants that escape marBST2 do not show significant differences in viral particle release as it would be expected, and do not produce an increase in cell-free viral infectivity. Instead, the observed changes increase viral fitness and modestly favor cell-to-cell transmission of the virus, allowing viral escape from this RF. Although the exact mechanism of escape is still not clear, our results show that adapted Env mutants do not have the same molecular escape mechanism from BST2 than HIV-1 Vpu, HIV-2 Env or SIVs Nef, suggesting the presence of a new mechanism of escape from BST2 that does not seem to directly target marBST2.

In addition, we have observed the presence of post-entry blocks to the early phase of HIV-1 infection in peripheral blood lymphocytes (PBLs) and a B lymphocytic cell line (B-LCL). The blocks present in these cells are dominant and phenotypically different from each other. In PBLs, the block occurs at the level of reverse transcription, reducing the accumulation of early and late transcripts, similar to the block imposed by TRIM5alpha. However, we have found that marmoset TRIM5alpha does not block HIV-1. In contrast, the restriction factor present in B-LCLs blocks HIV-1 replication at a later step, after nuclear entry, and inhibits integration. Additionally, we have identified an HIV-1 capsid mutant that is able to escape the restriction in the marmoset B-LCLs. Our results suggest that the factors responsible for the blocks present in marmoset PBLs and B-LCLs are different. We propose the existence of at least two new restriction factors able to block HIV-1 infection in marmoset lymphocytes. Up to date we have been able to identify one protein that could be responsible, at least in part, for the restriction of HIV-1 in marmoset primary cells.

• Public health relevance:

Currently there are approximately 37 million people living with HIV-1 infection and it is estimated that about 1-2 million deaths from AIDS and an equal number of new infections occur annually. In spite of the advances in the fight against HIV-1 since the isolation of the virus in 1983, there is still no vaccine available for the prevention of HIV-1 infection, and so far the available therapies have failed to eradicate the virus.

The use of different animal models for the development of vaccines, testing of new antiviral drugs, and studies of pathogenesis has been invaluable. However, the available animal models used in AIDS research have limitations. The development of new animal models involving infection with more complete HIV-1-like viruses is a necessity for future progress in these areas. Our studies may help pave the way towards a non-human primate model of permissive HIV-infection. Altogether, our studies provide new insights into the viral-host interactions and the mechanisms that the virus uses to evade innate immune activity. Knowledge and characterization of the mechanism of action of these restriction factors can help to better understand HIV pathology and assist the discovery of novel therapies against the infection.