Antagonism of BST-2/Tetherin Is a Conserved Function of the Env Glycoprotein of Primary HIV-2 Isolates

Abstract

Although HIV-2 does not encode a vpu gene, the ability to antagonize bone marrow stromal antigen 2 (BST-2) is conserved in some HIV-2 isolates, where it is controlled by the Env glycoprotein. We previously reported that a single-amino-acid difference between the laboratory-adapted ROD10 and ROD14 Envs controlled the enhancement of virus release (referred to here as Vpulike) activity. Here, we investigated how conserved the Vpu-like activity is in primary HIV-2 isolates. We found that half of the 34 tested primary HIV-2 Env isolates obtained from 7 different patients enhanced virus release. Interestingly, most HIV-2 patients harbored a mixed population of viruses containing or lacking Vpu-like activity. Vpu-like activity and Envelope functionality varied significantly among Env isolates; however, there was no direct correlation between these two functions, suggesting they evolved independently. In comparing the Env sequences from one HIV-2 patient, we found that similar to the ROD10/ROD14 Envs, a single-amino-acid change (T568I) in the ectodomain of the TM subunit was sufficient to confer Vpu-like activity to an inactive Env variant. Surprisingly, however, absence of Vpu-like activity was not correlated with absence of BST-2 interaction. Taken together, our data suggest that maintaining the ability to antagonize BST-2 is of functional relevance not only to HIV-1 but also to HIV-2 as well. Our data show that as with Vpu, binding of HIV-2 Env to BST-2 is important but not sufficient for antagonism. Finally, as observed previously, the Vpu-like activity in HIV-2 Env can be controlled by single-residue changes in the TM subunit. IMPORTANCE Lentiviruses such as HIV-1 and HIV-2 encode accessory proteins whose function is to overcome host restriction mechanisms.Vpu is a well-studied HIV-1 accessory protein that enhances virus release by antagonizing the host restriction factor BST-2. HIV-2 does not encode a vpu gene. Instead, the HIV-2 Env glycoprotein was found to antagonize BST-2 in some isolates. Here, we cloned multiple Env sequences from 7 HIV-2-infected patients and found that about half were able to antagonize BST-2. Importantly, most HIV-2 patients harbored a mixed population of viruses containing or lacking the ability to antagonize BST-2. In fact, in comparing Env sequences from one patient combined with site-directed mutagenesis, we were able to restore BST-2 antagonism to an inactive Env protein by a single-amino-acid change. Our data suggest that targeting BST-2 by HIV-2 Env is a dynamic process that can be regulated by simple changes in the Env sequence.

Preadaptation of Simian Immunodeficiency Virus SIVsmm Facilitated Env-Mediated Counteraction of Human Tetherin by Human Immunodeficiency Virus Type 2

“…Several studies have mapped residues in HIV-2 group A Env proteins that are essential for efficient counteraction of human tetherin. These comprise a tyrosinebased endocytosis motif in the cytoplasmic domain, as well as several residues in the extracellular domain (i.e., K422, I568, A598, and N659) (36,38,66,67,80,81). Most of them, including N659, which is required for direct interaction of Env with human tetherin (67), are already present in SIVsmm Envs and conserved among different groups of HIV-2.…”

“…This is even more surprising as inactive Env proteins may act in a transdominant manner and prevent tetherin counteraction by otherwise active Env proteins (66). Notably, however, a previous study showed that only about 50% of all primary HIV-2 group A isolates encode an Env protein that antagonizes this restriction factor (38). Thus, although Env-mediated anti-tetherin activity can be found in different groups of HIV-2, it may be less conserved than Vpu-mediated tetherin counteraction in HIV-1 group M. One likely explanation is the accessibility and antigenicity of the viral envelope protein.…”

“…Amino acids known to be required for the anti-tetherin activity of HIV-2 A Env are shown in green. Please note that the numbering of the amino acid positions (I568, A598, and N659) used in previous publications (38,66,67) is different from the one in this alignment. Dashes represent gaps that were introduced to improve the alignment (TMD, transmembrane domain).…”

“…, and CRF01_AB Env counteract human tetherin. Since the Env proteins of some HIV-2 group A strains exert anti-tetherin activity in human cells (36)(37)(38), we tested whether other groups of HIV-2 are also capable of Env-mediated tetherin counteraction. To analyze the effects of Env on tetherin restriction in infected cells, we inserted heterologous HIV-2 and SIV env sequences into the proviral HIV-2 clone 7312A (Fig.…”

“…While the different groups of HIV-1 are relatively well characterized, most studies analyzing immune evasion mechanisms and human-specific adaptations of HIV-2 focused on the characterization of epidemic group A and (to a lesser extent) group B viruses. For example, it has been shown that HIV-2 group A mastered the tetherin hurdle by switching from Nef to Env to counteract this restriction factor (36)(37)(38). However, it has remained unclear whether/how other groups of HIV-2 antagonize human tetherin.…”