Type I interferon protects cells from virus infection through the induction of a group of genes collectively named interferon-stimulated genes (ISGs). In this study, we utilized short hairpin RNA (shRNA) to deplete ISGs in SupT1 cells in order to identify ISGs that suppress the production of human immunodeficiency virus type 1 (HIV-1). Among the ISG candidates thus identified were interferon-induced transmembrane (IFITM) proteins, including IFITM1, IFITM2, and IFITM3, that potently inhibit HIV-1 replication at least partially through interfering with virus entry. Further mutagenesis analysis shows that the intracellular region, rather than the N- and C-terminal extracellular domains, is essential for the antiviral activity of IFITM1. Altogether, these data suggest that the IFITM proteins serve as important components of the innate immune system to restrict HIV-1 infection.

Cas9 cleaves specific DNA sequences with the assistance of a programmable single guide RNA (sgRNA). Repairing this broken DNA by the cell's error-prone non-homologous end joining (NHEJ) machinery leads to insertions and deletions (indels) that often impair DNA function. Using HIV-1, we have now demonstrated that many of these indels are indeed lethal for the virus, but that others lead to the emergence of replication competent viruses that are resistant to Cas9/sgRNA. This unexpected contribution of Cas9 to the development of viral resistance is facilitated by some indels that are not deleterious for viral replication, but that are refractory to recognition by the same sgRNA as a result of changing the target DNA sequences. This observation illustrates two opposite outcomes of Cas9/sgRNA action, i.e., inactivation of HIV-1 and acceleration of viral escape, thereby potentially limiting the use of Cas9/sgRNA in HIV-1 therapy.

Highly active antiretroviral therapy (HAART) has transformed HIV-1 infection from a deadly disease to a manageable chronic illness, albeit does not provide a cure. The recently developed genome editing system called CRISPR/Cas9 offers a new tool to inactivate the integrated latent HIV-1 DNA and may serve as a new avenue toward cure. We tested 10 sites in HIV-1 DNA that can be targeted by CRISPR/Cas9. The engineered CRISPR/Cas9 system was introduced into the JLat10.6 cells that are latently infected by HIV-1. The sequencing results showed that each target site in HIV-1 DNA was efficiently mutated by CRISPR/Cas9 with the target site in the second exon of Rev (called T10) exhibiting the highest degree of mutation. As a result, HIV-1 gene expression and virus production were significantly diminished with T10 causing a 20-fold reduction. The CRISPR/Cas9 complex efficiently mutates and deactivates HIV-1 proviral DNA in latently infected Jurkat cells. Our results also revealed a highly efficient Cas9 target site within the second exon of Rev that represents a promising target to be further explored in the CRISPR/Cas9-based cure strategy.

Interferon-inducible transmembrane (IFITM) protein family members IFITM1, -2, and -3 restrict the infection of multiple enveloped viruses. Significant enrichment of a minor IFITM3 allele was recently reported for patients who were hospitalized for seasonal and 2009 H1N1 pandemic flu. This IFITM3 allele lacks the region corresponding to the first amino-terminal 21 amino acids and is unable to inhibit influenza A virus. In this study, we found that deleting this 21-amino-acid region relocates IFITM3 from the endosomal compartments to the cell periphery. This finding likely underlies the lost inhibition of influenza A virus that completes its entry exclusively within endosomes at low pH. Yet, wild-type IFITM3 and the mutant with the 21-amino-acid deletion inhibit HIV-1 replication equally well. Given the pH-independent nature of HIV-1 entry, our results suggest that IFITM3 can inhibit viruses that enter cells via different routes and that its N-terminal region is specifically required for controlling pH-dependent viruses.

The human myxovirus-resistance protein B (MxB, also called Mx2) was recently reported to inhibit HIV-1 infection by impeding the nuclear import and integration of viral DNA. However, it is currently unknown whether there exist MxB-resistant HIV-1 strains in the infected individuals. Answer to this question should address whether MxB exerts an inhibitory pressure on HIV-1 in vivo and whether HIV-1 has evolved to evade MxB inhibition. We have examined ten transmitted founder (T/F) HIV-1 strains for their sensitivity to MxB inhibition by infecting CD4+ T cell lines SupT1 and PM1 that were stably transduced to express MxB. Two T/F stains, CH040.c and RHPA.c, were found resistant and this resistance phenotype was mapped to the amino acid positions 87 and 208 in viral capsid. The H87Q mutation is located in the cyclophilin A (CypA) binding loop and has a prevalence of 21% in HIV-1 sequences registered in HIV database. This finding prompted us to test other frequent amino acid variants in the CypA-binding region and the results revealed MxB-resistant mutations at amino acid positions 86, 87, 88 and 92 in capsid. All these mutations diminished the interaction of HIV-1 capsid with CypA. Our results demonstrate the existence of MxB-resistant T/F HIV-1 strains. The high prevalence of MxB-resistant mutations in the CypA-binding loop indicates the significant selective pressure of MxB on HIV-1 replication in vivo especially given that this viral resistance mechanism operates at expense of losing CypA.

Members of the interferon‐induced transmembrane (IFITM) protein family inhibit the entry of a wide range of viruses. Viruses often exploit the endocytosis pathways to invade host cells and escape from the endocytic vesicles often in response to low pH. Localization to these endocytic vesicles is essential for IFITM3 to interfere with the cytosolic entry of pH‐dependent viruses. However, the nature of the sorting signal that targets IFITM3 to these vesicles is poorly defined. In this study, we report that IFITM3 possesses a YxxΦ sorting motif, i.e. 20‐YEML‐23, that enables IFITM3 to undergo endocytosis through binding to the μ2 subunit of the AP‐2 complex. IFITM3 accumulates at the plasma membrane as a result of either mutating 20‐YEML‐23, depleting the μ2 subunit or overexpressing μ2 mutants. Importantly, blocking endocytosis of IFITM3 abrogates its ability to inhibit pH‐dependent viruses. We have therefore identified a critical sorting signal, namely 20‐YEML‐23, that controls both the endocytic trafficking and the antiviral action of IFITM3. This finding also reveals that as an endocytic protein, IFITM3 first arrives at the plasma membrane before it is endocytosed and further traffics to the late endosomes where it acts to impede virus entry.