Unique Phenotypic Characteristics of Recently Transmitted HIV-1 Subtype C Envelope Glycoprotein gp120: Use of CXCR6 Coreceptor by Transmitted Founder Viruses

To prescreen or not to prescreen for broadly neutralizing antibody sensitivity in HIV cure-related trials

The use of broadly neutralizing antibodies (bNAbs) as a cure-related research strategy for human immunodeficiency virus (HIV) has gained attention from the scientific community. bNAbs are specialized antibodies that target HIV-1 by binding to proteins on the surface of the virus, preventing the infection of human cells. In HIV-1 clinical studies assessing the use of bNAbs, it has been common practice to prescreen potential participants for bNAb sensitivity. However, the use of pre-screening in HIV-1 bNAb clinical trials is a topic of ongoing debate, with regard to its potential benefits and limitations. In this paper, we examine the possible benefits and limitations of pre-screening for bNAb sensitivity in HIV-1 cure-related studies, and suggest alternative methods which may be more effective or efficient at saving costs and time. Ultimately, the decision to use pre-screening in HIV-1 bNAb clinical trials should be based on a careful assessment of the potential benefits and limitations of this approach, as well as the specific needs, goals, design, and population of the study in question.

Generation and characterization of infectious molecular clones of transmitted/founder HIV-1 subtype C viruses

The genetic diversity of HIV impedes vaccine development. Identifying the viral properties of transmitted/founder (T/F) variants may provide a common vaccine target. To study the biological nature of T/F viruses, we constructed full-length clones from women detected during Fiebig stage I acute HIV-1 infection (AHI) from heterosexual male-to-female (MTF) transmission; and clones after one year of infection using In-Fusion-based cloning. Eighteen full-length T/F clones were generated from 9 women and six chronic infection clones were from 2 individuals. All clones but one were non-recombinant subtype C. Three of the 5 T/F clones and 3 chronic clones tested replicated efficiently in PBMCs and utilised CCR5 coreceptor for cell entry. Transmitted/founder and chronic infection clones displayed heterogenous in vitro replicative capacity and resistance to type I interferon. T/F viruses had shorter Env glycoproteins and fewer N-linked glycosylation sites in Env. Our findings suggest MTF transmission may select viruses with compact envelopes.

Inferring the multiplicity of founder variants initiating HIV-1 infection: a systematic review and individual patient data meta-analysis

BACKGROUND

HIV-1 infections initiated by multiple founder variants are characterised by a higher viral load and a worse clinical prognosis than those initiated with single founder variants, yet little is known about the routes of exposure through which transmission of multiple founder variants is most probable. Here we used individual patient data to calculate the probability of multiple founders stratified by route of HIV exposure and study methodology.

METHODS

We conducted a systematic review and meta-analysis of studies that estimated founder variant multiplicity in HIV-1 infection, searching MEDLINE, Embase, and Global Health databases for papers published between Jan 1, 1990, and Sept 14, 2020. Eligible studies must have reported original estimates of founder variant multiplicity in people with acute or early HIV-1 infections, have clearly detailed the methods used, and reported the route of exposure. Studies were excluded if they reported data concerning people living with HIV-1 who had known or suspected superinfection, who were documented as having received pre-exposure prophylaxis, or if the transmitting partner was known to be receiving antiretroviral treatment. Individual patient data were collated from all studies, with authors contacted if these data were not publicly available. We applied logistic meta-regression to these data to estimate the probability that an HIV infection is initiated by multiple founder variants. We calculated a pooled estimate using a random effects model, subsequently stratifying this estimate across exposure routes in a univariable analysis. We then extended our model to adjust for different study methods in a multivariable analysis, recalculating estimates across the exposure routes. This study is registered with PROSPERO, CRD42020202672.

FINDINGS

We included 70 publications in our analysis, comprising 1657 individual patients. Our pooled estimate of the probability that an infection is initiated by multiple founder variants was 0·25 (95% CI 0·21-0·29), with moderate heterogeneity (Q=132·3, p<0·0001, I²=64·2%). Our multivariable analysis uncovered differences in the probability of multiple variant infection by exposure route. Relative to a baseline of male-to-female transmission, the predicted probability for female-to-male multiple variant transmission was significantly lower at 0·13 (95% CI 0·08-0·20), and the probabilities were significantly higher for transmissions in people who inject drugs (0·37 [0·24-0·53]) and men who have sex with men (0·30 [0·33-0·40]). There was no significant difference in the probability of multiple variant transmission between male-to-female transmission (0·21 [0·14-0·31]), post-partum transmission (0·18 [0·03-0·57]), pre-partum transmission (0·17 [0·08-0·33]), and intra-partum transmission (0·27 [0·14-0·45]).

INTERPRETATION

We identified that transmissions in people who inject drugs and men who have sex with men are significantly more likely to result in an infection initiated by multiple founder variants, and female-to-male infections are significantly less probable. Quantifying how the routes of HIV infection affect the transmission of multiple variants allows us to better understand how the evolution and epidemiology of HIV-1 determine clinical outcomes.

FUNDING

Medical Research Council Precision Medicine Doctoral Training Programme and a European Research Council Starting Grant.

Identification of early-induced broadly neutralizing activities against transmitted founder HIV strains

OBJECTIVES

Broadly neutralizing antibodies have been proposed as key actors for HIV vaccine development. However, they display features of highly matured antibodies, hampering their induction by vaccination. As protective broadly neutralizing antibodies should be induced rapidly after vaccination and should neutralize the early-transmitted founder (T/F) viruses, we searched whether such antibodies may be induced following HIV infection.

DESIGN

Sera were collected during acute infection (Day 0) and at viral set point (Month 6/12) and the neutralizing activity against T/F strains was investigated. Neutralizing activity in sera collected from chronic progressor was analyzed in parallel.

METHODS

We compared neutralizing activity against T/F strains with neutralizing activity against non-T/F strains using the conventional TZM-bL neutralizing assay.

RESULTS

We found neutralizing antibodies (nAbs) preferentially directed against T/F viruses in sera collected shortly after infection. This humoral response evolved by shifting to nAbs directed against non-T/F strains.

CONCLUSION

Although features associated with nAbs directed against T/F viruses need further investigations, these early-induced nAbs may display lesser maturation characteristics; therefore, this might increase their interest for future vaccine designs.

Transmission and Drug Resistance Characteristics of Human Immunodeficiency Virus-1 Strain Using Medical Information Data Retrieval System

This study was aimed at exploring the transmission and drug resistance characteristics of acquired immunodeficiency syndrome (AIDS) caused by human immunodeficiency virus-1 (HIV-1). The query expansion algorithm based on Candecomp Parafac (CP) decomposition was adopted to construct a data information retrieval system for semantic web and tensor decomposition. In the latent variable model based on tensor decomposition, the three elements in the triples generated feature vectors to calculate the training samples. The HIV patient data set was selected to evaluate the performance of the system, and then, the HIV gene resistance of 213 patients was retrospectively analyzed based on the electronic medical records. 43 cases showed failure of ribonucleic acid drug resistance, the ART virological failure rate was 24.43% (43/213), and one case was not reported. There was 1 case of RNA hemolysis that could not be detected. There were 50 resistant cases of nonnucleotide reverse transcriptase inhibitors (NNRTI), accounting for 29.94% (50/167), and there were 17 resistant cases of nucleotide reverse transcriptase inhibitors (NRTI), accounting for 10.18% (17/167) of all mutation cases. Among the HIV-1 strains, 19 cases failed the detection of drug resistance sites in the integrase region, and mutations in the integrase region were significantly more than those in the protease region. There were 12 types of HIV-1 strains with drug-resistant mutations. The fusion technical scheme constructed in this study showed excellent performance in medical information retrieval. In this study, the characteristics of HIV-1 of AIDS patients were analyzed from different directions, and effective treatment was performed for patients, so as to provide reference for clinical diagnosis of AIDS patients.

Specific properties of shRNA-mediated CCR5 downregulation that enhance the inhibition of HIV-1 infection in combination with shRNA targeting HIV-1 rev

Treatment with RNAi against HIV-1 transcripts efficiently inhibits viral replication but induces selection of escape mutants; therefore, the CCR5 coreceptor was suggested as an additional target. Blocking viral and host transcripts improved the antiviral effect. We have used short hairpin RNA (shRNA) targeting the human CCR5 (shCCR5) or the HIV-1 rev (shRev) transcripts to demonstrate distinctive properties of anti-CCR5 shRNA: shCCR5 induced more sustained protection than shRev; partial reduction in CCR5 expression substantially decreased HIV-1 infection, and shCCR5 performed better than shRev in the mixed shRNA-treated and untreated cultures. These observations indicate that CCR5 inhibitors should be conveniently included in HIV-1 gene silencing treatment schedules when only a certain cell fraction is protected to further reduce endogenous virus in a properly ART-treated HIV-1 infected individual.

Deciphering DNA Methylation in HIV Infection

With approximately 38 million people living with HIV/AIDS globally, and a further 1.5 million new global infections per year, it is imperative that we advance our understanding of all factors contributing to HIV infection. While most studies have focused on the influence of host genetic factors on HIV pathogenesis, epigenetic factors are gaining attention. Epigenetics involves alterations in gene expression without altering the DNA sequence. DNA methylation is a critical epigenetic mechanism that influences both viral and host factors. This review has five focal points, which examines (i) fluctuations in the expression of methylation modifying factors upon HIV infection (ii) the effect of DNA methylation on HIV viral genes and (iii) host genome (iv) inferences from other infectious and non-communicable diseases, we provide a list of HIV-associated host genes that are regulated by methylation in other disease models (v) the potential of DNA methylation as an epi-therapeutic strategy and biomarker. DNA methylation has also been shown to serve as a robust therapeutic strategy and precision medicine biomarker against diseases such as cancer and autoimmune conditions. Despite new drugs being discovered for HIV, drug resistance is a problem in high disease burden settings such as Sub-Saharan Africa. Furthermore, genetic therapies that are under investigation are irreversible and may have off target effects. Alternative therapies that are nongenetic are essential. In this review, we discuss the potential role of DNA methylation as a novel therapeutic intervention against HIV.

Potent Induction of Envelope-Specific Antibody Responses by Virus-Like Particle (VLP) Immunogens Based on HIV-1 Envelopes from Patients with Early Broadly Neutralizing Responses

Longitudinal studies in HIV-1-infected individuals have indicated that 2 to 3 years of infection are required to develop broadly neutralizing antibodies. However, we have previously identified individuals with broadly neutralizing activity (bNA) in early HIV-1 infection, indicating that a vaccine may be capable of bNA induction after short periods of antigen exposure. Here, we describe 5 HIV-1 envelope sequences from individuals who have developed bNA within the first 100 days of infection (early neutralizers) and selected two of them to design immunogens based on HIV-1-Gag virus-like particles (VLPs). These VLPs were homogeneous and incorporated the corresponding envelopes (7 to 9 μg of gp120 in 10¹⁰ VLPs). Both envelopes (Envs) bound to well-characterized broadly neutralizing antibodies (bNAbs), including trimer-specific antibodies (PGT145, VRC01, and 35022). For immunogenicity testing, we immunized rabbits with the Env-VLPs or with the corresponding stabilized soluble envelope trimers. A short immunization protocol (105 days) was used to recapitulate the early nAb induction observed after HIV-1 infection in these two individuals. All VLP and trimeric envelope immunogens induced a comparably strong anti-gp120 response despite having immunized rabbits with 30 times less gp120 in the case of the Env-VLPs. In addition, animals immunized with VLP-formulated Envs induced antibodies that cross-recognized the corresponding soluble stabilized trimer and vice versa, even though no neutralizing activity was observed. Nevertheless, our data may provide a new platform of immunogens, based on HIV-1 envelopes from patients with early broadly neutralizing responses, with the potential to generate protective immune responses using vaccination protocols similar to those used in classical preventive vaccines.

IMPORTANCE It is generally accepted that an effective HIV-1 vaccine should be able to induce broad-spectrum neutralizing antibodies. Since most of these antibodies require long periods of somatic maturation in vivo, several groups are developing immunogens, based on the HIV envelope protein, that require complex and lengthy immunization protocols that would be difficult to implement in the general population. Here, we show that rabbits immunized with new envelopes (VLP formulated) from two individuals who demonstrated broadly neutralizing activity very early after infection induced specific HIV-1 antibodies after a short immunization protocol. This evidence provides the basis for generating protective immune responses with classic vaccination protocols with vaccine prototypes based on HIV envelope sequences from individuals who have developed early broadly neutralizing responses.

Complement-Opsonized HIV Modulates Pathways Involved in Infection of Cervical Mucosal Tissues: A Transcriptomic and Proteomic Study

Genital mucosal transmission is the most common route of HIV spread. The initial responses triggered at the site of viral entry are reportedly affected by host factors, especially complement components present at the site, and this will have profound consequences on the outcome and pathogenesis of HIV infection. We studied the initial events associated with host-pathogen interactions by exposing cervical biopsies to free or complement-opsonized HIV. Opsonization resulted in higher rates of HIV acquisition/infection in mucosal tissues and emigrating dendritic cells. Transcriptomic and proteomic data showed a significantly more pathways and higher expression of genes and proteins associated with viral replication and pathways involved in different aspects of viral infection including interferon signaling, cytokine profile and dendritic cell maturation for the opsonized HIV. Moreover, the proteomics data indicate a general suppression by the HIV exposure. This clearly suggests that HIV opsonization alters the initial signaling pathways in the cervical mucosa in a manner that promotes viral establishment and infection. Our findings provide a foundation for further studies of the role these early HIV induced events play in HIV pathogenesis.

HIV co-receptor-tropism: cellular and molecular events behind the enigmatic co-receptor switching

Recognition of cell-surface receptors and co-receptors is a crucial molecular event towards the establishment of HIV infection. HIV exists as several variants that differentially recognize the principal co-receptors, CCR5 and CXCR4, in different cell types, known as HIV co-receptor-tropism. The relative levels of these variants dynamically adjust to the changing host selection pressures to infect a vast repertoire of cells in a stage-specific manner. HIV infection sets in through immune cells such as dendritic cells, macrophages, and T-lymphocytes in the acute stage, while a wide range of other cells, including astrocytes, glial cells, B-lymphocytes, and epithelial cells, are infected during chronic stages. A change in tropism occurs during the transition from acute to a chronic phase, termed as co-receptor switching marked by a change in disease severity. The cellular and molecular events leading to co-receptor switching are poorly understood. This review aims to collate our present understanding of the dynamics of HIV co-receptor-tropism vis-à-vis host and viral factors, highlighting the cellular and molecular events involved therein. We present the possible correlations between virus entry, cell tropism, and co-receptor switching, speculating its consequences on disease progression, and proposing new scientific pursuits to help in an in-depth understanding of HIV biology.

HIV-1 Entry and Prospects for Protecting against Infection

Human Immunodeficiency Virus type-1 (HIV-1) establishes a latent viral reservoir soon after infection, which poses a major challenge for drug treatment and curative strategies. Many efforts are therefore focused on blocking infection. To this end, both viral and host factors relevant to the onset of infection need to be considered. Given that HIV-1 is most often transmitted mucosally, strategies designed to protect against infection need to be effective at mucosal portals of entry. These strategies need to contend also with cell-free and cell-associated transmitted/founder (T/F) virus forms; both can initiate and establish infection. This review will discuss how insight from the current model of HIV-1 mucosal transmission and cell entry has highlighted challenges in developing effective strategies to prevent infection. First, we examine key viral and host factors that play a role in transmission and infection. We then discuss preventive strategies based on antibody-mediated protection, with emphasis on targeting T/F viruses and mucosal immunity. Lastly, we review treatment strategies targeting viral entry, with focus on the most clinically advanced entry inhibitors.

Deep Profiling Identifies Selection of Nonsynonymous Amino Acid Substitutions in HIV-1 Envelope During Early Infection

Understanding the evolutionary dynamics of the viruses within an individual at or near the moment of transmission can provide critical inputs for the design of an effective vaccine for HIV infection. In this study, high-throughput sequencing technology was employed to analyze the evolutionary rate in viruses obtained at a single time point from drug-naive recently infected infants and adults in the chronic stage of disease. Gene-wise nonsynonymous (pN) and synonymous (pS) mutation rates were estimated and compared between the two groups. Significant differences were observed in the evolutionary rates between viruses in the early and late stages of infection. Higher rates of adaptive mutations in the HIV-1 envelope gene (env) were found in the chronic viruses as compared with those in the early stages of HIV infection. Conversely, percentage of nonsynonymous substitutions in env was found to be higher in recently transmitted viruses. In addition, a positive correlation was found between mutation and the evolutionary rate, and infectivity titer in recent infection. Despite the small sample size, the study identified useful information about viral evolution on transmission-associated bottlenecks. The effect of intraindividual HIV-1 evolution at the population level was highly contemporary, and the higher percentage of nonsynonymous substitutions seen in env during recent HIV-1 infection has suggested a pattern of convergent evolution leading to a positive selection for survival fitness and disease progression.

Infection with multiple HIV-1 founder variants is associated with lower viral replicative capacity, faster CD4+ T cell decline and increased immune activation during acute infection

HIV-1 transmission is associated with a severe bottleneck in which a limited number of variants from a pool of genetically diverse quasispecies establishes infection. The IAVI protocol C cohort of discordant couples, female sex workers, other heterosexuals and men who have sex with men (MSM) present varying risks of HIV infection, diverse HIV-1 subtypes and represent a unique opportunity to characterize transmitted/founder viruses (TF) where disease outcome is known. To identify the TF, the HIV-1 repertoire of 38 MSM participants' samples was sequenced close to transmission (median 21 days post infection, IQR 18-41) and assessment of multivariant infection done. Patient derived gag genes were cloned into an NL4.3 provirus to generate chimeric viruses which were characterized for replicative capacity (RC). Finally, an evaluation of how the TF virus predicted disease progression and modified the immune response at both acute and chronic HIV-1 infection was done. There was higher prevalence of multivariant infection compared with previously described heterosexual cohorts. A link was identified between multivariant infection and replicative capacity conferred by gag, whereby TF gag tended to be of lower replicative capacity in multivariant infection (p = 0.02) suggesting an overall lowering of fitness requirements during infection with multiple variants. Notwithstanding, multivariant infection was associated with rapid CD4+ T cell decline and perturbances in the CD4+ T cell and B cell compartments compared to single variant infection, which were reversible upon control of viremia. Strategies aimed at identifying and mitigating multivariant infection could contribute toward improving HIV-1 prognosis and this may involve strategies that tighten the stringency of the transmission bottleneck such as treatment of STI. Furthermore, the sequences and chimeric viruses help with TF based experimental vaccine immunogen design and can be used in functional assays to probe effective immune responses against TF.

Sub-attomole detection of HIV-1 using padlock probes and rolling circle amplification combined with microfluidic affinity chromatography

Despite significant progress in diagnostics and disease management during the past decades, human immunodeficiency virus (HIV) infections are still responsible for nearly 1 million deaths every year, mostly in resource-limited settings. Thus, novel, accurate and cost-effective tools for viral load monitoring become crucial to allow specific diagnostics and the effective monitoring of the associated antiviral therapies. Herein, we report an effective combination of a (1) padlock probe (PLP)-mediated rolling circle amplification (RCA) bioassay and an (2) agarose bead-based microfluidic device for the affinity chromatography-based capture and detection of RCA products (RCPs) pre-labelled simultaneously with biotin and an organic fluorophore. This method allowed the efficient capture of ~1 μm-sized RCPs followed by their quantification either as discrete signals or an average fluorescence signal, thus being compatible with both high-resolution imaging for maximum sensitivity as well as simpler optical detection setups. A limit of detection < 30 fM was obtained for HIV-1 synthetic target with just a single round of RCA, comparable to recently reported procedures requiring technically complex amplification strategies such as hyperbranching and/or enzymatic digestion/amplification. Furthermore, targeting a set of five conserved regions in the HIV-1 gag gene, the method could specifically detect HIV-1 in 293T cell culture supernatants, as well as a set of 11 HIV-1 NIH reference samples with four different subtypes. The reported method provides simplicity of operation, unique versatility of signal transduction (i.e. average or discrete signals), and potential coupling with previously reported miniaturized photodetectors. These combined features hold promise for bringing RCA-based molecular diagnostics closer to the point-of-care.

An in vitro dataset on infectious potential of transmitted/founder (TF) and non-TF (NT) HIV-1 viruses generated from Interferon alpha-treated primary CD4+ T cells

This data article describes the infectivity of transmitted/founder (TF) and non-TF (NT) HIV-1 viruses derived from primary CD4⁺ T cells treated with or without IFN-α, over a period of 12 days. TF and NT viruses described in this article were derived from the same individual (one of each from 8 infants who acquired HIV infection through mother-to-child transmission (MTCT). IFN-α resistance to both TF and NT viruses was studied by infecting TZM-bl cells and measuring luciferase expression (expressed as relative light units, RLU). Measurement of luciferase expression is extremely sensitive and allows quantification of even small changes in gene expression at the transcriptional level.

Understanding the mechanisms driving the spread of subtype C HIV-1

Human immunodeficiency virus type 1 (HIV-1) subtype C (C-HIV) is the most prevalent form of HIV-1 globally, accounting for approximately 50% of infections worldwide. C-HIV is the predominant and near-exclusive subtype in the low resource regions of India and Southern Africa. Given the vast diversity of HIV-1 subtypes, it is curious as to why C-HIV constitutes such a large proportion of global infections. This enriched prevalence may be due to phenotypic differences between C-HIV isolates and other viral strains that permit enhanced transmission efficiency or, pathogenicity, or might due to the socio-demographics of the regions where C-HIV is endemic. Here, we compare the mechanisms of C-HIV pathogenesis to less prominent HIV-1 subtypes, including viral genetic and phenotypic characteristics, and host genetic variability, to understand whether evolutionary factors drove C-HIV to predominance.

In vitro replicative fitness of early Transmitted founder HIV-1 variants and sensitivity to Interferon alpha

Type I interferons, particularly interferon-alpha (IFN-α), play a vital role in the host's anti-viral defenses by interfering with viral replication. However, the virus rapidly evolves to exploit the IFN-α response for its replication, spread, and pathogenic function. In this study, we attempted to determine IFN-α susceptibility and productivity of infectious transmitted/founder (TF) (n = 8) and non-transmitted (NT) viruses (n = 8) derived from HIV-1 infected infants. Independent experiments were carried out to determine IFN-α resistance, replication fitness, and viral productivity in CD4⁺ T cells over a short period. In vitro studies showed that TF viruses were resistant to IFN-α during the very near moment of transmission, but in the subsequent time points, they became susceptible to IFN-α. We did not observe much difference in replicative fitness of the TF viruses in cultures treated with and without IFN-α, but the difference was significant in the case of NT viruses obtained from the same individual. Despite increased susceptibility to IFN-α, NT viruses produced more viral particles than TF viruses. Similar results were also obtained in cultures treated with maraviroc (MVC). The study identified unique characteristics of TF viruses thus prompting further investigation into virus-host interaction occurring during the early stages of HIV infection.

Coreceptor Tropism and Maraviroc Sensitivity of Clonally Derived Ethiopian HIV-1C Strains Using an in-house Phenotypic Assay and Commonly Used Genotypic Methods

OBJECTIVES

Genotypic Tropism Testing (GTT) tools are generally developed based on HIV-1 subtype B (HIV-1B) and used for HIV-1C as well but with a large discordance of prediction between different methods. We used an established phenotypic assay for comparison with GTT methods and for the determination of in vitro maraviroc sensitivity of pure R5-tropic and dual-tropic HIV-1C.

METHODS

Plasma was obtained from 58 HIV-1C infected Ethiopians. Envgp120 was cloned into a luciferase tagged NL4-3 plasmid. Phenotypic tropism was determined by in house method and the V3 sequences were analysed by five GTT methods. In vitro maraviroc sensitivity of R5-tropic and dual-tropic isolates were compared in the TZMbl cell-line.

RESULTS

The phenotypes were classified as R5 in 92.4% and dual tropic (R5X4) in 7.6% of 79 clones. The concordance between phenotype and genotype ranged from 64.7% to 84.3% depending on the GTT method. Only 46.9% of the R5 phenotypes were predicted as R5 by all GTT tools while R5X4 phenotypes were predicted as X4 by four methods, but not by Raymond's method. All six tested phenotypic R5 clones, as well as five of six of dual tropic clones, showed a dose response to maraviroc.

CONCLUSION

There is a high discordance between GTT methods, which underestimates the presence of R5 and overestimates X4 strains compared to a phenotypic assay. Currently available GTT algorithms should be further improved for tropism prediction in HIV-1C. Maraviroc has an in vitro activity against most HIV-1C viruses and could be considered as an alternative regimen in individuals infected with CCR5-tropic HIV-1C viruses.

Variable infectivity and conserved engagement in cell-to-cell viral transfer by HIV-1 Env from Clade B transmitted founder clones

HIV-1 transmission is usually initiated by a single viral strain called transmitted/ founder (T/F) virus. In in vitro models, HIV-1 can efficiently spread via cell-free and virological synapse (VS)-mediated cell-to-cell infection. Both modes of infection require the viral glycoprotein Envelope (Env). The efficiency with which T/F Envs initiate VS and mediate cell-to-cell infection has not been well characterized. Here we tested a panel of isogenic HIV-1 molecular clones that carry different Clade B T/F Envs. We found that despite variable infectivity among different Env clones in the two modes of infection, T/F Envs generally mediated efficient VS formation and subsequent cell-to-cell transfer. In contrast, in vitro infectivity of the T/F Env clones was more variable and strongly correlated with intrinsic fusogenicity of various Envs. We speculate that the conservation of cell-to-cell transfer by T/F Env is indicative of a biologically important function of Env.