Virus-associated host CD62L increases attachment of human immunodeficiency virus type 1 to endothelial cells and enhances trans infection of CD4+ T lymphocytes

Host Molecule Incorporation into HIV Virions, Potential Influences in HIV Pathogenesis

During the last phase of HIV viral production, nascent HIV virions acquire a fraction of the cellular lipid membrane to create the external lipid envelope, a process by which cellular proteins present on the surface of the infected cell can be incorporated along with Env trimers. Interestingly, several studies indicated that these incorporated host molecules could conserve their biological activity and consequently contribute to HIV pathogenesis either by enhancing the infectivity of HIV virions, their tissue tropism or by affecting immune cell functions. The following review will describe the main approaches used to characterize membrane bound host molecule incorporation into HIV virions, the proposed mechanisms involved, and the role of a non-exhaustive list of incorporated molecules.

Active PD-L1 incorporation within HIV virions functionally impairs T follicular helper cells

The limited development of broadly neutralizing antibodies (BnAbs) during HIV infection is classically attributed to an inadequate B-cell help brought by functionally impaired T follicular helper (Tfh) cells. However, the determinants of Tfh-cell functional impairment and the signals contributing to this condition remain elusive. In the present study, we showed that PD-L1 is incorporated within HIV virions through an active mechanism involving p17 HIV matrix protein. We subsequently showed that in vitro produced PD-L1high but not PD-L1low HIV virions, significantly reduced Tfh-cell proliferation and IL-21 production, ultimately leading to a decreased of IgG1 secretion from GC B cells. Interestingly, Tfh-cell functions were fully restored in presence of anti-PD-L1/2 blocking mAbs treatment, demonstrating that the incorporated PD-L1 proteins were functionally active. Taken together, the present study unveils an immunovirological mechanism by which HIV specifically exploits the regulatory potential of PD-L1 to suppress the immune system during the course of HIV infection.

During HIV infection, the development of effective BnAbs remains a rare phenomenon, occurring in only 15–20% of HIV-infected individuals after years of infection. Although multiple mechanisms may be involved, recent studies have suggested that functional impairment of Tfh cells, through immune checkpoint (IC)/IC-Ligand (IC-L) interactions, may lead to a decrease in B-cell help leading to low BnAbs production. Our laboratory recently showed that PD-L1 was predominantly expressed on lymph node (LN) migratory dendritic cells located predominantly in extra-follicular areas, implying that the source of IC-L contributing to Tfh-cell functional impairment may be independent of cellular expression of IC-L. These observations prompted us to investigate the potential contribution of IC-L incorporated within HIV virion envelope to Tfh-cell functional impairment. We subsequently demonstrated that PD-L1 was incorporated into a large fraction of HIV virions in the plasma of viremic HIV-infected individuals. Interestingly, PD-L1 remains active when incorporated into HIV virions envelope and could impaired Tfh-cell proliferation, resulting in decreased IgG1 production by B cells in vitro. These findings demonstrate an unsuspected mechanism contributing to the regulation of Tfh-cell function, which may contribute to the low production of BnAbs by B cells during HIV infection.

P-selectin glycoprotein ligand-1 (PSGL-1/CD162) is incorporated into clinical HIV-1 isolates and can mediate virus capture and subsequent transfer to permissive cells

Background

P-selectin glycoprotein ligand-1 (PSGL-1/CD162) has been studied extensively for its role in mediating leukocyte rolling through interactions with its cognate receptor, P-selectin. Recently, PSGL-1 was identified as a novel HIV-1 host restriction factor, particularly when expressed at high levels in the HIV envelope. Importantly, while the potent antiviral activity of PSGL-1 has been clearly demonstrated in various complementary model systems, the breadth of PSGL-1 incorporation across genetically diverse viral isolates and clinical isolates has yet to be described. Additionally, the biological activity of virion-incorporated PSGL-1 has also yet to be shown.

Results

Herein we assessed the levels of PSGL-1 on viruses produced through transfection with various amounts of PSGL-1 plasmid DNA (0–250 ng), compared to levels of PSGL-1 on viruses produced through infection of T cell lines and primary PBMC. We found that very low levels of PSGL-1 plasmid DNA (< 2.5 ng/well) were necessary to generate virus models that could closely mirror the phenotype of viruses produced via infection of T cells and PBMC. Unique to this study, we show that PSGL-1 is incorporated in a broad range of HIV-1 and SIV isolates and that virions with incorporated PSGL-1 are detectable in plasma from viremic HIV-1-infected individuals, corroborating the relevance of PSGL-1 in natural infection. Additionally, we show that PSGL-1 on viruses can bind its cognate selectin receptors, P-, E-, and L-selectins. Finally, we show viruses with endogenous levels of PSGL-1 can be captured by P-selectin and transferred to HIV-permissive bystander cells, highlighting a novel role for PSGL-1 in HIV-1 infection. Notably, viruses which contained high levels of PSGL-1 were noninfectious in our hands, in line with previous findings reporting the potent antiviral activity of PSGL-1.

Conclusions

Our results indicate that levels of PSGL-1 incorporation into virions can vary widely among model systems tested, and that careful tailoring of plasmid levels is required to recapitulate physiological systems when using pseudovirus models. Taken together, our data suggest that PSGL-1 may play diverse roles in the physiology of HIV-1 infection, particularly due to the functionally active state of PSGL-1 on virion surfaces and the breadth of PSGL-1 incorporation among a wide range of viral isolates.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12977-022-00593-5.

Platelet activation by SARS-CoV-2 implicates the release of active tissue factor by infected cells

Platelets are hyperactivated in coronavirus disease 2019 (COVID-19). However, the mechanisms promoting platelet activation by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are not well understood. This may be due to inherent challenges in discriminating the contribution of viral vs host components produced by infected cells. This is particularly true for enveloped viruses and extracellular vesicles (EVs), as they are concomitantly released during infection and share biophysical properties. To study this, we evaluated whether SARS-CoV-2 itself or components derived from SARS-CoV-2-infected human lung epithelial cells could activate isolated platelets from healthy donors. Activation was measured by the surface expression of P-selectin and the activated conformation of integrin αIIbβ3, degranulation, aggregation under flow conditions, and the release of EVs. We find that neither SARS-CoV-2 nor purified spike activates platelets. In contrast, tissue factor (TF) produced by infected cells was highly potent at activating platelets. This required trace amounts of plasma containing the coagulation factors FX, FII, and FVII. Robust platelet activation involved thrombin and the activation of protease-activated receptor (PAR)-1 and -4 expressed by platelets. Virions and EVs were identified by electron microscopy. Through size-exclusion chromatography, TF activity was found to be associated with a virus or EVs, which were indistinguishable. Increased TF messenger RNA (mRNA) expression and activity were also found in lungs in a murine model of COVID-19 and plasma of severe COVID-19 patients, respectively. In summary, TF activity from SARS-CoV-2–infected cells activates thrombin, which signals to PARs on platelets. Blockade of molecules in this pathway may interfere with platelet activation and the coagulation characteristic of COVID-19.

Endothelial Cells Promote Productive HIV Infection of Resting CD4+ T Cells by an Integrin-Mediated Cell Adhesion-Dependent Mechanism

Resting CD4⁺ T cells are primary targets of early HIV infection events in vivo, but do not readily support HIV replication in vitro. This barrier to infection can be overcome by exposing resting CD4⁺ T cells to endothelial cells (ECs). ECs line blood vessels and direct T cell trafficking into inflamed tissues. Cell trafficking pathways have been shown to have overlapping roles in facilitating HIV replication, but their relevance to EC-mediated enhancement of HIV susceptibility in resting CD4⁺ T cells has not previously been examined. We characterized the phenotype of primary human resting CD4⁺ T cells that became productively infected with HIV when cocultured with primary human blood and lymphatic ECs. The infected CD4⁺ T cells were primarily central memory cells enriched for high expression of the integrins LFA-1 and VLA-4. ICAM-1 and VCAM-1, the cognate ligands for LFA-1 and VLA-4, respectively, were expressed by the ECs in the coculture. Blocking LFA-1 and VLA-4 on resting CD4⁺ T cells inhibited infection by 65.4%–96.9%, indicating that engagement of these integrins facilitates EC-mediated enhancement of productive HIV infection in resting CD4⁺ T cells. The demonstration that ECs influence cellular HIV susceptibility of resting memory CD4⁺ T cells through cell trafficking pathways engaged during the transmigration of T cells into tissues highlights the physiological relevance of these findings for HIV acquisition and opportunities for intervention.

Identification of the SHREK Family of Proteins as Broad-Spectrum Host Antiviral Factors

Mucins and mucin-like molecules are highly glycosylated, high-molecular-weight cell surface proteins that possess a semi-rigid and highly extended extracellular domain. P-selectin glycoprotein ligand-1 (PSGL-1), a mucin-like glycoprotein, has recently been found to restrict HIV-1 infectivity through virion incorporation that sterically hinders virus particle attachment to target cells. Here, we report the identification of a family of antiviral cellular proteins, named the Surface-Hinged, Rigidly-Extended Killer (SHREK) family of virion inactivators (PSGL-1, CD43, TIM-1, CD34, PODXL1, PODXL2, CD164, MUC1, MUC4, and TMEM123) that share similar structural characteristics with PSGL-1. We demonstrate that SHREK proteins block HIV-1 infectivity by inhibiting virus particle attachment to target cells. In addition, we demonstrate that SHREK proteins are broad-spectrum host antiviral factors that block the infection of diverse viruses such as influenza A. Furthermore, we demonstrate that a subset of SHREKs also blocks the infectivity of a hybrid alphavirus-SARS-CoV-2 (Ha-CoV-2) pseudovirus. These results suggest that SHREK proteins may be a part of host innate immunity against enveloped viruses.

Identification of the SHREK family of proteins as broad-spectrum host antiviral factors

Mucins and mucin-like molecules are highly glycosylated, high-molecular-weight cell surface proteins that possess a semi-rigid and highly extended extracellular domain. P-selectin glycoprotein ligand-1 (PSGL-1), a mucin-like glycoprotein, has recently been found to restrict HIV-1 infectivity through virion incorporation that sterically hinders virus particle attachment to target cells. Here, we report the identification of a family of antiviral cellular proteins, named the Surface-Hinged, Rigidly-Extended Killer (SHREK) family of virion inactivators (PSGL-1, CD43, TIM-1, CD34, PODXL1, PODXL2, CD164, MUC1, MUC4, and TMEM123), that share similar structural characteristics with PSGL-1. We demonstrate that SHREK proteins block HIV-1 infectivity by inhibiting virus particle attachment to target cells. In addition, we demonstrate that SHREK proteins are broad-spectrum host antiviral factors that block the infection of diverse viruses such as influenza A. Furthermore, we demonstrate that a subset of SHREKs also blocks the infectivity of a hybrid alphavirus-SARS-CoV-2 virus-like particle. These results suggest that SHREK proteins may be a part of host innate immunity against enveloped viruses.

The Incorporation of Host Proteins into the External HIV-1 Envelope

The incorporation of biologically active host proteins into HIV-1 is a well-established phenomenon, particularly due to the budding mechanism of viral egress in which viruses acquire their external lipid membrane directly from the host cell. While this mechanism might seemingly imply that host protein incorporation is a passive uptake of all cellular antigens associated with the plasma membrane at the site of budding, this is not the case. Herein, we review the evidence indicating that host protein incorporation can be a selective and conserved process. We discuss how HIV-1 virions displaying host proteins on their surface can exhibit a myriad of altered phenotypes, with notable impacts on infectivity, homing, neutralization, and pathogenesis. This review describes the canonical and emerging methods to detect host protein incorporation, highlights the well-established host proteins that have been identified on HIV-1 virions, and reflects on the role of these incorporated proteins in viral pathogenesis and therapeutic targeting. Despite many advances in HIV treatment and prevention, there remains a global effort to develop increasingly effective anti-HIV therapies. Given the broad range of biologically active host proteins acquired on the surface of HIV-1, additional studies on the mechanisms and impacts of these incorporated host proteins may inform the development of novel treatments and vaccine designs.

Prior mucosal exposure to heterologous cells alters the pathogenesis of cell-associated mucosal feline immunodeficiency virus challenge

BACKGROUND

Several lines of research suggest that exposure to cellular material can alter the susceptibility to infection by HIV-1. Because sexual contact often includes exposure to cellular material, we hypothesized that repeated mucosal exposure to heterologous cells would induce an immune response that would alter the susceptibility to mucosal infection. Using the feline immunodeficiency virus (FIV) model of HIV-1 mucosal transmission, the cervicovaginal mucosa was exposed once weekly for 12 weeks to 5,000 heterologous cells or media (control) and then cats were vaginally challenged with cell-associated or cell-free FIV.

RESULTS

Exposure to heterologous cells decreased the percentage of lymphocytes in the mucosal and systemic lymph nodes (LN) expressing L-selectin as well as the percentage of CD4+ CD25+ T cells. These shifts were associated with enhanced ex-vivo proliferative responses to heterologous cells. Following mucosal challenge with cell-associated, but not cell-free, FIV, proviral burden was reduced by 64% in cats previously exposed to heterologous cells as compared to media exposed controls.

CONCLUSIONS

The pathogenesis and/or the threshold for mucosal infection by infected cells (but not cell-free virus) can be modulated by mucosal exposure to uninfected heterologous cells.

Human hepatoma cells transmit surface bound HIV-1 to CD4+ T cells through an ICAM-1/LFA-1-dependent mechanism

BACKGROUND

During the viremic phase of human immunodeficiency virus type-1 (HIV-1) infection, hepatocytes are likely to be constantly exposed to circulating virions. Knowing that a contact between hepatocytes and CD4+ T lymphocytes is favoured by the local slow blood flow present within the liver, we hypothesize that hepatic cells can act as a viral reservoir and thus contribute to HIV-1 propagation.

RESULTS

We report that human hepatoma cells bind and internalize HIV-1 particles. Infection of CD4+ T cells was found to be much more efficient following a contact with virus-loaded hepatocytes than with cell-free virus. Additional studies suggest that infection of CD4+ T cells in trans with hepatocytes carrying virus is primarily due to surface bound HIV-1 particles and relies on LFA-1/ICAM-1 interactions.

CONCLUSION

This work represents the first demonstration by which circulating CD4+ T cells can be potentially infected with HIV-1 through a contact with hepatocytes in the liver.

Cellular proteins detected in HIV-1

It has been known for some time that HIV-1 virions contain cellular proteins in addition to proteins encoded by the viral genome. Recent studies have vastly increased the number of host proteins detected in HIV-1. This review summarises the current findings on several cellular proteins present in these virions, including some functional studies on their potential roles in the viral replication cycle and pathogenesis. Because retroviruses require extensive assistance from host proteins and pathways, the data from biochemical characterisations of HIV-1 serve as an important starting point for understanding the role of cellular proteins that act in or influence the biology of HIV-1. Additionally, a better understanding of the interactions between cellular proteins and viral components might provide more targets for anti-HIV therapeutic intervention and provide for a better understanding of how HIV-1 alters the immune system. The extensive study of HIV-1 has already brought new insights to the fields of immunology and vaccine science. In the same way, knowledge of viral--cellular protein interactions might assist our understanding of important cellular pathways.