PSGL-1 restricts HIV-1 infectivity by blocking virus particle attachment to target cells | Yajing Fu et al. | ResearchPod

Transcript: PSGL-1 restricts HIV-1 infectivity by blocking virus particle attachment to target cells

Alex: HIV particles can leave a cell and still fail to attach to their next target. This study finds that a host protein carried on those particles can prevent that first contact. The distinction is between making virus particles and making particles that can infect.

Sam: That puts the problem between one infected cell and the next. Where does the finding come from, and who should care?

Alex: It comes from researchers reporting an experimental study in PNAS, titled “PSGL-1 restricts HIV-1 infectivity by blocking virus particle attachment to target cells.” For researchers studying viral entry or host defenses, it identifies a restriction that particle-production measurements alone could miss.

Sam: So we'll follow how they separate particle production from infectivity, what supports the attachment mechanism, and where the evidence stops.

Alex: First, the protein. P-selectin glycoprotein ligand-1, shortened to PSGL-1, normally helps immune cells adhere and move into inflamed tissues. It has a long, heavily sugar-coated portion projecting outside the cell. The study asks what happens when that protein gets incorporated into newly assembled HIV particles.

Sam: Earlier work already called it an antiviral factor. What was unresolved enough to need these experiments?

Alex: Previous studies reported that it inhibited HIV replication and reduced the infectivity of newly produced particles. One report also placed an effect early after entry, during reverse transcription, when HIV copies its genetic material into DNA. How the protein disabled newly made particles remained unclear.

Sam: Those possibilities put the action in different cells. How did the authors distinguish protection of the receiving cell from damage to the virus it produces?

Alex: They separated those roles experimentally. They first infected cells engineered to express PSGL-1 with virus made without it. In an infection restricted to a single cycle, expression in the receiving cells did not inhibit infection in their system.

Sam: But those cells could still produce less-infectious offspring. A spreading infection would reveal that even if the first round looked normal.

Alex: That was the pattern. PSGL-1 inhibited spreading infection over fourteen days. But the single-cycle experiment showed no block through particle assembly and release. The key question became whether the released particles were defective.

Sam: If one preparation simply contained fewer particles, it would look less infectious. How did they account for that?

Alex: They matched preparations using p24, an HIV structural protein used here to normalize virus input. Then they applied those matched inputs to reporter cells that signal infection. PSGL-1 expression in the producer cells almost completely abolished the infectivity of released particles at higher tested doses.

PSGL-1 restricts HIV-1 infectivity by blocking virus particle attachment to target cells

Yajing Fu, Sijia He, Abdul A. Waheed, Deemah Dabbagh, Zheng Zhou, Benjamin Trinité, Zhao Wang, Jieshi Yu, Dan Wang, Feng Li, David N. Levy, Hong Shang, Eric O. Freed, Yuntao Wu

Apr 28, 2020·8:00·zotero·Original Paper

PSGL-1HIV-1 InfectivityAntiviral Host FactorVpuLeukocyte TetheringHIV-1 Restriction

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8 min

Summary

Research Question

P-selectin glycoprotein ligand-1 (PSGL-1) is a surface glycoprotein expressed on blood CD4 T cells that is known to be up-regulated during inflammation. While previous research has established that PSGL-1 expression correlates with reduced HIV-1 replication, the precise mechanism behind this antiviral effect remained unclear. This study investigated how PSGL-1 interferes with the HIV-1 life cycle and whether this mechanism extends to other viruses.

Approach

The researchers used a variety of assays, including flow cytometry, Western blotting, and viral infectivity measurements, to monitor the interaction between PSGL-1 and HIV-1. By stable expression of PSGL-1 in virus-producer cells and knockdown of endogenous PSGL-1 in T-cell lines, they examined the impact on virion assembly, release, and subsequent infectivity. They also used domain-deletion mutagenesis to map the functional regions of the PSGL-1 protein required for its antiviral activity and performed cross-virus experiments using murine leukemia virus (MLV) and influenza A to test for broad-spectrum activity.

Main Findings

Mechanism of Inhibition: PSGL-1 does not inhibit the release of HIV-1 particles from infected cells. Instead, it is incorporated into the virions themselves. Once in the virion, the elongated and heavily glycosylated extracellular domain of PSGL-1 acts as a steric hinderance, physically blocking the virus from attaching to target cells.
Broad-Spectrum Activity: This restrictive mechanism is not unique to HIV-1. PSGL-1 successfully inhibited the infectivity of other enveloped viruses, including MLV and influenza A, suggesting a common vulnerability among viruses that acquire surface proteins from host cells.
Viral Evasion: HIV-1 has evolved mechanisms to counteract this restriction. The viral accessory protein Vpu is primarily responsible for inducing the degradation of PSGL-1, while Nef also contributes to reducing its cell-surface expression, allowing the virus to partially bypass the restrictive barrier.
Shared Selectin Ligands: The researchers found that CD43, a related selectin-binding glycoprotein, mimics this activity and inhibits HIV-1 infectivity with similar potency to PSGL-1, pointing to a shared property of these mucin-like proteins in blocking viral attachment.

Why It Matters

This study reveals a novel, mechanical form of intrinsic immunity where a host factor acts as a physical roadblock to viral entry. Understanding how PSGL-1 restricts such a wide range of enveloped viruses provides important insights into the host-pathogen arms race and suggests that targeting the interaction between viruses and these host restriction factors could lead to new therapeutic strategies.

Key Terms and Definitions

PSGL-1 — A dimeric, mucin-like glycoprotein that normally mediates leukocyte tethering to endothelium but also acts as an antiviral restriction factor by blocking viral particle attachment.
Steric Hindrance — A mechanism where the physical size and bulky, elongated structure of a molecule (such as PSGL-1 on a virion surface) prevents necessary biological interactions, like viral binding to target cells.
Vpu — An HIV-1 accessory protein that targets host proteins like PSGL-1 for degradation to assist in viral escape from host defenses.
Virion Infectivity — The capacity of a released virus particle to successfully enter and initiate replication in a new target cell.
Selectin Ligand — A class of glycoproteins (including PSGL-1 and CD43) that bind to selectins and are characterized by extensive glycosylation and extended, rod-like structures.

Abstract

Significance PSGL-1 and CD43 are surface glycoproteins expressed on blood CD4 T cells to bind to selectins for T cell tethering, rolling, and migration into inflamed tissues. The PSGL-1 level is greatly up-regulated during inflammation. Here we found that PSGL-1 and CD43 expression inhibits HIV spreading infection. Mechanistically, PSGL-1 blocks the binding of virus particles to target cells. PSGL-1–mediated suppression of virus infectivity extends to another retrovirus—murine leukemia virus—and to influenza A virus. These results further our understanding of virus–host interactions and help elucidate mechanisms by which cellular host factors regulate viral infection and pathogenesis. , P-selectin glycoprotein ligand-1 (PSGL-1) is a dimeric, mucin-like, 120-kDa glycoprotein that binds to P-, E-, and L-selectins. PSGL-1 is expressed primarily on the surface of lymphoid and myeloid cells and is up-regulated during inflammation to mediate leukocyte tethering and rolling on the surface of endothelium for migration into inflamed tissues. Although it has been reported that PSGL-1 expression inhibits HIV-1 replication, the mechanism of PSGL-1–mediated anti-HIV activity remains to be elucidated. Here we report that PSGL-1 in virions blocks the infectivity of HIV-1 particles by preventing the binding of particles to target cells. This inhibitory activity is independent of the viral glycoprotein present on the virus particle; the binding of particles bearing the HIV-1 envelope glycoprotein or vesicular stomatitis virus G glycoprotein or even lacking a viral glycoprotein is impaired by PSGL-1. Mapping studies show that the extracellular N-terminal domain of PSGL-1 is necessary for its anti–HIV-1 activity, and that the PSGL-1 cytoplasmic tail contributes to inhibition. In addition, we demonstrate that the PSGL-1–related monomeric E-selectin–binding glycoprotein CD43 also effectively blocks HIV-1 infectivity. HIV-1 infection, or expression of either Vpu or Nef, down-regulates PSGL-1 from the cell surface; expression of Vpu appears to be primarily responsible for enabling the virus to partially escape PSGL-1–mediated restriction. Finally, we show that PSGL-1 inhibits the infectivity of other viruses, such as murine leukemia virus and influenza A virus. These findings demonstrate that PSGL-1 is a broad-spectrum antiviral host factor with a unique mechanism of action.

PSGL-1 restricts HIV-1 infectivity by blocking virus particle attachment to target cells: Research Paper Podcast