Platelet-Released Factors: Their Role in Viral Disease and Applications for Extracellular Vesicle (EV) Therapy

Abstract

Platelets, which are small anuclear cell fragments, play important roles in thrombosis and hemostasis, but also actively release factors that can both suppress and induce viral infections. Platelet-released factors include sCD40L, microvesicles (MVs), and alpha granules that have the capacity to exert either pro-inflammatory or anti-inflammatory effects depending on the virus. These factors are prime targets for use in extracellular vesicle (EV)-based therapy due to their ability to reduce viral infections and exert anti-inflammatory effects. While there are some studies regarding platelet microvesicle-based (PMV-based) therapy, there is still much to learn about PMVs before such therapy can be used. This review provides the background necessary to understand the roles of platelet-released factors, how these factors might be useful in PMV-based therapy, and a critical discussion of current knowledge of platelets and their role in viral diseases.

Keywords: microparticles; microvesicle; platelets; therapy; vesicles; virus.

Figure 1

Viral entry and activation of platelets. 1. Human immunodeficiency virus-1 (HIV-1), dengue virus (DENV), influenza A virus (IAV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) bind receptors on the platelet plasma membrane as well as within endosomes. Receptors involved include CXCR/CCR receptors, DC-SIGN, CLEC 2, FCγRIIα, ACE2, and TLRs 3/4/7/8. 2. Upon viral binding, viruses are internalized into the platelets. 3. Both viral binding and virus internalization lead to platelet activation. 4. Platelet activation results in platelet effector functions, such as 4a. α-granule release, 4b. release of soluble CD40L (sCD40L), and 4c. extracellular vesicle (EV) and microvesicle (MV) release. Created with BioRender.com (30 December 2021).

Figure 2

Role of sCD40L in Viral Infections. One major source of sCD40L in human plasma is platelets. In human immunodeficiency virus (HIV) infection, sCD40L increases leukocyte adhesion to the blood brain barrier (BBB), perhaps leading to extravasation and increases in regulatory T cell differentiation, which potentially leads to a reduction in T-cell responses. The administration of combination anti-retroviral therapy (cART) reduces plasma sCD40L levels. In dengue virus (DENV) infection, high levels of plasma sCD40L is associated with severe hemorrhagic dengue with plasma leakage. In influenza A virus (IAV), a high plasma sCD40L level is associated with severe disease and influenza-associated encephalopathy. In SARS-CoV-2 infection, high levels of plasma sCD40L was found in intensive care unit (ICU) cases compared to lower plasma sCD40L levels found in non-ICU cases. Created with BioRender.com (9 February 2022).

Figure 3

The Role of Platelet Microvesicles (PMVs) in Viral Infection. PMVs are a type of extracellular vesicle derived from activated platelets. In human immunodeficiency virus (HIV) infection, PMVs expressing CXCR4 have been shown in vitro to deliver CXCR4 upon fusion to other cells. Expressing CXCR4 on their surface, these cells are potentially more susceptible to HIV infection, perpetuating disease progression. In dengue virus (DENV) infection, PMVs are associated with increases in neutrophil functions, such as netosis as well as an increase in pro-inflammatory cytokines. Additionally, circulating PMVs are associated with non-severe DENV infection and are reduced in patients with severe disease, suggesting a beneficial role of PMVs in disease resolution. In both influenza A virus (IAV) and SARS-CoV-2 infections, mortality is associated with an increase in PMVs, suggesting a role of PMVs in promoting disease severity. Created with BioRender.com (9 February 2022).

Figure 4

Role of α-Granules in Viral Infection. α-granule released from activated platelets contain CXCL4, a cytokine. In human immunodeficiency virus (HIV) infection, CXCL4 binds the major HIV surface protein gp120, which protects against cell infection. In dengue virus (DENV) infection, CXCL4 is known to promote inflammation and severe dengue. Anti-CXCL4 reduces inflammation and DENV infection in vitro. In influenza A virus (IAV) infection, CXCL4 recruits neutrophils to the lungs, which helps fight against infection. In SARS-CoV-2 infection, decreased plasma CXCL4 is correlated with poor disease progression, indicating that CXCL4 plays a protective role. Created with BioRender.com (9 February 2022).

Figure 5

Platelet Extracellular Vesicle (pEV) Therapy for Treating Viral Diseases. 1. Platelet rich plasma (PRP) is isolated from healthy donors. 2. Platelets are isolated from PRP. 3. Surface engineering of platelets could involve the incorporation of receptors aimed at carrying out specific functions to treat a disease. In vitro culture methods are used to generate pEVs. 4. pEV cargo loading involves small drugs, molecules, and RNA. EV surface targeting includes antibodies, inflammatory cytokine receptors, and proteins that specifically target certain organs. After cargo loading, pEVs would be isolated to a clinical grade-level and administered to treat patients with severe viral infections. Created with BioRender.com (9 February 2022).