Manipulation of Oxidative Stress Responses by Non-Thermal Plasma to Treat Herpes Simplex Virus Type 1 Infection and Disease

Abstract

Herpes simplex virus type 1 (HSV-1) is a contagious pathogen with a large global footprint, due to its ability to cause lifelong infection in patients. Current antiviral therapies are effective in limiting viral replication in the epithelial cells to alleviate clinical symptoms, but ineffective in eliminating latent viral reservoirs in neurons. Much of HSV-1 pathogenesis is dependent on its ability to manipulate oxidative stress responses to craft a cellular environment that favors HSV-1 replication. However, to maintain redox homeostasis and to promote antiviral immune responses, the infected cell can upregulate reactive oxygen and nitrogen species (RONS) while having a tight control on antioxidant concentrations to prevent cellular damage. Non-thermal plasma (NTP), which we propose as a potential therapy alternative directed against HSV-1 infection, is a means to deliver RONS that affect redox homeostasis in the infected cell. This review emphasizes how NTP can be an effective therapy for HSV-1 infections through the direct antiviral activity of RONS and via immunomodulatory changes in the infected cells that will stimulate anti-HSV-1 adaptive immune responses. Overall, NTP application can control HSV-1 replication and address the challenges of latency by decreasing the size of the viral reservoir in the nervous system.

Keywords: adaptive immunity; antioxidant; antiviral therapy; immunomodulation; innate immunity; oxidative stress; plasma; reactive oxygen and nitrogen species; redox homeostasis.

Figure 1

The HSV-1 lifecycle is characterized by the cycle between lytic infection and latent infection over the course of a patient’s life. Infection with HSV-1 is known for causing cold sores that can appear around the mouth, eyes, and genitalia. These clinical manifestations are characteristic of lytic infection, when HSV-1 is actively replicating and producing new viruses in mucosal epithelial cells in these areas. It is at this point in infection where antiviral therapies are administered to alleviate symptoms and shorten the course of virus infection. As clinical symptoms disappear, HSV-1 virions travel to nearby nerves and establish viral latency in peripheral sensory neurons. Latent infection is characterized by negligible viral gene expression, which prevents detection by the host immune system. During the life of the infected individual, HSV-1 lytic infection can become reactivated by stress stimuli. Reactivation can lead to the recurrence of lesions caused by productive, lytic infection, as well as an increased risk of more serious disease.

Figure 2

During infection of an epithelial cell, HSV-1 depends on the cell for endosome trafficking and its replication machinery. Due to this dependence, HSV-1 infection is invasive and must continuously avoid immune clearance from the cell. To overcome this, HSV-1 manipulates the cellular redox environment by upregulating oxidative stress responses to overwhelm the cell’s capacity for balancing redox, crafting an environment that promotes HSV-1 replication. This allows the virus to avoid sensing by immune receptors and the subsequent innate responses directed against HSV-1.

Figure 3

In a cell, RONS can be produced by various organelles during normal metabolic processes including cellular respiration in the mitochondria, protein folding in the ER, and in the peroxisome. Alternatively, enzymes such as NOX and NOS can generate RONS by transporting free electrons across membranes to reduce molecular oxygen. To avoid the accumulation of RONS, cells can oxidize RONS through antioxidants or enzymatically remove them by converting RONS into non-reactive products.

Figure 4

Overview of how cellular immune responses are stimulated during HSV-1 lytic infection. Inside the infected cell, PRRs involving TLRs, DNA/RNA sensors, and integrins sense components of HSV-1. Upon activation of sensors by HSV-1 or HSV-1-induced oxidative stress, immune signaling pathways such as NF-κB can be stimulated to promote the expression of proinflammatory cytokines. When released by the infected cell, these cytokines aid in innate immune cell (e.g., macrophage, dendritic cell, neutrophil) recruitment to promote phagocytosis and uptake of HSV-1 antigens for presentation to the immune system and stimulation of an adaptive anti-HSV-1 immune response (e.g., CD8+ T cells). Alternatively, epithelial cells can also present CD8+ T cells with HSV-1 antigens to prompt elimination of the infected cell.

Figure 5

NTP (DBD electrode shown) is a partially ionized gas that generates charged species (electrons, and positive- and negative-charged ions), radiation, electromagnetic fields, and reactive species generated at ambient temperatures. Through collisions between electrons from NTP, and atoms and molecules in the gas and liquid phase, RONS are generated. Following their generation, RONS such as superoxide, nitric oxide, singlet oxygen, ozone, and hydrogen peroxide are delivered to biological targets, which are said to mediate much of NTP’s antiviral effect. The exact composition of RONS delivered can depend on the plasma device and application modality.

Figure 6

Summary of the roles and effects of oxidative stress in cellular immune responses, HSV-1 infection of host cells, and NTP exposure. The Venn diagram illustrates the unique and overlapping roles of RONS and oxidative stress in each aspect of a putative NTP-based treatment for HSV-1 infection.

Figure 7

NTP is a potential antiviral and immunomodulatory agent for HSV-1 infection. We propose that the exposure of HSV-1-infected cells to NTP will directly (red) result in the inactivation of HSV-1 virion components and promote the display of SAMPs on the cell surface. Additionally, NTP-generated RONS can directly modify signaling proteins to activate immune signaling and balance redox levels through antioxidant transcription. Based on these effects, we believe NTP will indirectly (green) promote and expand the breadth of stimulation of both innate and adaptive immune responses against HSV-1. In return, this will decrease the amount of virus that can travel into the nervous system to establish viral latency in peripheral ganglionic neurons, ultimately reducing the frequency of reactivation over time.