Immune response of T cells during herpes simplex virus type 1 (HSV-1) infection

Abstract

Herpes simplex virus type 1 (HSV-1), a neurotropic member of the alphaherpes virus family, is among the most prevalent and successful human pathogens. HSV-1 can cause serious diseases at every stage of life including fatal disseminated disease in newborns, cold sores, eye disease, and fatal encephalitis in adults. HSV-1 infection can trigger rapid immune responses, and efficient inhibition and clearance of HSV-1 infection rely on both the innate and adaptive immune responses of the host. Multiple strategies have been used to restrict host innate immune responses by HSV-1 to facilitate its infection in host cells. The adaptive immunity of the host plays an important role in inhibiting HSV-1 infections. The activation and regulation of T cells are the important aspects of the adaptive immunity. They play a crucial role in host-mediated immunity and are important for clearing HSV-1. In this review, we examine the findings on T cell immune responses during HSV-1 infection, which hold promise in the design of new vaccine candidates for HSV-1.

1. Introduction

Herpes simplex virus type 1 (HSV-1), from the alphaherpes virus subfamily, is an enveloped, nuclear-replicating, and large double-stranded DNA virus. The genome of HSV-1 is an about 152 kb linear double-stranded GC-rich DNA sequence, and contains two unique regions called the long unique region (U_L) and the short unique region (U_S) (Fig. 1a), which encodes at least 84 proteins (Kieff et al., 1971). The genome of HSV-1 is located within the nucleocapsid, which is surrounded by a group of tegument proteins. The nucleocapsid and tegument proteins are surrounded by a lipid envelope studded with glycoproteins which are important for binding to and entry into new susceptible cells (Egan et al., 2013). The major steps of the life cycle of HSV-1 are: entry into the host cell, viral gene expression, genome replication, virion assembly, and release of new infectious virus (Fig. 1b) (Kukhanova et al., 2014). Three classes of genes of HSV-1 are expressed in a consecutive manner, including immediate early (IE) genes, early genes, and late genes. The products of IE genes regulate the expressions of early genes and late genes (Harkness et al., 2014).

Fig. 1.

Genome information and life cycle of HSV-1

(a) Structure of the HSV-1 DNA. The unique long (U_L) region is flanked by the terminal repeat (TR_L) and the internal repeat (IR_L). The unique short (U_S) region is bounded by the terminal repeat (TR_S) and the internal repeat (IR_S). (b) HSV-1 life cycle. 1: entry into the host cell; 2: viral gene expression; 3: genome replication; 4: virion assembly; 5: release of new infectious virus

The primary infection of HSV-1 is mainly in epithelial or mucosal cells, and then establishes a latent infection when it is transported to the sensory ganglia (Nicoll et al., 2012). During HSV-1 latent infection, the genome transcription is inhibited with the exception of a sequence encoding the latency-associated transcripts (LATs) (Wagner and Bloom, 1997; Preston, 2000; Efstathiou and Preston, 2005). The renewed lytic infection at epithelial or mucosal cells happens when there is reactivation of latent HSV-1 (Wuest and Carr, 2008).

HSV-1 infection is widespread, and its seropositivity may cover more than 70% of the world population. In developing countries, HSV-1 infection is universal, and acquired from intimate contact with family in early childhood (Whitley et al., 1988). In developed countries, some data suggest that acquisition of HSV-1 is delayed from early childhood to young adulthood (Hashido et al., 1999; Mertz et al., 2003). In the United States, 65% of people have antibodies to HSV-1, which is similar to the epidemiology in Europe (Xu et al., 2002). HSV-1 infection can cause clinical disease in various parts of the human body, such as genitalia, eye, oral, and central nervous system (CNS). The diseases associated with HSV-1 are listed in Table 1.

Table 1.

Diseases associated with HSV-1 infection

Infected body part	Disease	Reference
Skin	Cutaneous herpes	Zendri et al., 2005; Faron et al., 2016
	Genital herpes	Nieuwenhuis et al., 2006; Khoury-Hanold et al., 2016
Ocular	Herpes simplex keratitis (HSK)	Burrel et al., 2013; Tsatsos et al., 2016
	Uveitis	Krichevskaia et al., 2005; van Velzen et al., 2013
	Acute retinal necrosis	Mora et al., 2009; Fong et al., 2014
Orolabial	Cold sores	Richardson et al., 2013; Chi et al., 2015
	Oral ulcers	Sepulveda et al., 2005; Nicolatou-Galitis et al., 2006
CNS	Encephalitis	Bradshaw and Venkatesan, 2016; Eriksson et al., 2016
	Meningitis	Eisenstein et al., 2004; Azadfar et al., 2014
	Alzheimer’s disease	Beffert et al., 1998; Itzhaki et al., 1998

Inhibition of viral infection and clearance of the virus from infected cells rely on the innate and adaptive immunity of the host. The host innate immune system has evolved soluble components and specialized cells to block viral infection, replication, and shedding (Medzhitov and Janeway, 2000; Kawai and Akira, 2006). During viral infection, pattern recognition receptors (PRRs) have a role in detecting the viral pathogen-associated molecular patterns (PAMPs) in infected cells. The activated PRRs, such as Toll-like receptors (TLRs) and retinoic acid inducible gene-I (RIG-I)-like receptors (RLRs), will induce interferon production and cytokine release (Akira et al., 2006). The cellular PRRs to detect HSV-1 PAMPs have been reviewed extensively (Paludan et al., 2011; Melchjorsen, 2012). The type I interferon (IFN) signal pathway is the important first line of defense for the host against HSV-1. The innate immune cells including monocytes, neutrophils, dendritic cells (DCs), macrophages, and natural killer (NK) cells also play a crucial role in inhibiting HSV-1 infection (Kodukula et al., 1999; Barr et al., 2007; Murphy et al., 2008; Zheng et al., 2008; Mott et al., 2011; 2014; Frank et al., 2012; Kim et al., 2012; Molesworth-Kenyon et al., 2012; Swiecki et al., 2013; Vogel et al., 2014; Menasria et al., 2015). During infection, HSV-1 has developed multiple mechanisms to evade innate host immune responses and attenuate host antiviral elements (Paladino and Mossman, 2009; Suazo et al., 2015; Su et al., 2016). Although the functions of host innate immunity such as type I IFN and innate immune cell activity are need to suppress HSV-1 replication and infection, the generation of both CD8⁺ and CD4⁺ T cells is ultimately required to inhibit viral infection, drive HSV-1 into latency, and repress reactivation. This review will summarize and discuss current findings of the T cell immune response during HSV-1 infection, which could facilitate the attempts of more effective vaccine development for treating HSV-1 infection.

2. Immune responses of CD8⁺ T cells during HSV-1 infection

Among the immune cells involved in the immunity induced by pathogen invasion, CD8⁺ T cells play a central role in host adaptive immunity against many intracellular pathogens and clearing the viruses from the host (Wiesel et al., 2009; Kalia et al., 2010). Following pathogen recognition in the context of major histocompatibility complex class I (MHC-I) on antigen presenting cells (APCs), the naive CD8⁺ T cells could be differentiated into Tc1, Tc2, or Tc17 cells (Mosmann et al., 1997; Lee et al., 2011; Zhang and Bevan, 2011). During viral infection, the immune response of CD8⁺ T cells was divided into three characteristic phases, which are the initial activation and expansion, a contraction phase, and the establishment and maintenance of memory (Kaech et al., 2002a). During the acute phase of viral infection in humans, the robust immune responses of CD8⁺ T cells have also been observed (Callan et al., 1998). At the peak of proliferation, there is up to 10⁴-to 10⁵-fold expansion of CD8⁺ T cells, which divide approximately every 6 to 8 h, and then undergo activation and differentiation after this dramatic proliferation (Murali-Krishna et al., 1998). Upon antigenic stimulation, the expressions of granzymes and perforin are up-regulated by CD8⁺ T cells, and then these cells become cytolytic and gain the ability to enter non-lymphoid tissues (Bachmann et al., 1999; Cerwenka et al., 1999; Kaech et al., 2002a; 2002b; Wherry et al., 2003). CD8⁺ T cells also secret the IFN-γ when responding to viral infection, and IFN-γ promotes presentation of antigens to CD8⁺ T cells through enhanced processing of viral peptides for loading into MHC-I, which promotes immune response and inhibits viral infection (Groettrup et al., 1996; Schroder et al., 2004).

Previous studies indicated that CD8⁺ T cells might play a crucial role in restricting HSV-1 infection (Marrack and Kappler, 1987; Simmons, 1989). In line with the classic paradigm that HSV-1 infection is controlled by CD8⁺ T cells, MHC-I-restricted CD8⁺ T cells can be recovered from lymph nodes draining into herpetic lesions after HSV-1 infection on Day 4 (Nash et al., 1980). CD8⁺ T cells also have an ability to shut down HSV-1 infection in the trigeminal ganglia (TG) and prevent neurologic damage, and the response of CD8⁺ T cells could be divided into acute and latent phases (Simmons and Tscharke, 1992; Liu et al., 2000). In the mouse model, the infiltration peaks of CD8⁺ T cells in mouse ganglia occurred on Day 12, and a great number of CD8⁺ T cells persisted in the ganglia for up to 90 d. Using a murine flank scarification model, CD8⁺ T cells were observed to be involved in inhibiting HSV-1 replication in the draining ganglia (Simmons et al., 1992). In terms of effector molecules that are associated with CD8⁺ T cells mediating immunity, alternatively it was reported that IFN-γ not only might exert an antiviral effect and block the replication of numerous viruses in vitro, but also has an antiviral activity in controlling HSV-1 infection in primary human neurons and astrocytes (Li et al., 2011; 2012). Furthermore, another possible effector molecule is granzyme A (GrA), and the animals deficient in GrA show the decreased viral clearance from infection sites after peripheral HSV-1 inoculation (Pereira et al., 2001).

The CD8⁺ T cells might interact with many APCs during HSV-1 infection, such as DCs and ganglionic cells. The type and condition of APC used to characterize CD8⁺ T cell responses are also critically important for the inhibition of HSV-1 infection. Compared with B cells, the fibroblasts are more susceptible to HSV-1-mediated down-regulation of human leukocyte antigen class I (HLA-I) and they were observed to poorly re-stimulate memory CD8⁺ T cell responses (Yasukawa and Zarling, 1984; Kohl, 1991; Tigges et al., 1996).

Tissue-resident memory T cells (T_RM cells) are a subtype of memory lymphocytes and reside in nonlymphoid tissues in humans and mice (Schenkel and Masopust, 2014). CD8⁺ T_RM cells, a novel class of CD8⁺ memory T cells, have been well characterized (Krzysiek et al., 2013). During HSV-1 infection, virus-specific CD8⁺ T_RM cells are created in both ganglia and mucosa (Khanna et al., 2003; Gebhardt et al., 2009; Ariotti et al., 2014). CD8⁺ T_RM cells exist in non-lymphoid tissue compartments for long periods, and now these cells are also found in brain, kidney, joints, and other non-barrier tissues, which can trigger protective innate and adaptive immunity (Schenkel et al., 2014). CD8⁺ T_RM cells could express the effector molecules IFN-γ and granzyme B (GrB), and more these cells are not replenished from the circulating CD8⁺ T cell pool (Mackay et al., 2012). The roles of CD8⁺ T_RM cells are listed in Fig. 2.

Fig. 2.

Function of CD8⁺ T_RM cells

3. Immune response of CD4⁺ T cells during HSV-1 infection

In the host adaptive immune system, CD4⁺ T cells are another important branch, which not only have the ability to regulate an effective immune response to pathogens, but also have a role in control of host survival. Naive CD4⁺ T cells could differentiate into Th1, Th2, Th17, and induced regulatory T (iTreg) cells through interaction with antigen-MHC complex, and CD4⁺ T cell differentiation depends on the cytokines of the microenvironment (Luckheeram et al., 2012; Zheng, 2013). CD4⁺ T cells have ability to restrict and eliminate acute viral infection, and depend on the distinct phenotypes with their respective cytokine profiles. CD4⁺ T cells also modulate the functions of adaptive immunity and the innate immune cells (Sant and McMichael, 2012). Th1 cells have been regarded as critical for immunity to intracellular microorganisms and Th2 cells for immunity to many extracellular pathogens, including helminthes (Mosmann and Coffman, 1989; Paul and Seder, 1994). Treg cells function is needed to limit the extent of virus-induced inflammatory lesions, which implies that expanding and activating Tregs could be therapeutically valuable (Sehrawat and Rouse, 2011). During HSV-1 infection, the expression of IL-17 was up-regulated mainly through the Th17 cells in host cornea. Th1 cells are also responsible for orchestrating herpes stromal keratitis (HSK) during viral infection (Suryawanshi et al., 2011).

In the HSV-1 corneal infection model, HSV-1 replication is largely eliminated by 4–6 day post infection (dpi), and the CD4⁺ T cells that mediate HSK infiltrate the cornea around 7 dpi (Lepisto et al., 2006; Yun et al., 2014). Previous study has shown that the CD4⁺ T cells are important for preventing genital disease in a mice model during HSV-1 infection (Kuklin et al., 1998). When the mice are inoculated intravaginally (i.vag.) with HSV-1, CD4⁺ T cells can accumulate and persist within the spinal cords and dorsal root ganglia, which suggested that CD4⁺ T cells have the ability to clear virus from both neural and genital sites after HSV-1 primary infection (Johnson et al., 2008). It had been demonstrated that the CD4⁺CD25⁺ Tregs participate in T cell immune response to HSV-1 (Suvas et al., 2003). Following i.vag. inoculation of HSV-1, the number of CD4⁺ T cells from iliac lymph nodes, spinal cords, or dorsal root ganglia generally peaked around 6 or 8 dpi, and these cells increased expression of the activation marker CD25, CD44, or CD69 (Johnson et al., 2008). CD4⁺ T cells are expanded in the draining lymph nodes (DLNs) and re-stimulated in the infected cornea to regulate the destructive inflammatory disease after HSV-1 corneal infection (Buela and Hendricks, 2015). Recruitment of memory CD4⁺ T cells after infection of HSV-1 is also involved in the development of Th1 cells (Sin et al., 2000). DCs are resident in the DLNs and account for the expanding of naive HSV-specific CD4⁺ T cells in DLNs and the re-stimulating of the CD4⁺ T effector cells that infiltrate the cornea to mediate HSK (Hendricks et al., 1992). CD4⁺ T cells contribute to protection against HSV-1 in mice and ablation of CD4⁺ T cells increased susceptibility in naive animals (Manickan et al., 1995a; 1995b). In the mice model, CD4⁺ T cells are sufficient and have a ability to inhibit and clear HSV-1 from both neural and genital sites after primary infection (Johnson et al., 2008).

4. Infiltration of CD8⁺ and CD4⁺ T cells into central nervous system during HSV-1 infection

Despite the fact that the CNS of a host generally has the restrictive nature of the blood-brain barrier (BBB), CD8⁺ and CD4⁺ T cells can infiltrate into CNS during a variety of disease states and viral infection (Stohlman et al., 1998; Marten et al., 2003). When animals are exposed to glucocorticoids, the CNSs of them are shown to be vulnerable, which could enhance the susceptibility to viral infection. In the mouse model, psychological stress can induce the production of glucocorticoid, which enhances the infection of HSV-1 in mice and causes the development of HSV-1 encephalitis (HSE) (Nair et al., 2007). In a mouse model of HSE, the HSV-1-specific T cells infiltrated into CNS, and the brain of the mouse had an increase in CD8⁺ and CD4⁺ T cells during HSV-1 infection (Anglen et al., 2003). Multifocal brain demyelination (MBD) has been reported in susceptible mouse strains upon lip inoculation with HSV-1 and immunosuppression prevents the development of such lesions (Kastrukoff et al., 1987; 1993). CD8⁺ T cells appear to be involved in the focal lesions of the brain and the depletion of such cells prevents lesion development (Hudson and Streilein, 1994). Due to the limited evidence, it still remains unclear whether CD8⁺ T cells are responsible for limitation of HSV-1 replication and spreading within the CNS prior to an infection, or the delayed entrance of CD8⁺ T cells could result in pathology (Anglen et al., 2003).

The microglia, mstrocytes, perivascular macrophages, DCs, and endothelial cells of the brain have a function in presenting antigen to T cells in CNS (Aloisi, 1999; 2001; Aloisi et al., 2000). During HSV-1 infection, the dendritic-like cells and macrophage-like cells increase the expression of H-2K^b, which indicates a potential role of these cells to prime of HSV-1-specific CD8⁺ T cells in host CNS (Nair et al., 2007).

5. Exhaustion of CD8⁺ T cells during HSV-1 infection

The CD8⁺ T cells can be exhausted during chronic viral infection. These exhausted CD8⁺ T cells exhibit lost memory potential and poor effector function (Zajac et al., 1998; Wherry, 2011; Schietinger and Greenberg, 2014). The first report on CD8⁺ T cells exhaustion was found in mice when infected with lymphocytic choriomeningitis virus (LCMV), and virus-specific CD8⁺ T cells become less functional, as the infection persists, and may gradually lose their effector function, including proliferation, cytotoxicity, and cytokine production (Zajac et al., 1998). In addition, the exhaustion of CD8⁺ T cell function has been demonstrated in several human chronic virus infections, including hepatitis C virus (HCV), hepatitis B virus (HBV), and human immunodeficiency virus (HIV). The inhibitory receptors are expressed in exhausted CD8⁺ T cells, such as programmed cell death protein 1 (PD-1), T-cell immunoglobulin domain and mucin domain-3 (Tim-3), cluster of differentiation 244 (CD244), B-and T-lymphocyte attenuator (BTLA), cytotoxic T-lymphocyte associated protein 4 (CTLA-4), CD160, and killer cell lectin-like receptor subfamily G member 1 (KLRG1) (Bengsch et al., 2010; Wherry, 2011).

HSV-1 can reactivate from the TG, where there is a sizable pool of virus-specific CD8⁺ T cells. This phenomenon may depend on exhausting CD8⁺ T cells (Hoshino et al., 2007; Chentoufi et al., 2010). PD-1 is the most common inhibitory marker expressed in exhausted CD8⁺ T cells (Barber et al., 2006; Petrovas et al., 2007; Fourcade et al., 2010; Sakuishi et al., 2010). Besides the expression of PD-1, the high expressions of Tim-3 and CTLA-4 were also detected on the HSV-1-specific CD8⁺ T cells from LAT⁺ TG, which helped to discriminate between exhaustion and activation (Srivastava et al., 2016). During HSV-1 latency infection, LAT influences the CD8⁺ T cell levels and exhaustion. LAT⁺ HSV-1 can present more viral agent to CD8⁺ T cells, and this leads to increase in the expression of PD-1/Tim-3 and CD8⁺ T cell exhaustion. There are three subsets of exhausted CD8⁺ T cells in the TG of mice latently infected by HSV-1, the first one expressing PD-1, the second expressing Tim-3, and the third expressing both PD-1 and Tim-3 (Allen et al., 2011). The GrB of CD8⁺ T cells plays a major role in cytotoxic lytic granule-mediated apoptosis of cells. HSV-1 LAT not only restricts the GrB-induced cell apoptosis, but also inhibits the GrB-induced cleavage of caspase-3 (Jiang et al., 2011).

6. Vaccine for HSV-1

HSV-1 has the ability to build a primary and latent infection in human bodies, and causes serious diseases in human beings (Laing et al., 2012). It will benefit public health worldwide by reducing HSV-1 infection in human beings, and the efficient and effective pathway to control viral infectious disease is to inject vaccines. However, many of people are infected by HSV-1 during childhood, and the most of infected persons never undergo recurrent herpetic disease (Khanna et al., 2004). Therefore, the efficient and effective approach to prevent the infection of HSV-1 is prophylactic vaccination. The method to produce a therapeutic vaccine is targeting the viral proteins during HSV-1 latent infection, which proves more efficacious in restricting recurrent disease in human beings.

The advantages and disadvantages of experimental vaccine formats, which are mixture of viral proteins, peptides, attenuated replication-competent viruses, and replication-defective viruses, are reviewed (Stanberry et al., 2000). Many vaccine types including prophylactic and therapeutic containing viral DNA, glycoproteins, or replication-defective virus have been built in the past ten years. Nowadays, the approaches to develop vaccines are focused on the mechanism of viral evasion of the host immune system, and combination with the use of new and more specific adjuvants (Coleman and Shukla, 2013).

Researchers are interested in exploiting T cells to develop the vaccines, because the T cells of the host have specific and long-term immunologic memory and the ability to clear the virus in vitro and in animals (Laing et al., 2012). It is known that T cells have taken part in protective immunity after vaccination and correlate with viral reactivation in animal manipulation (Noisakran and Carr, 1999; Liu et al., 2000; Sheridan et al., 2009). Some studies suggest that T cell epitope specificities vary with different clinical presentations of HSV-1, and the epitope discovery has facilitated investigation of HSV-1-specific T cells in the search for a natural immune response (Chentoufi et al., 2008). T cells respond to a complex and serious pathogen with HSV-1, which has been decoded with a linked set of cellular and molecular tools to reveal novel candidate vaccine antigens (Jing et al., 2012). During HSV-1 cornea infection in mouse models, it has been shown that the CD8⁺ T cells might have no influence on the immunopathology in HSV-1. However, CD8⁺ T cells are protective at peripheral sites of infection and in the TG of the host, so a vaccine targeting CD8⁺ T cells might be particularly efficient and effective (Khanna et al., 2004).

Based on the protective immune response of T cells, it is important to develop a vaccine that elevates T cells. The best protection of the host during viral infection is to induce stronger cell-mediated immunity as well as better humoral responses, because viruses have used clever immune evasion strategies to inhibit the neutralizing antibody response (Coleman and Shukla, 2013). Nowadays, there has no specific vaccine or immunization strategy for HSV-1, but fascinating recent work comparing HSV-1-seropositive persons with and without histories of symptomatic orolabial herpes raises the possibility that humoral response patterns to specific proteins, or T-cell response patterns to defined epitopes within HSV-1 glycoproteins, may correlate with clinical severity, offering a set of criteria for rational down-selection of vaccine candidates (Chentoufi et al., 2008; Dasgupta et al., 2012).

7. Conclusions

In summary, HSV-1 is one of the persistent pathogens of human beings, and can cause a variety of diseases, such as cold sores, cutaneous herpes, genital herpes, and encephalitis. HSV-1 can establish a latent infection in TG of host, while CD4⁺ and CD8⁺ T cells can control the reactivation of the HSV-1 by surrounding latently infected neurons. The interactions between the host immunity and HSV-1 are very complicated. There is a battle between the host and HSV-1, and construction of a potent HSV-1-specific T cell immunity relies on multifaceted DCs priming of T cells as well as both CD4⁺ and CD8⁺ T cell cooperation at several stages and anatomic sites. Thus, favored vaccines elicit complex T cell responses, and likely also B cell responses, to generate long-lasting virus-neutralizing antibodies. Now the approaches to design the new vaccine against HSV-1 are by attracting and retaining T_RM cells in peripheral tissue locations. In any case, development of a useful HSV-1 vaccine needs to overcome the challenges posed by HSV-1 to traditional vaccine strategies.