Cutting edge: a NK complex-linked locus governs acute versus latent herpes simplex virus infection of neurons

The Dilemma of HSV-1 Oncolytic Virus Delivery: The Method Choice and Hurdles

Oncolytic viruses (OVs) have emerged as effective gene therapy and immunotherapy drugs. As an important gene delivery platform, the integration of exogenous genes into OVs has become a novel path for the advancement of OV therapy, while the herpes simplex virus type 1 (HSV-1) is the most commonly used. However, the current mode of administration of HSV-1 oncolytic virus is mainly based on the tumor in situ injection, which limits the application of such OV drugs to a certain extent. Intravenous administration offers a solution to the systemic distribution of OV drugs but is ambiguous in terms of efficacy and safety. The main reason is the synergistic role of innate and adaptive immunity of the immune system in the response against the HSV-1 oncolytic virus, which is rapidly cleared by the body's immune system before it reaches the tumor, a process that is accompanied by side effects. This article reviews different administration methods of HSV-1 oncolytic virus in the process of tumor treatment, especially the research progress in intravenous administration. It also discusses immune constraints and solutions of intravenous administration with the intent to provide new insights into HSV-1 delivery for OV therapy.

Ginkgolic Acid Inhibits Herpes Simplex Virus Type 1 Skin Infection and Prevents Zosteriform Spread in Mice

Herpes simplex virus type 1 (HSV-1) causes a lifelong latent infection with an estimated global prevalence of 66%. Primary and recurrent HSV infections are characterized by a tingling sensation, followed by an eruption of vesicles, which can cause painful erosions. Commonly used antiviral drugs against HSV infection are nucleoside analogues including acyclovir (ACV), famciclovir, and valacyclovir. Although these nucleoside analogues reduce morbidity and mortality in immunocompetent individuals, ACV-resistant HSV strains (ACV^R-HSV) have been isolated from immunocompromised patients. Thus, ACV^R-HSV infection poses a critical emerging public health concern. Recently, we reported that ginkgolic acid (GA) inhibits HSV-1 by disrupting viral structure, blocking fusion, and inhibiting viral protein synthesis. Additionally, we showed GA affords a broad spectrum of fusion inhibition of all three classes of fusion proteins, including those of HIV, Ebola, influenza A and Epstein Barr viruses. Here we report GA’s antiviral activity against HSV-1 skin infection in BALB/cJ mice. GA-treated mice demonstrated a significantly reduced mortality rate and decreased infection scores compared to controls treated with dimethylsulfoxide (DMSO)-vehicle. Furthermore, GA efficiently inhibited ACV^R-HSV-1 strain 17+ in vitro and in vivo. Since GA’s mechanism of action includes virucidal activity and fusion inhibition, it is expected to work alone or synergistically with other anti-viral drugs, and we anticipate it to be effective against additional cutaneous and potentially systemic viral infections.

The recruitment of peripheral blood leukocytes to the brain is delayed in susceptible BALB/c compared to resistant C57BL/6 mice during herpes simplex virus encephalitis

The cerebral immune response induced by herpes simplex virus (HSV) encephalitis (HSE) was evaluated in susceptible BALB/c and resistant C57BL/6 mice. BALB/c and C57BL/6 (named C57BL/6-high) mice were respectively infected intranasally with 1 × 10³ and 5 × 10⁵ plaque-forming units (PFUs) of HSV-1. C57BL/6 mice (named C57BL/6-low) infected with a low inoculum (1 × 10³ PFUs) of HSV-1 were tested in parallel. Mice were monitored for weight loss, sickness signs, and survival for 21 days. The viral load, infectious titers, cytokine/chemokine levels, and peripheral leukocyte infiltration were determined in brain homogenates on days 0 (non-infected), 4, 6, and 8 post-infection (p.i.) by qPCR, plaque assay, ELISA/Luminex™, and flow cytometry, respectively. Our results showed that the mortality of BALB/c mice (67%) was higher compared to those of C57BL/6-low (0%; P ≤ 0.01) and C57BL/6-high (20%; P ≤ 0.05) animals. This higher mortality was associated with increased infectious titers and cytokine/chemokine levels in the brains of BALB/c compared to C57BL/6 mice. Recruitment of inflammatory monocytes, dendritic cells, natural killer, and natural killer T cells to the brain was higher in C57BL/6-high compared to BALB/c animals on day 4 p.i. Infiltration of inflammatory monocytes and T cells in the brain of BALB/c mice was seen on day 6 p.i. Our data suggest that a rapid, sustained, and coordinated recruitment of peripheral leukocytes to the brain of C57BL/6-high mice results in an effective control of viral replication and inflammation whereas the delayed infiltration of immune cells in the brain of BALB/c mice was associated with an exacerbated inflammatory response during HSE.

Natural killer cells in herpesvirus infections

Natural killer (NK) cells are potent innate cytotoxic lymphocytes for the destruction of infected and transformed cells. Although they were originally considered to be ready-made assassins after their hematopoietic development, it has recently become clear that their activity is regulated by mechanisms such as repertoire composition, licensing, priming, and adaptive memory-like differentiation. Some of these mechanisms are influenced by infectious disease agents, including herpesviruses. In this review, we will compare expansion, stimulation, and effector functions of NK cell populations after infections with β- and γ ₁-herpesviruses because, though closely related, these pathogens seem to drive completely opposite NK cell responses. The discussed findings suggest that different NK cell subsets expand and perform protective functions during infectious diseases and might be used diagnostically to predict resistance to the causative pathogens as well as treat them by adoptive transfer of the respective populations.

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

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.

Mapping Murine Corneal Neovascularization and Weight Loss Virulence Determinants in the Herpes Simplex Virus 1 Genome and the Detection of an Epistatic Interaction between the UL and IRS/US Regions

Herpes simplex virus 1 (HSV-1) most commonly causes recrudescent labial ulcers; however, it is also the leading cause of infectious blindness in developed countries. Previous research in animal models has demonstrated that the severity of HSV-1 ocular disease is influenced by three main factors: host innate immunity, host immune response, and viral strain. We have previously shown that mixed infection with two avirulent HSV-1 strains (OD4 and CJ994) results in recombinants with a wide range of ocular disease phenotype severity. Recently, we developed a quantitative trait locus (QTL)-based computational approach (vQTLmap) to identify viral single nucleotide polymorphisms (SNPs) predicted to influence the severity of the ocular disease phenotypes. We have now applied vQTLmap to identify HSV-1 SNPs associated with corneal neovascularization and mean peak percentage weight loss (MPWL) using 65 HSV-1 OD4-CJ994 recombinants. The vQTLmap analysis using Random Forest for neovascularization identified phenotypically meaningful nonsynonymous SNPs in the ICP4, UL41 (VHS), UL42, UL46 (VP11/12), UL47 (VP13/14), UL48 (VP22), US3, US4 (gG), US6 (gD), and US7 (gI) coding regions. The ICP4 gene was previously identified as a corneal neovascularization determinant, validating the vQTLmap method. Further analysis detected an epistatic interaction for neovascularization between a segment of the unique long (UL) region and a segment of the inverted repeat short (IRS)/unique short (US) region. Ridge regression was used to identify MPWL-associated nonsynonymous SNPs in the UL1 (gL), UL2, UL4, UL49 (VP22), UL50, and ICP4 coding regions. The data provide additional insights into virulence gene and epistatic interaction discovery in HSV-1.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) typically causes recurrent cold sores; however, it is also the leading source of infectious blindness in developed countries. Corneal neovascularization is critical for the progression of blinding ocular disease, and weight loss is a measure of infection severity. Previous HSV-1 animal virulence studies have shown that the severity of ocular disease is partially due to the viral strain. In the current study, we used a recently described computational quantitative trait locus (QTL) approach in conjunction with 65 HSV-1 recombinants to identify viral single nucleotide polymorphisms (SNPs) involved in neovascularization and weight loss. Neovascularization SNPs were identified in the ICP4, VHS, UL42, VP11/12, VP13/14, VP22, gG, US3, gD, and gI genes. Further analysis revealed an epistatic interaction between the UL and US regions. MPWL-associated SNPs were detected in the UL1 (gL), UL2, UL4, VP22, UL50, and ICP4 genes. This approach will facilitate future HSV virulence studies.

Von Willebrand Factor Gene Variants Associate with Herpes simplex Encephalitis

Herpes simplex encephalitis (HSE) is a rare complication of Herpes simplex virus type-1 infection. It results in severe parenchymal damage in the brain. Although viral latency in neurons is very common in the population, it remains unclear why certain individuals develop HSE. Here we explore potential host genetic variants predisposing to HSE. In order to investigate this we used a rat HSE model comparing the HSE susceptible SHR (Spontaneously Hypertensive Rats) with the asymptomatic infection of BN (Brown Norway). Notably, both strains have HSV-1 spread to the CNS at four days after infection. A genome wide linkage analysis of 29 infected HXB/BXH RILs (recombinant inbred lines-generated from the prior two strains), displayed variable susceptibility to HSE enabling the definition of a significant QTL (quantitative trait locus) named Hse6 towards the end of chromosome 4 (160.89-174Mb) containing the Vwf (von Willebrand factor) gene. This was the only gene in the QTL with both cis-regulation in the brain and included several non-synonymous SNPs (single nucleotide polymorphism). Intriguingly, in human chromosome 12 several SNPs within the intronic region between exon 43 and 44 of the VWF gene were associated with human HSE pathogenesis. In particular, rs917859 is nominally associated with an odds ratio of 1.5 (95% CI 1.11-2.02; p-value = 0.008) after genotyping in 115 HSE cases and 428 controls. Although there are possibly several genetic and environmental factors involved in development of HSE, our study identifies variants of the VWF gene as candidates for susceptibility in experimental and human HSE.

Quantitative Trait Locus Based Virulence Determinant Mapping of the HSV-1 Genome in Murine Ocular Infection: Genes Involved in Viral Regulatory and Innate Immune Networks Contribute to Virulence

Herpes simplex virus type 1 causes mucocutaneous lesions, and is the leading cause of infectious blindness in the United States. Animal studies have shown that the severity of HSV-1 ocular disease is influenced by three main factors; innate immunity, host immune response and viral strain. We previously showed that mixed infection with two avirulent HSV-1 strains (OD4 and CJ994) resulted in recombinants that exhibit a range of disease phenotypes from severe to avirulent, suggesting epistatic interactions were involved. The goal of this study was to develop a quantitative trait locus (QTL) analysis of HSV-1 ocular virulence determinants and to identify virulence associated SNPs. Blepharitis and stromal keratitis quantitative scores were characterized for 40 OD4:CJ994 recombinants. Viral titers in the eye were also measured. Virulence quantitative trait locus mapping (vQTLmap) was performed using the Lasso, Random Forest, and Ridge regression methods to identify significant phenotypically meaningful regions for each ocular disease parameter. The most predictive Ridge regression model identified several phenotypically meaningful SNPs for blepharitis and stromal keratitis. Notably, phenotypically meaningful nonsynonymous variations were detected in the UL24, UL29 (ICP8), UL41 (VHS), UL53 (gK), UL54 (ICP27), UL56, ICP4, US1 (ICP22), US3 and gG genes. Network analysis revealed that many of these variations were in HSV-1 regulatory networks and viral genes that affect innate immunity. Several genes previously implicated in virulence were identified, validating this approach, while other genes were novel. Several novel polymorphisms were also identified in these genes. This approach provides a framework that will be useful for identifying virulence genes in other pathogenic viruses, as well as epistatic effects that affect HSV-1 ocular virulence.

In addition to causing recurrent labial lesions, herpes simplex virus type 1 (HSV-1) is also the primary source of infectious blindness in the United States. Animal studies have shown that the severity of infection is influenced by several factors, including viral strain. Conventional studies investigating the genetics of viral virulence have focused on characterizing a naturally occurring strain, and engineering mutations into viruses. The purpose of this study was to develop a quantitative trait locus (QTL) computational analysis of HSV-1 genome to identify ocular virulence determinants and associated viral SNPs. Notably, phenotypically meaningful variations were detected in the UL24, UL29 (ICP8), UL41 (VHS), UL53 (gK), UL54 (ICP27), UL56, ICP4, US1 (ICP22), US3 and gG genes. Several genes previously implicated in virulence were identified, validating this approach, while other genes were novel. This is the first time a QTL based approach has been applied to a herpesvirus and it will also be valuable in future virulence, epistasis, and protein-protein interaction studies.

A NK complex-linked locus restricts the spread of herpes simplex virus type 1 in the brains of C57BL/6 mice

The most frequent cause of sporadic viral encephalitis in western countries is Herpes simplex virus (HSV). Despite treatment, mortality rates reach 20-30% while survivors often suffer from significant morbidity. In mice, resistance to lethal Herpes simplex encephalitis (HSE) is multifactorial and influenced by mouse and virus strain as well as route of infection. The ability to restrict viral spread in the brain is one factor contributing to resistance. After infection of the oral mucosa with HSV type 1 (HSV-1), virus spreads throughout the brains of susceptible strains but is restricted in resistant C57BL/6 mice. To further investigate restriction of viral spread in the brain, mendelian analysis was combined with studies of congenic, intra-natural killer complex (intra-NKC) recombinant and antibody-depleted mice. Results from mendelian analysis support the restriction of viral spread as a dominant trait and consistent with a single gene effect. In congenic mice, the locus maps to the NKC on chromosome 6 and is provisionally termed Herpes Resistance Locus 2 (Hrl2). In intra-NKC recombinants, the locus is further mapped to the segment Cd69 through D6Wum34; a different location from previously identified loci (Hrl and Rhs1) also associated with HSV-1 infection. Studies with antibody-depleted mice indicate the effect of this locus is mediated by NK1.1(+) expressing cells. This model increases our knowledge of lethal HSE, which may lead to new treatment options.

Genome-wide mouse mutagenesis reveals CD45-mediated T cell function as critical in protective immunity to HSV-1

Herpes simplex encephalitis (HSE) is a lethal neurological disease resulting from infection with Herpes Simplex Virus 1 (HSV-1). Loss-of-function mutations in the UNC93B1, TLR3, TRIF, TRAF3, and TBK1 genes have been associated with a human genetic predisposition to HSE, demonstrating the UNC93B-TLR3-type I IFN pathway as critical in protective immunity to HSV-1. However, the TLR3, UNC93B1, and TRIF mutations exhibit incomplete penetrance and represent only a minority of HSE cases, perhaps reflecting the effects of additional host genetic factors. In order to identify new host genes, proteins and signaling pathways involved in HSV-1 and HSE susceptibility, we have implemented the first genome-wide mutagenesis screen in an in vivo HSV-1 infectious model. One pedigree (named P43) segregated a susceptible trait with a fully penetrant phenotype. Genetic mapping and whole exome sequencing led to the identification of the causative nonsense mutation L3X in the Receptor-type tyrosine-protein phosphatase C gene (Ptprc(L3X)), which encodes for the tyrosine phosphatase CD45. Expression of MCP1, IL-6, MMP3, MMP8, and the ICP4 viral gene were significantly increased in the brain stems of infected Ptprc(L3X) mice accounting for hyper-inflammation and pathological damages caused by viral replication. Ptprc(L3X) mutation drastically affects the early stages of thymocytes development but also the final stage of B cell maturation. Transfer of total splenocytes from heterozygous littermates into Ptprc(L3X) mice resulted in a complete HSV-1 protective effect. Furthermore, T cells were the only cell population to fully restore resistance to HSV-1 in the mutants, an effect that required both the CD4⁺ and CD8⁺ T cells and could be attributed to function of CD4⁺ T helper 1 (Th1) cells in CD8⁺ T cell recruitment to the site of infection. Altogether, these results revealed the CD45-mediated T cell function as potentially critical for infection and viral spread to the brain, and also for subsequent HSE development.

The calcitonin receptor gene is a candidate for regulation of susceptibility to herpes simplex type 1 neuronal infection leading to encephalitis in rat

Herpes simplex encephalitis (HSE) is a fatal infection of the central nervous system (CNS) predominantly caused by Herpes simplex virus type 1. Factors regulating the susceptibility to HSE are still largely unknown. To identify host gene(s) regulating HSE susceptibility we performed a genome-wide linkage scan in an intercross between the susceptible DA and the resistant PVG rat. We found one major quantitative trait locus (QTL), Hse1, on rat chromosome 4 (confidence interval 24.3–31 Mb; LOD score 29.5) governing disease susceptibility. Fine mapping of Hse1 using recombinants, haplotype mapping and sequencing, as well as expression analysis of all genes in the interval identified the calcitonin receptor gene (Calcr) as the main candidate, which also is supported by functional studies. Thus, using unbiased genetic approach variability in Calcr was identified as potentially critical for infection and viral spread to the CNS and subsequent HSE development.

Herpes simplex encephalitis (HSE) is a rare, but severe infection of the central nervous system (CNS) caused by Herpes simplex virus type 1. We have previously characterized a model for HSE in the inbred DA rat which resembles human HSE. Interestingly the inbred PVG rat is completely resistant to the disease and displays reduced or no uptake of viral particles into the peripheral and central nerve compartments respectively. To identify the gene(s) regulating HSE pathogenesis, we crossed the susceptible DA and the resistant PVG.A rats for two generations and infected 239 rats of the F2 (DAxPVG.A) cohort with HSV-1. A genome-wide linkage scan demonstrated one strong quantitative trait locus (QTL), Hse1, on rat chromosome 4 regulating disease susceptibility. Fine mapping, haplotype mapping, sequencing and expression analysis of the genes in the Hse1 interval collectively support the underlying genetic variation to be located in, or adjacent to the calcitonin receptor gene (Calcr). Further support for a role of CalcR in regulating HSV-1 replication and propagation is provided by strain-dependent differences in the calcitonin receptor protein tissue localization and in functional studies. Using an unbiased genetic mapping approach this study identifies Calcr as a candidate for regulating susceptibility to HSE.

Host genetic factors in susceptibility to herpes simplex type 1 virus infection: contribution of polymorphic genes at the interface of innate and adaptive immunity

HSV-1 establishes life-long latency that can result in clinical relapses or in asymptomatic virus shedding. Although virtually all adults have been exposed to HSV-1, the clinical course varies remarkably. Genetic host variability could be related to this clinical diversity. In this study, we analyzed the contribution of gene families in chromosomes 1, 6, 12, and 19, which encode key regulators of the innate and adaptive immunity, in a cohort of 302 individuals. Class I and class II alleles of the HLA system, the copy-number variation of NK cell receptor genes (KIR and NKG2C), the combinations of killer cell Ig-like receptor and their HLA ligands, and CD16A and CD32A allotypes of variable affinity for IgG subclasses were all studied. Although no major susceptibility locus for HSV-1 was identified, our results show that the risk of suffering clinical HSV-1 infection is modified by MHC class I allotypes (B*18, C*15, and the group of alleles encoding A19), the high-affinity receptor/ligand pair KIR2DL2/HLA-C1, and the CD16A-158V/F dimorphism. Conversely, HLA class II and CD32A polymorphisms and NKG2C deletion did not seem to influence the clinical course of herpetic infection. Collectively, these findings support an important role in host defense against herpetic infection for several polymorphic genes implicated in adaptive immunity and in surveillance of its subversion. They confirm the crucial role of cytotoxic cells (CTL and NK) and the contribution of genetic diversity to the clinical course of HSV-1 infection.

Innate and adaptive immune responses to herpes simplex virus

Immune responses against HSV-1 and HSV-2 are complex and involve a delicate interplay between innate signaling pathways and adaptive immune responses. The innate response to HSV involves the induction of type I IFN, whose role in protection against disease is well characterized in vitro and in vivo. Cell types such as NK cells and pDCs contribute to innate anti-HSV responses in vivo. Finally, the adaptive response includes both humoral and cellular components that play important roles in antiviral control and latency. This review summarizes the innate and adaptive effectors that contribute to susceptibility, immune control and pathogenesis of HSV, and highlights the delicate interplay between these two important arms of immunity.

Natural killer cells in immunodefense against infective agents

Following the discovery of innate immune receptors, the topics of innate immunity and its role in defense against infective agents have recently blossomed into very active research fields, after several decades of neglect. Among innate immune cells, natural killer (NK) cells are endowed with the unique ability to recognize and kill cells infected with a variety of pathogens, irrespective of prior sensitization to these microbes. NK cells have a number of other functions, including cytokine production and immunoregulatory activities. Major advances have recently been made in the understanding of the role of NK cells in the physiopathology of infectious diseases. The cellular and molecular mechanisms regulating the acquisition of effector functions by NK cells and their triggering upon pathogenic encounters are being unraveled. The possibility that the power of NK cells could be harnessed for the design of innovative treatments against infections is a major incentive for biologists to further explore NK cell subset complexity and to identify the ligands that activate NK cell receptors.

Ly49h-deficient C57BL/6 mice: a new mouse cytomegalovirus-susceptible model remains resistant to unrelated pathogens controlled by the NK gene complex

Cmv1 was the first mouse cytomegalovirus (MCMV) resistance locus identified in C57BL/6 mice. It encodes Ly49H, a NK cell-activating receptor that specifically recognizes the m157 viral protein at the surface of MCMV-infected cells. To dissect the effect of the Ly49h gene in host-pathogen interactions, we generated C57BL/6 mice lacking the Ly49h region. We found that 36 h after MCMV infection, the lack of Ly49h resulted in high viral replication in the spleen and dramatically enhanced proinflammatory cytokine production in the serum and spleen. At later points in time, we observed that MCMV induced a drastic loss in CD8(+) T cells in B6.Ly49h(-/-) mice, probably reflecting severe histological changes in the spleen. Overall, our results indicate that Ly49H(+) NK cells contain a systemic production of cytokines that may contribute to the MCMV-induced pathology and play a central role in maintaining normal spleen cell microarchitecture. Finally, we tested the ability of B6.Ly49h(-/-) mice to control replication of Leishmania major and ectromelia virus. Resistance to these pathogens has been previously mapped within the NK gene complex. We found that the lack of Ly49H(+) NK cells is not associated with an altered resistance to L. major. In contrast, absence of Ly49H(+) NK cells seems to afford additional protection against ectromelia infection in C57BL/6 mice, suggesting that Ly49H may recognize ectromelia-infected cells with detrimental effects. Taken together, these results confirm the pivotal role of the Ly49H receptor during MCMV infection and open the way for further investigations in host-pathogen interactions.

Natural killer cells as novel helpers in anti-herpes simplex virus immune response

Innate defenses help to eliminate infection, but some of them also play a major role in shaping the magnitude and efficacy of the adaptive immune response. With regard to influencing subsequent adaptive immunity, NK cells aided by dendritic cells may be the most relevant components of the innate reaction to herpes simplex virus (HSV) infection. We confirm that mice lacking or depleted of NK cells are susceptible to HSV-induced lesions. The quantity and quality of CD8(+) cytotoxic T lymphocytes generated in the absence of NK cells were diminished, thereby contributing to susceptibility to HSV-induced encephalitis. We demonstrate a novel helper role for NK cells, in that NK cells compensate for the loss of CD4 helper T cells and NK cell supplementation enhances the function of wild type anti-HSV CD8 T cells. In addition, NK cells were able to partially rescue the dysfunctional CD8(+) T cells generated in the absence of CD4 T helper cells, thereby performing a novel rescue function. Hence, NK cells may well be exploited for enhancing and rescuing the T-cell response in situations where the CD4 helper response is affected.

Evolutionary struggles between NK cells and viruses

Natural killer (NK) cells are well recognized for their ability to provide a first line of defence against viral pathogens and they are increasingly being implicated in immune responses against certain bacterial and parasitic infections. Reciprocally, viruses have devised numerous strategies to evade the activation of NK cells and have influenced the evolution of NK-cell receptors and their ligands. NK cells contribute to host defence by their ability to rapidly secrete cytokines and chemokines, as well as to directly kill infected host cells. In addition to their participation in the immediate innate immune response against infection, interactions between NK cells and dendritic cells shape the nature of the subsequent adaptive immune response to pathogens.

Genetic susceptibility to herpes simplex virus 1 encephalitis in mice and humans

PURPOSE OF REVIEW

Herpes simplex encephalitis is a rare complication of herpes simplex virus 1 infection that strikes otherwise healthy individuals. Its pathogenesis has long remained elusive. We highlight the investigations dealing with the genetic basis of herpes simplex encephalitis in mice and humans.

RECENT FINDINGS

Mouse models have revealed the impact of various host genes on protective immunity to herpes simplex encephalitis through strain-dependent variability (forward genetics) and via targeted knockouts (reverse genetics). These studies established in particular the crucial role of IFNalpha/beta in immunity to herpes simplex virus 1, paving the way towards the elucidation of the genetic cause of human herpes simplex encephalitis. Two children with rare, specific STAT1 or NEMO mutations displayed a broad impairment of IFNalpha/beta and IFNlambda-mediated immunity and predisposition to several infectious diseases including herpes simplex encephalitis. In contrast, children with UNC93B1 and TLR3 mutations displayed a selective impairment of dsRNA-induced IFNalpha/beta and IFNlambda production and predisposition to isolated herpes simplex encephalitis.

SUMMARY

Herpes simplex encephalitis results from a series of monogenic primary immunodeficiencies that impair the TLR3 and UNC-93B-dependent production of IFNalpha/beta and IFNlambda in the central nervous system, at least in a fraction of children. This is not only crucial for the understanding of immunity to herpes simplex virus 1, but also for the diagnosis and treatment of herpes simplex encephalitis.

Cmv1 and natural killer cell responses to murine cytomegalovirus infection

The dissection of genetic resistance to murine cytomegalovirus infection in inbred laboratory mouse strains led to the identification of a natural killer cell activation receptor that recognizes a virus-encoded protein. Herein, we summarize the genetic approach and findings that have provided novel insights into innate immune control of virus infections.

IL-18, but not IL-12, regulates NK cell activity following intranasal herpes simplex virus type 1 infection

Infection of the respiratory tract with HSV type 1 (HSV-1) can have severe clinical complications, yet little is known of the immune mechanisms that control the replication and spread of HSV-1 in this site. The present study investigated the protective role of IL-12 and IL-18 in host defense against intranasal HSV-1 infection. Both IL-12 and IL-18 were detected in lung fluids following intranasal infection of C57BL/6 (B6) mice. IL-18-deficient (B6.IL-18(-/-)) mice were more susceptible to HSV-1 infection than wild-type B6 mice as evidenced by exacerbated weight loss and enhanced virus growth in the lung. IL-12-deficient (B6.IL-12(-/-)) mice behaved similarly to B6 controls. Enhanced susceptibility of B6.IL-18(-/-) mice to HSV-1 infection correlated with a profound impairment in the ability of NK cells recovered from the lungs to produce IFN-gamma or to mediate cytotoxic activity ex vivo. The weak cytotoxic capacity of NK cells from the lungs of B6.IL-18(-/-) mice correlated with reduced expression of the cytolytic effector molecule granzyme B. Moreover, depletion of NK cells from B6 or B6.IL-12(-/-) mice led to enhanced viral growth in lungs by day 3 postinfection; however, this treatment had no effect on viral titers in lungs of B6.IL-18(-/-) mice. Together these studies demonstrate that IL-18, but not IL-12, plays a key role in the rapid activation of NK cells and therefore in control of early HSV-1 replication in the lung.

A role for plasmacytoid dendritic cells in the rapid IL-18-dependent activation of NK cells following HSV-1 infection

Natural killer (NK) cells play a crucial role in the initial response to viral infections but the mechanisms controlling their activation are unclear. We show a rapid and transient activation of NK cells that results in the production of IFN-gamma immediately following infection with herpes simplex virus type 1 (HSV-1). Activation of NK cells leading to synthesis of IFN-gamma was not mediated by a direct interaction with virus but required the presence of additional cell types and was largely dependent on the cytokine IL-18, but not IL-12. HSV-1-induced IFN-gamma expression by NK cells in vitro was impaired in spleen cultures depleted of CD11c(+) cells. Conversely, coculture of NK cells with virus-exposed conventional DC or plasmacytoid (p)DC restored the production of IFN-gamma, indicating that multiple DC subsets could mediate NK cell activation. While conventional DC populations stimulated NK cells independently of IL-18, they were less effective than pDC in promoting NK cell IFN-gamma expression. In contrast, the potent stimulation of NK cells by pDC was dependent on IL-18 as pDC from IL-18-deficient mice only activated a similar proportion of NK cells as conventional DC. These data identify IL-18 as a crucial factor for pDC-mediated NK cell regulation.

NK cell-mediated destruction of influenza A virus-infected peripheral but not central neurones

Peripheral neurones have the potential to transmit infectious agents to the central nervous system (CNS). This raises the possibility of existing host defence mechanisms that may prevent such spread. Natural killer (NK) cells can target infected cells, and by this ability serve to limit spread of infection prior to the development of adaptive immune responses. To address directly if NK cells can target infected peripheral neurones, we examined the expression of NK cell-activating ligands and susceptibility to NK cell-mediated cytolytic effects in ex vivo cultures of mouse peripheral dorsal root ganglia (DRG) neurones prior to and after infection with a neurotropic strain of influenza A virus, WSN/33. In infected DRG cultures, retinoic acid early inducible gene-1 (RAE-1) transcripts were induced and exposure to interleukin (IL)-2-activated NK cells resulted in a total destruction of neurites. Studies on cultures from interferon (IFN)-alpha/betaR-deficient mice suggest that the infection engages an IFN-alpha/beta-dependent signalling pathway to induce RAE-1 transcripts. In contrast, induction of RAE-1 transcripts or NK cell-mediated neurite destructions was not observed in central hippocampal neurones. This reveals distinct properties between peripheral DRG and central hippocampal neurones with respect to the ability to signal for immune destruction following infection.

CD4+ T cell migration into the cornea is reduced in CXCL9 deficient but not CXCL10 deficient mice following herpes simplex virus type 1 infection

The role of CXCL9 and CXCL10 in the ocular immune response to herpes simplex virus type 1 (HSV-1) infection was investigated using mice deficient in either CXCL9 or CXCL10. CXCL10 but not CXCL9 deficient mice showed an increase in sensitivity to ocular virus infection as measured by an elevation in virus titer recovered in the tear film and corneal tissue. The increase in virus was associated with an increase in the expression of the chemokine CCL2 but no significant change in the infiltration of CD4(+) T cells or NK cells into the corneal stroma. In contrast, a significant reduction in CD4(+) T cell infiltration into the cornea was found in CXCL9 deficient mice following HSV-1 infection consistent with the absence of CXCL9 expression and reduction in expression of other chemokines including CCL3, CCL5, CXCL1, and CXCL10. Collectively, the results suggest a non-redundant role for CXCL9 and CXCL10 in response to ocular HSV-1 infection in terms of controlling virus replication and recruitment of CD4(+) T cells into the cornea.

DAP12 signaling directly augments proproliferative cytokine stimulation of NK cells during viral infections

NK cells vigorously proliferate during viral infections. During the course of murine CMV infection, this response becomes dominated by the preferential proliferation of NK cells that express the activation receptor Ly49H. The factors driving such selective NK cell proliferation have not been characterized. In this study, we demonstrate that preferential NK cell proliferation is dependent on DAP12-mediated signaling following the binding of Ly49H to its virally encoded ligand, m157. Ly49H signaling through DAP12 appears to directly augment NK cell sensitivity to low concentrations of proproliferative cytokines such as IL-15. The impact of Ly49H-mediated signaling on NK cell proliferation is masked in the presence of high concentrations of proproliferative cytokines that nonselectively drive all NK cells to proliferate.

Critical Residues at the Ly49 Natural Killer Receptor’s Homodimer Interface Determine Functional Recognition of m157, a Mouse Cytomegalovirus MHC Class I-Like Protein

NK cell function is regulated by Ly49 receptors in mice and killer cell Ig-like receptors in humans. Although inhibitory Ly49 and killer cell Ig-like receptors predominantly ligate classical MHC class I molecules, recent studies suggest that their activating counterparts recognize infection. The quintessential example is resistance to the mouse CMV in C57BL/6 mice, which depends on the functional recognition of m157, a mouse CMV-encoded MHC class I-like molecule, by Ly49H, an activating NK cell receptor. We have taken advantage of the natural variation in closely related members of the Ly49C-like receptors and the availability of Ly49 crystal structures to understand the molecular determinants of the Ly49H-m157 interaction and to identify amino acid residues discriminating between m157 binding and nonbinding receptors. Using a site-directed mutagenesis approach, we have targeted residues conserved in receptors binding to m157 (Ly49H and Ly49I(129)) but different from receptors lacking m157 recognition (Ly49C, Ly49I(B6), and Ly49U). Wild-type and mutant receptors were transfected into reporter cells, and physical binding as well as functional activation by m157 was studied. Our findings suggested that the Ly49 MHC class I contact "site 2," I226, may not be involved in m157 binding. In contrast, residue Y146 and G151, mapping at the receptor homodimer interface, are likely critical for functional recognition of the m157 glycoprotein. Our combined functional and three-dimensional modeling approach suggested that the architecture of the Ly49H dimer is crucial to accessing m157, but not MHC class I. These results link Ly49 homodimerization variability to the direct recognition of pathogen products.

Quantitative analysis of herpes simplex virus type 1-specific memory B cells generated by different routes of infection

We compared the herpes simplex virus type 1 (HSV)-specific memory B cell (MBC) populations generated by footpad and intranasal infection in mice. Both routes of infection generated transient antibody-secreting cell responses in the draining lymph nodes and spleen, and sustained circulating IgG. HSV-specific IgG MBCs, analyzed by limiting dilution assay approximately 8 weeks after infection, were distributed in a range of lymph nodes and in the spleen and Peyer's patches. Overall, the route of infection had little effect on the MBC frequency in each anatomical location. Interestingly, after both routes of infection there was a trend towards preferential MBC accumulation in the mediastinal lymph node. Intravaginal challenge of mice primed by footpad or intranasal infection generated similar secondary IgG responses. Our findings indicate that the widespread dispersion of MBCs to lymphoid tissues throughout the body is largely independent of the route of infection, but may be influenced by tissue-specific factors.

Natural killer cell immune responses

Natural killer (NK) cells play a vital role in innate immune responses to infection; they express activation receptors that recognize virus-infected cells. Highly related to receptors recognizing tumor cells, the activation receptors trigger cytotoxicity and cytokine production. NK cells also express inhibitory receptors for major histocompatibility complex (MHC) class I molecules that block the action of the activation receptors. Although many ligands for NK cell receptors have MHC class I folds, recent studies also indicate ligands resembling the NK cell receptors themselves. A combination of immunologic, genetic, biophysical, and in vivo approaches is being employed to understand fully how these receptors contribute to NK cell activities in innate immunity to pathogens and tumors.

Recognition of vaccinia virus-infected cells by human natural killer cells depends on natural cytotoxicity receptors

Natural Killer (NK) cells are important in the immune response to a number of viruses; however, the mechanisms used by NK cells to discriminate between healthy and virus-infected cells are only beginning to be understood. Infection with vaccinia virus provokes a marked increase in the susceptibility of target cells to lysis by NK cells, and we show that recognition of the changes in the target cell induced by vaccinia virus infection depends on the natural cytotoxicity receptors NKp30, NKp44, and NKp46. Vaccinia virus infection does not induce expression of ligands for the activating NKG2D receptor, nor does downregulation of major histocompatibility complex class I molecules appear to be of critical importance for altered target cell susceptibility to NK cell lysis. The increased susceptibility to lysis by NK cells triggered upon poxvirus infection depends on a viral gene, or genes, transcribed early in the viral life cycle and present in multiple distinct orthopoxviruses. The more general implications of these data for the processes of innate immune recognition are discussed.

Herpesviruses and the innate immune response

Herpesvirus infection leads to the rapid induction of an innate immune response. A central aspect of this host response is the production and secretion of type I interferon. The current model of virus-mediated interferon production includes three stages: sensitization, induction, and amplification. A key mediator of all three stages is the cellular transcription factor interferon regulatory factor 3 (IRF3). Although the precise details of IRF3 activation and interferon production in response to herpesvirus infection are still being elucidated, viral proteins that block components of the interferon pathway, particularly IRF3, have been identified and characterized. In vivo studies have shown that in addition to type I interferon, interleukin-15 (IL-15) and natural killer (NK) cells also play an important role in mediating resistance to herpesvirus infection. Recent investigations have demonstrated a strong association between IRF3, interferon, IL-15, and NK cells. This review will focus on herpesvirus-mediated induction of innate immunity, the central role of the type I interferon response and mechanisms used by herpesviruses to block host antiviral immunity.

Re-evaluating the role of natural killer cells in innate resistance to herpes simplex virus type 1

Background

Interferon-γ acts to multiply the potency with which innate interferons (α/β) suppress herpes simplex virus type 1 (HSV-1) replication. Recent evidence suggests that this interaction is functionally relevant in host defense against HSV-1. However, it is not clear which WBCs of the innate immune system, if any, limit HSV-1 spread in an IFN-γ dependent manner. The current study was initiated to determine if natural killer (NK) cells provide innate resistance to HSV-1 infection, and if so to determine if this resistance is IFN-γ-dependent.

Results

Lymphocyte-deficient scid or rag2^-/- mice were used to test four predictions of the central hypothesis, and thus determine if innate resistance to HSV-1 is dependent on 1. NK cell cytotoxicity, 2. NK cells, 3. WBCs, or 4. the IFN-activated transcription factor, Stat 1. Loss of NK cell cytotoxic function or depletion of NK cells had no effect on the progression of HSV-1 infection in scid mice. In contrast, viral spread and pathogenesis developed much more rapidly in scid mice depleted of WBCs. Likewise, loss of Stat 1 function profoundly impaired the innate resistance of rag2^-/- mice to HSV-1.

Conclusion

Lymphocyte-deficient mice possess a very tangible innate resistance to HSV-1 infection, but this resistance is not dependent upon NK cells.

Viral modulation of NK cell immunity

Natural killer (NK) cells have been implicated in innate immune responses against viruses such as herpesviruses, which cause persistent infections in the host. In response to the selective pressure that is exerted by NK cells, many viruses have evolved strategies either to evade detection by NK cells or to modulate the activity of NK cells. Here, we review the unique relationship that exists between NK cells and viruses, with a focus on herpesviruses.

Natural killer cells utilize both perforin and gamma interferon to regulate murine cytomegalovirus infection in the spleen and liver

Natural killer (NK) cells are critical for innate regulation of the acute phase of murine cytomegalovirus (MCMV) infection and have been reported to utilize perforin (Pfp)- and gamma interferon (IFN-gamma)-dependent effector mechanisms in an organ-specific manner to regulate MCMV infection in the spleen and liver. In this study, we further examined the roles of NK cells, Pfp, and IFN-gamma in innate immunity to MCMV infection. With the recently described NK cell-deficient (NKD) mouse, we confirmed previous findings that NK cells, but not NKT cells, are required for control of the acute phase of MCMV infection in spleen and liver cells. Interestingly, we found that Pfp and IFN-gamma are each important for regulating MCMV replication in both the spleen and the liver. Moreover, NK cells can regulate MCMV infection in the spleens and livers of Pfp(-/-) mice in a Pfp-independent manner and can use an IFN-gamma-independent mechanism to control MCMV infection in IFN-gamma(-/-) mice. Thus, contrary to previous reports, NK cells utilize both Pfp and IFN-gamma to control MCMV infection in the spleen and liver.

Mapping of the BALB/c Ly49 cluster defines a minimal natural killer cell receptor gene repertoire

The BALB/c inbred mouse strain is one of the most commonly used for immunological studies and is an animal model for natural killer (NK) cell function during pathogen infection and tumorigenesis. To understand better NK cell function in this strain, the complete BALB/c Ly49 haplotype was deduced. The BALB/c haplotype spans approximately 300 kb with a gene order and content of Ly49q, e, x, i, g, l, c, and a. Functional BALB/c alleles of Ly49q and e were isolated and found to be conserved. The BALB/c cluster represents a minimal haplotype as it contains many fewer functional genes than the 129 or B6 mouse strains. The small number of BALB/c Ly49 genes is due mainly to an absent group of genes (relative to B6 and 129) between Ly49x and i, although other smaller deletions are present. These gene deletions provide a genetic basis for the lack of certain Ly49-associated NK cell functions in this mouse strain. Finally, the mapping of a third Ly49 haplotype reveals that the basic murine Ly49 repertoire is composed of three framework gene pairs (Ly49q and e, Ly49i and g, and Ly49c and a) that are interspersed with variable numbers of strain-specific Ly49.

Re-evaluating natural resistance to herpes simplex virus type 1

It is often stated that individuals of a species can differ significantly in their innate resistance to infection with herpes simplex virus type 1 (HSV-1). Three decades ago Lopez reported that C57BL/6 mice could survive a 5,000-fold-higher inoculum of HSV-1 given intraperitoneally than mice of the A or BALB/c strain (Nature 258:152-153, 1975). Susceptible strains of mice died of encephalitis-like symptoms, suggesting that viral spread to the central nervous system was the cause of death. Although Lopez's study documented that C57BL/6 mice were resistant to the development of HSV-1 encephalitis and mortality, the resistance of C57BL/6 mice to other steps of the HSV-1 infection process was not assessed. The results of the present study extend these observations to clarify the difference between resistance to (i) HSV-1 pathogenesis, (ii) HSV-1 replication, (iii) HSV-1 spread, and (iv) the establishment of latent HSV-1 infection. Although C57BL/6 mice are more resistant to HSV-1 pathogenesis than BALB/c mice, the results of the present study establish that HSV-1 enters, replicates, spreads, and establishes latent infections with virtually identical efficiencies in C57BL/6 and BALB/c mice. These observations raise questions about the validity of the inference that differences in natural resistance are relevant in explaining what differentiates humans with recurrent herpetic disease from the vast majority of asymptomatic carriers of HSV-1 and HSV-2.

The host response to herpes simplex virus infection

PURPOSE OF REVIEW

Infection with herpes simplex virus remains a significant cause of disease. The host immune system plays an important role in containing viral replication, and there has been considerable progress in defining which components of immunity are key to the resolution of infection. Nevertheless, effective immunoprophylaxis or immunotherapy has not yet been achieved.

RECENT FINDINGS

Recent work has focused on understanding the early events leading to the herpes simplex virus-specific immune response, in particular on the role of antigen-presenting dendritic cells. Herpes simplex virus has evolved a number of ways of interfering with antigen presentation by dendritic cells, thus presumably impeding or delaying the host immune response. Nevertheless, herpes simplex virus triggers strong cellular and humoral immunity. The ability of dendritic cells to take up dead or dying infected cells and cross-present them to cognate T cells may be the key to resolving this apparent paradox. Interaction between dendritic cell subsets, and particularly the virus-induced release of type I interferons may be essential to drive efficient antigen cross-presentation and subsequent T-cell activation.

SUMMARY

A greater understanding of the importance of dendritic cells in driving viral immunity, and of the ligands that activate these cells and the cytokines they secrete, has provided novel vaccination strategies. The delivery of immunomodulatory genes together with viral antigens, for example by DNA vaccination, may harness the full potential of dendritic cells, and achieve the goal of effective immunological control of herpes simplex virus.

Herpes simplex virus-specific CD8+ T cells can clear established lytic infections from skin and nerves and can partially limit the early spread of virus after cutaneous inoculation

HSV infects skin or mucosal epithelium as well as entering the sensory nerves and ganglia. We have used TCR-transgenic T cells specific for the immunodominant class I-restricted determinant from HSV glycoprotein B (gB) combined with a flank zosteriform model of infection to examine the ability of CD8+ T cells to deal with infection. During the course of zosteriform disease, virus rapidly spreads from the primary inoculation site in the skin to sensory dorsal root ganglia and subsequently reappears in the distal flank. Virus begins to be cleared from all sites about 5 days after infection when gB-specific CD8+ T cells first appear within infected tissues. Although activated gB-specific effectors can partially limit virus egress from the skin, they do so only at the earliest times after infection and are ineffective at halting the progression of zosteriform disease once virus has left the inoculation site. In contrast, these same T cells can completely clear ongoing lytic replication if transferred into infected immunocompromised RAG-1-/- mice. Therefore, we propose that the role of CD8+ T cells during the normal course of disease is to clear replicating virus after infection is well established rather than limit the initial spread of HSV from the primary site of inoculation.

Independent Control ofLy49gAlleles: Implications for NK Cell Repertoire Selection and Tumor Cell Killing

A novel murine NK cell-reactive mAb, AT8, was generated. AT8 recognizes Ly49G from 129/J, BALB/c, and related mouse strains, but does not bind to Ly49G(B6). Costaining with AT8 and a Ly49G(B6)-restricted Ab (Cwy-3) provides the first direct evidence that Ly49G protein is expressed from both alleles on a significant proportion of NK cells from four different types of F(1) hybrid mice. The observed level of biallelic Ly49G expression reproducibly followed the product rule in both freshly isolated and cultured NK cells. Surprisingly, the percentage of NK cells expressing both Ly49G alleles could be dramatically increased in vitro and in vivo through IL-2R- and IFN receptor-dependent signaling pathways, respectively. Unexpectedly, Ly49G(B6+) NK cells in an H-2(d), but not H-2(b), background were more likely to lyse D(d+) and Chinese hamster ovary tumor cells than Ly49G(BALB/129+) NK cells. Furthermore, Ly49G(B6+) NK cells also proliferated to a higher degree in response to poly(I:C) than NK cells expressing a non-Ly49G(B6) allele in an H-2(d), but not H-2(b), background. These results suggest that Ly49G(B6) has a lower affinity for H-2D(d) than Ly49G(BALB/129), and the genetic background calibrates the responsiveness of NK cells bearing self-specific Ly49. Other H-2D(d) receptors on the different Ly49G(+) NK cell subsets were unequally coexpressed, possibly explaining the disparate responses of Ly49G(B6+) NK cells in different hybrid mice. These data indicate that the stochastic mono- and biallelic expression of divergent Ly49G alleles increases the range of MHC affinities and the functional potential in the total NK cell population of heterozygous mice.

A locus on mouse chromosome 6 that determines resistance to herpes simplex virus also influences reactivation, while an unlinked locus augments resistance of female mice

During studies to determine a role for tumor necrosis factor (TNF) in herpes simplex virus type 1 (HSV-1) infection using TNF receptor null mutant mice, we discovered a genetic locus, closely linked to the TNF p55 receptor (Tnfrsf1a) gene on mouse chromosome 6 (c6), that determines resistance or susceptibility to HSV-1. We named this locus the herpes resistance locus, Hrl, and showed that it also mediates resistance to HSV-2. Hrl has at least two alleles, Hrl(r), expressed by resistant strains like C57BL/6 (B6), and Hrl(s), expressed by susceptible strains like 129S6 (129) and BALB/c. Although Hrl is inherited as an autosomal dominant gene, resistance to HSV-1 is strongly sex biased such that female mice are significantly more resistant than male mice. Analysis of backcrosses between resistant B6 and susceptible 129 mice revealed that a second locus, tentatively named the sex modifier locus, Sml, functions to augment resistance of female mice. Besides determining resistance, Hrl is one of several genes involved in the control of HSV-1 replication in the eye and ganglion. Remarkably, Hrl also affects reactivation of HSV-1, possibly by interaction with some unknown gene(s). We showed that Hrl is distinct from Cmv1, the gene that determines resistance to murine cytomegalovirus, which is encoded in the major NK cell complex just distal of p55 on c6. Hrl has been mapped to a roughly 5-centimorgan interval on c6, and current efforts are focused on obtaining a high-resolution map for Hrl.

Immune functions encoded by the natural killer gene complex

There has been marked progress in our understanding of the role of natural killer (NK) cells in immune responses, mainly due to the identification of NK-cell receptors and their ligands. The genes encoding many NK-cell receptors are located in the NK-gene complex (NKC). Here, we review the properties of NKC-encoded receptors, and provide a genomic and conceptual framework for an insight into NK-cell function and biology.

Transgenic expression of the activating natural killer receptor Ly49H confers resistance to cytomegalovirus in genetically susceptible mice

Natural resistance to infection with mouse cytomegalovirus (MCMV) is controlled by a dominant locus, Cmv1. Cmv1 is linked to the Ly49 family of natural killer receptors on distal chromosome 6. While some studies localized Cmv1 as distal to the Ly49 gene cluster, genetic and functional analysis identified Ly49h as a pivotal factor in resistance to MCMV. The role of these two independent genomic domains in MCMV resistance was evaluated by functional complementation using transgenesis of bacterial artificial chromosomes (BAC) in genetically susceptible mice. Phenotypic and genetic characterization of the transgenic animals traced the resistance gene to a single region spanning the Ly49h gene. The appearance of the Ly49H protein in NK cells of transgenic mice coincided with the emergence of MCMV resistance, and there was a threshold Ly49H protein level associated with full recovery. Finally, transgenic expression of Ly49H in the context of either of the two independent susceptibility alleles, Cmv1(sBALB) or Cmv1(sFVB), conferred resistance to MCMV infection. These results demonstrate that Ly49h is necessary and sufficient to confer MCMV resistance, and formally demonstrate allelism between Cmv1 and Ly49h. This panel of transgenic animals provides a unique resource to study possible pleiotropic effect of Cmv1.

Impaired clearance of herpes simplex virus type 1 from mice lacking CD1d or NKT cells expressing the semivariant V alpha 14-J alpha 281 TCR

Ag-presenting molecule CD1 and CD1-restricted NKT cells are known to contribute to defense against a range of infectious pathogens, including some viruses. CD1-restricted NKT cells, a distinct subpopulation of T cells, have striking and rapid effector functions that contribute to host defense, including rapid production of IFN-gamma and IL-4, and activation of NK cells. Consideration of the important contributions of innate and adaptive immunity to clearance of HSV prompted us to investigate the role of CD1 and of NKT cells expressing the V alpha 14-J alpha 281 TCR in the pathogenesis of HSV infection. To address this issue, we compared infection in wild-type mice with that in CD1 gene knockout (GKO) and J alpha 281 GKO mice. In this study, we report impaired clearance of virus and viral Ags, and more florid acute infection in mice lacking CD1 (and by inference, CD1-restricted T cells), in comparison with parental C57BL6 mice. In J alpha 281 GKO mice there was also impairment of virus clearance, resembling that seen in CD1 GKO mice. These results imply roles for the V alpha 14-J alpha 281 subset of NKT cells and for CD1d in control of HSV infection.

Recent progress in herpes simplex virus immunobiology and vaccine research

Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) cause prevalent, chronic infections that have serious outcomes in some individuals. Neonatal herpes may occur when the infant traverses the cervix during maternal genital herpes. Genital herpes is a major risk factor for human immunodeficiency virus type 1 transmission. Considerable efforts have been made to design and test vaccines for HSV, focusing on genital infection with HSV-2. Several protein subunit vaccines based on HSV-2 envelope glycoproteins have reached advanced-phase clinical trials. These antigens were chosen because they are the targets of neutralizing-antibody responses and because they elicit cellular immunity. Encouraging results have been reported in studies of treatment of HSV-seronegative women with a vaccine consisting of truncated glycoprotein D of HSV-2 and a novel adjuvant. Because most sexual HSV transmission occurs during asymptomatic shedding, it is important to evaluate the impact of vaccination on HSV-2 infection, clinically apparent genital herpes, and HSV shedding among vaccine recipients who acquire infection. There are several other attractive formats, including subunit vaccines that target cellular immune responses, live attenuated virus strains, and mutant strains that undergo incomplete lytic replication. HSV vaccines have also been evaluated for the immunotherapy of established HSV infection.

Natural killer cell activation receptors in innate immunity to infection

Natural killer (NK) cells are best known for their capacity to kill tumors but they are also critical in early innate responses to infection, especially herpesviruses. Recent studies indicate that NK cell receptors involved in tumor target specificity are also involved in responses to viral infections.

Direct evidence for interferon-gamma production by effector-memory-type intraepidermal T cells residing at an effector site of immunopathology in fixed drug eruption

Effector-memory T cells are strategically placed to epithelial tissues to provide frontline immune protection against pathogens. Their detrimental effects, however, have been rarely examined because of difficulty in sampling these T cells in pathological settings. Our previous studies suggested persistence of a similar subset of intraepidermal CD8(+) T cells at high frequencies in the lesions of fixed drug eruption, a localized variant of drug-induced dermatoses. In situ activation of this subset resulting in localized epidermal injury can be traced in the lesions after antigen challenge by paired immunohistochemical staining, reverse transcriptase-polymerase chain reaction in situ, and flow cytometry of dispersed cells. Here we show that effector-memory T cells were greatly enriched in these intraepidermal CD8(+) T cells, but not dermal and circulating counterparts, and that they constitutively express an early activation marker CD69 even before challenge. Surprisingly, a large proportion of these T cells expressed immediate effector function as evidenced by the rapid production of high levels of interferon-gamma in situ with much faster kinetics than their counterparts at the mRNA and protein levels after challenge. This was followed by localized epidermal injury. The intracellular cytokine assay ex vivo shows that the great majority of these dispersed T cells produce interferon-gamma. This study provides the first in situ description of the detrimental effects specifically mediated by effector-memory T cells residing at the effector site of immunopathology.

Successful control of viruses by NK cells--a balance of opposing forces?

Natural killer (NK) cells play a crucial role in limiting the severity of diseases caused by a range of viruses. Recent data have shown that the effector functions of NK cells can be specifically stimulated when NK cell activation receptors engage cellular major histocompatibility complex (MHC) class I-like ligands induced after infection or by specific viral gene products. However, to counter this NK cell response viruses have evolved an array of strategies to subvert efficient NK cell activation. These data indicate that the balance of host NK cell responses and viral NK cell escape mechanisms can be strategically poised as each strives for survival.