Interleukin-15 and natural killer and NKT cells play a critical role in innate protection against genital herpes simplex virus type 2 infection

Interleukin-21 receptor signalling is important for innate immune protection against HSV-2 infections

Interleukin (IL) -21 is produced by Natural Killer T (NKT) cells and CD4(+) T cells and is produced in response to virus infections, where IL-21 has been shown to be essential in adaptive immune responses. Cells from the innate immune system such as Natural Killer (NK) cells and macrophages are also important in immune protection against virus. These cells express the IL-21 receptor (IL-21R) and respond to IL-21 with increased cytotoxicity and cytokine production. Currently, however it is not known whether IL-21 plays a significant role in innate immune responses to virus infections. The purpose of this study was to investigate the role of IL-21 and IL-21R in the innate immune response to a virus infection. We used C57BL/6 wild type (WT) and IL-21R knock out (KO) mice in a murine vaginal Herpes Simplex Virus type 2 (HSV-2) infection model to show that IL-21 - IL-21R signalling is indeed important in innate immune responses against HSV-2. We found that the IL-21R was expressed in the vaginal epithelium in uninfected (u.i) WT mice, and expression increased early after HSV-2 infection. IL-21R KO mice exhibited increased vaginal viral titers on day 2 and 3 post infection (p.i.) and subsequently developed significantly higher disease scores and a lower survival rate compared to WT mice. In addition, WT mice infected with HSV-2 receiving intra-vaginal pre-treatment with murine recombinant IL-21 (mIL-21) had decreased vaginal viral titers on day 2 p.i., significantly lower disease scores, and a higher survival rate compared to infected untreated WT controls. Collectively our data demonstrate the novel finding that the IL-21R plays a critical role in regulating innate immune responses against HSV-2 infection.

Role of type 1 natural killer T cells in pulmonary immunity

Mucosal sites are populated by a multitude of innate lymphoid cells and "innate-like" T lymphocytes expressing semiconserved T-cell receptors. Among the latter group, interest in type I natural killer T (NKT) cells has gained considerable momentum over the last decade. Exposure to NKT cell antigens is likely to occur continuously at mucosal sites. For this reason, and as they rapidly respond to stress-induced environmental cytokines, NKT cells are important contributors to immune and inflammatory responses. Here, we review the dual role of mucosal NKT cells during immune responses and pathologies with a particular focus on the lungs. Their role during pulmonary acute and chronic inflammation and respiratory infections is outlined. Whether NKT cells might provide a future attractive therapeutic target for treating human respiratory diseases is discussed.

A Thr/Ser dual residue motif in the cytoplasmic tail of human CD1d is important for the down-regulation of antigen presentation following a herpes simplex virus 1 infection

CD1d-restricted T (natural killer T; NKT) cells are important for controlling herpesvirus infections. Interestingly, herpes simplex virus (HSV) can down-regulate CD1d-mediated activation of NKT cells. We have previously shown that the Thr322 residue in the cytoplasmic tail of human CD1d is important for its intracellular trafficking and functional expression. We proposed that the phosphorylation of T322 is a signal for CD1d lysosomal targeting and subsequent degradation. In the current study, we generated dual mutants by substituting the T322 and S323 residues of wild-type (WT) CD1d with Ala (non-phosphorylatable) or Asp (mimicking phosphorylation) and ectopically expressed them in human embryonic kidney 293 cells. We found that the surface expression levels of the CD1d mutants was in this order: T322AS323A > WT > T322A > S323A > S323D > T322D > T322DS323D. Our results therefore suggest that mimicking the phosphorylation of both T322 and S323 has a cumulative negative effect on the functional expression of CD1d. As previously reported, we also found that upon an HSV infection, antigen presentation by WT CD1d is reduced and the CD1d molecule is degraded. Interestingly, the T322A/S323A double mutation inhibited CD1d degradation and rescued CD1d-mediated antigen presentation following an HSV-1 infection. This suggests that the T322/S323 dyad may be phosphorylated, which then targets CD1d for lysosomal degradation post-infection as a means of immune evasion, explaining (at least in part) the reduced antigen presentation observed. Hence, our findings strongly suggest that T322 and S323 form a dual residue motif that can regulate the functional expression of CD1d during a viral infection.

Immunological control of herpes simplex virus infections

Herpes simplex virus type 1 (HSV-1) is capable of causing a latent infection in sensory neurons that lasts for the lifetime of the host. The primary infection is resolved following the induction of the innate immune response that controls replication of the virus until the adaptive immune response can clear the active infection. HSV-1-specific CD8⁺ T cells survey the ganglionic regions containing latently infected neurons and participate in preventing reactivation of HSV from latency. The long-term residence and migration dynamics of the T cells in the trigeminal ganglia appear to distinguish them from the traditional memory T cell subsets. Recently described tissue resident memory (T_RM) T cells establish residence and survive for long periods in peripheral tissue compartments following antigen exposure. This review focuses on the immune system response to HSV-1 infection. Particular emphasis is placed on the evidence pointing to the HSV-1-specific CD8⁺ T cells in the trigeminal belonging to the T_RM class of memory T cells and the role of T_RM cells in virus infection, pathogenesis, latency, and disease.

The role of invariant natural killer T cells in microbial immunity

Invariant natural killer T cells (iNKT cells) are unique lymphocytes with characteristic features, such as expression of an invariant T-cell antigen receptor (TCR) α-chain, recognition of glycolipid antigens presented by CD1d molecules, and ability to rapidly produce large amounts of cytokines, including interferon-γ (IFN-γ) and interleukin 4 (IL-4) upon TCR stimulation. Many studies have demonstrated that iNKT cells participate in immune response against diverse microbes, including bacteria, fungi, protozoan parasites, and viruses. Generally, these cells play protective roles in host defense against infections. However, in some contexts they play pathogenic roles, by inducing or augmenting inflammation. Recent reports show that iNKT cells recognize glycolipid antigens from pathogenic bacteria including Streptococcus pneumoniae, and they contribute to host defense against infection. iNKT cell responses to these microbial glycolipid antigens are highly conserved between rodents and humans, suggesting that iNKT cells are evolutionally conserved because their invariant TCR is useful in detecting certain pathogens. Furthermore, glycolipid-mediated iNKT cell activation during immunization has adjuvant activity, enhancing humoral and cell-mediated responses. Therefore, iNKT cell activation is an attractive target for developing new vaccines for infectious diseases.

TLR3 impairment in human newborns

Newborns are highly susceptible to viral infections. We hypothesized that this susceptibility could be due to a dysregulated expression of innate virus-sensing receptors, i.e., TLR3, TLR7, TLR8, and TLR9 and the cytosolic receptors retinoic acid-inducible gene I, melanoma differentiation-associated protein 5, protein kinase R, and IFN-γ-inducible protein 16. Cord blood mononuclear cells (CBMCs) expressed mRNA for all these receptors except for TLR3. In peripheral blood mononuclear cells (PBMCs), TLR3 mRNA was preferentially expressed in cytotoxic cells, particularly CD56(dim) NK cells. Cord NK cells in contrast showed low TLR3 mRNA expression and lacked TLR3 protein expression. Cord NK cells did not produce IFN-γ in response to polyinosinic-polycytidylic acid [poly(I:C)], whereas strong IFN-γ production was observed in poly(I:C)-stimulated adult NK cells. Cord NK cells had poor cytotoxic function that was only marginally enhanced by exposure to the TLR3 ligand poly(I:C). Opposite to NK cells from adults, their cytotoxicity was not improved by herpes simplex virus (HSV) exposure and they were unable to kill HSV-infected cells. There were no differences in the TLR3 mRNA levels among men, women, and pregnant women, implying that TLR3 is not under sex hormone control. However, decidual NK cells expressed low levels of TLR3 mRNA, which was attributed to their CD56(bright) phenotype. Our data show that cord blood NK cells have deficient TLR3 expression associated with an inability to respond to poly(I:C) and HSV activation and to kill HSV-infected cells. This might explain why newborns are particularly sensitive to neonatal HSV infections.

HSV-2 regulates monocyte inflammatory response via the Fas/FasL pathway

Monocytic cells represent important cellular elements of the innate and adaptive immune responses in viral infections. We assessed the role of Fas/FasL in promoting monocyte apoptosis during HSV-2 infection by using an in vitro model based on the murine RAW 264.7 monocytic cell line and an in vivo murine model of HSV-2 infection applied to C57BL6, MRL-Fas^lpr/J (Fas−/−) and C3-Fasl^gld/J (FasL−/−) mice. HSV-2 infection of the monocytic cell line led to early induction of apoptosis, with no protective expression of anti-apoptotic Bcl-2. HSV-2 infected monocytes up-regulated Fas and FasL expression early during in vitro infection but were susceptible to Fas induced apoptosis. The vaginal monocytes in the HSV-2 murine model of infection up-regulated FasL expression and were susceptible to Fas induced apoptosis. HSV-2 infection of Fas and FasL- deficient mice led to decreased apoptosis of monocytes and impaired recruitment of NK, CD4+ and CD8+ T cells within the infection sites. The vaginal lavages of HSV-2 infected Fas and FasL- deficient showed decreased production of CXCL9, CXCL10 and TNF-α in comparison to HSV-2 infected wild-type mice strain. The decreased recruitment of immune competent cells was accompanied by delayed virus clearance from the infected tissue. Triggering of the Fas receptor on HSV-2 infected monocytes in vitro up-regulated the expression of CXCL9 chemokines and the cytokine TNF-α. Our study provides novel insights on the role of Fas/FasL pathway not only in apoptosis of monocytes but also in regulating local immune response by monocytes during HSV-2 infection.

Promoting immunity during chronic infection--the therapeutic potential of common gamma-chain cytokines

The continued global burden wrought by chronic infectious disease is unrelenting. Current therapies have curbed the severity of disease for patients, but Human Immunodeficiency Virus (HIV) and Hepatitis B (HBV) infection remain incurable and Mycobacterium tuberculosis (MTB) is rapidly becoming resistant to our existing antibiotics. Much attention has been given to enhancing T cell immunity through the use of certain common gamma-chain cytokines, which have proven to be essential and necessary for T cell survival and function. This article reviews the pre-clinical and clinical literature surrounding IL-2, IL-7, IL-15 and IL-21 and we comment on the potential therapeutic promise of these cytokines as adjuvant treatments for chronic infectious diseases.

HSV-2 specifically down regulates HLA-C expression to render HSV-2-infected DCs susceptible to NK cell killing

Both NK cells and CTLs kill virus-infected and tumor cells. However, the ways by which these killer cells recognize the infected or the tumorigenic cells are different, in fact almost opposite. CTLs are activated through the interaction of the TCR with MHC class I proteins. In contrast, NK cells are inhibited by MHC class I molecules. The inhibitory NK receptors recognize mainly MHC class I proteins and in this regard practically all of the HLA-C proteins are recognized by inhibitory NK cell receptors, while only certain HLA-A and HLA-B proteins interact with these receptors. Sophisticated viruses developed mechanisms to avoid the attack of both NK cells and CTLs through, for example, down regulation of HLA-A and HLA-B molecules to avoid CTL recognition, leaving HLA-C proteins on the cell surface to inhibit NK cell response. Here we provide the first example of a virus that through specific down regulation of HLA-C, harness the NK cells for its own benefit. We initially demonstrated that none of the tested HSV-2 derived microRNAs affect NK cell activity. Then we show that surprisingly upon HSV-2 infection, HLA-C proteins are specifically down regulated, rendering the infected cells susceptible to NK cell attack. We identified a motif in the tail of HLA-C that is responsible for the HSV-2-meduiated HLA-C down regulation and we show that the HLA-C down regulation is mediated by the viral protein ICP47. Finally we show that HLA-C proteins are down regulated from the surface of HSV-2 infected dendritic cells (DCs) and that this leads to the killing of DC by NK cells. Thus, we propose that HSV-2 had developed this unique and surprising NK cell-mediated killing strategy of infected DC to prevent the activation of the adaptive immunity.

Approximately 20% of all humans are latently and asymptomatically infected with HSV-2. This suggests that the virus developed mechanisms to avoid immune cell detection; many of which are still unknown. Infected cells are killed mainly by two lymphocyte populations; NK cells and CTLs that belong to the innate and the adaptive immunity, respectively. While the killing machinery of these two cell types is similar, almost identical, the ways by which they discriminate between infected and uninfected cells is different. CTLs are activated, primarily by DCs, to become effector cells. They then recognize virus-derived peptides in the groove of MHC class I molecules and eliminate the virally infected cells. In contrast, NK cells recognize infected cells through several NK cell activating receptors, while the recognition of MHC class I proteins by NK cells leads to inhibition of NK cell killing. Viruses, such as HIV, developed mechanisms to interfere with the function of both NK cells and CTLs via targeting of specific MHC class I proteins. Here we show that HSV-2 developed a MHC class I-dependent mechanism in which the virus, through specific targeting of HLA-C by the viral protein ICP47, harness the NK cells for its own benefit, probably to avoid the activation of adaptive immune response.

Lack of PD-L1 expression by iNKT cells improves the course of influenza A infection

There is evidence indicating that invariant Natural Killer T (iNKT) cells play an important role in defense against influenza A virus (IAV). However, the effect of inhibitory receptor, programmed death-1 (PD-1), and its ligands, programmed death ligand (PD-L) 1 and 2 on iNKT cells in protection against IAV remains to be elucidated. Here we investigated the effects of these co-stimulatory molecules on iNKT cells in the response to influenza. We discovered that compare to the wild type, PD-L1 deficient mice show reduced sensitivity to IAV infection as evident by reduced weight loss, decreased pulmonary inflammation and cellular infiltration. In contrast, PD-L2 deficient mice showed augmented weight loss, pulmonary inflammation and cellular infiltration compare to the wild type mice after influenza infection. Adoptive transfer of iNKT cells from wild type, PD-L1 or PD-L2 deficient mice into iNKT cell deficient mice recapitulated these findings. Interestingly, in our transfer system PD-L1^−/−-derived iNKT cells produced high levels of interferon-gamma whereas PD-L2^−/−-derived iNKT cells produced high amounts of interleukin-4 and 13 suggesting a role for these cytokines in sensitivity to influenza. We identified that PD-L1 negatively regulates the frequency of iNKT cell subsets in the lungs of IAV infected mice. Altogether, these results demonstrate that lack of PD-L1 expression by iNKT cells reduces the sensitivity to IAV and that the presence of PD-L2 is important for dampening the deleterious inflammatory responses after IAV infection. Our findings potentially have clinical implications for developing new therapies for influenza.

HSV-1 strain McKrae is more neuroinvasive than HSV-1 KOS after corneal or vaginal inoculation in mice

Strains of HSV-1 have been noted to vary in their pathogenesis. We compared the replication of strains KOS and McKrae in mice by two routes of infection, ocular and vaginal. Peripheral replication of KOS was similar (cornea) or attenuated over time (vagina) compared with McKrae; however, McKrae replicated in the nervous system to significantly higher levels than KOS after inoculation by either route. Host genetic background strongly influenced the capacity for virus entry into the nervous system from the vagina. KOS and McKrae replicated equivalently after intracranial inoculation, indicating that McKrae's pathogenic phenotype is linked to neuroinvasiveness rather than neurovirulence.

HSV-2 vaccine: current state and insights into development of a vaccine that targets genital mucosal protection

HSV-2 is one of the most prevalent sexually transmitted infections that result in significant morbidity and financial burden on health systems around the world. Recurrent and asymptomatic re-activation accompanied by viral shedding is common among sero-positive individuals, leading to relatively high efficiency of transmission. Prophylactic HSV-2 vaccines are the best and cheapest option to address the problems associated with HSV-2 infections globally. However, despite persistent efforts, the search for an efficacious vaccine for HSV-2 remains elusive. In this review, the current state of HSV-2 vaccines and the outcome of past human trials are examined. Furthermore, we discuss the evidence and strategies from experimental mouse models that have been successful in inducing protective immunity in the genital tract against HSV-2, following immunization. Future vaccination strategies that focus on induction of robust mucosal immunity in the genital tract may hold the key for a successful vaccine against HSV-2.

Hiding lipid presentation: viral interference with CD1d-restricted invariant natural killer T (iNKT) cell activation

The immune system plays a major role in protecting the host against viral infection. Rapid initial protection is conveyed by innate immune cells, while adaptive immunity (including T lymphocytes) requires several days to develop, yet provides high specificity and long-lasting memory. Invariant natural killer T (iNKT) cells are an unusual subset of T lymphocytes, expressing a semi-invariant T cell receptor together with markers of the innate NK cell lineage. Activated iNKT cells can exert direct cytolysis and can rapidly release a variety of immune-polarizing cytokines, thereby regulating the ensuing adaptive immune response. iNKT cells recognize lipids in the context of the antigen-presenting molecule CD1d. Intriguingly, CD1d-restricted iNKT cells appear to play a critical role in anti-viral defense: increased susceptibility to disseminated viral infections is observed both in patients with iNKT cell deficiency as well as in CD1d- and iNKT cell-deficient mice. Moreover, viruses have recently been found to use sophisticated strategies to withstand iNKT cell-mediated elimination. This review focuses on CD1d-restricted lipid presentation and the strategies viruses deploy to subvert this pathway.

Invariant NKT cells: regulation and function during viral infection

Natural killer T cells (NKT cells) represent a subset of T lymphocytes that express natural killer (NK) cell surface markers. A subset of NKT cells, termed invariant NKT cells (iNKT), express a highly restricted T cell receptor (TCR) and respond to CD1d-restricted lipid ligands. iNKT cells are now appreciated to play an important role in linking innate and adaptive immune responses and have been implicated in infectious disease, allergy, asthma, autoimmunity, and tumor surveillance. Advances in iNKT identification and purification have allowed for the detailed study of iNKT activity in both humans and mice during a variety of chronic and acute infections. Comparison of iNKT function between non-pathogenic simian immunodeficiency virus (SIV) infection models and chronic HIV-infected patients implies a role for iNKT activity in controlling immune activation. In vitro studies of influenza infection have revealed novel effector functions of iNKT cells including IL-22 production and modulation of myeloid-derived suppressor cells, but ex vivo characterization of human iNKT cells during influenza infection are lacking. Similarly, as recent evidence suggests iNKT involvement in dengue virus pathogenesis, iNKT cells may modulate responses to a number of emerging pathogens. This Review will summarize current knowledge of iNKT involvement in responses to viral infections in both human and mouse models and will identify critical gaps in knowledge and opportunities for future study. We will also highlight recent efforts to harness iNKT ligands as vaccine adjuvants capable of improving vaccination-induced cellular immune responses.

B cell and T cell immunity in the female genital tract: potential of distinct mucosal routes of vaccination and role of tissue-associated dendritic cells and natural killer cells

The female genital mucosa constitutes the major port of entry of sexually transmitted infections. Most genital microbial pathogens represent an enormous challenge for developing vaccines that can induce genital immunity that will prevent their transmission. It is now established that long-lasting protective immunity at mucosal surfaces has to involve local B-cell and T-cell effectors as well as local memory cells. Mucosal immunization constitutes an attractive way to generate systemic and genital B-cell and T-cell immune responses that can control early infection by sexually transmitted pathogens. Nevertheless, no mucosal vaccines against sexually transmitted infections are approved for human use. The mucosa-associated immune system is highly compartmentalized and the selection of any particular route or combinations of routes of immunization is critical when defining vaccine strategies against genital infections. Furthermore, mucosal surfaces are complex immunocompetent tissues that comprise antigen-presenting cells and also innate immune effectors and non-immune cells that can act as 'natural adjuvants' or negative immune modulators. The functions of these cells have to be taken into account when designing tissue-specific antigen-delivery systems and adjuvants. Here, we will discuss data that compare different mucosal routes of immunization to generate B-cell and T-cell responses in the genital tract, with a special emphasis on the newly described sublingual route of immunization. We will also summarize data on the understanding of the effector and induction mechanisms of genital immunity that may influence the development of vaccine strategies against genital infections.

T cell memory in the context of persistent herpes viral infections

The generation of a functional memory T cell pool upon primary encounter with an infectious pathogen is, in combination with humoral immunity, an essential process to confer protective immunity against reencounters with the same pathogen. A prerequisite for the generation and maintenance of long-lived memory T cells is the clearance of antigen after infection, which is fulfilled upon resolution of acute viral infections. Memory T cells play also a fundamental role during persistent viral infections by contributing to relative control and immuosurveillance of active replication or viral reactivation, respectively. However, the dynamics, the phenotype, the mechanisms of maintenance and the functionality of memory T cells which develop upon acute/resolved infection as opposed to chronic/latent infection differ substantially. In this review we summarize current knowledge about memory CD8 T cell responses elicited during α-, β-, and γ-herpes viral infections with major emphasis on the induction, maintenance and function of virus-specific memory CD8 T cells during viral latency and we discuss how the peculiar features of these memory CD8 T cell responses are related to the biology of these persistently infecting viruses.

Functions of IL-15 in anti-viral immunity: multiplicity and variety

An effective immune response to an invading viral pathogen requires the combined actions of both innate and adaptive immune cells. For example, NK cells and cytotoxic CD8 T cells are capable of the direct engagement of infected cells and the mediation of antiviral responses. Both NK and CD8 T cells depend on common gamma chain (γc) cytokine signals for their development and homeostasis. The γc cytokine IL-15 is very well characterized for its role in promoting the development and homeostasis of NK cells and CD8 T cells, but emerging literature suggests that IL-15 mediates the anti-viral responses of these cell populations during an active immune response. Both NK cells and CD8 T cells must become activated, migrate to sites of infection, survive at those sites, and expand in order to maximally exert effector functions, and IL-15 can modulate each of these processes. This review focuses on the functions of IL-15 in the regulation of multiple aspects of NK and CD8 T cell biology, investigates the mechanisms by which IL-15 may exert such diverse functions, and discusses how these different facets of IL-15 biology may be therapeutically exploited to combat viral diseases.

Contact-dependent interference with invariant NKT cell activation by herpes simplex virus-infected cells

Invariant CD1d-restricted NKT (iNKT) cells play important roles in generating protective immune responses against infections. In this study, we have investigated the role of human iNKT cells in HSV-1 infection and their interaction with epidermal keratinocytes. These cells express CD1d and are the primary target of the virus. Keratinocytes loaded with α-galactosyl ceramide (α-GalCer) could stimulate IFN-γ production and CD25 upregulation by iNKT cells. However, both α-GalCer-dependent and cytokine-dependent activation of iNKT cells was impaired after coculture with HSV-1-infected cells. Notably, CD1d downregulation was not observed on infected keratinocytes, which were also found to inhibit TCR-independent iNKT cell activation. Further examination of the cytokine profile of iNKT-keratinocyte cocultures showed inhibition of IFN-γ, IL-5, IL-10, IL-13, and IL-17 secretion but upregulation of IL-4 and TNF-α after the infection. Moreover, cell-to-cell contact between infected keratinocytes and iNKT cells was required for the inhibition of activation, as the cell-free supernatants containing virus did not affect activation. Productive infection of iNKT cells was however not required for the inhibitory effect. After coculture with infected cells, iNKT cells were no longer responsive to further stimulation with α-GalCer-loaded CD1d-expressing cells. We found that exposure to HSV-1-infected cells resulted in impaired TCR signaling downstream of ZAP70. Additionally, infected cells upregulated the expression of the negative T cell regulator, galectin-9; however, blocking experiments indicated that the impairment of iNKT cell responses was independent of galectin-9. Thus, interference with activation of human iNKT cells by HSV-1 may represent a novel immunoevasive strategy used by the virus to avoid immune clearance.

Porcine reproductive and respiratory syndrome virus induces interleukin-15 through the NF-κB signaling pathway

Porcine reproductive and respiratory syndrome virus (PRRSV) mainly infects macrophages/dendritic cells and modulates cytokine expression in these cells. Interleukin-15 (IL-15) is a pleiotropic cytokine involved in wide range of biological activities. It has been shown to be essential for the generation, activation, and proliferation of NK and NKT cells and for the survival and activation of CD8(+) effector and memory T cells. In this study, we discovered that PRRSV infection upregulated IL-15 production at both the mRNA and protein levels in porcine alveolar macrophages (PAMs), blood monocyte-derived macrophages (BMo), and monocyte-derived dendritic cells (DCs). We subsequently demonstrated that the NF-κB signaling pathway was essential for PRRSV infection-induced IL-15 production. First, addition of an NF-κB inhibitor drastically reduced PRRSV infection-induced IL-15 production. We then found that NF-κB was indeed activated upon PRRSV infection, as evidenced by IκB phosphorylation and degradation. Moreover, we revealed an NF-κB binding motif in the cloned porcine IL-15 (pIL-15) promoter, deletion of which abrogated the pIL-15 promoter activity in PRRSV-infected alveolar macrophages. In addition, we demonstrated that PRRSV nucleocapsid (N) protein had the ability to induce IL-15 production in porcine alveolar macrophage cell line CRL2843 by transient transfection, which was mediated by its multiple motifs, and it also activated NF-κB. These data indicated that PRRSV infection-induced IL-15 production was likely through PRRSV N protein-mediated NF-κB activation. Our findings provide new insights into the molecular mechanisms underling the IL-15 production induced by PRRSV infection.

Critical role of natural killer cells in lung immunopathology during influenza infection in mice

Influenza viral infection results in excessive pulmonary inflammation that has been linked to the damage caused by immune responses and viral replication. The multifunctional cytokine interleukin (IL-15), influences the proliferation and maintenance of immune cells such as CD8(+) T cells and natural killer (NK) cells. Here we show that IL-15(-/-) mice are protected from lethal influenza infection. Irrespective of the mouse strains, the protection observed was linked to the lack of NK cells. Increased survival in the IL-15(-/-) or NK1.1(+) cell-depleted wild-type mice was associated with significantly lower lung lesions as well as decreased mononuclear cells and neutrophils in the airway lumen. Levels of interleukin 10 were significantly higher and levels of proinflammatory cytokines, including interleukin 6 and interleukin 12, were significantly lower in the bronchoalveolar lavage fluid from IL-15(-/-) and NK1.1(+) cell-depleted wild-type mice than in that from control mice. Our data suggest that NK cells significantly augment pulmonary inflammation, contributing to the pathogenesis of influenza infection.

Comparison of the host immune response to herpes simplex virus 1 (HSV-1) and HSV-2 at two different mucosal sites

A study was undertaken to compare the host immune responses to herpes simplex virus 1 (HSV-1) and HSV-2 infection by the ocular or genital route in mice. Titers of HSV-2 from tissue samples were elevated regardless of the route of infection. The elevation in titers of HSV-2, including cell infiltration and cytokine/chemokine levels in the central nervous system relative to those found following HSV-1 infection, was correlative with inflammation. These results underscore a dichotomy between the host immune responses to closely related alphaherpesviruses.

Genital HSV-2 infection induces short-term NK cell memory

NK cells are known as innate immune cells that lack immunological memory. Recently, it has been shown that NK cells remember encounters with chemical haptens that induce contact hypersensitivity and cytomegalovirus infection. Here, we show the existence of NK cell memory following HSV-2 infection. Stimulation with HSV-2 Ags led to higher IFNγ production in NK cells that were exposed 30 days previously to HSV-2, compared to NK cells from naïve mice. More importantly, this increased production of IFNγ in NK cells was independent of B- and T- lymphocytes and specific for the HSV-2 Ags. We also showed that previously exposed NK cells in a B- and T-lymphocyte free environment mediate protection against HSV-2 infection and they are necessary for the protection of mice against HSV-2 infection. Collectively, NK cells remember prior HSV-2 encounters independent of B- and T- lymphocytes leading to protection against HSV-2 mediated morbidity and mortality upon re-exposure.

Interleukin-22 is produced by invariant natural killer T lymphocytes during influenza A virus infection: potential role in protection against lung epithelial damages

Invariant natural killer T (iNKT) cells are non-conventional lipid-reactive αβ T lymphocytes that play a key role in host responses during viral infections, in particular through the swift production of cytokines. Their beneficial role during experimental influenza A virus (IAV) infection has recently been proposed, although the mechanisms involved remain elusive. Here we show that during in vivo IAV infection, mouse pulmonary iNKT cells produce IFN-γ and IL-22, a Th17-related cytokine critical in mucosal immunity. Although permissive to viral replication, IL-22 production by iNKT cells is not due to IAV infection per se of these cells but is indirectly mediated by IAV-infected dendritic cells (DCs). We show that activation of the viral RNA sensors TLR7 and RIG-I in DCs is important for triggering IL-22 secretion by iNKT cells, whereas the NOD-like receptors NOD2 and NLRP3 are dispensable. Invariant NKT cells respond to IL-1β and IL-23 provided by infected DCs independently of the CD1d molecule to release IL-22. In vitro, IL-22 protects IAV-infected airway epithelial cells against mortality but has no role on viral replication. Finally, during early IAV infection, IL-22 plays a positive role in the control of lung epithelial damages. Overall, IAV infection of DCs activates iNKT cells, providing a rapid source of IL-22 that might be beneficial to preserve the lung epithelium integrity.

The role of interleukin-15 in inflammation and immune responses to infection: implications for its therapeutic use

Interleukin-15 (IL-15) is a pleiotropic cytokine with a broad range of biological functions in many diverse cell types. It plays a major role in the development of inflammatory and protective immune responses to microbial invaders and parasites by modulating immune cells of both the innate and adaptive immune systems. This review provides an overview of the mechanisms by which IL-15 modulates the host response to infectious agents and its utility as a cytokine adjuvant in vaccines against infectious pathogens.

Herpes simplex virus-2 in the genital mucosa: insights into the mucosal host response and vaccine development

PURPOSE OF REVIEW

Herpes simplex virus (HSV)-2 is the predominant cause of genital herpes and has been implicated in HIV infection and transmission. Thus far, vaccines developed against HSV-2 have been clinically ineffective in preventing infection. This review aims to summarize the innate and adaptive immune responses against HSV-2 and examines the current status of vaccine development.

RECENT FINDINGS

Both innate and adaptive immune responses are essential for an effective primary immune response and the generation of immunity. The innate response involves Toll-like receptors, natural killer cells, plasmacytoid dendritic cells, and type I, II, and III interferons. The adaptive response requires a balance between CD4+ and CD8+ T-cells for optimal viral clearance. T-regulatory cells may be involved, although their exact function has yet to be determined. Current vaccine development involves the use of HSV-2 peptides or attenuated/replication-defective HSV-2 to generate adaptive anti-HSV-2 immune responses, however the generation of innate responses may also be an important consideration.

SUMMARY

Although vaccine development has primarily focused on the adaptive response, arguments for innate involvement are emerging. A greater understanding of the innate and adaptive processes underlying the response to HSV-2 infection will provide the foundation for the development of an effective vaccine.

Primary immunodeficiency associated with defects in CD1 and CD1-restricted T cells

CD1 is a family of atypical MHC class I molecules that present various endogenous and exogenous lipid antigens to CD1-restricted T cells. While little is known about the function of CD1a-, CD1b-, and CD1c-restricted lipid-reactive T cells due to their absence in mice, CD1d-restricted natural killer T (NKT) cells have been extensively studied since their description almost 20 years ago. NKT cells, effector memory cells that share characteristics of innate and adaptive lymphocytes, are among the earliest responders in immune reactions and have broad effects on the activation of other immune cell lineages, including NK cells, T cells, and B cells. Accordingly, studies in mice have revealed critical roles of NKT cells in infectious, malignant, and autoimmune diseases. The recent description of primary immunodeficiencies associated with defects in CD1 and CD1-restricted T cells has provided a unique opportunity to study the biological role of lipid antigen presentation in human disease. Intriguingly, these studies revealed that defects in lipid immunity are associated with susceptibility to selected infectious and malignant diseases but not with broad immunodeficiency.

Disseminated varicella infection caused by varicella vaccine strain in a child with low invariant natural killer T cells and diminished CD1d expression

BACKGROUND

Live attenuated varicella vaccine is considered a safe vaccine with serious adverse effects reported only in immunocompromised children. We describe a severe life-threatening infection with varicella vaccine virus causing rash and pneumonitis in a 6-year-old boy with no apparent immunodeficiency.

METHODS AND RESULTS

Polymerase chain reaction (PCR) analysis of vesicle swab samples demonstrated varicella zoster virus (VZV). Sequencing of the PCR product demonstrated 100% homology with human herpesvirus 3 strain VZV-Oka ORF62 gene. Routine immunologic investigations failed to demonstrate any abnormality. Total leukocyte, lymphocyte, and neutrophil counts and lymphocyte subsets were normal. Immunoglobulins, C3, C4, and CH50 were intact. Specific IgG to protein and polysaccharide antigens and to Epstein-Barr virus and cytomegalovirus were present. Normal lymphocyte proliferation to phytohemagglutinin and VZV antigens was detected. Neutrophil function and natural killer (NK) cell activity were normal. The analysis of invariant NK T (iNKT) cell numbers and function revealed diminished iNKT cells, reported once previously and unique to our patient, deficient expression of the cognate receptor, CD1d.

CONCLUSIONS

This report provides a further link between deficiency of the iNKT/CD1d pathway and increased susceptibility to varicella vaccine virus, suggesting an important role of this innate pathway in host defense against yet another member of the herpesvirus family.

A detrimental role for invariant natural killer T cells in the pathogenesis of experimental dengue virus infection

Dengue virus (DENV), a member of the mosquito-borne flaviviruses, is a serious public health problem in many tropical countries. We assessed the in vivo physiologic contribution of invariant natural killer T (iNKT) cells, a population of nonconventional lipid-reactive αβ T lymphocytes, to the host response during experimental DENV infection. We used a mouse-adapted DENV serotype 2 strain that causes a disease that resembles severe dengue in humans. On DENV challenge, splenic and hepatic iNKT cells became activated insofar as CD69 and Fas ligand up-regulation and interferon-γ production. C57BL/6 mice deficient in iNKT cells (Jα18(-/-)) were more resistant to lethal infection than were wild-type animals, and the phenotype was reversed by adoptive transfer of iNKT cells to Jα18(-/-) animals. The absence of iNKT cells in Jα18(-/-) mice was associated with decreased systemic and local inflammatory responses, less liver injury, diminished vascular leak syndrome, and reduced activation of natural killer cells and neutrophils. iNKT cell functions were not necessary for control of primary DENV infection, after either natural endogenous activation or exogenous activation with the canonical iNKT cell agonist α-galactosylceramide. Together, these data reveal a novel and critical role for iNKT cells in the pathogenesis of severe experimental dengue disease.

Low numbers and altered phenotype of invariant natural killer T cells in recurrent varicella zoster virus infection

Invariant natural killer T (iNKT) cells play an important role in the immune response against various infectious agents. In this study we investigated their role in human defense against the varicella zoster virus. We observed decreased numbers of iNKT cells in patients who failed to control latent varicella zoster virus infection, e.g. underwent several reactivations of the virus. The residual population of iNKT cells expressed significantly higher levels of inhibitory receptor CD158a that was further up-regulated in the course of acute viral infection. Both of these abnormalities might contribute to impaired control of varicella zoster virus in human.

Clinical and immunological aspects of recurrent, bacterial and viral skin affections

The authors analyzed the subpopulation composition of lymphocytes in the peripheral blood of 69 patients suffering from chronic recurrent furunculosis and herpes at the stage of remission. They revealed similar changes in the form of an increased content of T-helpers and immunoregulatory index. In herpes patients such changes are accompanied with the reduced number of natural killers while in furunculosis patients changes are related to the reduced amount of activated T-lymphocytes. These changes do not depend on the relapse duration or frequency but are related to the number of exacerbations in the course of the disease.

Potential role of invariant NKT cells in the control of pulmonary inflammation and CD8+ T cell response during acute influenza A virus H3N2 pneumonia

Influenza A virus (IAV) infection results in a highly contagious respiratory illness leading to substantial morbidity and occasionally death. In this report, we assessed the in vivo physiological contribution of invariant NKT (iNKT) lymphocytes, a subset of lipid-reactive αβ T lymphocytes, on the host response and viral pathogenesis using a virulent, mouse-adapted, IAV H3N2 strain. Upon infection with a lethal dose of IAV, iNKT cells become activated in the lungs and bronchoalveolar space to become rapidly anergic to further restimulation. Relative to wild-type animals, C57BL/6 mice deficient in iNKT cells (Jα18(-/-) mice) developed a more severe bronchopneumonia and had an accelerated fatal outcome, a phenomenon reversed by the adoptive transfer of NKT cells prior to infection. The enhanced pathology in Jα18(-/-) animals was not associated with either reduced or delayed viral clearance in the lungs or with a defective local NK cell response. In marked contrast, Jα18(-/-) mice displayed a dramatically reduced IAV-specific CD8(+) T cell response in the lungs and in lung-draining mediastinal lymph nodes. We further show that this defective CD8(+) T cell response correlates with an altered accumulation and maturation of pulmonary CD103(+), but not CD11b(high), dendritic cells in the mediastinal lymph nodes. Taken together, these findings point to a role for iNKT cells in the control of pneumonia as well as in the development of the CD8(+) T cell response during the early stage of acute IAV H3N2 infection.

The Role of Plasmacytoid Dendritic Cells in Innate and Adaptive Immune Responses against Alpha Herpes Virus Infections

In 1999, two independent groups identified plasmacytoid dendritic cells (PDC) as major type I interferon- (IFN-) producing cells in the blood. Since then, evidence is accumulating that PDC are a multifunctional cell population effectively coordinating innate and adaptive immune responses. This paper focuses on the role of different immune cells and their interactions in the surveillance of alpha herpes virus infections, summarizes current knowledge on PDC surface receptors and their role in direct cell-cell contacts, and develops a risk factor model for the clinical implications of herpes simplex and varicella zoster virus reactivation. Data from studies involving knockout mice and cell-depletion experiments as well as human studies converge into a "spider web", in which the direct and indirect crosstalk between many cell populations tightly controls acute, latent, and recurrent alpha herpes virus infections. Notably, cells involved in innate immune regulations appear to shape adaptive immune responses more extensively than previously thought.

NK cells require type I IFN receptor for antiviral responses during genital HSV-2 infection

Type I interferon (IFN) signalling, NK cells and NK cell-derived IFN-γ are critical in the early control of genital HSV-2 infection. We have recently reported that NK cells are the source of early IFN-γ in the genital tract in response to HSV-2. However, the response of NK cells to genital HSV-2 infection is not well defined in the context of type I IFN signalling. Here we show that HSV-2 replication was significantly higher in mice deficient in the type I IFN receptor or NK cells compared to wild type controls. There was no detectable IFN-γ production in the genital washes from IFN-α/βR(-/-) mice or NK cell depleted mice in response to HSV-2 infection compared to control mice. Absence of the type I IFN receptor does not alter homing of NK cells to the genital mucosa. Moreover, the absence of IL-12 had no significant effect on NK cell-derived IFN-γ. Surprisingly, IFN-α/βR(-/-) mice had more IL-15 positive cells in the genital mucosa in response to HSV-2 infection compared to control mice. We then examined the expression of IL-15 receptors on NK cells. There was no significant differences in the levels of IL-15 receptor expression on NK cells from IFN-α/βR(-/-) or control mice. Our data clearly suggest that type I IFN receptor signalling is essential for NK cell activation in response to genital HSV-2 infection, and propose that NK cell activation by IL-15 may involve type I IFNs.

A critical role for IL-15 in TLR-mediated innate antiviral immunity against genital HSV-2 infection

Innate antiviral immunity, particularly at mucosal surfaces, has a critical role in early control of viral infections. Both type I interferons (IFNs) and interleukin-15 (IL-15) are essential components of innate antiviral immunity. It has been shown that toll-like receptor (TLR) ligand-induced innate antiviral immunity requires IFN-α/β and -λ receptor signaling. However, it is not known if IL-15 has a role in TLR ligand-mediated antiviral responses. Here, we report that ligands for TLR-3 and TLR-9 cannot confer protection against genital herpes simplex virus-2 (HSV-2) in the absence of IL-15 in vivo. Interestingly, wild-type mice depleted of natural killer (NK) cells and treated with TLR ligands are protected upon HSV-2 challenge, suggesting that the critical role of IL-15 is independent of NK cell-mediated activity. To examine the cytokine response in the absence of IL-15, we investigated TLR ligand-induced IFN-β and -λ production in the vaginal washes, but found no impairment in IL-15(-/-) mice. Finally, we report no impairment in the expression of the IFN-stimulated genes in IL-15(-/-) mice. Collectively, the data suggest that TLR ligands induce an IFN-mediated response in the vaginal tract of both wild-type and IL-15(-/-) mice, but its induction is insufficient for providing protection against HSV-2 in the absence of IL-15.

IL-15 and type I interferon are required for activation of tumoricidal NK cells by virus-infected dendritic cells

There is increasing evidence that natural killer (NK) cells play an important role in antitumor immunity following dendritic cell (DC) vaccination. Little is known, however, about the optimal stimulation of DCs that favors NK activation in tumor-bearing hosts. In this study, we demonstrate that treatment with toll-like receptor (TLR) ligands and infection with a mutant vesicular stomatitis virus (VSV-ΔM51) both induced DC maturation. Further, inoculation of these DCs led to robust NK-mediated protection against tumor challenge. Strikingly, only VSV-ΔM51-infected DCs were capable of suppressing the growth of established tumors, suggesting that additional signals provided by viral infection may be required to activate tumoricidal NK cells in tumor-bearing hosts. VSV-ΔM51 infection of DCs induced greater type I interferon (IFN I) production than TLR ligand treatment, and disruption of the IFN I pathway in DCs eliminated their ability to induce NK activation and tumor protection. However, further studies indicated that IFN I alone was not sufficient to activate NK cells, especially in the presence of a tumor, and DC-derived IL-15 was additionally required for tumoricidal NK activation. These results suggest that induction of IFN I by VSV-ΔM51 allows DCs to overcome tumor-associated immunosuppression and facilitate IL-15-mediated priming of tumoricidal NK cells. Thus, the mode of DC maturation should be carefully considered when designing DC-based cancer immunotherapies.

Loss of the type I interferon pathway increases vulnerability of mice to genital herpes simplex virus 2 infection

The mouse model of genital herpes relies on medoxyprogesterone treatment of female mice to render the vaginal lumen susceptible to inoculation with herpes simplex virus 2 (HSV-2). In the present study, we report that mice deficient in the A1 chain of the type I interferon receptor (CD118(-/-)) are susceptible to HSV-2 in the absence of medroxyprogesterone preconditioning. In the absence of hormone pretreatment, 2,000 PFU of a clinical isolate of HSV-2 was sufficient to establish a productive infection in the vagina of 75% ± 17% and in the spinal cord of 71% ± 14% of CD118(-/-) mice, whereas the same dose of HSV-2 replicated to detectable levels in only 13% ± 13% of vaginal samples and 0% of spinal cord samples from wild-type mice, as determined at day 5 postinfection. The susceptibility to HSV-2 infection in the CD118(-/-) mice was associated with a significant reduction in the infiltration of HSV-specific cytotoxic T lymphocytes into the vaginal tissue, the local production of gamma interferon (IFN-γ), and the expression of T cell-recruiting chemokines CCL5, CXCL9, and CXCL10. Collectively, the results underscore the significant contribution of type I IFNs in resistance to genital HSV-2 infection.

The TLR7 agonist R848 alleviates allergic inflammation by targeting invariant NKT cells to produce IFN-gamma

It has been documented that TLR7 stimulation triggers not only antiviral responses, but also alleviates experimental asthma. Considering the implication of invariant NKT (iNKT) cells in both situations, we postulated that they might contribute to the anti-inflammatory effect of TLR7 ligands. We show in this study that spleen cells activated by the TLR7 agonist resiquimod (R848) attenuate allergic inflammation upon adoptive transfer when they are recovered from wild-type, but not from iNKT cell-deficient Jα18(-/-) mice, which proves the specific involvement of this regulatory population. Furthermore, we provide evidence that IFN-γ is critical for the protective effect, which is lost when transferred iNKT cells are sorted from IFN-γ-deficient mice. In support of a direct activation of iNKT cells through TLR7 signaling in vivo, we observed a prompt increase of serum IFN-γ levels, associated with upregulation of CD69 expression on iNKT cells. Moreover, we demonstrate that iNKT cells effectively express TLR7 and respond to R848 in vitro by producing high levels of IFN-γ in the presence of IL-12, consistent with the conclusion that their contribution to the alleviation of allergic inflammation upon treatment with TLR7 ligands is mediated through IFN-γ.

Impact of valency of a glycoprotein B-specific monoclonal antibody on neutralization of herpes simplex virus

Herpes simplex virus (HSV) glycoprotein B (gB) is an integral part of the multicomponent fusion system required for virus entry and cell-cell fusion. Here we investigated the mechanism of viral neutralization by the monoclonal antibody (MAb) 2c, which specifically recognizes the gB of HSV type 1 (HSV-1) and HSV-2. Binding of MAb 2c to a type-common discontinuous epitope of gB resulted in highly efficient neutralization of HSV at the postbinding/prefusion stage and completely abrogated the viral cell-to-cell spread in vitro. Mapping of the antigenic site recognized by MAb 2c to the recently solved crystal structure of the HSV-1 gB ectodomain revealed that its discontinuous epitope is only partially accessible within the observed multidomain trimer conformation of gB, likely representing its postfusion conformation. To investigate how MAb 2c may interact with gB during membrane fusion, we characterized the properties of monovalent (Fab and scFv) and bivalent [IgG and F(ab')(2)] derivatives of MAb 2c. Our data show that the neutralization capacity of MAb 2c is dependent on cross-linkage of gB trimers. As a result, only bivalent derivatives of MAb 2c exhibited high neutralizing activity in vitro. Notably, bivalent MAb 2c not only was capable of preventing mucocutaneous disease in severely immunodeficient NOD/SCID mice upon vaginal HSV-1 challenge but also protected animals even with neuronal HSV infection. We also report for the first time that an anti-gB specific monoclonal antibody prevents HSV-1-induced encephalitis entirely independently from complement activation, antibody-dependent cellular cytotoxicity, and cellular immunity. This indicates the potential for further development of MAb 2c as an anti-HSV drug.

Heterogeneity in phenotype and function of CD8+ and CD4/CD8 double-negative Natural Killer T cell subsets in sooty mangabeys

BACKGROUND

We have recently reported the presence of CD8(+) and CD4/8 double-negative (DN) natural killer T (NKT) lymphocytes in sooty mangabeys. To investigate differences in the two NKT cell subsets, we compared the phenotype and function of sooty mangabey CD8(+) and DN NKT cells.

METHODS

Flow-sorted NKT lymphocytes from one SIV-negative sooty mangabey were subjected to limiting dilution cloning. Invariant NKT clones were characterized by flow cytometry and cytokine ELISA.

RESULTS

The majority of NKT clones displayed an effector memory phenotype and expressed CXCR3 and NKG2D. While CD8(+) NKT subsets expressed significantly higher levels of granzyme B and perforin and produced more IFN-gamma, the DN NKT subsets secreted significantly more IL-4, IL-13, and IL-10.

CONCLUSIONS

The Th1 and Th2 cytokine bias of CD8(+) and DN NKT cells, respectively, indicates the presence of functionally heterogeneous populations of NKT cells in sooty mangabeys.

Paucity of CD4+ natural killer T (NKT) lymphocytes in sooty mangabeys is associated with lack of NKT cell depletion after SIV infection

Lack of chronic immune activation in the presence of persistent viremia is a key feature that distinguishes nonpathogenic simian immunodeficiency virus (SIV) infection in natural hosts from pathogenic SIV and HIV infection. To elucidate novel mechanisms downmodulating immune activation in natural hosts of SIV infection, we investigated natural killer T (NKT) lymphocytes in sooty mangabeys. NKT lymphocytes are a potent immunoregulatory arm of the innate immune system that recognize glycolipid antigens presented on the nonpolymorphic MHC-class I-like CD1d molecules. In a cross-sectional analysis of 50 SIV-negative and 50 naturally SIV-infected sooty mangabeys, ligand alpha-galactosylceramide loaded CD1d tetramers co-staining with Valpha24-positive invariant NKT lymphocytes were detected at frequencies >or=0.002% of circulating T lymphocytes in approximately half of the animals. In contrast to published reports in Asian macaques, sooty mangabey NKT lymphocytes consisted of CD8(+) and CD4/CD8 double-negative T lymphocytes that were CXCR3-positive and CCR5-negative suggesting that they trafficked to sites of inflammation without being susceptible to SIV infection. Consistent with these findings, there was no difference in the frequency or phenotype of NKT lymphocytes between SIV-negative and SIV-infected sooty mangabeys. On stimulation with alpha-galactosylceramide loaded on human CD1d molecules, sooty mangabey NKT lymphocytes underwent degranulation and secreted IFN-gamma, TNF-alpha, IL-2, IL-13, and IL-10, indicating the presence of both effector and immunoregulatory functional capabilities. The unique absence of CD4(+) NKT lymphocytes in sooty mangabeys, combined with their IL-10 cytokine-secreting ability and preservation following SIV infection, raises the possibility that NKT lymphocytes might play a role in downmodulating immune activation in SIV-infected sooty mangabeys.

The role of dendritic cells in alphaherpesvirus infections: archetypes and paradigms

Dendritic cells (DCs) play a critical role in orchestrating both innate and adaptive components of the immune system and are therefore of pivotal importance in the initiation of immune responses to control and eliminate viral infections. A major focus of this review is to give an overview on the recent findings that point out the importance of DCs in controlling alphaherpesvirus infections, but also indicate that these viruses have evolved several strategies to inhibit and/or exploit DC functions to delay or escape elimination by the immune system. In addition, we point out the common features and interspecies differences between DCs from man and animal, and discuss the potential use of animal alphaherpesvirus homologues to gain further insights into the interaction between alphaherpesviruses and DCs in their natural virus-host environment. Finally, recent knowledge on the potential of alphaherpesviruses as vectors for DC stimulation and their use for immunotherapy is presented.

NKT cells: friend or foe during viral infections?

NKT cells are innate-like T lymphocytes that are found in rodents and primates. They are non-conventional T cells restricted by the CD1d molecule that presents self and exogenous glycolipids. NKT cells are unique in their ability to promptly secrete copious amounts of cytokines such as IFN-gamma and IL-4. Once activated, NKT cells can provide maturation signals to downstream cells, including DC, NK cells, and lymphocytes, thereby contributing to both innate and acquired immunity. Accordingly, NKT cells can influence a wide array of immune responses, including tumor surveillance, maintenance of self-tolerance and anti-infectious defenses. Studies performed with NKT-cell-deficient mice have shown that these cells are critical for the clearance of various pathogens. During bacterial infections, NKT cells can be activated either indirectly by DC or directly by bacterial lipid antigens presented by CD1d. Although viruses do not contain lipid antigens, NKT cells have also been implicated in antiviral responses. The capacity of NKT cells to regulate viral immune-surveillance, either constitutively or post-activation, makes them an attractive clinical target. In this review, we summarize recent publications dealing with the functions and relevance of NKT cells in the context of viral infections, both in murine models and in humans.

Direct CD1d-mediated stimulation of APC IL-12 production and protective immune response to virus infection in vivo

CD1d-restricted NKT cells rapidly stimulate innate and adaptive immunity through production of Th1 and/or Th2 cytokines and induction of CD1d(+) APC maturation. However, therapeutic exploitation of NKT cells has been hampered by their paucity and defects in human disease. NKT cell-APC interactions can be modeled by direct stimulation of human APCs through CD1d in vitro. We have now found that direct ligation with multiple CD1d mAbs also stimulated bioactive IL-12 release from CD1d(+) but not CD1d knockout murine splenocytes in vitro. Moreover, all of the CD1d mAbs tested also induced IL-12 as well as both IFN-gamma and IFN-alpha in vivo from CD1d(+) but not CD1d-deficient recipients. Unlike IFN-gamma, CD1d-induced IFN-alpha was at least partially dependent on invariant NKT cells. Optimal resistance to infection with picornavirus encephalomyocarditis virus is known to require CD1d-dependent APC IL-12-induced IFN-gamma as well as IFN-alpha. CD1d ligation in vivo enhanced systemic IL-12, IFN-gamma, and IFN-alpha and was protective against infection by encephalomyocarditis virus, suggesting an alternative interpretation for previous results involving CD1d "blocking" in other systems. Such protective responses, including elevations in Th1 cytokines, were also seen with CD1d F(ab')(2)s in vivo, whereas an IgM mAb (with presumably minimal tissue penetration) was comparably effective at protection in vivo as well as cytokine induction both in vivo and in vitro. Although presumably acting immediately "downstream," CD1d mAbs were protective later during infection than the invariant NKT cell agonist alpha-galactosylceramide. These data indicate that NKT cells can be bypassed with CD1d-mediated induction of robust Th1 immunity, which may have therapeutic potential both directly and as an adjuvant.

Herpes simplex virus (HSV)-specific T cells activated in the absence of IFN-gamma express alternative effector functions but are not protective against genital HSV-2 infection

Interferon gamma (IFNgamma) is important for immune resistance to herpes simplex virus (HSV) infection. To examine the influence of IFNgamma on the development of HSV-specific immune responses and test for IFNgamma-independent adaptive immune mechanisms of protection, IFNgamma-deficient mice (IFNgamma(-/-)) were immunized with thymidine kinase-deficient HSV-2 (HSV-2 333tk(-)). HSV-specific cellular and humoral responses were elicited in immunized IFNgamma(-/-) mice resulting in increased resistance relative to non-immune C57BL/6J (B6) mice following challenge with fully virulent HSV-2. CD8(+) T cells from IFNgamma(-/-) mice displayed cytotoxic activity and secreted TNFalpha. HSV-specific CD4(+) T cells from immunized IFNgamma(-/-) mice secreted IL-4, TNFalpha, and IL-17, but unlike T cells from HSV-immune B6 mice, could not clear virus from genital tissue following adoptive transfer. HSV-immune IFNgamma(-/-) mice produced predominantly IgG(1) HSV-specific antibodies while immune B6 mice produced predominantly IgG(2c) antibodies. Transfer of equivalent amounts of HSV-specific antibodies from either strain to naïve mice imparted equivalent early resistance against infection of the genital epithelia. However, protection against neurological symptoms mediated by immune B6 antibodies was superior late in infection. Taken together, these results demonstrate that the limited resistance of HSV-immune IFNgamma(-/-) mice to HSV-2 infection resulted from the action of HSV-specific Ab rather than IFNgamma-independent effector functions of T cells. Further, protection against neurological manifestations of HSV-2 infection was superior in mice receiving Ab from immune B6 mice suggesting that Ab-mediated protective mechanisms involving IFNgamma-induced IgG subclasses were more effective once virus had spread to neural tissues.

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.