Granzymes and caspase 3 play important roles in control of gammaherpesvirus latency

The innate and T-cell mediated immune response during acute and chronic gammaherpesvirus infection

Immediately after entry into host cells, viruses are sensed by the innate immune system, leading to the activation of innate antiviral effector mechanisms including the type I interferon (IFN) response and natural killer (NK) cells. This innate immune response helps to shape an effective adaptive T cell immune response mediated by cytotoxic T cells and CD4+ T helper cells and is also critical for the maintenance of protective T cells during chronic infection. The human gammaherpesvirus Epstein-Barr virus (EBV) is a highly prevalent lymphotropic oncovirus that establishes chronic lifelong infections in the vast majority of the adult population. Although acute EBV infection is controlled in an immunocompetent host, chronic EBV infection can lead to severe complications in immunosuppressed patients. Given that EBV is strictly host-specific, its murine homolog murid herpesvirus 4 or MHV68 is a widely used model to obtain in vivo insights into the interaction between gammaherpesviruses and their host. Despite the fact that EBV and MHV68 have developed strategies to evade the innate and adaptive immune response, innate antiviral effector mechanisms still play a vital role in not only controlling the acute infection but also shaping an efficient long-lasting adaptive immune response. Here, we summarize the current knowledge about the innate immune response mediated by the type I IFN system and NK cells, and the adaptive T cell-mediated response during EBV and MHV68 infection. Investigating the fine-tuned interplay between the innate immune and T cell response will provide valuable insights which may be exploited to design better therapeutic strategies to vanquish chronic herpesviral infection.

Widespread discrepancy in Nnt genotypes and genetic backgrounds complicates granzyme A and other knockout mouse studies

Granzyme A (GZMA) is a serine protease secreted by cytotoxic lymphocytes, with Gzma-/- mouse studies having informed our understanding of GZMA's physiological function. We show herein that Gzma-/- mice have a mixed C57BL/6J and C57BL/6N genetic background and retain the full-length nicotinamide nucleotide transhydrogenase (Nnt) gene, whereas Nnt is truncated in C57BL/6J mice. Chikungunya viral arthritis was substantially ameliorated in Gzma-/- mice; however, the presence of Nnt and the C57BL/6N background, rather than loss of GZMA expression, was responsible for this phenotype. A new CRISPR active site mutant C57BL/6J GzmaS211A mouse provided the first insights into GZMA's bioactivity free of background issues, with circulating proteolytically active GZMA promoting immune-stimulating and pro-inflammatory signatures. Remarkably, k-mer mining of the Sequence Read Archive illustrated that ≈27% of Run Accessions and ≈38% of BioProjects listing C57BL/6J as the mouse strain had Nnt sequencing reads inconsistent with a C57BL/6J genetic background. Nnt and C57BL/6N background issues have clearly complicated our understanding of GZMA and may similarly have influenced studies across a broad range of fields.

The immunoregulatory function of polyphenols: implications in cancer immunity

Polyphenols have demonstrated several potential biological activities, notably antitumoral activity dependent on immune function. In the present review, we describe studies that investigated antitumor immune responses influenced by polyphenols and the mechanisms by which polyphenols improve the immune response. We also discuss the limitations in related areas, especially unexplored areas of research, and next steps required to develop a therapeutic approach utilizing polyphenols in oncology.

Staying alive: cell death in antiviral immunity

Programmed cell death is an integral part of host defense against invading intracellular pathogens. Apoptosis, programmed necrosis, and pyroptosis each serve to limit pathogen replication in infected cells, while simultaneously promoting the inflammatory and innate responses that shape effective long-term host immunity. The importance of carefully regulated cell death is evident in the spectrum of inflammatory and autoimmune disorders caused by defects in these pathways. Moreover, many viruses encode inhibitors of programmed cell death to subvert these host responses during infection, thereby facilitating their own replication and persistence. Thus, as both virus and cell vie for control of these pathways, the battle for survival has shaped a complex host-pathogen interaction. This review will discuss the multifaceted role that programmed cell death plays in maintaining the immune system and its critical function in host defense, with a special emphasis on viral infections.

Is murine gammaherpesvirus-68 (MHV-68) a suitable immunotoxicological model for examining immunomodulatory drug-associated viral recrudescence?

Immunosuppressive agents are used for treatment of a variety of autoimmune diseases including rheumatoid arthritis (RA), systemic lupus erythematosis (SLE), and psoriasis, as well as for prevention of tissue rejection after organ transplantation. Recrudescence of herpesvirus infections, and increased risk of carcinogenesis from herpesvirus-associated tumors are related with immunosuppressive therapy in humans. Post-transplant lymphoproliferative disorder (PTLD), a condition characterized by development of Epstein Barr Virus (EBV)-associated B-lymphocyte lymphoma, and Kaposi's Sarcoma (KS), a dermal tumor associated with Kaposi Sarcoma-associated virus (KSHV), may develop in solid organ transplant patients. KS also occurs in immunosuppressed Acquired Immunodeficiency (AIDS) patients. Kaposi Sarcoma-associated virus (KSHV) is a herpes virus genetically related to EBV. Murine gammaherpes-virus-68 (MHV-68) is proposed as a mouse model of gammaherpesvirus infection and recrudescence and may potentially have relevance for herpesvirus-associated neoplasia. The pathogenesis of MHV-68 infection in mice mimics EBV/KSHV infection in humans with acute lytic viral replication followed by dissemination and establishment of persistent latency. MHV-68-infected mice may develop lymphoproliferative disease that is accelerated by disruption of the immune system. This manuscript first presents an overview of gammaherpesvirus pathogenesis and immunology as well as factors involved in viral recrudescence. A description of different types of immunodeficiency then follows, with particular focus on viral association with lymphomagenesis after immunosuppression. Finally, this review discusses different gammaherpesvirus animal models and describes a proposed MHV-68 model to further examine the interplay of immunomodulatory agents and gammaherpesvirus-associated neoplasia.

Ataxia telangiectasia mutated kinase controls chronic gammaherpesvirus infection

Gammaherpesviruses, such as Epstein-Barr virus (EBV), are ubiquitous cancer-associated pathogens that interact with DNA damage response, a tumor suppressor network. Chronic gammaherpesvirus infection and pathogenesis in a DNA damage response-insufficient host are poorly understood. Ataxia-telangiectasia (A-T) is associated with insufficiency of ataxia-telangiectasia mutated (ATM), a critical DNA damage response kinase. A-T patients display a pattern of anti-EBV antibodies suggestive of poorly controlled EBV replication; however, parameters of chronic EBV infection and pathogenesis in the A-T population remain unclear. Here we demonstrate that chronic gammaherpesvirus infection is poorly controlled in an animal model of A-T. Intriguingly, in spite of a global increase in T cell activation and numbers in wild-type (wt) and ATM-deficient mice in response to mouse gammaherpesvirus 68 (MHV68) infection, the generation of an MHV68-specific immune response was altered in the absence of ATM. Our finding that ATM expression is necessary for an optimal adaptive immune response against gammaherpesvirus unveils an important connection between DNA damage response and immune control of chronic gammaherpesvirus infection, a connection that is likely to impact viral pathogenesis in an ATM-insufficient host.

Zinc finger antiviral protein inhibits murine gammaherpesvirus 68 M2 expression and regulates viral latency in cultured cells

Zinc finger antiviral protein (ZAP) is a host factor that specifically inhibits the replication of certain viruses by binding to specific viral mRNAs and repressing mRNA expression. Here we report that ZAP inhibits expression of murine gammaherpesvirus 68 (MHV-68) M2, which plays important roles in establishment and maintenance of viral latency. Downregulation of endogenous ZAP in cells harboring latent MHV-68 promoted lytic replication of the virus. These results suggest that ZAP inhibits M2 expression and regulates the maintenance of MHV-68 latency.

Granzyme B regulates antiviral CD8+ T cell responses

CTLs and NK cells use the perforin/granzyme cytotoxic pathway to kill virally infected cells and tumors. Human regulatory T cells also express functional granzymes and perforin and can induce autologous target cell death in vitro. Perforin-deficient mice die of excessive immune responses after viral challenges, implicating a potential role for this pathway in immune regulation. To further investigate the role of granzyme B in immune regulation in response to viral infections, we characterized the immune response in wild-type, granzyme B-deficient, and perforin-deficient mice infected with Sendai virus. Interestingly, granzyme B-deficient mice, and to a lesser extent perforin-deficient mice, exhibited a significant increase in the number of Ag-specific CD8(+) T cells in the lungs and draining lymph nodes of virally infected animals. This increase was not the result of failure in viral clearance because viral titers in granzyme B-deficient mice were similar to wild-type mice and significantly less than perforin-deficient mice. Regulatory T cells from WT mice expressed high levels of granzyme B in response to infection, and depletion of regulatory T cells from these mice resulted in an increase in the number of Ag-specific CD8(+) T cells, similar to that observed in granzyme B-deficient mice. Furthermore, granzyme B-deficient regulatory T cells displayed defective suppression of CD8(+) T cell proliferation in vitro. Taken together, these results suggest a role for granzyme B in the regulatory T cell compartment in immune regulation to viral infections.

A quarter century of granzymes

Granzymes (Grs) were discovered just over a quarter century ago. They are produced by cytotoxic T cells and natural killer cells and are released upon interaction with target cells. Intensive biochemical, genetic, and biological studies have been performed in order to study their roles in immunity and inflammation. This review summarizes research on the family of Grs.

Pathogenesis and host control of gammaherpesviruses: lessons from the mouse

Gammaherpesviruses are lymphotropic viruses that are associated with the development of lymphoproliferative diseases, lymphomas, as well as other nonlymphoid cancers. Most known gammaherpesviruses establish latency in B lymphocytes. Research on Epstein-Barr virus (EBV) and murine gammaherpesvirus 68 (MHV68/γHV68/MHV4) has revealed a complex relationship between virus latency and the stage of B cell differentiation. Available data support a model in which gammaherpesvirus infection drives B cell proliferation and differentiation. In general, the characterized gammaherpesviruses exhibit a very narrow host tropism, which has severely limited studies on the human gammaherpesviruses EBV and Kaposi's sarcoma-associated herpesvirus. As such, there has been significant interest in developing animal models in which the pathogenesis of gammaherpesviruses can be characterized. MHV68 represents a unique model to define the effects of chronic viral infection on the antiviral immune response.

Orphan granzymes find a home

Cytotoxic lymphocytes are armed with granules that are released in the granule-exocytosis pathway to kill tumor cells and virus-infected cells. Cytotoxic granules contain the pore-forming protein perforin and a family of structurally homologues serine proteases called granzymes. While perforin facilitates the entry of granzymes into a target cell, the latter initiate distinct apoptotic routes. Granzymes are also implicated in extracellular functions such as extracellular matrix degradation, immune regulation, and inflammation. The family of human granzymes consists of five members, of which granzyme A and B have been studied most extensively. Recently, elucidation of the specific characteristics of the other three human granzymes H, K, and M, also referred to as orphan granzymes, have started. In this review, we summarize and discuss what is currently known about the biology of the human orphan granzymes.

Granzyme C supports efficient CTL-mediated killing late in primary alloimmune responses

It is well established that granzymes A and B play a role in CTL killing of target cells by the perforin-dependent granule exocytosis pathway. The functions of multiple additional granzymes expressed in CTL are less well defined. In the present studies, CTL generated from mice deficient in dipeptidyl peptidase 1 (DPP1) were used to investigate the contribution of granzyme C to CTL killing of allogeneic target cells. DPP1 is required for activation of granzymes A and B by proteolytic removal of their N-terminal dipeptide prodomains while a significant portion of granzyme C is processed normally in the absence of DPP1. Cytotoxicity of DPP1(-/-) CTL generated in early (5-day) MLC in vitro and in peritoneal exudate cells 5 days after initial allogeneic sensitization in vivo was significantly impaired compared with wild-type CTL. Following 3 days of restimulation with fresh allogeneic stimulators however, cytotoxicity of these DPP1(-/-) effector cells was comparable to that of wild-type CTL. Killing mediated by DPP1(-/-) CTL following restimulation was rapid, perforin dependent, Fas independent and associated with early mitochondrial injury, phosphatidyl serine externalization, and DNA degradation, implicating a granzyme-dependent apoptotic pathway. The increased cytotoxicity of DPP1(-/-) CTL following restimulation coincided with increased expression of granzyme C. Moreover, small interfering RNA inhibition of granzyme C expression during restimulation significantly decreased cytotoxicity of DPP1(-/-) but not wild-type CTL. These results indicate that during late primary alloimmune responses, granzyme C can support CTL-mediated killing by the granule exocytosis pathway in the absence of functional granzymes A or B.

Antibody-independent control of gamma-herpesvirus latency via B cell induction of anti-viral T cell responses

B cells can use antibody-dependent mechanisms to control latent viral infections. It is unknown whether this represents the sole function of B cells during chronic viral infection. We report here that hen egg lysozyme (HEL)-specific B cells can contribute to the control of murine gamma-herpesvirus 68 (gammaHV68) latency without producing anti-viral antibody. HEL-specific B cells normalized defects in T cell numbers and proliferation observed in B cell-/- mice during the early phase of gammaHV68 latency. HEL-specific B cells also reversed defects in CD8 and CD4 T cell cytokine production observed in B cell-/- mice, generating CD8 and CD4 T cells necessary for control of latency. Furthermore, HEL-specific B cells were able to present virally encoded antigen to CD8 T cells. Therefore, B cells have antibody independent functions, including antigen presentation, that are important for control of gamma-herpesvirus latency. Exploitation of this property of B cells may allow enhanced vaccine responses to chronic virus infection.

Immune control of HSV-1 latency

A hallmark of the herpes family of viruses is their ability to cause recurrent disease. Upon primary infection, Herpes Simplex virus (HSV) establishes a latent infection in sensory neurons that persists for the life of the individual. Reactivation of these latent viral genomes with virion formation is the source of virus for most HSV recurrent disease. This review details recent exciting findings supporting a role for the host immune system, particularly CD8+ T cells in maintaining HSV-1 in a latent state.

Cow milk is not responsible for most gastrointestinal immune-like syndromes--evidence from a population-based study

BACKGROUND

Gastrointestinal hypersensitivity to cow milk (CM) may be more common among school-aged children and young adults than previously thought.

OBJECTIVE

The objective was to study various gastrointestinal complaints and the immunologic mechanisms associated with food-related, especially CM-related, gastrointestinal disorders in young adults.

DESIGN

Of 827 subjects aged 16-21 y who completed a questionnaire on food-related gastrointestinal symptoms, 49 symptomatic subjects agreed to a clinical examination, including an interview, blood tests, a lactose-maldigestion test, a blinded CM challenge and, in severely symptomatic subjects (n = 12), an endoscopic examination. Twenty-nine subjects served as controls.

RESULTS

Approximately 10% of the subjects reported having major gastrointestinal symptoms, mainly food-related (n = 70 of 86), during the preceding year. Specific organic disease was found in 2 symptomatic subjects: 1 case of celiac disease and 1 of colitis. The result of the lactose-maldigestion test was positive in 16 of the remaining 47 symptomatic subjects, but only 4 carried the C/C(-13910) genotype for adult-type hypolactasia. The symptomatic subjects had restricted their consumption of certain foods, particularly CM. However, in a blinded challenge, CM-induced symptoms were rare. The symptomatic subjects had higher plasma soluble intercellular adhesion molecule 1 (P = 0.007) and lower granzyme A (P = 0.001) concentrations than did the control subjects. Duodenal biopsy samples tended to have higher intraepithelial CD3(+) cell counts (P = 0.065) and a higher expression of transforming growth factor beta (P = 0.073) and interleukin 12p35 messenger RNA (P = 0.075) than did the control subjects.

CONCLUSIONS

In an unselected cohort of young adults, 8% reported food-related gastrointestinal symptoms. The finding of immunologic activity implied the existence of a food-related gastrointestinal syndrome but not one induced by CM.

Alpha/beta interferons regulate murine gammaherpesvirus latent gene expression and reactivation from latency

Alpha/beta interferon (IFN-alpha/beta) protects the host from virus infection by inhibition of lytic virus replication in infected cells and modulation of the antiviral cell-mediated immune response. To determine whether IFN-alpha/beta also modulates the virus-host interaction during latent virus infection, we infected mice lacking the IFN-alpha/beta receptor (IFN-alpha/betaR(-/-)) and wild-type (wt; 129S2/SvPas) mice with murine gammaherpesvirus 68 (gammaHV68), a lymphotropic gamma-2-herpesvirus that establishes latent infection in B cells, macrophages, and dendritic cells. IFN-alpha/betaR(-/-) mice cleared low-dose intranasal gammaHV68 infection with wt kinetics and harbored essentially wt frequencies of latently infected cells in both peritoneum and spleen by 28 days postinfection. However, latent virus in peritoneal cells and splenocytes from IFN-alpha/betaR(-/-) mice reactivated ex vivo with >40-fold- and 5-fold-enhanced efficiency, respectively, compared to wt cells. Depletion of IFN-alpha/beta from wt mice during viral latency also significantly increased viral reactivation, demonstrating an antiviral function of IFN-alpha/beta during latency. Viral reactivation efficiency was temporally regulated in both wt and IFN-alpha/betaR(-/-) mice. The mechanism of IFN-alpha/betaR action was distinct from that of IFN-gammaR, since IFN-alpha/betaR(-/-) mice did not display persistent virus replication in vivo. Analysis of viral latent gene expression in vivo demonstrated specific upregulation of the latency-associated gene M2, which is required for efficient reactivation from latency, in IFN-alpha/betaR(-/-) splenocytes. These data demonstrate that an IFN-alpha/beta-induced pathway regulates gammaHV68 gene expression patterns during latent viral infection in vivo and that IFN-alpha/beta plays a critical role in inhibiting viral reactivation during latency.

A surface groove essential for viral Bcl-2 function during chronic infection in vivo

Antiapoptotic Bcl-2 family proteins inhibit apoptosis in cultured cells by binding BH3 domains of proapoptotic Bcl-2 family members via a hydrophobic BH3 binding groove on the protein surface. We investigated the physiological importance of the BH3 binding groove of an antiapoptotic Bcl-2 protein in mammals in vivo by analyzing a viral Bcl-2 family protein. We show that the γ-herpesvirus 68 (γHV68) Bcl-2 family protein (γHV68 v-Bcl-2), which is known to inhibit apoptosis in cultured cells, inhibits both apoptosis in primary lymphocytes and Bax toxicity in yeast. Nuclear magnetic resonance determination of the γHV68 v-Bcl-2 structure revealed a BH3 binding groove that binds BH3 domain peptides from proapoptotic Bcl-2 family members Bax and Bak via a molecular mechanism shared with host Bcl-2 family proteins, involving a conserved arginine in the BH3 peptide binding groove. Mutations of this conserved arginine and two adjacent amino acids to alanine (SGR to AAA) within the BH3 binding groove resulted in a properly folded protein that lacked the capacity of the wild-type γHV68 v-Bcl-2 to bind Bax BH3 peptide and to block Bax toxicity in yeast. We tested the physiological importance of this v-Bcl-2 domain during viral infection by engineering viral mutants encoding a v-Bcl-2 containing the SGR to AAA mutation. This mutation resulted in a virus defective for both efficient reactivation of γHV68 from latency and efficient persistent γHV68 replication. These studies demonstrate an essential functional role for amino acids in the BH3 peptide binding groove of a viral Bcl-2 family member during chronic infection.

Viruses can manipulate their hosts by expressing proteins that structurally and functionally resemble host cellular proteins. One important cellular process manipulated by viruses is apoptosis, a cell death program that is regulated by a family of Bcl-2-like proapoptotic and antiapoptotic proteins. Gammaherpesviruses encode Bcl-2 family proteins (v-Bcl-2) that may contribute to their ability to cause tumors and persist for the lifetime of their hosts. The authors solved the structure of the murine γ-herpesvirus 68 (γHV68) v-Bcl-2 and found that it is similar to cellular antiapoptotic proteins and that v-Bcl-2 uses the same mechanism as cellular Bcl-2 to bind to peptides from proapoptotic Bcl-2 family proteins. Furthermore, they found that a γHV68 virus expressing a mutated form of v-Bcl-2 that cannot bind to peptides from proapoptotic Bcl-2 family proteins is defective in its ability to cause chronic viral infection in mice. Thus, a specific structural feature and molecular mechanism of the v-Bcl-2 that is shared with host antiapoptotic Bcl-2 proteins is important for the function of this protein during viral infection. These findings enhance our understanding of the molecular mechanisms of chronic γ-herpesvirus infection, and suggest that targeting the functions of the v-Bcl-2 protein might have therapeutic benefit.

Functional Analysis of Granzyme M and Its Role in Immunity to Infection

Cytotoxic lymphocytes express a large family of granule serine proteases, including one member, granzyme (Grz)M, with a unique protease activity, restricted expression, and distinct gene locus. Although a number of Grzs, including GrzM, have been shown to mediate target cell apoptosis in the presence of perforin, the biological activity of Grz has been restricted to control of a number of viral pathogens, including two natural mouse pathogens, ectromelia, and murine CMV (MCMV). In this article, we describe the first reported gene targeting of GrzM in mice. GrzM-deficient mice display normal NK cell/T cell development and homeostasis and intact NK cell-mediated cytotoxicity of tumor targets as measured by membrane damage and DNA fragmentation. GrzM-deficient mice demonstrated increased susceptibility to MCMV infection typified by the presence of more viral inclusions and transiently higher viral burden in the visceral organs of GrzM-deficient mice compared with wild-type (WT) mice. The cytotoxicity of NK cells from MCMV-infected GrzM-deficient mice remained unchanged and, like WT control mice, GrzM-deficient mice eventually effectively cleared MCMV infection from the visceral organs. In contrast, GrzM-deficient mice were as resistant as WT control mice to mouse pox ectromelia infection, as well as challenge with a number of NK cell-sensitive tumors. These data confirm a role for GrzM in the host response to MCMV infection, but suggest that GrzM is not critical for NK cell-mediated cytotoxicity.

Granzyme B and the Downstream Granzymes C and/or F Are Important for Cytotoxic Lymphocyte Functions

Although the functions of granzyme A (GzmA) and GzmB are well-defined, a number of orphan granzymes of unknown function are also expressed in cytotoxic lymphocytes. Previously, we showed that a targeted loss-of-function mutation for GzmB was associated with reduced expression of several downstream orphan granzyme genes in the lymphokine-activated killer cell compartment. To determine whether this was caused by the retained phosphoglycerate kinase I gene promoter (PGK-neo) cassette in the GzmB gene, we retargeted the GzmB gene with a LoxP-flanked PGK-neo cassette, then removed the cassette in embryonic stem cells by transiently expressing Cre recombinase. Mice homozygous for the GzmB null mutation containing the PGK-neo cassette (GzmB-/-/+PGK-neo) displayed reduced expression of the closely linked GzmC and F genes in their MLR-derived CTLs and lymphokine-activated killer cells; removal of the PGK-neo cassette (GzmB-/-/DeltaPGK-neo) restored the expression of both genes. Cytotoxic lymphocytes derived from mice with the retained PGK-neo cassette (GzmB-/-/+PGK-neo) had a more severe cytotoxic defect than those deficient for GzmB only (GzmB-/-/DeltaPGK-neo). Similarly, GzmB-/-/+PGK-neo mice displayed a defect in the allogeneic clearance of P815 tumor cells, whereas GzmB-/-/DeltaPGK-neo mice did not. These results suggest that the retained PGK-neo cassette in the GzmB gene causes a knockdown of GzmC and F expression, and also suggest that these granzymes are relevant for the function of cytotoxic lymphocytes in vitro and in vivo.