Noncytotoxic inhibition of cytomegalovirus replication through NK cell protease granzyme M-mediated cleavage of viral phosphoprotein 71

Granzymes in health and diseases: the good, the bad and the ugly

Granzymes are a family of serine proteases, composed of five human members: GA, B, H, M and K. They were first discovered in the 1980s within cytotoxic granules released during NK cell- and T cell-mediated killing. Through their various proteolytic activities, granzymes can trigger different pathways within cells, all of which ultimately lead to the same result, cell death. Over the years, the initial consideration of granzymes as mere cytotoxic mediators has changed due to surprising findings demonstrating their expression in cells other than immune effectors as well as new intracellular and extracellular activities. Additional roles have been identified in the extracellular milieu, following granzyme escape from the immunological synapse or their release by specific cell types. Outside the cell, granzyme activities mediate extracellular matrix alteration via the degradation of matrix proteins or surface receptors. In certain contexts, these processes are essential for tissue homeostasis; in others, excessive matrix degradation and extensive cell death contribute to the onset of chronic diseases, inflammation, and autoimmunity. Here, we provide an overview of both the physiological and pathological roles of granzymes, highlighting their utility while also recognizing how their unregulated presence can trigger the development and/or worsening of diseases.

Widespread and dynamic expression of granzyme C by skin-resident antiviral T cells

After recognition of cognate antigen (Ag), effector CD8+ T cells secrete serine proteases called granzymes in conjunction with perforin, allowing granzymes to enter and kill target cells. While the roles for some granzymes during antiviral immune responses are well characterized, the function of others, such as granzyme C and its human ortholog granzyme H, is still unclear. Granzyme C is constitutively expressed by mature, cytolytic innate lymphoid 1 cells (ILC1s). Whether other antiviral effector cells also produce granzyme C and whether it is continually expressed or responsive to the environment is unknown. To explore this, we analyzed granzyme C expression in different murine skin-resident antiviral lymphocytes. At steady-state, dendritic epidermal T cells (DETCs) expressed granzyme C while dermal γδ T cells did not. CD8+ tissue-resident memory T cells (TRM) generated in response to cutaneous viral infection with the poxvirus vaccinia virus (VACV) also expressed granzyme C. Both DETCs and virus-specific CD8+ TRM upregulated granzyme C upon local VACV infection. Continual Ag exposure was not required for maintained TRM expression of granzyme C, although re-encounter with cognate Ag boosted expression. Additionally, IL-15 treatment increased granzyme C expression in both DETCs and TRM. Together, our data demonstrate that granzyme C is widely expressed by antiviral T cells in the skin and that expression is responsive to both environmental stimuli and TCR engagement. These data suggest that granzyme C may have functions other than killing in tissue-resident lymphocytes.

Natural killer cells induce HIV-1 latency reversal after treatment with pan-caspase inhibitors

The establishment of a latency reservoir is the major obstacle for a cure of HIV-1. The shock-and-kill strategy aims to reactivate HIV-1 replication in HIV -1 latently infected cells, exposing the HIV-1-infected cells to cytotoxic lymphocytes. However, none of the latency reversal agents (LRAs) tested so far have shown the desired effect in people living with HIV-1. We observed that NK cells stimulated with a pan-caspase inhibitor induced latency reversal in co-cultures with HIV-1 latently infected cells. Synergy in HIV-1 reactivation was observed with LRAs prostratin and JQ1. The supernatants of the pan-caspase inhibitor-treated NK cells activated the HIV-1 LTR promoter, indicating that a secreted factor by NK cells was responsible for the HIV-1 reactivation. Assessing changes in the secreted cytokine profile of pan-caspase inhibitor-treated NK cells revealed increased levels of the HIV-1 suppressor chemokines MIP1α (CCL3), MIP1β (CCL4) and RANTES (CCL5). However, these cytokines individually or together did not induce LTR promoter activation, suggesting that CCL3-5 were not responsible for the observed HIV-1 reactivation. The cytokine profile did indicate that pan-caspase inhibitors induce NK cell activation. Altogether, our approach might be-in combination with other shock-and-kill strategies or LRAs-a strategy for reducing viral latency reservoirs and a step forward towards eradication of functionally active HIV-1 in infected individuals.

Oxidative and Non-Oxidative Antimicrobial Activities of the Granzymes

Cell-mediated cytotoxicity is an essential immune defense mechanism to fight against viral, bacterial or parasitic infections. Upon recognition of an infected target cell, killer lymphocytes form an immunological synapse to release the content of their cytotoxic granules. Cytotoxic granules of humans contain two membrane-disrupting proteins, perforin and granulysin, as well as a homologous family of five death-inducing serine proteases, the granzymes. The granzymes, after delivery into infected host cells by the membrane disrupting proteins, may contribute to the clearance of microbial pathogens through different mechanisms. The granzymes can induce host cell apoptosis, which deprives intracellular pathogens of their protective niche, therefore limiting their replication. However, many obligate intracellular pathogens have evolved mechanisms to inhibit programed cells death. To overcome these limitations, the granzymes can exert non-cytolytic antimicrobial activities by directly degrading microbial substrates or hijacked host proteins crucial for the replication or survival of the pathogens. The granzymes may also attack factors that mediate microbial virulence, therefore directly affecting their pathogenicity. Many mechanisms applied by the granzymes to eliminate infected cells and microbial pathogens rely on the induction of reactive oxygen species. These reactive oxygen species may be directly cytotoxic or enhance death programs triggered by the granzymes. Here, in the light of the latest advances, we review the antimicrobial activities of the granzymes in regards to their cytolytic and non-cytolytic activities to inhibit pathogen replication and invasion. We also discuss how reactive oxygen species contribute to the various antimicrobial mechanisms exerted by the granzymes.

Noncytotoxic functions of killer cell granzymes in viral infections

Cytotoxic lymphocytes produce granules armed with a set of 5 serine proteases (granzymes (Gzms)), which, together with the pore-forming protein (perforin), serve as a major defense against viral infections in humans. This granule-exocytosis pathway subsumes a well-established mechanism in which target cell death is induced upon perforin-mediated entry of Gzms and subsequent activation of various (apoptosis) pathways. In the past decade, however, a growing body of evidence demonstrated that Gzms also inhibit viral replication and potential reactivation in cell death–independent manners. For example, Gzms can induce proteolysis of viral or host cell proteins necessary for the viral entry, release, or intracellular trafficking, as well as augment pro-inflammatory antiviral cytokine response. In this review, we summarize current evidence for the noncytotoxic mechanisms and roles by which killer cells can use Gzms to combat viral infections, and we discuss the potential thereof for the development of novel therapies.

Cheap and Commonplace: Making the Case for BCG and γδ T Cells in COVID-19

Antigen-specific vaccines developed for the COVID-19 pandemic demonstrate a remarkable achievement and are currently being used in high income countries with much success. However, new SARS-CoV-2 variants are threatening this success via mutations that lessen the efficacy of antigen-specific antibodies. One simple approach to assisting with this issue is focusing on strategies that build on the non-specific protection afforded by the innate immune response. The BCG vaccine has been shown to provide broad protection beyond tuberculosis disease, including against respiratory viruses, and ongoing studies are investigating its efficacy as a tool against SARS-CoV-2. Gamma delta (γδ) T cells, particularly the Vδ2 subtype, undergo rapid expansion after BCG vaccination due to MHC-independent mechanisms. Consequently, γδ T cells can produce diverse defenses against virally infected cells, including direct cytotoxicity, death receptor ligands, and pro-inflammatory cytokines. They can also assist in stimulating the adaptive immune system. BCG is affordable, commonplace and non-specific, and therefore could be a useful tool to initiate innate protection against new SARS-CoV-2 variants. However, considerations must also be made to BCG vaccine supply and the prioritization of countries where it is most needed to combat tuberculosis first and foremost.

The CD8+ T Cell Noncytotoxic Antiviral Responses

The CD8+ T cell noncytotoxic antiviral response (CNAR) was discovered during studies of asymptomatic HIV-infected subjects more than 30 years ago. In contrast to CD8+ T cell cytotoxic lymphocyte (CTL) activity, CNAR suppresses HIV replication without target cell killing. This activity has characteristics of innate immunity: it acts on all retroviruses and thus is neither epitope specific nor HLA restricted. The HIV-associated CNAR does not affect other virus families. It is mediated, at least in part, by a CD8+ T cell antiviral factor (CAF) that blocks HIV transcription. A variety of assays used to measure CNAR/CAF and the effects on other retrovirus infections are described. Notably, CD8+ T cell noncytotoxic antiviral responses have now been observed with other virus families but are mediated by different cytokines. Characterizing the protein structure of CAF has been challenging despite many biologic, immunologic, and molecular studies. It represents a low-abundance protein that may be identified by future next-generation sequencing approaches. Since CNAR/CAF is a natural noncytotoxic activity, it could provide promising strategies for HIV/AIDS therapy, cure, and prevention.

Cytotoxic Lymphocytes Target HIV-1 Gag Through Granzyme M-Mediated Cleavage

Untreated HIV-1 infection leads to a slow decrease in CD4⁺ T cell lymphocytes over time resulting in increased susceptibility to opportunistic infections (acquired immunodeficiency syndrome, AIDS) and ultimately death of the infected individual. Initially, the host's immune response controls the infection, but cannot eliminate the HIV-1 from the host. Cytotoxic lymphocytes are the key effector cells in this response and can mediate crucial antiviral responses through the release of a set of proteases called granzymes towards HIV-1-infected cells. However, little is known about the immunological molecular mechanisms by which granzymes could control HIV-1. Since we noted that HIV-1 subtype C (HIV-1C) Gag with the tetrapeptide insertion PYKE contains a putative granzyme M (GrM) cleavage site (KEPL) that overlaps with the PYKE insertion, we analyzed the proteolytic activity of GrM towards Gag. Immunoblot analysis showed that GrM could cleave Gag proteins from HIV-1B and variants from HIV-1C of which the Gag-PYKE variant was cleaved with extremely high efficiency. The main cleavage site was directly after the insertion after leucine residue 483. GrM-mediated cleavage of Gag was also observed in co-cultures using cytotoxic lymphocytes as effector cells and this cleavage could be inhibited by a GrM inhibitor peptide. Altogether, our data indicate towards a noncytotoxic immunological mechanism by which GrM-positive cytotoxic lymphocytes target the HIV-1 Gag protein within infected cells to potentially control HIV-1 infection. This mechanism could be exploited in new therapeutic strategies to treat HIV-1-infected patients to improve immunological control of the infection.

How to Train Your Dragon: Harnessing Gamma Delta T Cells Antiviral Functions and Trained Immunity in a Pandemic Era

The emergence of viruses with pandemic potential such as the SARS-CoV-2 coronavirus causing COVID-19 poses a global health challenge. There is remarkable progress in vaccine technology in response to this threat, but their design often overlooks the innate arm of immunity. Gamma Delta (γδ) T cells are a subset of T cells with unique features that gives them a key role in the innate immune response to a variety of homeostatic alterations, from cancer to microbial infections. In the context of viral infection, a growing body of evidence shows that γδ T cells are particularly equipped for early virus detection, which triggers their subsequent activation, expansion and the fast deployment of antiviral functions such as direct cytotoxic pathways, secretion of cytokines, recruitment and activation of other immune cells and mobilization of a trained immunity memory program. As such, γδ T cells represent an attractive target to stimulate for a rapid and effective resolution of viral infections. Here, we review the known aspects of γδ T cells that make them crucial component of the immune response to viruses, and the ways that their antiviral potential can be harnessed to prevent or treat viral infection.

Modulation of Inflammation by Extracellular Granzyme A

Granzyme A (GrA) has long been recognized as one of the key players in the induction of cell death of neoplastic, foreign or infected cells after granule delivery by cytotoxic cells. While the cytotoxic potential of GrA is controversial in current literature, accumulating evidence now indicates roles for extracellular GrA in modulating inflammation and inflammatory diseases. This paper aims to explore the literature presenting current knowledge on GrA as an extracellular modulator of inflammation by summarizing (i) the presence and role of extracellular GrA in several inflammatory diseases, and (ii) the potential molecular mechanisms of extracellular GrA in augmenting inflammation.

Killer cell proteases can target viral immediate-early proteins to control human cytomegalovirus infection in a noncytotoxic manner

Human cytomegalovirus (HCMV) is the most frequent viral cause of congenital defects and can trigger devastating disease in immune-suppressed patients. Cytotoxic lymphocytes (CD8⁺ T cells and NK cells) control HCMV infection by releasing interferon-γ and five granzymes (GrA, GrB, GrH, GrK, GrM), which are believed to kill infected host cells through cleavage of intracellular death substrates. However, it has recently been demonstrated that the in vivo killing capacity of cytotoxic T cells is limited and multiple T cell hits are required to kill a single virus-infected cell. This raises the question whether cytotoxic lymphocytes can use granzymes to control HCMV infection in a noncytotoxic manner. Here, we demonstrate that (primary) cytotoxic lymphocytes can block HCMV dissemination independent of host cell death, and interferon-α/β/γ. Prior to killing, cytotoxic lymphocytes induce the degradation of viral immediate-early (IE) proteins IE1 and IE2 in HCMV-infected cells. Intriguingly, both IE1 and/or IE2 are directly proteolyzed by all human granzymes, with GrB and GrM being most efficient. GrB and GrM cleave IE1 after Asp³⁹⁸ and Leu⁴¹⁴, respectively, likely resulting in IE1 aberrant cellular localization, IE1 instability, and functional impairment of IE1 to interfere with the JAK-STAT signaling pathway. Furthermore, GrB and GrM cleave IE2 after Asp¹⁸⁴ and Leu¹⁷³, respectively, resulting in IE2 aberrant cellular localization and functional abolishment of IE2 to transactivate the HCMV UL112 early promoter. Taken together, our data indicate that cytotoxic lymphocytes can also employ noncytotoxic ways to control HCMV infection, which may be explained by granzyme-mediated targeting of indispensable viral proteins during lytic infection.

Human cytomegalovirus (HCMV) is the leading viral cause of congenital defects, can trigger disease in immune-compromised patients, and plays roles in cancer development. Cytotoxic lymphocytes kill HCMV-infected cells via releasing a set of five cytotoxic serine proteases called granzymes. However, the killing capacity of cytotoxic cells is limited and multiple T cell hits are required to kill a single virus-infected cell. This raises the question whether cytotoxic lymphocytes can use granzymes to control HCMV infection in a noncytotoxic manner. Here, we show that cytotoxic lymphocytes can also use granzymes to inhibit HCMV replication in absence of cell death. All five granzymes cleave and inactivate both viral immediate-early (IE1/2) proteins, which are essential players for initiating HCMV infection. Our data support the model that cytotoxic cells employ granzymes to dampen HCMV replication prior to accumulation of sufficient hits to kill the infected cell.

Expanding the Known Functional Repertoire of the Human Cytomegalovirus pp71 Protein

The human cytomegalovirus pp71 protein is packaged within the tegument of infectious virions and performs multiple functions in host cells to prime them for productive, lytic replication. Here we review the known and hypothesized functions of pp71 in regulating proteolysis, infection outcome (lytic or latent), histone deposition, transcription, translation, immune evasion, cell cycle progression, and pathogenesis. We also highlight recent advances in CMV-based vaccine candidates informed by an improved understanding of pp71 function.

Proteome analysis of human CD56neg NK cells reveals a homogeneous phenotype surprisingly similar to CD56dim NK cells

NK cells lacking CD56 (CD56^neg ) were first identified in chronic HIV-1 infection. However, CD56^neg NK cells also exist in healthy individuals, albeit in significantly lower numbers. Here, we provide an extensive proteomic characterisation of human CD56^neg peripheral blood NK cells of healthy donors and compare them to their CD56^dim and CD56^bright counterparts. Unbiased large-scale surface receptor profiling clustered CD56^neg cells as part of the main NK cell compartment and indicated an overall CD56^dim -like phenotype. Total proteome analyses of CD56^neg NK cells further confirmed their similarity with CD56^dim NK cells, and revealed a complete cytolytic inventory with high levels of perforin and granzyme H and M. In the present study, twelve proteins discriminated CD56^neg NK cells from CD56^dim NK cells with nine up-regulated and three down-regulated proteins in the CD56^neg NK cell population. Those proteins were functionally related to lytic granule composition and transport, interaction with the extracellular matrix, DNA transcription or repair, and proliferation. Corroborating these results, CD56^neg NK cells showed modest cytotoxicity, degranulation, and IFN-ɣ secretion as compared to CD56^dim NK cells. In conclusion, CD56^neg NK cells constitute functionally competent cells sharing many features of bona fide CD56^dim NK cells in healthy individuals, but with some distinct characteristics.

Epigenetics and Genetics of Viral Latency

Viral latency can be considered a metastable, nonproductive infection state that is capable of subsequent reactivation to repeat the infection cycle. Viral latent infections have numerous associated pathologies, including cancer, birth defects, neuropathy, cardiovascular disease, chronic inflammation, and immunological dysfunctions. The mechanisms controlling the establishment, maintenance, and reactivation from latency are complex and diversified among virus families, species, and strains. Yet, as examined in this review, common properties of latent viral infections can be defined. Eradicating latent virus has become an important but elusive challenge and will require a more complete understanding of the mechanisms controlling these processes.

Deep immune profiling by mass cytometry links human T and NK cell differentiation and cytotoxic molecule expression patterns

The elimination of infected or tumor cells by direct lysis is a key T and NK cell effector function. T and NK cells can kill target cells by coordinated secretion of cytotoxic granules containing one or both pore-forming proteins, perforin and granulysin and combinations of granzyme (Gzm) family effector proteases (in humans: Gzm A, B, K, M and H). Understanding the pattern of expression of cytotoxic molecules and the relationship to different states of T and NK cells may have direct relevance for immune responses in autoimmunity, infectious disease and cancer. Approaches capable of simultaneously evaluating expression of multiple cytotoxic molecules with detailed information on T and NK differentiation state, however, remain limited. Here, we established a high dimensional mass cytometry approach to comprehensively interrogate single cell proteomic expression of cytotoxic programs and lymphocyte differentiation. This assay identified a coordinated expression pattern of cytotoxic molecules linked to CD8 T cell differentiation stages. Coordinated high expression of perforin, granulysin, Gzm A, Gzm B and Gzm M was associated with markers of late effector memory differentiation and expression of chemokine receptor CX3CR1. However, classical gating and dimensionality reduction approaches also identified other discordant patterns of cytotoxic molecule expression in CD8 T cells, including reduced perforin, but high Gzm A, Gzm K and Gzm M expression. When applied to non-CD8 T cells, this assay identified different patterns of cytotoxic molecule co-expression by CD56^hi versus CD56^dim defined NK cell developmental stages; in CD4 T cells, low expression of cytotoxic molecules was found mainly in TH1 phenotype cells, but not in Tregs or T follicular helper cells (TFH). Thus, this comprehensive, single cell, proteomic assessment of cytotoxic protein co-expression patterns demonstrates specialized cytotoxic programs in T cells and NK cells linked to their differentiation stages. Such comprehensive cytotoxic profiling may identify distinct patterns of cytotoxic potential relevant for specific infections, autoimmunity or tumor settings.

Cytomegalovirus-Infected Cells Resist T Cell Mediated Killing in an HLA-Recognition Independent Manner

In order to explore the potential of HLA-independent T cell therapy for human cytomegalovirus (HCMV) infections, we developed a chimeric antigen receptor (CAR) directed against the HCMV encoded glycoprotein B (gB), which is expressed at high levels on the surface of infected cells. T cells engineered with this anti-gB CAR recognized HCMV-infected cells and released cytokines and cytotoxic granules. Unexpectedly, and in contrast to analogous approaches for HIV, Hepatitis B or Hepatitis C virus, we found that HCMV-infected cells were resistant to killing by the CAR-modified T cells. In order to elucidate whether this phenomenon was restricted to the use of CARs, we extended our experiments to T cell receptor (TCR)-mediated recognition of infected cells. To this end we infected fibroblasts with HCMV-strains deficient in viral inhibitors of antigenic peptide presentation and targeted these HLA-class I expressing peptide-loaded infected cells with peptide-specific cytotoxic T cells (CTLs). Despite strong degranulation and cytokine production by the T cells, we again found significant inhibition of lysis of HCMV-infected cells. Impairment of cell lysis became detectable 1 day after HCMV infection and gradually increased during the following 3 days. We thus postulate that viral anti-apoptotic factors, known to inhibit suicide of infected host cells, have evolved additional functions to directly abrogate T cell cytotoxicity. In line with this hypothesis, CAR-T cell cytotoxicity was strongly inhibited in non-infected fibroblasts by expression of the HCMV-protein UL37x1, and even more so by additional expression of UL36. Our data extend the current knowledge on Betaherpesviral evasion from T cell immunity and show for the first time that, beyond impaired antigen presentation, infected cells are efficiently protected by direct blockade of cytotoxic effector functions through viral proteins.

A High Yield and Cost-efficient Expression System of Human Granzymes in Mammalian Cells

When cytotoxic T lymphocytes (CTL) or natural killer (NK) cells recognize tumor cells or cells infected with intracellular pathogens, they release their cytotoxic granule content to eliminate the target cells and the intracellular pathogen. Death of the host cells and intracellular pathogens is triggered by the granule serine proteases, granzymes (Gzms), delivered into the host cell cytosol by the pore forming protein perforin (PFN) and into bacterial pathogens by the prokaryotic membrane disrupting protein granulysin (GNLY). To investigate the molecular mechanisms of target cell death mediated by the Gzms in experimental in-vitro settings, protein expression and purification systems that produce high amounts of active enzymes are necessary. Mammalian secreted protein expression systems imply the potential to produce correctly folded, fully functional protein that bears posttranslational modification, such as glycosylation. Therefore, we used a cost-efficient calcium precipitation method for transient transfection of HEK293T cells with human Gzms cloned into the expression plasmid pHLsec. Gzm purification from the culture supernatant was achieved by immobilized nickel affinity chromatography using the C-terminal polyhistidine tag provided by the vector. The insertion of an enterokinase site at the N-terminus of the protein allowed the generation of active protease that was finally purified by cation exchange chromatography. The system was tested by producing high levels of cytotoxic human Gzm A, B and M and should be capable to produce virtually every enzyme in the human body in high yields.

Granzyme M: behind enemy lines

The granule-exocytosis pathway is the major mechanism via which cytotoxic lymphocytes eliminate virus-infected and tumor cells. In this pathway, cytotoxic lymphocytes release granules containing the pore-forming protein perforin and a family of serine proteases known as granzymes into the immunological synapse. Pore-formation by perforin facilitates entry of granzymes into the target cell, where they can activate various (death) pathways. Humans express five different granzymes, of which granzymes A and B have been most extensively characterized. However, much less is known about granzyme M (GrM). Recently, structural analysis and advanced proteomics approaches have determined the primary and extended specificity of GrM. GrM functions have expanded over the past few years: not only can GrM efficiently induce cell death in tumor cells, it can also inhibit cytomegalovirus replication in a noncytotoxic manner. Finally, a role for GrM in lipopolysaccharide-induced inflammatory responses has been proposed. In this review, we recapitulate the current status of GrM expression, substrate specificity, functions, and inhibitors.

Granzyme M targets host cell hnRNP K that is essential for human cytomegalovirus replication

Human cytomegalovirus (HCMV) is the most frequent viral cause of congenital defects and HCMV infection in immunocompromised patients may trigger devastating disease. Cytotoxic lymphocytes control HCMV by releasing granzymes towards virus-infected cells. In mice, granzyme M (GrM) has a physiological role in controlling murine CMV infection. However, the underlying mechanism remains poorly understood. In this study, we showed that human GrM was expressed by HCMV-specific CD8(+) T cells both in latently infected healthy individuals and in transplant patients during primary HCMV infection. We identified host cell heterogeneous nuclear ribonucleoprotein K (hnRNP K) as a physiological GrM substrate. GrM most efficiently cleaved hnRNP K in the presence of RNA at multiple sites, thereby likely destroying hnRNP K function. Host cell hnRNP K was essential for HCMV replication not only by promoting viability of HCMV-infected cells but predominantly by regulating viral immediate-early 2 (IE2) protein levels. Furthermore, hnRNP K interacted with IE2 mRNA. Finally, GrM decreased IE2 protein expression in HCMV-infected cells. Our data suggest that targeting of hnRNP K by GrM contributes to the mechanism by which cytotoxic lymphocytes inhibit HCMV replication. This is the first evidence that cytotoxic lymphocytes target host cell proteins to control HCMV infections.

Tale of a tegument transactivator: the past, present and future of human CMV pp71

Herpesviruses assemble large virions capable of delivering to a newly infected cell not only the viral genome, but also viral proteins packaged within the tegument layer between the DNA-containing capsid and the lipid envelope. In this review, we describe the tegument transactivator of the β-herpesvirus human CMV, the pp71 protein. We present the known mechanistic features through which it activates viral gene expression during a lytic infection but fails to do so when the virus establishes latency, and describe how pp71 stimulates the cell cycle and may help infected cells avoid detection by the adaptive immune system. A historical overview of pp71 is extended with current perceptions of its roles during human CMV infections and suggestions for future avenues of experimentation.

NK cells: immune cross-talk and therapeutic implications

Increased evidence of cross-talk between NK cells and other immune cells has enhanced the possibilities of exploiting the interplay between the activation and inhibition of NK cells for immunotherapeutic purposes. The battery of receptors possessed by NK cells help them to efficiently detect aberrant and infected cells and embark on the signaling pathways necessary to eliminate them. Endogenous expansion of NK cells and their effector mechanisms are under exploration for enhancing adoptive immunotherapy prospects in combination with immunostimulatory and cell-death-sensitizing treatments against cancer, viral infections and other pathophysiological autoimmune conditions. Various modes of NK cell manipulation are being undertaken to overcome issues such as relapse and graft rejections associated with adoptive immunotherapy. While tracing the remarkable properties of NK cells and the major developments in this field, we highlight the role of immune cooperativity in the betterment of current immunotherapeutic approaches.

Cell death-independent functions of granzymes: hit viruses where it hurts

Granule exocytosis by cytotoxic lymphocytes is the key mechanism of our immune response to eliminate virus-infected cells. These lytic granules contain the pore-forming protein perforin and a set of five serine proteases called granzymes (GrA, GrB, GrH, GrK, GrM) that display distinct substrate specificities. Granzymes have mostly been studied for their ability to induce cell death. However, viruses have evolved many inhibitors to effectively block apoptosis. Evidence is emerging that granzymes also use noncytotoxic strategies to inhibit viral replication and potential viral reactivation from latency. Granzymes directly cleave viral or host cell proteins that are required in the viral life cycle. Furthermore, granzymes induce a pro-inflammatory cytokine response to create an antiviral environment. In this review, we summarize and discuss these novel strategies by which the immune system counteracts viral infections, and we will address the potential therapeutic applications that could emerge from this intriguing mechanism.

Tegument protein control of latent herpesvirus establishment and animation

Herpesviruses are successful pathogens that infect most vertebrates as well as at least one invertebrate species. Six of the eight human herpesviruses are widely distributed in the population. Herpesviral infections persist for the life of the infected host due in large part to the ability of these viruses to enter a non-productive, latent state in which viral gene expression is limited and immune detection and clearance is avoided. Periodically, the virus will reactivate and enter the lytic cycle, producing progeny virus that can spread within or to new hosts. Latency has been classically divided into establishment, maintenance, and reactivation phases. Here we focus on demonstrated and postulated molecular mechanisms leading to the establishment of latency for representative members of each human herpesvirus family. Maintenance and reactivation are also briefly discussed. In particular, the roles that tegument proteins may play during latency are highlighted. Finally, we introduce the term animation to describe the initiation of lytic phase gene expression from a latent herpesvirus genome, and discuss why this step should be separated, both molecularly and theoretically, from reactivation.