Can we really learn from model pathogens?

The Untold Story of Granzymes in Oncoimmunology: Novel Opportunities with Old Acquaintances

For more than 20 years perforin and granzymes (GZMs) have been recognized as key cell death executors of cytotoxic T (Tc) and natural killer (NK) cells during cancer immunosurveillance. In immune surveillance, perforin and GZMB, the most potent cytotoxic molecules, act mainly as antitumoral and anti-infectious factors. However, when expressed by immune regulatory cells they may contribute to immune evasion of specific cancer types. By contrast, the other major granzyme, GZMA, seems not to play a major role in Tc/NK cell-mediated cytotoxicity, but acts as a proinflammatory cytokine that might contribute to cancer development. Members of the GZM family also regulate other biological processes unrelated to cell death, such as angiogenesis, vascular integrity, extracellular matrix remodeling, and barrier function, all of which contribute to cancer initiation and progression. Thus, a new paradigm is emerging in the field of oncoimmunology. Can GZMs act as protumoral factors under some circumstances? We review the diverse roles of GZMs in cancer progression, and new therapeutic opportunities emerging from targeting these protumoral roles.

The contribution of rodent models to the pathological assessment of flaviviral infections of the central nervous system

Members of the genus Flavivirus are responsible for a spectrum of important neurological syndromes in humans and animals. Rodent models have been used extensively to model flavivirus neurological disease, to discover host-pathogen interactions that influence disease outcome, and as surrogates to determine the efficacy and safety of vaccines and therapeutics. In this review, we discuss the current understanding of flavivirus neuroinvasive disease and outline the host, viral and experimental factors that influence the outcome and reliability of virus infection of small-animal models.

The ectromelia virus SPI-2 protein causes lethal mousepox by preventing NK cell responses

Ectromelia virus (ECTV) is a natural pathogen of mice that causes mousepox, and many of its genes have been implicated in the modulation of host immune responses. Serine protease inhibitor 2 (SPI-2) is one of these putative ECTV host response modifier proteins. SPI-2 is conserved across orthopoxviruses, but results defining its mechanism of action and in vivo function are lacking or contradictory. We studied the role of SPI-2 in mousepox by deleting the SPI-2 gene or its serine protease inhibitor reactive site. We found that SPI-2 does not affect viral replication or cell-intrinsic apoptosis pathways, since mutant viruses replicate in vitro as efficiently as wild-type virus. However, in the absence of SPI-2 protein, ECTV is attenuated in mousepox-susceptible mice, resulting in lower viral loads in the liver, decreased spleen pathology, and substantially improved host survival. This attenuation correlates with more effective immune responses in the absence of SPI-2, including an earlier serum gamma interferon (IFN-γ) response, raised serum interleukin 18 (IL-18), increased numbers of granzyme B(+) CD8(+) T cells, and, most notably, increased numbers and activation of NK cells. Both virus attenuation and the improved immune responses associated with SPI-2 deletion from ECTV are lost when mice are depleted of NK cells. Consequently, SPI-2 renders mousepox lethal in susceptible strains by preventing protective NK cell defenses.

Course of infection with the emergent pathogen Brucella microti in immunocompromised mice

A new Brucella species, Brucella microti, has been isolated from wild rodents and found to be pathogenic in mice. The biological relevance of this new mouse pathogen is clear, as it allows us to study Brucella infection in a species-specific model. The course of infection in wild-type (wt) and immunodeficient mice that lack B (Jh), T and B (SCID), or T, B, and NK (SCID.Beige) cells was analyzed over 3 weeks. wt mice completely cleared bacteria from the liver and spleen after that time. However, SCID mice showed a much higher bacterial load in the spleen and liver than wt and Jh mice after 1 week and maintained the same level during the next 2 weeks. All mice tested survived for the 3 weeks. In contrast, the bacterial levels in mice that lacked NK cell activity progressively increased and these mice succumbed to infection after 16 to 18 days. Histopathology analysis of infected mice showed extensive areas of necrotic tissue and thrombosis in liver after 1 week in all infected SCID.Beige mice but were not seen in either SCID or wt animals. These processes were dramatically increased after 21 days, corresponding with the death of SCID.Beige animals. Our results indicate that T and/or B cells are required for the control of infection with the mouse pathogen Brucella microti in liver and spleen but that NK cells are crucial for survival in the absence of B and T cells. In addition, they suggest that controlled granuloma formation is critical to clear this type of infection in wt mice.

Pivotal role of antibody and subsidiary contribution of CD8+ T cells to recovery from infection in a murine model of Japanese encephalitis

The immunological correlates for recovery from primary Japanese encephalitis virus (JEV) infection in humans and experimental animals remain poorly defined. To investigate the relative importance of the adaptive immune responses, we have established a mouse model for Japanese encephalitis in which a low-dose virus inoculum was administered into the footpads of adult C57BL/6 mice. In this model, ~60% of the mice developed a fatal encephalitis and a virus burden in the central nervous system (CNS). Using mice lacking B cells (μMT(-/-) mice) and immune B cell transfer to wild-type mice, we show a critically important role for humoral immunity in preventing virus spread to the CNS. T cell help played an essential part in the maintenance of an effective antibody response necessary to combat the infection, since mice lacking major histocompatibility complex class II showed truncated IgM and blunted IgG responses and uniformly high lethality. JEV infection resulted in extensive CD8(+) T cell activation, judged by upregulation of surface markers CD69 and CD25 and cytokine production after stimulation with a JEV NS4B protein-derived H-2D(b)-binding peptide and trafficking of virus-immune CD8(+) T cells into the CNS. However, no significant effect of CD8(+) T cells on the survival phenotype was found, which was corroborated in knockout mice lacking key effector molecules (Fas receptor, perforin, or granzymes) of cytolytic pathways triggered by T lymphocytes. Accordingly, CD8(+) T cells are mostly dispensable for recovery from infection with JEV. This finding highlights the conflicting role that CD8(+) T cells play in the pathogenesis of JEV and closely related encephalitic flaviviruses such as West Nile virus.

The level of friend retrovirus replication determines the cytolytic pathway of CD8+ T-cell-mediated pathogen control

Cytotoxic T cells (CTL) play a central role in the control of viral infections. Their antiviral activity can be mediated by at least two cytotoxic pathways, namely, the granule exocytosis pathway, involving perforin and granzymes, and the Fas-FasL pathway. However, the viral factor(s) that influences the selection of one or the other pathway for pathogen control is elusive. Here we investigate the role of viral replication levels in the induction and activation of CTL, including their effector potential, during acute Friend murine leukemia virus (F-MuLV) infection. F-MuLV inoculation results in a low-level infection of adult C57BL/6 mice that is enhanced about 500-fold upon coinfection with the spleen focus-forming virus (SFFV). Both the low- and high-level F-MuLV infections generated CD8+ effector T cells that were essential for the control of viral replication. However, the low-level infection induced CD8+ T cells expressing solely FasL but not the cytotoxic molecules granzymes A and B, whereas the high-level infection resulted in induction of CD8+ effector T cells secreting molecules of the granule exocytosis pathway. By using knockout mouse strains deficient in one or the other cytotoxic pathway, we found that low-level viral replication was controlled by CTL that expressed FasL but control of high-level viral replication required perforin and granzymes. Additional studies, in which F-MuLV replication was enhanced experimentally in the absence of SFFV coinfection, supported the notion that only the replication level of F-MuLV was the critical factor that determined the differential expression of cytotoxic molecules by CD8+ T cells and the pathway of CTL cytotoxicity.

CD8(+) T cell-mediated immune responses in West Nile virus (Sarafend strain) encephalitis are independent of gamma interferon

The flavivirus West Nile virus (WNV) can cause fatal encephalitis in humans and mice. It has recently been demonstrated, in an experimental model using WNV strain Sarafend and C57BL/6 mice, that both virus- and immune-mediated pathology is involved in WNV encephalitis, with CD8(+) T cells being the dominant subpopulation of lymphocyte infiltrates in the brain. Here, the role of activated WNV-immune CD8(+) T cells in mouse WNV encephalitis was investigated further. Passive transfer of WNV-immune CD8(+) T cells reduced mortality significantly and prolonged survival times of mice infected with WNV. Early infiltration of WNV-immune CD8(+) T cells into infected brains is shown, suggesting a beneficial contribution of these lymphocytes to recovery from encephalitis. This antiviral function was not markedly mediated by gamma interferon (IFN-gamma), as a deficiency in IFN-gamma did not affect mortality to two strains of WNV (Sarafend and Kunjin) or brain virus titres significantly. The cytolytic potential, as well as precursor frequency, of WNV-immune CD8(+) T cells were not altered by the absence of IFN-gamma. This was reflected in transfer experiments of WNV-immune CD8(+) T cells from IFN-gamma(-/-) mice into WNV-infected wild-type mice, which showed that IFN-gamma-deficient T cells were as effective as those from WNV-immune wild-type mice in ameliorating disease outcome. It is speculated here that one of the pleiotropic functions of IFN-gamma is mimicked by WNV-Sarafend-mediated upregulation of cell-surface expression of major histocompatibility complex antigens, which may explain the lack of phenotype of IFN-gamma(-/-) mice in response to WNV.

Exhaustion of type I interferon response following an acute viral infection

Viral infections often cause a period of heightened susceptibility to a secondary infection but the cause of this phenomenon is unknown. We found that a primary viral infection in mice rapidly triggers an IFN-I-dependent partial activation state in the majority of B and T lymphocytes, which reverts to a resting phenotype within 5 days. When a secondary infection with an unrelated virus occurred 5 to 9 days after the primary infection, no recurrence of marked activation of lymphocytes was observed. This was not due to an inherent inability of the previously activated cells to undergo renewed partial activation, because they responded when challenged with virus after transfer into "naive" recipients. Instead, the failure to respond optimally resided in the original host's incapacity to mount an IFN-I response to the secondary infection during this time period. Thus, transient immunosuppression through exhaustion of IFN-I production during an acute viral infection creates a time period of enhanced susceptibility to secondary infection.

Protection against polyoma virus-induced tumors is perforin-independent

CD8 T cells are necessary for controlling tumors induced by mouse polyoma virus (PyV), but the effector mechanism(s) responsible have not been determined. We examined the PyV tumorigenicity in C57BL/6 mice mutated in Fas or carrying targeted disruptions in the perforin gene or in both TNF receptor type I and type II genes. Surprisingly, none of these mice developed tumors. Perforin/Fas double-deficient radiation bone marrow chimeric mice were also resistant to PyV-induced tumors. Anti-PyV CD8 T cells in perforin-deficient mice were found not to differ from wild type mice with respect to phenotype, capacity to produce cytokines or maintenance of memory T cells, indicating that perforin does not modulate the PyV-specific CD8 T cell response. In addition, virus was cleared and persisted to similar extents in wild type and perforin-deficient mice. In summary, perforin/granzyme exocytosis is not an essential effector pathway for protection against PyV infection or tumorigenesis.

Immunogenicity of cytopathic and noncytopathic viral vectors

The impact of cytolytic versus noncytolytic viral infections on host responses is not well understood, due to limitations of the systems that have been used to address this issue. Using paired cytopathic and noncytopathic rabies viruses that differ by only two amino acids, we investigated several fundamental aspects of the immune response to these viral vectors. Greater cytopathic capacity translated into a greater degree of cross-priming to CD8(+) T cells (T(CD8)(+)) and more-robust short-term humoral and cellular responses. However, long-term responses to the two viruses were similar, suggesting that direct priming drives the bulk of the T(CD8)(+) antirabies response and that enhanced acute responses associated with greater virally mediated cellular destruction were balanced by other factors, such as prolonged antigen expression associated with noncytopathic virus. Such compensatory mechanisms may be in place to ensure comparable immunologic memories to various pathogens.

Perforin-deficient CD8+ T cells mediate fatal lymphocytic choriomeningitis despite impaired cytokine production

Intracerebral (i.c.) infection with lymphocytic choriomeningitis virus (LCMV) is one of the most studied models for virus-induced immunopathology, and based on results from perforin-deficient mice, it is currently assumed that fatal disease directly reflects perforin-mediated cell lysis. However, recent studies have revealed additional functional defects within the effector T cells of LCMV-infected perforin-deficient mice, raising the possibility that perforin may not be directly involved in mediating lethal disease. For this reason, we decided to reevaluate the role of perforin in determining the outcome of i.c. infection with LCMV. We confirmed that the expansion of virus-specific CD8(+) T cells is unimpaired in perforin-deficient mice. However, despite the fact that the virus-specific CD8(+) effector T cells in perforin-deficient mice are broadly impaired in their effector function, these mice invariably succumb to i.c. infection with LCMV strain Armstrong, although a few days later than matched wild-type mice. Upon further investigation, we found that this delay correlates with the delayed recruitment of inflammatory cells to the central nervous system (CNS). However, CD8(+) effector T cells were not kept from the CNS by sequestering in infected extraneural organ sites such as liver or lungs. Thus, the observed dysfunctionality regarding the production of proinflammatory mediators probably results in the delayed recruitment of effector cells to the CNS, and this appears to be the main explanation for the delayed onset of fatal disease in perforin-deficient mice. However, once accumulated in the CNS, virus-specific CD8(+) T cells can induce fatal CNS pathology despite the absence of perforin-mediated lysis and reduced capacity to produce several key cytokines.

The ‘supervirus’? Lessons from IL-4-expressing poxviruses

Members of the Poxviridae family are particularly adept at avoiding the host immune system, encoding a plethora of immunomodulatory proteins that subvert host defense. With their large genome, poxviruses are also useful for studying the effect of exogenous genes on virus-host interactions and immune responses. The insertion of the Th2 cytokine interleukin-4 (IL-4) into several poxviruses significantly increases the efficiency of the recombinant virus as a pathogen by directly inhibiting the development of Th1 immunity, which is crucial for viral clearance. In an age in which the fear of genetically modified weaponized pathogens exists, the understanding of how to make viruses more pathogenic further blurs the distinction between fundamental academic research and bioweapons development. Here, the extent of immune evasion by IL-4-expressing poxviruses will be explored, as will the consequences of this increased pathogenicity on protective immune responses.