Relative contributions of NK and CD8 T cells to IFN-gamma mediated innate immune protection against Listeria monocytogenes

IL-22 production is regulated by IL-23 during Listeria monocytogenes infection but is not required for bacterial clearance or tissue protection

Listeria monocytogenes (LM) is a gram-positive bacterium that is a common contaminant of processed meats and dairy products. In humans, ingestion of LM can result in intracellular infection of the spleen and liver, which can ultimately lead to septicemia, meningitis, and spontaneous abortion. Interleukin (IL)-23 is a cytokine that regulates innate and adaptive immune responses by inducing the production of IL-17A, IL-17F, and IL-22. We have recently demonstrated that the IL-23/IL-17 axis is required for optimal recruitment of neutrophils to the liver, but not the spleen, during LM infection. Furthermore, these cytokines are required for the clearance of LM during systemic infection. In other infectious models, IL-22 induces the secretion of anti-microbial peptides and protects tissues from damage by preventing apoptosis. However, the role of IL-22 has not been thoroughly investigated during LM infection. In the present study, we show that LM induces the production of IL-22 in vivo. Interestingly, IL-23 is required for the production of IL-22 during primary, but not secondary, LM infection. Our findings suggest that IL-22 is not required for clearance of LM during primary or secondary infection, using both systemic and mucosal models of infection. IL-22 is also not required for the protection of LM infected spleens and livers from organ damage. Collectively, these data indicate that IL-22 produced during LM infection must play a role other than clearance of LM or protection of tissues from pathogen- or immune-mediated damage.

Alternative memory in the CD8 T cell lineage

A prominent population of innate CD8+ T cells develops in the thymus of several gene-deficient mouse strains, including Itk, KLF2, CBP and Id3. These cells have the phenotype and function of memory CD8+ T cells, without previous exposure to antigen. Surprisingly, the cytokine IL-4 plays a key role in their development. As this developmental mechanism was discovered, it came to light that innate CD8+ T cells exist also in normal mice and in humans. In this review, we discuss how these cells develop, compare and contrast them to other CD8 memory cells, and discuss their potential physiological relevance.

A novel immunoregulatory function for IL-23: Inhibition of IL-12-dependent IFN-γ production

Most studies investigating the function of IL-23 have concluded that it promotes IL-17-secreting T cells. Although some reports have also characterized IL-23 as having redundant pro-inflammatory effects with IL-12, we have instead found that IL-23 antagonizes IL-12-induced secretion of IFN-γ. When splenocytes or purified populations of T cells were cultured with IL-23, IFN-γ secretion in response to IL-12 was dramatically reduced. The impact of IL-23 was most prominent in CD8(+) T cells, but was also observed in NK and CD4(+) T cells. Mechanistically, the IL-23 receptor was not required for this phenomenon, and IL-23 inhibited signaling through the IL-12 receptor by reducing IL-12-induced signal transducer and activator of transcription 4 (STAT4) phosphorylation. IL-23 was also able to reduce IFN-γ secretion by antagonizing endogenously produced IL-12 from Listeria monocytogenes (LM)-infected macrophages. In vivo, LM infection induced higher serum IFN-γ levels and a greater percentage of IFN-γ(+)CD8(+) T cells in IL-23p19-deficient mice as compared with WT mice. This increase in IFN-γ production coincided with increased LM clearance at days 2 and 3 post-infection. Our data suggest that IL-23 may be a key factor in determining the responsiveness of lymphocytes to IL-12 and their subsequent secretion of IFN-γ.

Myostatin expression, lymphocyte population, and potential cytokine production correlate with predisposition to high-fat diet induced obesity in mice

A strong relationship exists between increased inflammatory cytokines and muscle insulin resistance in obesity. This study focused on identifying a relationship between metabolic propensity and myostatin expression in muscle and spleen cells in response to high-fat diet intake. Using a comparative approach, we analyzed the effects of high-fat diet intake on myostatin and follistatin expression, spleen cell composition, and potential cytokine expression in high-fat diet induced obesity (HFDIO) resistant (SWR/J) and susceptible (C57BL/6) mice models. Results demonstrated overall increased myostatin expression in muscle following high-fat diet intake in HFDIO-susceptible mice, while myostatin expression levels decreased initially in muscle from high-fat diet fed resistant mice. In HFDIO-resistant mice, myostatin expression decreased in spleen, while myostatin increased in spleen tissue from HFDIO-susceptible mice. Proinflammatory cytokine (IL-17, IL-1β, and IFNγ) potential increased in splenocytes from HFDIO-susceptible mice. In comparison, C57BL/6 mice fed a high-fat diet exhibited higher frequencies of CD4⁺/CD44^hi and CD8⁺/CD44^hi cells in the spleen compared to control fed mice. Together, these results suggest that susceptibility to high-fat diet induced obesity could be influenced by local myostatin activity in a tissue-specific manner and that splenocytes exhibit differential cytokine production in a strain-dependent manner. This study sets the stage for future investigations into the interactions between growth, inflammation, and metabolism.

IFN-gamma triggers CCR2-independent monocyte entry into the brain during systemic infection by virulent Listeria monocytogenes

Listeria monocytogenes (Lm) is a bacterial pathogen that infects the brain via parasitized monocytes. CCR2 is important for monocyte migration into the brain after it is infected, but the degree of CCR2 involvement in monocyte migration to the CNS during systemic infection is less clear. Our recent data demonstrate that systemic infection with non-neuroinvasive DeltaactA Lm mutants triggers IFN-gamma-dependent brain influxes of Ly-6C(high) monocytes. Studies presented here tested the extent to which CCR2 and IFN-gamma are essential for monocyte migration to the brain during systemic infection with virulent Lm. For this, we assessed expression of monocyte-attracting chemokines in brains of normal and IFN-gamma mice during infection and tested the degree to which brain influxes of Ly-6C(high) monocytes were inhibited in chemokine- and chemokine receptor-deficient mice. In normal mice, systemic infection induced up-regulation of CCR2-binding (CCL2, CCL7, CCL8, CCL12) and CXCR3-binding chemokines (CXCL9, CXCL10). IFN-gamma mice had negligible mRNA and protein expression of CXCR3-binding chemokines, whereas expression of CCR2-binding chemokines was reduced, but remained significant. In addition, infection-triggered monocyte influxes were significantly reduced in IFN-gamma mice. Remarkably, brain monocyte influxes were normal during infection of CXCR3-, CCL2-, CCR1-, CCR5-, and CX3CR1-deficient mice. Influxes were transiently reduced in CCR2(-/-) mice, corresponding with retention of monocytes in the bone marrow but this was eventually overcome during infection. These data show that IFN-gamma is critical for triggering brain influxes of Ly-6C(high) monocytes during systemic infection with virulent Lm. This initial burst of monocyte migration is largely independent of individual chemokine receptors.

Natural killer cells promote tissue injury and systemic inflammatory responses during fatal Ehrlichia-induced toxic shock-like syndrome

Human monocytotropic ehrlichiosis is caused by Ehrlichia chaffeensis, a Gram-negative bacterium lacking lipopolysaccharide. We have shown that fatal murine ehrlichiosis is associated with CD8(+)T cell-mediated tissue damage, tumor necrosis factor-alpha, and interleukin (IL)-10 overproduction, and CD4(+)Th1 hyporesponsiveness. In this study, we examined the relative contributions of natural killer (NK) and NKT cells in Ehrlichia-induced toxic shock. Lethal ehrlichial infection in wild-type mice induced a decline in NKT cell numbers, and late expansion and migration of activated NK cells to the liver, a main infection site that coincided with development of hepatic injury. The spatial and temporal changes in NK and NKT cells in lethally infected mice correlated with higher NK cell cytotoxic activity, higher expression of cytotoxic molecules such as granzyme B, higher production of interferon-gamma and tumor necrosis factor-alpha, increased hepatic infiltration with CD8alphaCD11c(+) dendritic cells and CD8(+)T cells, decreased splenic CD4(+)T cells, increased serum concentrations of IL-12p40, IL-18, RANTES, and monocyte chemotactic protein-1, and elevated production of IL-18 by liver mononuclear cells compared with nonlethally infected mice. Depletion of NK cells prevented development of severe liver injury, decreased serum levels of interferon-gamma, tumor necrosis factor-alpha, and IL-10, and enhanced bacterial elimination. These data indicate that NK cells promote immunopathology and defective anti-ehrlichial immunity, possibly via decreasing the protective immune response mediated by interferon-gamma producing CD4(+)Th1 and NKT cells.

T cells expressing the transcription factor PLZF regulate the development of memory-like CD8+ T cells

Several gene deficiency models promote the development of “innate CD8⁺ T cells” that have diverse TCRs, but display a memory phenotype and rapidly produce cytokines. We here demonstrate that similar cells develop in Kruppel-Like Factor 2 (KLF2) deficient mice. However, this is not due to intrinsic deficiency in KLF2, but rather to interleukin 4 (IL-4) produced by an expanded population of T cells expressing the PLZF transcription factor. The development of innate CD8⁺ T cells in ITK and CBP transcription factor deficient mice is also attributable to this IL-4-dependent mechanism. Finally, we show that the same mechanism drives innate CD8⁺ T cell differentiation in BALB/c mice. These findings reveal a novel mechanism of regulation of CD8⁺ T cells via PLZF⁺ T cell production of IL-4.

Innate IFN-gamma production by subsets of natural killer cells, natural killer T cells and gammadelta T cells in response to dying bacterial-infected macrophages

Interferon-gamma (IFN-gamma) activation of macrophages is a crucial step in the early innate defence against bacterial infection. This innate IFN-gamma is thought to be produced mainly by natural killer (NK) cells through activation with interleukin (IL)-12p70 secreted by macrophages and dendritic cells (DCs) that have sensed bacterial products. However, a number of reports have shown that bacterial stimuli are unable to induce macrophages and/or DCs to produce sufficient amounts of IL-12p70 unless these cells are primed by IFN-gamma. It remains, therefore, unsettled how initial IFN-gamma is produced. In a previous study, we reported a novel IFN-gamma production pathway that was associated with cell death in macrophages caused by intracellular bacteria like Listeria monocytogenes (LM) and Shigella flexneri. In this study, we showed that cell death of bone-marrow-derived macrophage (BMM) cells following in vitro infection with Staphylococcus aureus (SA), an extracellular bacterium, can also stimulate this IFN-gamma production pathway. We also unequivocally demonstrated by using BMM cells from IL-12-deficient mice that the bacterial-infected macrophage cell death-mediated IFN-gamma production can occur without IL-12 although the magnitude of the response is much smaller than that in the presence of IL-12. The enhancing effect of IL-12 on this response proved to be attributable to the negligible amounts (0.5 approximately 1.5 pg/ml) of IL-12p70 but not to the large amounts of IL-12p40 that were both secreted by SA- and LM-infected macrophages. Taken all together, we propose that macrophage cell death caused by bacteria may trigger the initial IFN-gamma production at an early stage of bacterial infection.

CD4+CD25+Foxp3+ regulatory T cells optimize diversity of the conventional T cell repertoire during reconstitution from lymphopenia

The functional capacity of the adaptive immune system is dependent on the size and the diversity of the T cell population. In states of lymphopenia, T cells are driven to proliferate to restore the T cell population size. However, different T cell clones proliferate at different rates, and some T cells experience burst-like expansion called spontaneous lymphopenia-induced proliferation (LIP). These T cells are likely receiving stimulation from cognate Ags and are most responsible for inflammatory pathology that can emerge in lymphopenic states. Foxp3(+) regulatory T cells (Tregs) selectively inhibit spontaneous LIP, which may contribute to their ability to prevent lymphopenia-associated autoimmunity. We hypothesized that another potential negative consequence of unrestrained spontaneous LIP is constriction of the total T cell repertoire. We demonstrate that the absence of Foxp3(+) Tregs during the period of immune reconstitution results in the development of TCR repertoire "holes" and the loss of Ag-specific responsiveness to infectious microorganisms. In contrast, the presence of Tregs during the period of immune reconstitution preserves optimal TCR diversity and foreign Ag responsiveness. This finding contrasts with the generally accepted immunosuppressive role of Tregs and provides another example of Treg activity that actually enhances immune function.

IL-23 is required for protection against systemic infection with Listeria monocytogenes

Listeria monocytogenes (LM) is a Gram-positive, intracellular bacterium that can induce spontaneous abortion, septicemia, and meningitis. Although it is known that neutrophils are required for elimination of the bacteria and for survival of the host, the mechanisms governing the recruitment of neutrophils to LM-infected tissues are not fully understood. We demonstrate here that IL-23 and the IL-17 receptor A (IL-17RA), which mediates both IL-17A and IL-17F signaling, are necessary for resistance against systemic LM infection. LM-infected IL-23p19 knockout (KO) mice have decreased production of IL-17A and IL-17F, while IFN-gamma production is not altered by the lack of IL-23. LM induces the production of IL-17A from gammadelta T cells, but not CD4, CD8, or NK cells. Furthermore, a lack of efficient neutrophil recruitment to the liver is evident in both IL-23p19 KO and IL-17RA KO mice during LM infection. Immunocytochemical analysis of infected livers revealed that neutrophils were able to localize with LM in IL-23p19 KO and IL-17RA KO mice, indicating that IL-23 and IL-17RA do not regulate the precise localization of neutrophils with LM. The importance of IL-23-induced IL-17A was demonstrated by injecting IL-23p19 KO mice with recombinant IL-17A. These mice had reduced LM bacterial burdens compared with IL-23p19 KO mice that did not receive IL-17A. These results indicate that during LM infection, IL-23 regulates the production of IL-17A and IL-17F from gammadelta T cells, resulting in optimal liver neutrophil recruitment and enhanced bacterial clearance.

Differential effector pathways regulate memory CD8 T cell immunity against Plasmodium berghei versus P. yoelii sporozoites

Malaria results in >1,000,000 deaths per year worldwide. Although no licensed vaccine exists, much effort is currently focused on subunit vaccines that elicit CD8 T cell responses directed against Plasmodium parasite liver stage Ags. Multiple immune-effector molecules play a role in antimicrobial immunity mediated by memory CD8 T cells, including IFN-gamma, perforin, TRAIL, Fas ligand, and TNF-alpha. However, it is not known which pathways are required for memory CD8 T cell-mediated immunity against liver stage Plasmodium infection. In this study, we used a novel immunization strategy to generate memory CD8 T cells in the BALB/c mouse model of P. berghei or P. yoelii sporozoite infection to examine the role of immune-effector molecules in resistance to the liver stage infection. Our studies reveal that endogenous memory CD8 T cell-mediated protection against both parasite species is, in part, dependent on IFN-gamma, whereas perforin was only critical in protection against P. yoelii. We further show that neutralization of TNF-alpha in immunized mice markedly reduces memory CD8 T cell-mediated protection against both parasite species. Thus, our studies identify IFN-gamma and TNF-alpha as important components of the noncytolytic pathways that underlie memory CD8 T cell-mediated immunity against liver stage Plasmodium infection. Our studies also show that the effector pathways that memory CD8 T cells use to eliminate liver stage infection are, in part, Plasmodium species specific.

Identification of an IFN-gamma-producing neutrophil early in the response to Listeria monocytogenes

IFN-gamma plays a critical role during the immune response to infection with Listeria monocytogenes. Early in the innate response NK cells are thought to be a primary source of IFN-gamma; however, protection can be mediated by the presence of significant numbers of primed IFN-gamma-secreting CD8(+) T cells. In this report, we examined the early response to Listeria and found that 18 h after infection spleens contain CD11b(+), Gr-1(high), or Ly6G(+) cells that produce significant IFN-gamma. Morphological analysis of sorted Gr-1(high)IFN-gamma(+) and Gr-1(low)IFN-gamma(+) or Ly6G(+)IFN-gamma(+) cells confirmed that these cells were neutrophils. The importance of IFN-gamma production by these cells was further tested using adoptive transfer studies. Transfer of purified neutrophils from Ifng(+/+) mice led to increased bacterial clearance in Ifng(-/-) mice. Transfer of Ifng(-/-) neutrophils provided no such protection. We conclude that neutrophils are an early source of IFN-gamma during Listeria infection and are important in providing immune protection.

Myosin IIA associates with NK cell lytic granules to enable their interaction with F-actin and function at the immunological synapse

NK cell cytotoxicity requires the formation of an actin-rich immunological synapse (IS) with a target cell and the polarization of perforin-containing lytic granules toward the IS. Following the polarization of lytic granules, they traverse through the actin-rich IS to join the NK cell membrane in order for directed secretion of their contents to occur. We examined the role of myosin IIA as a candidate for facilitating this prefinal step in lytic NK cell IS function. Lytic granules in and derived from a human NK cell line, or ex vivo human NK cells, were constitutively associated with myosin IIA. When isolated using density gradients, myosin IIA-associated NK cell lytic granules directly bound to F-actin and the interaction was sensitive to the presence of ATP under conditions of flow. In NK cells from patients with a truncation mutation in myosin IIA, NK cell cytotoxicity, lytic granule penetration into F-actin at the IS, and interaction of isolated granules with F-actin were all decreased. Similarly, inhibition of myosin function also diminished the penetration of lytic granules into F-actin at the IS, as well as the final approach of lytic granules to and their dynamics at the IS. Thus, NK cell lytic granule-associated myosin IIA enables their interaction with actin and final transit through the actin-rich IS to the synaptic membrane, and can be defective in the context of naturally occurring human myosin IIA mutation.

Critical roles of NK and CD8+ T cells in central nervous system listeriosis

Listeria monocytogenes (LM) causes a life-threatening infectious disease affecting the brain of humans and domestic animals. Unfortunately, no adequate murine models for CNS listeriosis exist. Using intraparenchymal injection, we have established a new murine model for CNS listeriosis. Injection of a small volume of bacterial suspension limits the bacteria to the brain parenchyma with no leakage into the ventricular system. This new method enabled us to investigate the progression of and recovery from listerial brain infection, revealing roles for both innate and adaptive immune cells in CNS listeriosis. In the early phase of CNS listeriosis, NK cell-derived IFN-gamma is a critical cytokine in the limitation of bacterial growth by the host defense. During the later phase, CD8(+) but not CD4(+) T cells play a critical role and LM-specific CD8(+) T cells kill LM-infected microglia. Thus, innate and adaptive immune responses combine to successfully eliminate bacteria from the brain.

CD8 T Cells in old mice contribute to the innate immune response to Mycobacterium tuberculosis via interleukin-12p70-dependent and antigen-independent production of gamma interferon

Elderly individuals have increased morbidity and mortality associated with infectious diseases due in part to the progressive age-associated decline in immune function. Despite this, the old mouse model of Mycobacterium tuberculosis infection has revealed a CD8- and gamma interferon (IFN-gamma)-dependent early resistance to infection. In this study, we investigated the mechanism by which CD8 T cells from old mice contributed to the early immune response to M. tuberculosis. Following a low-dose aerosol infection with M. tuberculosis, CD8 T cells were identified as being a dominant source of IFN-gamma expression in the lungs of old mice early after infection, before the typical onset of antigen-specific immunity. In addition, M. tuberculosis-induced IFN-gamma production by CD8 T cells isolated from naïve old mice was major histocompatibility complex class I independent but was dependent on interleukin-12p70, confirming an innate role of CD8 T cells during M. tuberculosis infection. Moreover, the ability of CD8 T cells from old mice to produce increased innate IFN-gamma levels in response to M. tuberculosis infection was defined as a unique function of CD8 T cells from old mice and not the aged lung environment. Finally, we have identified increased expression of SET as being one possible mechanism by which CD8 T cells from old mice produce enhanced levels of IFN-gamma. Additional characterizations of the signaling events that lead to enhanced innate IFN-gamma production by CD8 T cells in old mice may lead to novel strategies to further enhance or perpetuate beneficial immune responses in the elderly.

Human CD4+ memory T cells are preferential targets for bystander activation and apoptosis

There is much evidence that T cells may be activated via mechanisms that act independently of direct TCR ligation. Despite this, the question of whether such forms of bystander T cell activation occur during immune responses is hotly debated. To address some outstanding questions, we set up an in vitro system within which to analyze bystander T cell activation in human T cells, in the absence of the possibility for TCR cross-reactivity. In addition, we have investigated the genetic, phenotypic, and functional characteristics of bystander-activated T cells. In this study, we show that bystander T cell activation is, indeed, observed during a specific immune response, and that it occurs preferentially among CD4(+) memory T cells. Furthermore, bystander-activated T cells display a distinct gene expression profile. The mechanism for bystander T cell activation involves soluble factors, and the outcome is an elevated level of apoptosis. This may provide an explanation for the attrition of T cell memory pools of heterologous specificity during immune responses to pathogens such as viruses.

Interleukin-12 is sufficient to promote antigen-independent interferon-gamma production by CD8 T cells in old mice

Numerous functional defects have been identified in naive T cells from aged mice, including deficiencies in proliferation, cytokine production and signal transduction. It is well documented that the ratio of naïve to memory T cells significantly decreases with age resulting in the majority of T cells from aged hosts expressing activated/memory T-cell markers (CD44(hi)), yet it is unclear whether T cells with a CD44(hi) phenotype in aged hosts are functionally equivalent to T cells with a similar phenotype in young hosts. We have identified a population of CD44(hi) CD8 T cells in old mice that are capable of secreting interferon-gamma (IFN-gamma) in response to interleukin-12 (IL-12) stimulation. This occurred in the absence of T-cell receptor engagement, a function that was not observed in CD8 T cells from young mice. This phenotype was associated with increased IL-12 receptor beta2 gene expression and IL-12 induced signal transducer and activator of transcription 4 (STAT-4) activation, even when CD8 T-cell numbers from young and old mice were normalized for CD44(hi) expression. Furthermore, we demonstrate that IL-12-induced STAT-4 activation was required for T helper type 1 (Th1) cytokine-induced IFN-gamma production in CD8 T cells. These data illustrate that old mice possess a specialized subset of CD44(hi) CD8 T cells with an enhanced responsiveness to IL-12, enabling these cells to produce substantial amounts of IFN-gamma in response to Th1 cytokine stimulation. We have therefore identified a functional difference in the populations of CD44(hi) CD8 T cells from young and old mice, and believe that understanding age-associated immunological changes is essential for helping the elderly combat deadly diseases.

A novel IL-17-dependent mechanism of cross protection: respiratory infection with mycoplasma protects against a secondary listeria infection

Immune responses to pathogens occur within the context of current and previous infections. Cross protection refers to the phenomena where infection with a particular pathogen provides enhanced resistance to a subsequent unrelated pathogen in an antigen-independent manner. Proposed mechanisms of antigen-independent cross protection have involved the secretion of IFN-gamma, which activates macrophages, thus providing enhanced innate immunity against the secondary viral or bacterial pathogen. Here we provide evidence that a primary infection with the chronic respiratory pathogen, Mycoplasma pulmonis, provides a novel form of cross protection against a secondary infection with Listeria monocytogenes that is not mediated by IFN-gamma, but instead relies upon IL-17 and mobilization of neutrophils. Mice infected with M. pulmonis have enhanced clearance of L. monocytogenes from the spleen and liver, which is associated with increased numbers of Gr-1(+)CD11b(+) cells and higher levels of IL-17. This enhanced clearance of L. monocytogenes was absent in mice depleted of Gr-1(+) cells or in mice deficient in the IL-17 receptor. Additionally, both the IL-17 receptor and neutrophils were essential for optimal clearance of M. pulmonis. Thus, a natural component of the immune response directed against M. pulmonis was able to enhance clearance of L. monocytogenes.

Multiple mechanisms contribute to the robust rapid gamma interferon response by CD8+ T cells during Listeria monocytogenes infection

A subset of CD8+ T cells can rapidly secrete gamma interferon (IFN-gamma) in an antigen-independent and interleukin-12 (IL-12)- and IL-18-dependent manner within 16 h of infection with the intracellular bacterial pathogen Listeria monocytogenes. This rapid IFN-gamma response is robust enough to be detected directly ex vivo and is not observed following infection with intracellular bacterial pathogens that remain sequestered within host cell vacuoles. We demonstrate here that three distinct pathways can lead to rapid secretion of IFN-gamma by CD8+ T cells during L. monocytogenes infection: (i) a direct cytokine-inducing activity encoded by the cholesterol-dependent cytolysin (CDC) listeriolysin O (LLO) acts within the infected cell, (ii) the pore-forming activity of LLO promotes cytosolic localization of bacterial products that trigger cytosol-specific signaling pathways, and (iii) the sustained presence of high concentrations of bacterial products can exogenously trigger cytokine production. Although it has been suggested that CDC protein toxins may act as Toll-like receptor 4 (TLR4) agonists to trigger proinflammatory cytokine secretion, we show in this report that TLR4 signaling is not required to induce a maximal rapid IFN-gamma response by CD8+ T cells. The results presented here indicate that multiple mechanisms contribute to the induction of rapid IFN-gamma secretion by CD8+ T cells during Listeria infection and that care must be taken when interpreting the results of in vitro assays, since the contribution of each pathway can vary depending on how the assay is performed.

IL-12 produced by dendritic cells augments CD8+ T cell activation through the production of the chemokines CCL1 and CCL17

IL-12 family members are an important link between innate and adaptive immunity. IL-12 drives Th1 responses by augmenting IFN-gamma production, which is key for clearance of intracellular pathogens. IL-23 promotes the development of IL-17-producing CD4(+) T cells that participate in the control of extracellular pathogens and the induction of autoimmunity. However, recent studies have shown that these cytokines can modulate lymphocyte migration and cellular interactions. Therefore, we sought to determine the individual roles of IL-12 and IL-23 in naive CD8(+) T cell activation by addressing their ability to influence IFN-gamma production and cellular interaction dynamics during priming by Listeria monocytogenes-infected dendritic cells (DC). We found that IL-12 was the major cytokine influencing the level of IFN-gamma production by CD8(+) T cells while IL-23 had little effect on this response. In addition, we observed that IL-12 promoted longer duration conjugation events between CD8(+) T cells and DC. This enhanced cognate interaction time correlated with increased production of the chemokines CCL1 and CCL17 by WT but not IL-12-deficient DC. Neutralization of both chemokines resulted in reduced interaction time and IFN-gamma production, demonstrating their importance in priming naive CD8(+) T cells. Our study demonstrates a novel mechanism through which IL-12 augments naive CD8(+) T cell activation by facilitating chemokine production, thus promoting more stable cognate interactions during priming.

Regulation of hierarchical clustering and activation of innate immune cells by dendritic cells

An early granulomatous response, characterized by collections of white blood cells at foci surrounding pathogens, occurs after infection by many intracellular organisms, including Listeria, but how these clusters become organized and for what purpose remain poorly understood. Here, we showed that dendritic cell (DC) activation by Listeria nucleated rapid clustering of innate cells, including granulocytes, natural killer (NK) cells, and monocytes, to sites of bacteria propagation where interleukin-12 was expressed in the spleen. Clustered NK cells expressed interferon-gamma (IFN-gamma), which was necessary for the activation and maturation of colocalized monocytes to tumor necrosis factor- and inducible nitric oxide synthase-producing DCs (TipDCs). NK cell clustering was necessary for IFN-gamma production and required pertussis-toxin-sensitive recruitment, in part mediated by the chemokine receptor CCR5, and MyD88 adaptor-mediated signaling. Thus, spatial organization of the immune response by DCs between 6 and 24 hr ensures functional activation of innate cells, which restricts pathogens before adaptive immunity is fully activated.

A subset of IL-10-producing gammadelta T cells protect the liver from Listeria-elicited, CD8(+) T cell-mediated injury

Although gammadelta T cells play a role in protecting tissues from pathogen-elicited damage to bacterial, viral and parasitic pathogens, the mechanisms involved in the damage and in the protection have not been clearly elucidated. This has been addressed using a murine model of listeriosis, which in mice lacking gammadelta T cells (TCRdelta(-/-)) is characterised by severe and extensive immune-mediated hepatic necrosis. We show that these hepatic lesions are caused by Listeria-elicited CD8(+) T cells secreting high levels of TNF-alpha that accumulate in the liver of Listeria-infected TCRdelta(-/-) mice. Using isolated populations of gammadelta T cells from wild-type and cytokine-deficient strains of mice to reconstitute TCRdelta(-/-) mice, the TCR variable gene 4 (Vgamma4)(+) subset of gammadelta T cells was shown to protect against liver injury. Hepatoprotection was dependent upon their ability to produce IL-10 after TCR-mediated interactions with Listeria-elicited macrophages and CD8(+) T cells. IL-10-producing Vgamma4(+) T cells also contribute to controlling CD8(+) T cell expansion and to regulating and reducing TNF-alpha secretion by activated CD8(+) T cells. This effect on TNF-alpha production was directly attributed to IL-10. These findings identify a novel mechanism by which pathogen-elicited CD8(+) T cells are regulated via interactions with, and activation of, IL-10-producing hepatoprotective gammadelta T cells.

Recruitment and activation of natural killer cells in vitro by a human dendritic cell vaccine

Recruitment of circulating natural killer (NK) cells into inflamed lymph nodes is known to provide a potent, IFN-gamma-dependent boost for Th1-polarized immune responses in mouse models. Such NK cell recruitment into draining lymph nodes is induced by certain s.c. injected adjuvants, including mature vaccine dendritic cells (DC), and is mediated by a CXCR3-dependent pathway. Here, we show that monocyte-derived immature human DCs stimulated with polyinosinic acid:polycytidylic acid, IFN-alpha, tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1 beta), and IFN-gamma, alpha-type 1-polarized DC (alpha DC1), secrete profuse amounts of the CXCR3 ligand CXCL9/MIG and substantial amounts of CXCL10/IP-10 and CXCL11/I-TAC after withdrawal of maturation stimuli. In sharp contrast, no measurable production of these chemokines was found in DCs after maturation with the current gold standard maturation cocktail for human DC-based cancer vaccines consisting of TNF-alpha, IL-1 beta, IL-6, and prostaglandin-E(2) (PGE(2)-DC). PGE(2)-DCs preferentially produced the Th2 and regulatory T-cell-attracting chemokines CCL17/TARC and CCL22/MDC, whereas only marginal levels of these chemokines were produced by alpha DC1s. Functional studies in vitro showed that supernatants from mature alpha DC1s actively recruited CD3(-)CD56(+) NK cells and that adding anti-CXCL9/MIG antibodies to the alpha DC1 supernatant substantially reduced this recruitment. Finally, alpha DC1s were able to induce IFN-gamma production when cocultured with resting autologous NK cells, but only if concurrent CD40 ligation was provided. These novel findings indicate that injected human alpha DC1-based vaccines have the potential to recruit and activate NK cells during their arrival to draining lymph nodes and that this feature may be of relevance for efficient priming of Th1 cells and CTLs.

The protein tyrosine phosphatase PTPN4/PTP-MEG1, an enzyme capable of dephosphorylating the TCR ITAMs and regulating NF-kappaB, is dispensable for T cell development and/or T cell effector functions

T cell receptor signaling processes are controlled by the integrated actions of families of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPases). Several distinct cytosolic protein tyrosine phosphatases have been described that are able to negatively regulate TCR signaling pathways, including SHP-1, SHP-2, PTPH1, and PEP. Using PTPase substrate-trapping mutants and wild type enzymes, we determined that PTPN4/PTP-MEG1, a PTPH1-family member, could complex and dephosphorylate the ITAMs of the TCR zeta subunit. In addition, the substrate-trapping derivative augmented basal and TCR-induced activation of NF-kappaB in T cells. To characterize the contribution of this PTPase in T cells, we developed PTPN4-deficient mice. T cell development and TCR signaling events were comparable between wild type and PTPN4-deficient animals. The magnitude and duration of TCR-regulated ITAM phosphorylation, as well as overall protein phosphorylation, was unaltered in the absence of PTPN4. Finally, Th1- and Th2-derived cytokines and in vivo immune responses to Listeria monocytogenes were equivalent between wild type and PTPN4-deficient mice. These findings suggest that additional PTPases are involved in controlling ITAM phosphorylations.

Innate responses to systemic infection by intracellular bacteria trigger recruitment of Ly-6Chigh monocytes to the brain

Blood borne Listeria monocytogenes enter the CNS via migration of parasitized Ly-6Chigh monocytes, but the signals that trigger this migration are not known. To understand more completely events leading to monocyte recruitment, experiments presented here combined microarray analysis of gene expression in the brains of experimentally infected mice with measurements of bacterial CFU and serum cytokines following i.v. infection with L. monocytogenes. At 24 and 48 h postinfection, the brain was sterile but there were significant changes in transcriptional activity related to serum proinflammatory cytokines. Real-time PCR confirmed mRNA up-regulation of genes related to IFN-gamma, IL-1, and TNF-alpha, although IFN-gamma itself was not up-regulated in the brain. Infection with Deltaacta, but not Deltahly mutants, increased serum concentrations of IFN-gamma, IL-6, and to a lesser extent TNF-alpha. The brain was not infected but there was widespread mRNA up-regulation in it and an influx of Ly-6Chigh monocytes in Deltaacta-infected mice. Moreover, DeltaactA-infected IFN-gamma-/- mice had no brain influx of Ly-6Chigh monocytes despite normal monocyte trafficking from bone marrow to blood and spleen. Additionally, IFN-gamma-/- mice showed diminished mRNA expression for monocyte-attracting chemokines, and significantly less CXCL9 and CXCL10 protein in the brain compared with normal mice. These data demonstrate that monocyte recruitment to the brain is independent of bacterial invasion of the CNS and is triggered by proinflammatory cytokines, in particular IFN-gamma, produced by the innate immune response to intracellular infection in peripheral organs.

Administration of a synthetic TLR4 agonist protects mice from pneumonic tularemia

Francisella tularensis is a Gram-negative intracellular pathogen that causes the zoonosis tularemia. Because F. tularensis LPS causes weak TLR4 activation, we hypothesized that administration of a synthetic TLR4 agonist, aminoalkyl glucosaminide phosphate (AGP), would boost the innate immune system and compensate for reduced TLR4 stimulation. Intranasal administration of AGPs induced intrapulmonary production of proinflammatory cytokines and chemokines. Mice treated with AGPs before and after inhalation of Francisella novicida exhibited augmented cytokine and inflammatory responses to infection; reduced bacterial replication in lung, liver, and spleen; and increased survival, whereas all PBS-treated control mice died within 4 days of infection, all AGP-treated mice showed prolonged time-to-death, and 30-60% of AGP-treated mice survived. The protective effect of AGP was lost in mice lacking IFN-gamma. Long-term survivors developed specific Th1 splenocyte responses and specific Abs dominated by IgG2 isotypes. Survivors were fully protected from rechallenge with aerosolized F. novicida. Thus, preventive administration of AGP successfully modulated innate immune responses to aerosolized F. novicida, leading to protective immunity to pneumonic tularemia. This is the first report of the protective effect of a TLR ligand on resistance to F. novicida-induced pneumonic tularemia.

Regulation of interferon-gamma during innate and adaptive immune responses

Interferon-gamma (IFN-gamma) is crucial for immunity against intracellular pathogens and for tumor control. However, aberrant IFN-gamma expression has been associated with a number of autoinflammatory and autoimmune diseases. This cytokine is produced predominantly by natural killer (NK) and natural killer T (NKT) cells as part of the innate immune response, and by Th1 CD4 and CD8 cytotoxic T lymphocyte (CTL) effector T cells once antigen-specific immunity develops. Herein, we briefly review the functions of IFN-gamma, the cells that produce it, the cell extrinsic signals that induce its production and influence the differentiation of naïve T cells into IFN-gamma-producing effector T cells, and the signaling pathways and transcription factors that facilitate, induce, or repress production of this cytokine. We then review and discuss recent insights regarding the molecular regulation of IFN-gamma, focusing on work that has led to the identification and characterization of distal regulatory elements and epigenetic modifications with the IFN-gamma locus (Ifng) that govern its expression. The epigenetic modifications and three-dimensional structure of the Ifng locus in naive CD4 T cells, and the modifications they undergo as these cells differentiate into effector T cells, suggest a model whereby the chromatin architecture of Ifng is poised to facilitate either rapid opening or silencing during Th1 or Th2 differentiation, respectively.

Innate Immune Activation of CD4 T Cells inSalmonella-Infected Mice Is Dependent on IL-18

Production of IFN-gamma by CD4 T cells is generally thought to be mediated by TCR triggering, however, Ag-nonspecific activation of effector CD8 T cells has been reported in infection models. In this study, we demonstrate that Ag-experienced CD4 T cells in the spleen of Salmonella-infected mice acquire the capacity to rapidly secrete IFN-gamma in response to stimulation with bacterial lysate or LPS. This innate responsiveness of T cells was transient and most apparent during, and immediately following, active Salmonella infection. Furthermore, innate T cell production of IFN-gamma in response to bacterial lysate or LPS was Ag independent and could be induced in Listeria-infected mice and in the absence of MHC class II expression. IL-18 was required for maximal innate responsiveness of CD4 T cells in Salmonella-infected mice and for optimal bacterial clearance in vivo. These data demonstrate that CD4 T cells acquire the capacity to respond to innate stimuli during active bacterial infection, a process that may contribute significantly to amplifying effector responses in vivo.

NK dendritic cells are innate immune responders to Listeria monocytogenes infection

NK dendritic cells (NKDC) are recently described immunologic cells that possess both lytic and Ag-presenting function and produce prolific quantities of IFN-gamma. The role of NKDC in innate immunity to bacterial infection is unknown. Because IFN-gamma is important in the immune response to Listeria monocytogenes (LM), we hypothesized that NKDC play a critical role during LM infection in mice. We found that LM increased the frequency and activation state of NKDC in vivo. Using in vivo intracellular cytokine analysis, we demonstrated that NKDC are a major source of early IFN-gamma during infection with LM. Adoptive transfer of wild-type NKDC into IFN-gamma-deficient recipients that were subsequently infected with LM decreased bacterial burden in the liver and spleen and prolonged survival. In contrast, NK cells were depleted early during LM infection, produced less IFN-gamma, and conferred less protection upon adoptive transfer into IFN-gamma-deficient mice. In vitro, LM induction of IFN-gamma secretion by NKDC depended on TLR9, in addition to IL-18 and IL-12. Our study establishes NKDC as innate immune responders to bacterial infection by virtue of their ability to secrete IFN-gamma.

Cytosolic Localization ofListeria monocytogenesTriggers an Early IFN-γ Response by CD8+T Cells That Correlates with Innate Resistance to Infection

IFN-gamma is critical for innate immunity against Listeria monocytogenes (L. monocytogenes), and it has long been thought that NK cells are the major source of IFN-gamma during the first few days of infection. However, it was recently shown that a significant number of CD44highCD8+ T cells also secrete IFN-gamma in an Ag-independent fashion within 16 h of infection with L. monocytogenes. In this report, we showed that infection with other intracellular pathogens did not trigger this early IFN-gamma response and that cytosolic localization of Listeria was required to induce rapid IFN-gamma production by CD44highCD8+ T cells. Infection of C57BL/6 mice with an Escherichia coli strain expressing listeriolysin O (LLO), a pore-forming toxin from L. monocytogenes, also resulted in rapid IFN-gamma expression by CD8+ T cells. These results suggest that LLO expression is essential for induction of the early IFN-gamma response, although it is not yet clear whether LLO plays a direct role in triggering a signal cascade that leads to cytokine production or whether it is required simply to release other bacterial product(s) into the host cell cytosol. Interestingly, mouse strains that displayed a rapid CD8+ T cell IFN-gamma response (C57BL/6, 129, and NZB) all had lower bacterial burdens in the liver 3 days postinfection compared with mouse strains that did not have an early CD8+ T cell IFN-gamma response (BALB/c, A/J, and SJL). These data suggest that participation of memory CD8+ T cells in the early immune response against L. monocytogenes correlates with innate host resistance to infection.

Listeria monocytogenes as a vector for tumor-associated antigens for cancer immunotherapy

As a facultative intracellular bacterium, Listeria monocytogenes has adapted to live within the cytosol of the host cell. It is actively taken up by antigen-presenting cells through phagocytosis, and as Listeria survive within these cells, it is an ideal vector for the delivery of antigens to be processed and presented through both the class I and II antigen-processing pathways. Once phagocytosed, Listeria produces virulence factors within the phagolysosome of the host cell, which allows it to break out of this organelle and live in the host cytosol. It is possible that these virulence factors can enhance the immunogenicity of tumor-associated antigens, which are poorly immunogenic. Recent progress in the development of this bacterium as a vaccine vector for tumor-associated antigens is discussed in the context of bacterial vectors in general. In several mouse models, Listeria-based vaccines have been demonstrated to be an effective method of influencing tumor growth and eliciting potent antitumor immune responses. Safety issues and the transition of Listeria into human clinical trials will also be discussed in this review.

Self-antigen maintains the innate antibacterial function of self-specific CD8 T cells in vivo

Self-specific CD8 T cells, which are selected by high-affinity interactions with self-Ags, develop into a lineage distinct from conventional CD8 T cells. We have previously shown that these self-specific cells acquire phenotypic and functional similarities to cells of the innate immune system including the expression of functional receptors associated with NK cells. In this study, we show that these self-specific cells have the ability to produce large amounts of IFN-gamma in response to infection with Listeria monocytogenes in a bystander fashion. The rapid production of IFN-gamma is associated with a dramatic reduction in the number of viable bacteria at the peak of infection. Self-specific CD8 T cells provide only marginal innate protection in the absence of self-Ag; however, the presence of self-Ag dramatically increases their protective ability. Exposure to self-Ag is necessary for the maintenance of the memory phenotype and responsiveness to inflammatory cytokines such as IL-15. Significantly, self-specific CD8 T cells are also more efficient in the production of IFN-gamma and TNF-alpha, thus providing more cytokine-dependent protection against bacterial infection when compared with NK cells. These findings illustrate that self-reactive CD8 T cells can play an important innate function in the early defense against bacterial infection.

Natural killer cells as an initial defense against pathogens

Natural killer (NK) cells serve as a crucial first line of defense against tumors and a diverse range of pathogens. Recognition of infection by NK cells is accomplished by the activation of receptors on the NK cell surface, which initiate NK cell effector functions. Many of the receptors and ligands involved in NK cell antimicrobial activity have been identified, and we are beginning to appreciate how they function during infection. In addition, NK cells are activated by cytokines (e.g. interleukin 12 and type I interferons), which are products of activated macrophages and dendritic cells. In response to these activating stimuli, NK cells secrete cytokines and chemokines and lyse target cells. Recent studies have focused on the mechanisms by which NK cells recognize and respond to viruses, parasites and bacteria, and on the unique role of NK cells in innate immunity to infection.

The role of CD8 T cells in innate immunity and in antigen non-specific protection

The role of CD8 T cells in adaptive immune responses is well understood. These lymphocytes respond through their T cell receptors to diverse antigens presented by MHC class I molecules by proliferating, secreting cytokines and chemokines, and directly lysing infected cells. Recently, a role for CD8 T cells in the innate immune response has become apparent. Independent of T cell receptor ligation, CD8 T cells can mount a response against pathogens by secreting cytokines and can defend against tumors by directly killing transformed cells. This innate response has been shown to be beneficial in controlling several types of bacterial infections. However, a subset of CD8 T cells that have innate non-antigen-specific capabilities has been implicated in self-reactivity, which could lead to autoimmunity.