Interleukin-12 is critical for induction of nitric oxide-mediated immunosuppression following vaccination of mice with attenuated Salmonella typhimurium

The dynamic immunomodulatory effects of vitamin D3 during Mycobacterium infection

Mycobacterium tuberculosis ( Mtb), is a highly infectious airborne bacterium. Previous studies have found vitamin D₃ to be a key factor in the defense against Mtb infection, through its regulation of the production of immune-related cytokines, chemokines and effector molecules. Mycobacterium smegmatis was used in our study as a surrogate of Mtb. We hypothesized that the continuous presence of vitamin D₃, as well as the level of severity of infection would differentially modulate host cell immune response in comparison with control and the vehicle, ethanol. We found that vitamin D₃ conditioning promotes increased bacterial clearance during low-level infection, intracellular containment during high-level infection, and minimizes host cytotoxicity. In the presence of vitamin D₃ host cell production of cytokines and effector molecules was infection-level dependent, most notably IL-12, which increased during high-level infection and decreased during low-level infection, and NO, which had a rate of change positively correlated to IL-12. Our study provides evidence that vitamin D₃ modulation is context-dependent and time-variant, as well as highly correlated to level of infection. This study furthers our mechanistic understanding of the dual role of vitamin D₃ as a regulator of bactericidal molecules and protective agent against host cell damage.

Nitric oxide and salmonella pathogenesis

Nitric oxide (NO) and its congeners contribute to the innate immune response to Salmonella. This enteric pathogen is exposed to reactive nitrogen species (RNS) in the environment and at different anatomical locations during its infectious cycle in vertebrate hosts. Chemical generation of RNS enhances the gastric barrier to enteropathogenic bacteria, while products of the Salmonella pathogenicity island 1 type III secretion system and Salmonella-associated molecular patterns stimulate transcription of inducible NO synthase (iNOS) by cells of the mononuclear phagocytic cell lineage. The resulting NO, or products that arise from its interactions with oxygen (O₂) or iron and low-molecular weight thiols, are preferentially bacteriostatic against Salmonella, while reaction of NO and superoxide (O2−) generates the bactericidal compound peroxynitrite (ONOO−). The anti-Salmonella activity of RNS emanates from the modification of redox active thiols and metal prosthetic groups of key molecular targets of the electron transport chain, central metabolic enzymes, transcription factors, and DNA and DNA-associated proteins. In turn, Salmonella display a plethora of defenses that modulate the delivery of iNOS-containing vesicles to phagosomes, scavenge and detoxify RNS, and repair biomolecules damaged by these toxic species. Traditionally, RNS have been recognized as important mediators of host defense against Salmonella. However, exciting new findings indicate that Salmonella can exploit the RNS produced during the infection to foster virulence. More knowledge of the primary RNS produced in response to Salmonella infection, the bacterial processes affected by these toxic species, and the adaptive bacterial responses that protect Salmonella from nitrosative and oxidative stress associated with NO will increase our understanding of Salmonella pathogenesis. This information may assist in the development of novel therapeutics against this common enteropathogen.

Dose-dependent lymphocyte apoptosis following respiratory infection with Vaccinia virus

Recently there has been renewed interest in poxvirus pathogenesis, especially with regard to infection via the respiratory route. Members of this family are known to produce a number of proteins that have the potential to negatively regulate the immune response. Vaccinia virus (VACV) has been used for a number of years as a model for the study of poxvirus infection. We have previously reported a dose-dependent decrease in virus-specific CD8(+) T cells following respiratory infection with VACV. In this study we have evaluated whether more generalized immunosuppressive effects are also observed following infection with a high dose of VACV. We have found that mice infected intranasally with a high, but non-lethal, dose of VACV exhibited significant weight loss as well as decreased thymocyte number. Although these mice mounted an immune response, there was a significant increase observed in bystander T and B cell apoptosis. While increased death was apparent in both naïve and activated/memory T cells populations, naïve T cells appeared more sensitive to this effect. These findings are important for our understanding of poxvirus regulation of the immune response and extends our previous understanding of VACV-mediated immunosuppression to include generalized apoptosis in the naïve and activated/memory repertoires.

In vivo-selected mutations in methyl-directed mismatch repair suppress the virulence attenuation of Salmonella dam mutant strains following intraperitoneal, but not oral, infection of naïve mice

Salmonella enterica serovar Typhimurium that lacks the DNA adenine methylase (Dam) ectopically expresses multiple genes that are preferentially expressed during infection, is attenuated for virulence, and confers heightened immunity in vaccinated hosts. The safety of dam mutant Salmonella vaccines was evaluated by screening within infected mice for isolates that have an increased capacity to cause disease relative to the attenuated parental strain. Since dam mutant strains are sensitive to the DNA base analog 2-aminopurine (2-AP), we screened for 2-AP-resistant (2-AP(r)) isolates in systemic tissues of mice infected with dam mutant Salmonella. Such 2-AP(r) derivatives were isolated following intraperitoneal but not oral administration and were shown to be competent for infectivity via intraperitoneal but not oral infection of naïve mice. These 2-AP(r) derivatives were deficient in methyl-directed mismatch repair and were resistant to nitric oxide, yet they retained the bile-sensitive phenotype of the parental dam mutant strain. Additionally, introduction of a mutH null mutation into dam mutant cells suppressed the inherent defects in intraperitoneal infectivity and nitric oxide resistance, as well as overexpression of SpvB, an actin cytotoxin required for Salmonella systemic survival. These data suggest that restoration of intraperitoneal virulence of dam mutant strains is associated with deficiencies in methyl-directed mismatch repair that correlate with the production of systemically related virulence functions.

T cell immunity evasion by virulent Salmonella enterica

Salmonella enterica are Gram-negative bacteria that cause systemic disease in their specific hosts. One of the recently appreciated features of Salmonella pathogenicity is the capacity of the bacteria to impair host adaptive immunity by interfering with DC function and T cell activation. It is likely that this feature of virulent Salmonella is needed to promote systemic dissemination in the host. Recent studies have suggested explanations for some of the molecular mechanisms developed by virulent Salmonella to impair DC and T cell function. Several of these mechanisms require the expression of virulence genes encoded within Salmonella pathogenicity islands. Targeted deletion of these genes diminishes Salmonella pathogenicity and leads to efficient activation of T cells by Salmonella-infected DCs. In this review, recent data that support the subversion of DC function by Salmonella as a means to evade host adaptive immunity and cause systemic infection are discussed. These new findings suggest a new pathogenesis model with DCs as key targets for Salmonella virulence factors.

Expression of the p60 autolysin enhances NK cell activation and is required for listeria monocytogenes expansion in IFN-gamma-responsive mice

Both peptidoglycan and muropeptides potently modulate inflammatory and innate immune responses. The secreted Listeria monocytogenes p60 autolysin digests peptidoglycan and promotes bacterial infection in vivo. Here, we report that p60 contributes to bacterial subversion of NK cell activation and innate IFN-gamma production. L. monocytogenes deficient for p60 (Deltap60) competed well for expansion in mice doubly deficient for IFNAR1 and IFN-gammaR1 or singly deficient for IFN-gammaR1, but not in wild-type, IFNAR1(-/-), or TLR2(-/-) mice. The restored competitiveness of p60-deficient bacteria suggested a specific role for p60 in bacterial subversion of IFN-gamma-mediated immune responses, since in vivo expansion of three other mutant L. monocytogenes strains (DeltaActA, DeltaNamA, and DeltaPlcB) was not complemented in IFN-gammaR1(-/-) mice. Bacterial expression of p60 was not required to induce socs1, socs3, and il10 expression in infected mouse bone marrow macrophages but did correlate with enhanced production of IL-6, IL-12p70, and most strikingly IFN-gamma. The primary source of p60-dependent innate IFN-gamma was NK cells, whereas bacterial p60 expression did not significantly alter innate IFN-gamma production by T cells. The mechanism for p60-dependent NK cell stimulation was also indirect, given that treatment with purified p60 protein failed to directly activate NK cells for IFN-gamma production. These data suggest that p60 may act on infected cells to indirectly enhance NK cell activation and increase innate IFN-gamma production, which presumably promotes early bacterial expansion through its immunoregulatory effects on bystander cells. Thus, the simultaneous induction of IFN-gamma production and factors that inhibit IFN-gamma signaling may be a common strategy for misdirection of early antibacterial immunity.

Cytokines in Salmonellosis

The recruitment and activation of phagocytic cells in infected tissues and the induction of T-cell- and B-cell-dependent acquired immunity are crucial for the control and resolution of Salmonella infections. These complex processes require the interaction of bacteria with a multitude of cell surface receptors and the controlled production of soluble mediators. The mechanisms of cytokine induction in response to Salmonella and the role of cytokine networks in Salmonella infections are the main foci of this review. Pathogen-associated molecular pattern receptors play an important role in recognition of bacteria by the host. Effective immunity against the bacterium therefore relies on the ability of the host to recruit phagocytes in the tissues and to enhance the antibacterial functions of these inflammatory cells. TNF-a, IFN-?, IL12, IL15, and IL18 are needed for the full expression of innate host resistance to Salmonella. The genes for mammalian cytokines can be cloned into suitable vectors and expressed in Salmonella as functional proteins. The in vivo production of cytokines by Salmonella carriers can have therapeutic applications and can modulate immune functions in the host. The possibility to modulate antigen-specific immune responses by expressing cytokines in Salmonella is illustrated by the increase in Salmonella-specific IgA responses induced by administration of IL-5-expressing bacteria. The same cytokines that are responsible for endotoxic shock are elevated in the late stages of lethal Salmonella infections, indicating that the toxicity of Salmonella lipopolysaccharide (LPS) may actually be contributing to the death of the host.

Adjuvant effect of chicken interferon-gamma for inactivated Salmonella Enteritidis antigen

The adjuvant effect of chicken interferon-gamma (ChIFN-gamma) was examined for protecting chickens against intestinal colonization of Salmonella Enteritidis (SE) following oral exposure. Ten 7-week-old chickens per group were immunized with inactivated SE twice with or without co-administration of ChIFN-gamma intramuscularly, and all chickens were challenged with SE. Sera collected from immunized groups with or without ChIFN-gamma, and from unimmunized group were measured for SE antibody by agglutination test. The levels of antibodies were raised by 1 week post-immunization and did not show any difference between groups with and without ChIFN-gamma. No antibodies were detected in unimmunized group before challenge. Fecal samples from each group were cultured at 1, 4, 7, and 13 days post-challenge to determine the incidence of intestinal colonization and the numbers of SE shed into the environment. Co-administration of ChIFN-gamma, significantly reduced the incidence of intestinal colonization (P<0.05). At 13 days post-challenge, the bacterial counts of SE in organs were also reduced in ChIFN-gamma administered group. These data suggest co-administration of ChIFN-gamma with SE antigen enhances protection against SE challenge without acceleration of antibody production.

High dose interleukin-12 exacerbates Bordetella pertussis infection and is associated with suppression of cell-mediated immunity in a murine aerosol challenge model

The in-vivo clearance of Bordetella pertussis infections in murine models in naive mice and animals vaccinated with whole-cell vaccine is considered to be via a Th-1-dependent mechanism in which interleukin-12 (IL)-12 may play a prominent role. It has also been demonstrated clearly that the treatment of animals with macrophage-derived IL-12 administered with an acellular vaccine can increase the efficacy of this vaccine preparation to levels seen with the whole-cell vaccine. However, the effects of exogenously added IL-12 on immune responses in non-vaccinated B. pertussis-challenged mice remain unclear, with two studies giving contradictory findings. In this study we have treated mice with escalating doses of mIL-12 (0.1-10 microg/mouse) prior to challenge with B. pertussis (using an aerosol challenge model of infection). The ability of mice to clear infection was assessed in IL-12 treated and in phosphate buffered saline (PBS) control animals at days 6 and 13 post-challenge. Lymphoid cells were isolated from spleen and cell-mediated immune responses assessed at days 1, 6 and 13 post-challenge. In addition, the direct effects of high-dose IL-12 on challenged mice was assessed by checking natural killer (NK) activity from isolated lung and spleen lymphoid cells as well as interferon-gamma (IFN-gamma) generation from isolated cells and serum at day 1 post-challenge. The results from this study show that bacterial colonization of the lungs is actually enhanced following treatment with high-dose IL-12. This is associated with impaired cellular immune responses. The mechanisms associated with the immunosuppressive effects of IL-12 are discussed.

Macrophage migration inhibitory factor (MIF) plays a pivotal role in immunity against Salmonella typhimurium

The cytokine macrophage migration inhibitory factor (MIF) exerts a multitude of biological functions. Notably, it induces inflammation at the interface between the immune system and the hypothalamus-pituitary-adrenal stress axis. The role of MIF in infectious diseases is not understood completely. Here, we show that MIF-deficient (MIF(-/-)) knockout mice fail to control an infection with wild-type Salmonella typhimurium. Increased susceptibility was accompanied by a reduced Th1 response, demonstrated by decreased levels of IL-12, IFNgamma, and tumor necrosis factor alpha. In Salmonella-infected MIF(-/-) mice, levels of IL-1beta were markedly increased. Additionally, infected MIF(-/-) mice showed elevated serum levels of nitric oxide and corticosterone as compared with control mice. Our results point to MIF as a key mediator in the host response to S. typhimurium. MIF not only promotes development of a protective Th1 response but ameliorates disease by altering levels of reactive nitrogen intermediates and corticosteroid hormones, which both exert immunosuppressive functions.

Candida albicans infection enhances immunosuppression induced by cyclophosphamide by selective priming of suppressive myeloid progenitors for NO production

Systemic infections caused by fungi after cytoreductive therapies are especially difficult to deal with in spite of currently available antimicrobials. However, little is known about the effects of fungi on the immune system of immunosuppressed hosts. We have addressed this by studying the in vitro T cell responses after systemic infection with Candida albicans in cyclophosphamide-treated mice. After cyclophosphamide treatment, a massive splenic colonization of the spleens, but not lymph nodes, by immature myeloid progenitor (Ly-6G(+)CD11b(+))cells is observed. These cells are able to suppress proliferation of T lymphocytes via a nitric oxide (NO)-dependent mechanism. Systemic infection with a sublethal dose of C. albicans did not cause immunosuppression per se but strongly increased NO-dependent suppression in cyclophosphamide-treated mice, by selective priming of suppressive myeloid progenitors (Ly-6G(+)CD11b(+)CD31(+)CD40(+)WGA(+)CD117(low/-)CD34(low/-)) for iNOS protein expression. The results indicate that systemic C. albicans infection can augment the effects of immunosuppressive therapies by promoting functional changes in immunosuppressive cells.

The quantity of nitric oxide released by macrophages regulates Chlamydia-induced disease

Intracellular bacteria of the genus Chlamydia cause numerous typically chronic diseases, frequently with debilitating sequelae. Genetic determinants of disease susceptibility after infection with Chlamydia bacteria are unknown. C57BL/6 mice develop severe pneumonia and poor immunity against Chlamydia after moderate respiratory infection whereas BALB/c mice are protected from disease and develop vigorous Th1 immunity. Here we show that infected C57BL/6 macrophages release more NO synthesized by NO synthase 2 (NOS2) than BALB/c macrophages and have lower mRNA concentrations of arginase II, a competitor of NOS2 for the common substrate, l-arginine. Reduction, but not elimination, of NO production by incomplete inhibition of NOS2 abolishes susceptibility of C57BL/6 mice to Chlamydia-induced disease. Thus, the quantity of NO released by infected macrophages is the effector mechanism that regulates between pathogenic and protective responses to chlamydial infection, and genes controlling NO production determine susceptibility to chlamydial disease.

Implications of Salmonella-induced nitric oxide (NO) for host defense and vaccines: NO, an antimicrobial, antitumor, immunosuppressive and immunoregulatory molecule

Attenuated Salmonella induce immunosuppressive, microbicidal and tumoricidal macrophages in mice. All three effects are mediated by activated macrophages producing nitric oxide (NO). NO is induced by the innate immune response pathway involving IL-12, NK cells and IFN-gamma in response to infection. NO has beneficial and detrimental effects on the host.

Paradoxical effects of IL-12 in leishmaniasis in the presence and absence of vaccinating antigen

Protective immunity against Leishmania major requires parasite-specific CD4+T helper cells, the development of which is promoted by interleukin 12 (IL-12). In this study we investigated the use of IL-12 DNA to enhance the protective immunity induced by prophylactic vaccination with the L. major Parasite Surface Antigen 2 (PSA-2) DNA. A plasmid was constructed in which the two murine IL-12 subunits p35 and p40 were secreted as a biologically active single chain cytokine. The immunomodulatory effects of this IL-12 DNA were examined by codelivery with PSA-2 DNA in susceptible BALB/c and resistant C3H/He mice and subsequent infection with L. major promastigotes. Surprisingly, administration of IL-12 DNA alone had a protective effect, while coadministration of IL-12 with PSA-2 DNA abrogated protection. This effect of IL-12 DNA was dose dependent and affected by the timing of administration in relation to PSA-2 DNA. The effect of IL-12 on protection was associated with a reduced number of INF-gamma-producing T cells early in infection. A further understanding of this paradoxical effect of IL-12 and possibly other cytokines on protective immunity may be important for their use as adjuvants for Leishmania DNA vaccines.

Inducible nitric oxide synthase and Salmonella infection

Salmonella infection is associated with the increased expression of inducible nitric oxide synthase in macrophages and other cells. This review summarizes current knowledge of the molecular mechanisms involved in the induction process, and discusses the functional significance of nitric oxide production in the context of host defense against Salmonella.

Nitric oxide-producing CD11b+Ly-6G(Gr-1)+CD31(ER-MP12)+cells in the spleen of cyclophosphamide–treated mice: implications for T-cell responses in immunosuppressed mice

During recovery from intensive chemotherapy with cyclophosphamide (CTX), mice suffer a severe but transitory impairment in spleen cell proliferation to T-cell mitogens (Con A or anti-CD3 plus IL-2). Although CTX treatment reduced spleen T-cell cellularity, this cannot fully account for T-cell unresponsiveness. The results showed that CTX induces the colonization of spleen by an immature myeloid CD11b+Ly-6G+CD31+ population. Its presence closely correlated with the maximum inhibition of T-cell proliferation. Moreover, this suppressive activity was dependent on nitric oxide (NO) production in cultures since (1) higher amounts of nitric oxide and inducible nitric oxide synthase (iNOS) mRNA were produced in CTX spleen cells (CTX-SC) than in control splenocyte cultures and (2) NOS inhibitors greatly improved the proliferation of T lymphocytes. Nitric oxide production and suppressive activity were also dependent on endogenous interferon-γ (IFN-γ) production since anti–IFN-γ abrogated both effects. Finally, iNOS protein expression was restricted to a heterogeneous population of CD31+cells in which CD11b+Ly-6G+ cells were required to suppress T-cell proliferation. These results indicated that CTX might also cause immunosuppression by a mechanism involving the presence of immature myeloid cells with suppressor activity. This may have implications in clinical praxis since inappropriate immunotherapies in patients treated with intensive chemotherapy could lead to deleterious T-cell responses. (Blood. 2000;95:212-220)

Nitric oxide-producing CD11b+Ly-6G(Gr-1)+CD31(ER-MP12)+cells in the spleen of cyclophosphamide–treated mice: implications for T-cell responses in immunosuppressed mice

During recovery from intensive chemotherapy with cyclophosphamide (CTX), mice suffer a severe but transitory impairment in spleen cell proliferation to T-cell mitogens (Con A or anti-CD3 plus IL-2). Although CTX treatment reduced spleen T-cell cellularity, this cannot fully account for T-cell unresponsiveness. The results showed that CTX induces the colonization of spleen by an immature myeloid CD11b+Ly-6G+CD31+ population. Its presence closely correlated with the maximum inhibition of T-cell proliferation. Moreover, this suppressive activity was dependent on nitric oxide (NO) production in cultures since (1) higher amounts of nitric oxide and inducible nitric oxide synthase (iNOS) mRNA were produced in CTX spleen cells (CTX-SC) than in control splenocyte cultures and (2) NOS inhibitors greatly improved the proliferation of T lymphocytes. Nitric oxide production and suppressive activity were also dependent on endogenous interferon-γ (IFN-γ) production since anti–IFN-γ abrogated both effects. Finally, iNOS protein expression was restricted to a heterogeneous population of CD31+cells in which CD11b+Ly-6G+ cells were required to suppress T-cell proliferation. These results indicated that CTX might also cause immunosuppression by a mechanism involving the presence of immature myeloid cells with suppressor activity. This may have implications in clinical praxis since inappropriate immunotherapies in patients treated with intensive chemotherapy could lead to deleterious T-cell responses. (Blood. 2000;95:212-220)

Role of protein kinase C in cyclic AMP-mediated suppression of T-lymphocyte activation following burn injury

Major burn injury induces T-lymphocyte dysfunction. Previous studies suggest that prostaglandin (PG) E2, which is elevated post-burn, is the causative factor via a cyclic AMP-dependent process. The present study was conducted to elucidate the mechanism by which cAMP induces T-lymphocyte dysfunction following burn injury. Splenocytes were isolated from mice 7 days after receiving a scald burn covering 25% of their total body surface or sham procedure. ConA-induced proliferation by splenocytes from burned mice was significantly suppressed. Macrophage depletion of the splenocyte cultures abrogated the suppression. Concanavalin A-stimulated proliferation by macrophage-depleted splenocytes was suppressed by PGE2 and dibutyryl cAMP in both groups. The IC50 of these cAMP-elevating agents, however, was approximately 100-fold lower for cells from burned mice, indicating an increased sensitivity to cAMP. PGE2 did not suppress PMA/Ca2+ ionophore-induced T-lymphocyte activation. Addition of PMA to ConA-stimulated cultures prevented the suppression of proliferative responses by PGE2, whereas Ca2+ ionophore had no effect. Thus, the suppression of T-lymphocyte activation following burn injury is macrophage-dependent, related to an increased sensitivity to cAMP and due to an uncoupling of cell surface receptors from protein kinase C activation.

The inflammatory and cytotoxic effects of a nitric oxide releasing cream on normal skin

We describe the pro-inflammatory and cytotoxic effects of nitric oxide in vivo in human skin. Nitrite and ascorbic acid were mixed on the skin of 12 normal volunteers, three times daily, to release nitric oxide. Exposure to nitric oxide was varied by randomizing the concentration of nitrite and duration of application. Nitric oxide treated skin showed significant increases in cells expressing CD3, CD4, CD8, CD68, neutrophil elastase, ICAM-1, VCAM-1, nitrosotyrosine, p53, and apoptotic cells compared with skin treated with ascorbic acid alone. There was no significant increase in mast cells. Following application of nitric oxide there were significantly fewer CD1a positive Langerhans cells in the epidermis. These appeared to lose dendritic morphology and migrate from the epidermis. There was no significant difference in staining for Ki-67, a marker related to proliferating cell nuclear antigen, between active and control skin but staining was greater after exposure to higher dose nitric oxide than the low dose. Apoptosis, cytotoxicity, and p53 staining were relatively greater after 48 h exposure than after 24 h. These results suggest that nitric oxide is pro-inflammatory and is toxic to DNA, leading to the accumulation of p53 and subsequent apoptosis. High-dose nitric oxide paradoxically led to a smaller increase in macrophages and T cells than low dose suggesting an immunosuppressive effect of higher levels.

Bacterial DNA or Oligonucleotides Containing Unmethylated CpG Motifs Can Minimize Lipopolysaccharide-Induced Inflammation in the Lower Respiratory Tract Through an IL-12-Dependent Pathway

To determine whether the systemic immune activation by CpG DNA could alter airway inflammation, we pretreated mice with either i.v. bacterial DNA (bDNA) or oligonucleotides with or without CpG motifs, exposed these mice to LPS by inhalation, and measured the inflammatory response systemically and in the lung immediately following LPS inhalation. Compared with non-CpG oligonucleotides, i.v. treatment with CpG oligonucleotides resulted in higher systemic concentrations of polymorphonuclear leukocytes, IL-10, and IL-12, but significantly reduced the concentration of total cells, polymorphonuclear leukocytes, TNF-α, and macrophage inflammatory protein-2 in the lavage fluid following LPS inhalation. The immunoprotective effect of CpG-containing oligonucleotides was dose-dependent and was most pronounced in mice pretreated between 2 and 4 h before the inhalation challenge, corresponding to the peak levels of serum cytokines. bDNA resulted in a similar immunoprotective effect, and methylation of the CpG motifs abolished the protective effect of CpG oligonucleotides. The protective effect of CpG oligonucleotides was observed in mice with either a disrupted IL-10 or IFN-γ gene, but release of cytokines in the lung was increased, especially in the mice lacking IFN-γ. In contrast, CpG DNA did not protect mice with a disrupted IL-12 gene against the LPS-induced cellular influx, even though CpG DNA reduced the release of TNF-α and macrophage inflammatory protein-2 in the lung. These findings indicate that CpG-containing oligonucleotides or bDNA are protected against LPS-induced cellular airway inflammation through an IL-12-dependent pathway, and that the pulmonary cytokine and cellular changes appear to be regulated independently.

Paclitaxel Enhances Macrophage IL-12 Production in Tumor-Bearing Hosts Through Nitric Oxide

Tumor-induced macrophages (Mφs) mediate immunosuppression, in part, through increased production of factors that suppress T cell responsiveness and underproduction of positive regulatory cytokines. Pretreatment of tumor-bearing host (TBH) Mφs with the anticancer agent paclitaxel (Taxol) partially reverses tumor-induced Mφ suppressor activity, suggesting that paclitaxel may restore TBH Mφ production of proimmune factors. Because paclitaxel demonstrates LPS-mimetic capabilities and increased production of the LPS-induced immunostimulatory cytokine IL-12 could account for enhanced T cell responsiveness, we investigated whether paclitaxel induces Mφ IL-12 production. Tumor growth significantly down-regulated Mφ IL-12 p70 production through selective dysregulation of IL-12 p40 expression. LPS stimulation failed to overcome tumor-induced dysregulation of p40 expression. In contrast, paclitaxel significantly enhanced both normal host and TBH Mφ IL-12 p70 production in vitro, although TBH Mφ IL-12 production was lower than that of similarly treated normal host Mφs. Paclitaxel enhanced p40 expression in a dose-dependent manner. Through reconstituted Mφ IL-12 expression, paclitaxel pretreatment relieved tumor-induced Mφ suppression of T cell alloreactivity. Blocking Mφ NO suppressed paclitaxel’s ability to induce IL-12 production. This suggests that paclitaxel-induced activities may involve a NO-mediated autocrine induction pathway. Collectively, these data demonstrate that paclitaxel restores IL-12 production in the TBH and ascribe a novel immunotherapeutic component to the pleiotropic activities of NO. Through its capacity to induce IL-12 production, paclitaxel may contribute to the correction of tumor-induced immune dysfunction.

Nitric oxide-mediated immunosuppression following murine Echinococcus multilocularis infection

In some parasitic infections immunosuppression is a prominent characteristic of the host-parasite interplay. We have used a murine alveolar echinococcosis (AE) model in susceptible C57BL/6 mice to document a suppressed splenocyte proliferative response to concanavalin A (Con A) at the early (1-month) stage and to Echinococcus multilocularis-crude antigen (Emc-antigen) at the late (4-6-month) stage of chronic infection. Despite proliferative suppression, splenic cytokine production [interleukin-2 (IL-2), IL-4 and interferon-gamma (IFN-gamma)] in response to Con A or Emc-antigen stimulation was not suppressed at 1 month postinfection (p.i.). Infection resulted in a strong Mac-1+ cell infiltration of the peritoneal cavity and spleen. Peritoneal cells (PEC) from mice infected at the 1-month stage were rich in macrophages and expressed significantly higher levels of transcripts for the inflammatory cytokine IL-1beta and for tumour necrosis factor-alpha and inducible nitric oxide synthase (iNOS), when compared with PEC from non-infected control mice. Conversely, the IL-10 transcript level remained low and did not change during infection. Spleen cells supplemented with PEC from infected mice induced a marked increase in the levels of nitrite in response to Con A and Emc-antigen stimulation, and also a complete suppression of splenic proliferation. The spleen cells from late-stage infected mice expressed only background levels of IL-10 but greatly increased levels of iNOS, when compared with normal spleen cells. This observation correlated with the immunosuppression demonstrated at the late stage of murine AE. Furthermore, the suppressed splenic proliferative responses observed at the early and late stage were reversed to a large extent by the addition of NG-monomethyl-l-arginine and partially by anti-IFN-gamma. Thus, our results demonstrated that the immunosuppression observed in chronic AE was not primarily dependent on IL-10 but rather on nitric oxide production by macrophages from infected animals.

Different Doses of Adenoviral Vector Expressing IL-12 Enhance or Depress the Immune Response to a Coadministered Antigen: the Role of Nitric Oxide

Joint immunization with two recombinant adenoviruses, one expressing hepatitis C virus (HCV) core and E1 proteins and another expressing IL-12 (RAdIL-12), strongly potentiates cellular immune response against HCV Ags in BALB/c mice when RAdIL-12 was used at doses of 1 × 105–1 × 107 plaque-forming units. However, cellular immunity against HCV Ags was abolished when higher doses (1 × 108 plaque-forming units) of RAdIL-12 were used. This immunosuppressive effect was associated with marked elevation of IFN-γ and nitric oxide in the serum and increased cell apoptosis in the spleen. Administration of N-nitro-l-arginine methyl ester (l-NAME), an inhibitor of nitric oxide synthase, to mice that received high doses of RAdIL-12 was lethal, whereas no apparent systemic toxicity by l-NAME was observed in those immunized with lower doses of the adenovirus. Interestingly, in mice immunized with recombinant adenovirus expressing core and E1 proteins of HCV in combination with RAdIL-12 at low doses (1 × 107 plaque-forming units), l-NAME inhibited T cell proliferation and CTL activity in response to HCV Ags and also production of Abs against adenoviral proteins. In conclusion, gene transfer of IL-12 can increase or abolish cell immunity against an Ag depending of the dose of the vector expressing the cytokine. IL-12 stimulates the synthesis of NO which is needed for the immunostimulating effects of IL-12, but apoptosis of T cells and immunosuppression ensues when IFN-γ and NO are generated at very high concentrations.

Role of gamma interferon in cellular immune response against murine Encephalitozoon cuniculi infection

Microsporidia are obligate intracellular protozoan parasites that cause a wide variety of opportunistic infection in patients with AIDS. Because it is able to grow in vitro, Encephalitozoon cuniculi is currently the best-studied microsporidian. T cells mediate protective immunity against this parasite. Splenocytes obtained from infected mice proliferate in vitro in response to irradiated parasites. A transient state of hyporesponsiveness to parasite antigen and mitogen was observed at day 17 postinfection. This downregulatory response could be partially reversed by addition of nitric oxide (NO) antagonist to the culture. Mice infected with E. cuniculi secrete significant levels of gamma interferon (IFN-gamma). Treatment with antibody to IFN-gamma or interleukin-2 (IL-12) was able to neutralize the resistance to the parasite. Mutant animals lacking the IFN-gamma or IL-12 gene were highly susceptible to infection. However, mice unable to secrete NO withstood high doses of parasite challenge, similar to normal wild-type animals. These studies describe an IFN-gamma-mediated protection against E. cuniculi infection that is independent of NO production.

In vivo blockage of nitric oxide with aminoguanidine inhibits immunosuppression induced by an attenuated strain of Salmonella typhimurium, potentiates Salmonella infection, and inhibits macrophage and polymorphonuclear leukocyte influx into the spleen

Our laboratory has previously shown that after immunization with a strain of Salmonella typhimurium, SL3235, made avirulent by a blockage in the pathway of aromatic synthesis, murine splenocytes were profoundly suppressed in their capacity to mount an in vitro antibody plaque-forming cell (PFC) response to sheep erythrocytes. Evidence indicated that suppression was mediated by nitric oxide (NO), since the in vitro addition of NG-monomethyl-L-arginine blocked suppression. The present studies examined the effect of blocking NO production on Salmonella-induced immunosuppression by in vivo administration of aminoguanidine hemisulfate (AG). AG was administered to C3HeB/FeJ mice in their drinking water (2.5% solution) for 7 days prior to intraperitoneal inoculation with SL3235. AG treatment inhibited the increase in nitrate and nitrite levels in plasma and nitrite levels in the spleen seen in immunized mice. Importantly, AG treatment completely blocked suppression of the splenic PFC response and markedly attenuated the suppression of the response to concanavalin A in immunized mice, providing further evidence that Salmonella-induced immunosuppression is mediated by NO. AG treatment also alleviated the majority of the splenomegaly associated with SL3235 inoculation, which correlated with a blockage of influx of neutrophils and macrophages into spleens, as assessed by flow cytometry. AG treatment unexpectedly resulted in 90% mortality in mice injected with the highly attenuated vaccine strain of Salmonella, SL3235. Increased mortality in AG-treated mice correlated with inability to clear organisms from the spleen by day 15 postinoculation and with persistent bacteremia, compared with control mice. Collectively, these in vivo results underscore the dual biological consequences of NO production following Salmonella infection, with NO being necessary for host defense, but also having the potentially adverse effect of immunosuppression. A unifying hypothesis to explain how these seemingly paradoxical effects could both result from NO production is presented.

Salmonella typhimurium infection in mice induces nitric oxide-mediated immunosuppression through a natural killer cell-dependent pathway

Splenocytes isolated from C57BL/6J female mice 3 to 7 days after inoculation with an attenuated strain of Salmonella typhimurium produced high levels of nitric oxide (39 to 77 microM) and gamma interferon (IFN-gamma). Additionally, spleen cell cultures from Salmonella-inoculated mice were markedly suppressed in their ability to generate an in vitro plaque-forming cell (PFC) response to sheep erythrocytes. Depletion of natural killer (NK) cells from the immune splenocyte population markedly reduced nitric oxide production, prevented suppression of PFC responses, and completely abrogated IFN-gamma release. Treatment of NK cell-depleted immune cells with IFN-gamma restored nitric oxide production to levels comparable to those of intact immune cells and also restored the immunosuppression. These results suggest that NK cells regulate the induction of nitric oxide-mediated immunosuppression following infection with S. typhimurium through the production of IFN-gamma.

Immune suppression by recombinant interleukin (rIL)-12 involves interferon gamma induction of nitric oxide synthase 2 (iNOS) activity: inhibitors of NO generation reveal the extent of rIL-12 vaccine adjuvant effect

Recombinant interleukin 12 (IL-12) can profoundly suppress cellular immune responses in mice. To define the underlying mechanism, recombinant murine (rm)IL-12 was given to C57BL/6 mice undergoing alloimmunization and found to transiently but profoundly suppress in vivo and in vitro allogeneic responses and in vitro splenocyte mitogenic responses. Use of neutralizing antibodies and genetically deficient mice showed that IFN-gamma (but not TNF-alpha) mediated rmIL-12-induced immune suppression. Splenocyte fractionation studies revealed that adherent cells from rmIL-12-treated mice suppressed the mitogenic response of normal nonadherent cells to concanavalin A and IL-2. Addition of an inhibitor of nitric oxide synthase (NOS) restored mitogenic responses, and inducible (i)NOS-/- mice were not immunosuppressed by rmIL-12. These results support the view that suppression of T cell responses is due to NO produced by macrophages responding to the high levels of IFN-gamma induced by rmIL-12. When a NOS inhibitor was given with rmIL-12 during vaccination of A/J mice with irradiated SCK tumor cells, immunosuppression was averted and the extent of rmIL-12's ability to enhance induction of protective antitumor immunity was revealed. This demonstrates that rmIL-12 is an effective vaccine adjuvant whose efficacy may be masked by its transient immunosuppressive effect.

Nitric oxide mediates immunosuppression induced by Listeria monocytogenes infection: quantitative studies

Our laboratory has shown that immunization of mice with an attenuated strain of Salmonella typhimuriuminduces profound suppression in the capacity of splenocytes to mount an in vitro antibody plaque-forming cell (PFC) response to sheep red blood cells (SRBC) and to proliferate in response to mitogens. In vitro addition of NG-monomethyl-L-arginine (NMMA), an inhibitor of nitric oxide (NO) synthase, to cell cultures from Salmonella-immunized mice completely blocked suppression of the PFC responses, implicating that NO is the suppressor factor. The present study quantified the role of nitric oxide in immunosuppression induced by Listeria monocytogenes, a gram positive intracellular pathogen of macrophages. Listeria infection resulted in suppression of the PFC assay at inoculating doses of greater than 6.5x10(3)colony forming units, with no suppression observed at lower doses. Suppression correlated with increased nitrite production. Addition of NMMA to spleen cell cultures taken from Listeria-infected mice completely blocked suppression of the PFC response, and returned nitrite production to baseline levels. In regard to Listeria-induced suppression of responses to the mitogen, Concanavalin A (Con A), the parameters were different from those observed for the PFC response. There was a direct correlation between the log10of the inoculating dose of Listeria and degree of immunosuppression, with suppression observed at doses as low as 1x10(3)cells. Addition of NMMA to the Con A-stimulated cultures resulted in reduced nitrite levels, but only partial restoration of the proliferative responses. Co-culture of splenocytes from Listeria inoculated mice with normal splenocytes in media with NMMA and reduced levels of L-arginine resulted in complete reversal of suppressed responses to Con A. Similar differences in ease of reversing suppression of the PFC response, as compared with responses to Con A, were previously noted using cells taken from Salmonella-infected mice. The present results show that a gram positive intracellular pathogen of macrophages, L. monocytogenes, induces immunosuppression in mouse spleen cells by a nitric oxide mediated mechanism that closely parallels that induced by the gram negative pathogen, S. typhimurium.

Interleukin-12 is required for control of the growth of attenuated aromatic-compound-dependent salmonellae in BALB/c mice: role of gamma interferon and macrophage activation

The attenuated S. typhimurium SL3261 (aroA) strain causes mild infections in BALB/c mice. We were able to exacerbate the disease by administering anti-interleukin-12 (IL-12) antibodies, resulting in bacterial counts in the spleens and livers of anti-IL-12-treated mice that were 10- to 100-fold higher than the ones normally observed in premortem mice; yet the animals showed only mild signs of illness. Nevertheless, they eventually died of a slow, progressive disease. Mice infected with salmonellae become hypersusceptible to endotoxin. We found that IL-12 neutralization prevented the death of infected mice following subcutaneous injection of lipopolysaccharide. Granulomatous lesions developed in the spleens and livers of control animals, as opposed to a widespread infiltration of mononuclear cells seen in the organs of anti-IL-12-treated mice. In the latter (heavily infected), salmonellae were seen within mononuclear cells, indicating an impairment of the bactericidal or bacteriostatic ability of the phagocytes in the absence of biologically active IL-12. Gamma interferon (IFN-gamma) levels were reduced in the sera and tissue homogenates from anti-IL-12-treated mice compared to those in control animals. Furthermore, fluorescence-activated cell sorter analysis on spleen cells showed that IL-12 neutralization impaired the upregulation of I-Ad/I-Ed antigens on macrophages from infected mice. Inducible nitric oxide synthase and IFN-gamma mRNA production was down-regulated in anti-IL-12-treated mice, which also showed an increased production of IL-10 mRNA and a decrease in nitric oxide synthase activity in the tissues. Administration of recombinant IFN-gamma to anti-IL-12-treated mice was able to restore host resistance, granuloma formation, and expression of major histocompatibility complex class II antigens in F4/80(+) and CD11b+ spleen cells.