Infection of HIV-1 transgenic mice with Mycobacterium avium induces the expression of infectious virus selectively from a Mac-1-positive host cell population

Infection of HIV-1-transgenic mice with Mycobacterium avium, a common opportunistic pathogen in AIDS patients, was shown to result in increased tissue expression of viral specific transcripts. Moreover, by coculturing splenocytes from the transgenic animals with human T cells it was possible to demonstrate that the elevation in HIV-1 mRNA triggered by M. avium infection reflects increased production of infectious virions. Viral immune activation was also shown to correlate with a marked elevation of p24 in supernatants of ex vivo-cultured tissues and, more importantly, in systemic increases in the HIV-1 protein in plasma. Interestingly, these tissue and systemic p24 responses were found to be differentially regulated. Thus, while in vitro p24 production by cultured splenocytes increased concurrently with bacterial loads during the first 6 wk of infection, levels of the Ag in plasma actually decreased. In situ localization experiments together with FACS analysis of HIV-1-expressing splenocytes indicated that virus production is restricted largely to cells of the monocyte/macrophage lineage. Indeed, in vitro p24 expression by cells from noninfected transgenic mice was up-regulated by polyclonal stimulation of macrophages but not T cells. Together these results underscore the importance of the macrophage reservoir in persistent virus expression and establish a convenient and relevant animal model for studying the factors responsible for immune activation of HIV-1 induced by mycobacterial as well as other common coinfections encountered by AIDS patients.

Infection of HIV-1 Transgenic Mice with Mycobacterium avium Induces the Expression of Infectious Virus Selectively from a Mac-1-Positive Host Cell Population

Infection of HIV-1-transgenic mice with Mycobacterium avium, a common opportunistic pathogen in AIDS patients, was shown to result in increased tissue expression of viral specific transcripts. Moreover, by coculturing splenocytes from the transgenic animals with human T cells it was possible to demonstrate that the elevation in HIV-1 mRNA triggered by M. avium infection reflects increased production of infectious virions. Viral immune activation was also shown to correlate with a marked elevation of p24 in supernatants of ex vivo-cultured tissues and, more importantly, in systemic increases in the HIV-1 protein in plasma. Interestingly, these tissue and systemic p24 responses were found to be differentially regulated. Thus, while in vitro p24 production by cultured splenocytes increased concurrently with bacterial loads during the first 6 wk of infection, levels of the Ag in plasma actually decreased. In situ localization experiments together with FACS analysis of HIV-1-expressing splenocytes indicated that virus production is restricted largely to cells of the monocyte/macrophage lineage. Indeed, in vitro p24 expression by cells from noninfected transgenic mice was up-regulated by polyclonal stimulation of macrophages but not T cells. Together these results underscore the importance of the macrophage reservoir in persistent virus expression and establish a convenient and relevant animal model for studying the factors responsible for immune activation of HIV-1 induced by mycobacterial as well as other common coinfections encountered by AIDS patients.

Interleukin-12 can directly induce T-helper 1 responses in interferon-gamma (IFN-gamma) receptor-deficient mice, but requires IFN-gamma signalling to downregulate T-helper 2 responses

An in vivo model of pulmonary granuloma formation around embolized schistosome eggs was investigated as an environment in which to analyse a role for interleukin-12 (IL-12) in the differentiation of T-helper 1 (Th1) and Th2 subsets. Specifically, mice deficient for the interferon-gamma receptor (IFN-gammaR-/-) were used to determine the role for IL-12 in the absence of IFN-gamma-mediated signalling. We show that recombinant IL-12 administered to IFN-gammaR-/- mice caused the up-regulation of mRNA for IFN-gamma in lung tissue, and the secretion of abundant IFN-gamma by in vitro-cultured lymph node cells in response to egg antigens. This indicates that IL-12 can act independently of IFN-gamma to induce the development of Th1 cells. Administration of rIL-12 to wild-type mice markedly reduced the secretion of Th2-associated cytokines, IL-4 and IL-5. However, these cytokines were not dramatically reduced in IFN-gammaR-/- mice treated with IL-12. We conclude that inhibition of these cytokines by IL-12 is primarily dependent upon effective IFN-gamma signalling, although abrogation of T-cell derived IL-10 appeared to be dependent upon IL-12. We also show that increases in mRNA for the beta2 subunit of the IL-12 receptor and the p40 subunit of IL-12 after rIL-12 treatment were lower in IFN-gammaR-/- mice, compared to wild-type mice, indicating that their expression was primarily dependent upon IFN-gamma with only a minor role for IL-12.

TNF-alpha, nitric oxide and IFN-gamma are all critical for development of necrosis in the small intestine and early mortality in genetically susceptible mice infected perorally with Toxoplasma gondii

We previously reported that genetic susceptibility of mice to peroral infection with T. gondii is associated with CD4+ T cell-dependent, interferon (IFN)-gamma-mediated necrosis of their small intestine. We examined the role of tumour necrosis factor (TNF)-alpha and nitric oxide (NO), in addition to IFN-gamma. At 7 days after infection, a marked increase in CD4+ T cells was observed in lamina propria mononuclear cells (LPC) of the small intestine as compared with normal mice, and significantly greater amounts of mRNA for IFN-gamma, TNF-alpha, and inducible NO synthase (iNOS) were detected in LPC of the small intestine of infected than uninfected animals. Treatment of infected mice with anti-TNF-alpha monoclonal antibody (mAb) or the iNOS inhibitor, aminoguanidine, prevented necrosis and prolonged time to death. Infected iNOS-targeted mutant mice did not develop the disease whereas infected, control mice did. Treatment with anti-TNF-alpha mAb did not affect the expression of IFN-gamma in the LPC but inhibited expression of iNOS in the infected mice, indicating the role of TNF-alpha in the induction of iNOS. These results suggest that NO induced by a combination of IFN-gamma and TNF-alpha through activation of iNOS is a critical mediator of intestinal pathology and contributes to early mortality in genetically susceptible mice.

Schistosome-infected IL-4 receptor knockout (KO) mice, in contrast to IL-4 KO mice, fail to develop granulomatous pathology while maintaining the same lymphokine expression profile

Th2 lymphocytes have been postulated to play a major role in the immunopathology induced by Schistosoma mansoni infection. Nevertheless, infected IL-4 knockout (KO) and wild-type (wt) mice develop egg granulomas comparable in size. To further investigate the function of the Th2 response in egg pathology we studied IL-4Ralpha-deficient mice, which are nonresponsive to both IL-4 and IL-13. In striking contrast to IL-4 KO animals, infected IL-4Ralpha KO mice developed only minimal hepatic granulomas and fibrosis despite the presence of CD3+ T cells in the residual egg lesions. Moreover, liver lymphokine mRNA levels in these animals and IL-4 KO mice were equivalent. In addition, infected IL-4Ralpha-deficient, IL-4-deficient, and wt animals developed similar egg Ag-specific IgG Ab titers, arguing that CD4-dependent Th activity is intact in KO mice. As expected, IFN-gamma secretion was strongly up-regulated in mesenteric lymph node cultures from both groups of deficient animals, a change reflected in increased serum IgG2a and IgG2b Ab levels. Surprisingly, Th2 cytokine production in infected IL-4Ralpha KO mice was not abolished but was only reduced and resembled that previously documented in IL-4 KO animals. This residual Th2 response is likely to explain the ability of IL-4 KO mice to generate egg granulomas, which cannot be formed in IL-4Ralpha-deficient animals because of their lack of responsiveness to the same cytokine ligands. Taken together, these findings argue that tissue pathology in schistosomiasis requires, in addition to egg-specific CD4+ lymphocytes, a previously unrecognized IL-4Ralpha+ non-T cell effector population.

Schistosome-Infected IL-4 Receptor Knockout (KO) Mice, in Contrast to IL-4 KO Mice, Fail to Develop Granulomatous Pathology While Maintaining the Same Lymphokine Expression Profile

Th2 lymphocytes have been postulated to play a major role in the immunopathology induced by Schistosoma mansoni infection. Nevertheless, infected IL-4 knockout (KO) and wild-type (wt) mice develop egg granulomas comparable in size. To further investigate the function of the Th2 response in egg pathology we studied IL-4Rα-deficient mice, which are nonresponsive to both IL-4 and IL-13. In striking contrast to IL-4 KO animals, infected IL-4Rα KO mice developed only minimal hepatic granulomas and fibrosis despite the presence of CD3+ T cells in the residual egg lesions. Moreover, liver lymphokine mRNA levels in these animals and IL-4 KO mice were equivalent. In addition, infected IL-4Rα-deficient, IL-4-deficient, and wt animals developed similar egg Ag-specific IgG Ab titers, arguing that CD4-dependent Th activity is intact in KO mice. As expected, IFN-γ secretion was strongly up-regulated in mesenteric lymph node cultures from both groups of deficient animals, a change reflected in increased serum IgG2a and IgG2b Ab levels. Surprisingly, Th2 cytokine production in infected IL-4Rα KO mice was not abolished but was only reduced and resembled that previously documented in IL-4 KO animals. This residual Th2 response is likely to explain the ability of IL-4 KO mice to generate egg granulomas, which cannot be formed in IL-4Rα-deficient animals because of their lack of responsiveness to the same cytokine ligands. Taken together, these findings argue that tissue pathology in schistosomiasis requires, in addition to egg-specific CD4+ lymphocytes, a previously unrecognized IL-4Rα+ non-T cell effector population.

Cytokine production in acute versus chronic human Schistosomiasis mansoni: the cross-regulatory role of interferon-gamma and interleukin-10 in the responses of peripheral blood mononuclear cells and splenocytes to parasite antigens

The contribution of interleukin (IL)-10 and interferon (IFN)-gamma to the regulation of type 1 and type 2 cytokine responses was investigated in Brazilians with different clinical forms of schistosomiasis mansoni. Cells from members of a family with acute intestinal schistosomiasis responded to schistosomal soluble egg antigen (SEA) or soluble adult worm antigen preparation (SWAP) with greater amounts of IFN-gamma than did cells from several patients with chronic intestinal schistosomiasis; IL-10 levels were similar. Neutralization of IL-10 had no effect on the SEA-specific IFN-gamma response in patients with acute infection, whereas SWAP-induced IFN-gamma was increased in both groups. Anti-IL-10 also up-regulated SEA-specific IFN-gamma protein and mRNA responses in most splenocyte cultures from hepatosplenic schistosomiasis patients but had no effect on antigen-specific IL-4 or IL-5 production. Neutralization of IFN-gamma resulted in a comparable increase in SWAP-specific IL-10 and IL-5, while IL-4 was not affected. These studies demonstrate that early disease in schistosomiasis is associated with a significant IFN-gamma response and that IL-10 contributes to the suppression of that response during both early and chronic infection.

Requirement for Tec kinases Rlk and Itk in T cell receptor signaling and immunity

T cell receptor (TCR) signaling requires activation of Zap-70 and Src family tyrosine kinases, but requirements for other tyrosine kinases are less clear. Combined deletion in mice of two Tec kinases, Rlk and Itk, caused marked defects in TCR responses including proliferation, cytokine production, and apoptosis in vitro and adaptive immune responses to Toxoplasma gondii in vivo. Molecular events immediately downstream from the TCR were intact in rlk-/-itk-/- cells, but intermediate events including inositol trisphosphate production, calcium mobilization, and mitogen-activated protein kinase activation were impaired, establishing Tec kinases as critical regulators of TCR signaling required for phospholipase C-gamma activation.

Effector cells of both nonhemopoietic and hemopoietic origin are required for interferon (IFN)-gamma- and tumor necrosis factor (TNF)-alpha-dependent host resistance to the intracellular pathogen, Toxoplasma gondii

Although interferon (IFN)-gamma-activated, mononuclear phagocytes are considered to be the major effectors of resistance to intracellular pathogens, it is unclear how they control the growth of microorganisms that reside in nonhemopoietic cells. Pathogens within such cells may be killed by metabolites secreted by activated macrophages or, alternatively, directly controlled by cytokine-induced microbicidal mechanisms triggered within infected nonphagocytic cells. To distinguish between these two basic mechanisms of cell-mediated immunity, reciprocal bone marrow chimeras were constructed between wild-type and IFN-gamma receptor-deficient mice and their survival assessed following infection with Toxoplasma gondii, a protozoan parasite that invades both hemopoietic and nonhemopoietic cell lineages. Resistance to acute and persistent infection was displayed only by animals in which IFN-gamma receptors were expressed in both cellular compartments. Parallel chimera experiments performed with tumor necrosis factor (TNF) receptor-deficient mice also indicated a codependence on hemopoietic and nonhemopoietic lineages for optimal control of the parasite. In contrast, in mice chimeric for inducible nitric oxide synthase (iNOS), an enzyme associated with IFN-gamma-induced macrophage microbicidal activity, expression by cells of hemopoietic origin was sufficient for host resistance. Together, these findings suggest that, in concert with bone marrow-derived effectors, nonhemopoietic cells can directly mediate, in the absence of endogenous iNOS, IFN-gamma- and TNF-alpha-dependent host resistance to intracellular infection.

The p47(phox-/-) mouse model of chronic granulomatous disease has normal granuloma formation and cytokine responses to Mycobacterium avium and Schistosoma mansoni eggs

Chronic granulomatous disease (CGD) is a genetic disorder of NADPH oxidase in which phagocytes are defective in generating reactive oxidants. CGD patients suffer from recurrent infections and exuberant and persistent tissue granuloma formation. We hypothesized that abnormal granulomata in CGD may result from aberrant T-cell-mediated cytokine responses. To assess Th-1-type cytokine responses and granulomata, we challenged p47(phox-/-) and wild-type mice with avirulent (SmD) or virulent (SmT) variants of Mycobacterium avium 2-151. To assess Th-2-type cytokine responses and granulomata, we used Schistosoma mansoni eggs (SME). Mononuclear cells were harvested, and cytokine responses were determined by enzyme-linked immunosorbent assay or reverse transcriptase PCR. Following SmD or SmT challenge, splenocytes from p47(phox-/-) and wild-type mice generated similar polar Th-1 responses (increased levels of gamma interferon and basal levels of interleukin 4 [IL-4] and IL-5). By 8 weeks after SmT challenge, exuberant splenic granulomata developed in p47(phox-/-) and wild-type mice. After SME challenge, thoracic lymph node mononuclear cells from p47(phox-/-) and wild-type mice generated similar mixed Th-1 and Th-2 cytokine responses to SME antigen and concanavalin A. Peak lung granuloma sizes and rates of regression were similar in p47(phox-/-) and wild-type mice. These results suggest that exuberant granulomatous inflammation in CGD is probably not the result of skewing of T-cell responses toward the Th-1 or Th-2 pole. Appropriate regression of established tissue granulomata in p47(phox-/-) mice challenged with SME suggests that abnormal granuloma formation in CGD is stimulus dependent and is not an invariant feature of the disease.

Role of cytokines in the formation and downregulation of hepatic circumoval granulomas and hepatic fibrosis in Schistosoma mansoni-infected mice

Schistosoma mansoni infections are associated with a strong Th2 cytokine response. Treatment of mice with IL-12 or anti-IL-2 or anti-IL-4 before i.v. injection of eggs increased IFN-gamma production and downregulated Th2 responses and pulmonary granuloma size. Conversely, anti-IFN-gamma antibody treatment increased Th2 responses and granuloma size. Similar manipulation produced less dramatic results in infected mice. However, sensitization of mice with eggs + IL-12 before infection augmented the Th1 response and decreased Th2 cytokines, granuloma size and fibrosis. Antisera to IFN-gamma, TNF-alpha or IL-12 during IL-12-egg immunization partly restored granuloma size and fibrosis following infection. Variations in the size of granulomas in acute (8 week) infections may be influenced primarily by the number and state of activation of T cells. In chronic (12-16 week) infections immunologic downmodulation proceeded normally in mice without functional CD8+ cells and in IFN-gamma KO mice but not in B cell KO (microMT) mice or in mice deficient in FcR expression in spite of the fact that these mice downregulated their T cell and cytokine responses. It is evident that the participation of cytokines in granuloma formation and regulation is complicated and that the mechanisms controlling both these phenomena are likely to involve both T cells and antibody/FcR interactions.

Optimal Vaccination Against Schistosoma mansoni Requires the Induction of Both B Cell- and IFN-γ-Dependent Effector Mechanisms

Mice immunized with radiation-attenuated cercariae of Schistosoma mansoni display resistance to challenge infection, which increases with multiple boosting. Protection in animals receiving a single vaccination is thought to involve a primarily cell-mediated, IFN-γ-dependent mechanism, while humoral immunity has been shown to contribute to challenge rejection in multiply (three times) immunized mice. To better understand the respective contribution of the B lymphocyte- and IFN-γ-dependent effector arms in host resistance, we compared vaccine-induced immunity in B cell-deficient (μMT) and IFN-γ knockout (GKO) animals. Unexpectedly, after a single vaccination, B cell knockout (KO) mice displayed reduced protection against challenge infection, although they developed a normal IFN-γ-dominated cytokine response. This defect in resistance was equivalent to that displayed by GKO animals. Moreover, whereas two additional vaccinations significantly increased the level of immunity in wild-type mice, the protection in B cell KO animals remained unchanged. In contrast, multiple vaccination resulted in increased but, nevertheless, defective resistance in GKO mice. Since FcR γ KO mice, which lack functional FcγRI, FcγRIII, and FcεRI, show no defects in vaccine-induced resistance after immunization either one or three times, the B cell-dependent mechanism of protection involved does not appear to require FcR signaling. Together, these findings indicate that effective vaccination against schistosomes depends on the simultaneous induction of both humoral and cell-mediated immunity, a conclusion that may explain the limited success of most subunit vaccine protocols designed to preferentially induce either B cell- or IFN-γ-dependent protective mechanisms.

Optimal vaccination against Schistosoma mansoni requires the induction of both B cell- and IFN-gamma-dependent effector mechanisms

Mice immunized with radiation-attenuated cercariae of Schistosoma mansoni display resistance to challenge infection, which increases with multiple boosting. Protection in animals receiving a single vaccination is thought to involve a primarily cell-mediated, IFN-gamma-dependent mechanism, while humoral immunity has been shown to contribute to challenge rejection in multiply (three times) immunized mice. To better understand the respective contribution of the B lymphocyte- and IFN-gamma-dependent effector arms in host resistance, we compared vaccine-induced immunity in B cell-deficient (muMT) and IFN-gamma knockout (GKO) animals. Unexpectedly, after a single vaccination, B cell knockout (KO) mice displayed reduced protection against challenge infection, although they developed a normal IFN-gamma-dominated cytokine response. This defect in resistance was equivalent to that displayed by GKO animals. Moreover, whereas two additional vaccinations significantly increased the level of immunity in wild-type mice, the protection in B cell KO animals remained unchanged. In contrast, multiple vaccination resulted in increased but, nevertheless, defective resistance in GKO mice. Since FcR gamma KO mice, which lack functional FcgammaRI, FcgammaRIII, and FcepsilonRI, show no defects in vaccine-induced resistance after immunization either one or three times, the B cell-dependent mechanism of protection involved does not appear to require FcR signaling. Together, these findings indicate that effective vaccination against schistosomes depends on the simultaneous induction of both humoral and cell-mediated immunity, a conclusion that may explain the limited success of most subunit vaccine protocols designed to preferentially induce either B cell- or IFN-gamma-dependent protective mechanisms.

Helicobacter hepaticus triggers colitis in specific-pathogen-free interleukin-10 (IL-10)-deficient mice through an IL-12- and gamma interferon-dependent mechanism

Mice rendered deficient in interleukin-10 (IL-10) by gene targeting (IL-10(-/-) mice) develop chronic enterocolitis resembling human inflammatory bowel disease (IBD) when maintained in conventional animal facilities. However, they display a minimal and delayed intestinal inflammatory response when reared under specific-pathogen-free (SPF) conditions, suggesting the involvement of a microbial component in pathogenesis. We show here that experimental infection with a single bacterial agent, Helicobacter hepaticus, induces chronic colitis in SPF-reared IL-10(-/-) mice and that the disease is accompanied by a type 1 cytokine response (gamma interferon [IFN-gamma], tumor necrosis factor alpha, and nitric oxide) detected by restimulation of spleen and mesenteric lymph node cells with a soluble H. hepaticus antigen (Ag) preparation. In contrast, wild-type (WT) animals infected with the same bacteria did not develop disease and produced IL-10 as the dominant cytokine in response to Helicobacter Ag. Strong H. hepaticus-reactive antibody responses as measured by Ag-specific total immunoglobulin G (IgG), IgG1, IgG2a, IgG2b, IgG3, and IgA were observed in both WT and IL-10(-/-) mice. In vivo neutralization of IFN-gamma or IL-12 resulted in a significant reduction of intestinal inflammation in H. hepaticus-infected IL-10(-/-) mice, suggesting an important role for these cytokines in the development of colitis in the model. Taken together, these microbial reconstitution experiments formally establish that a defined bacterial agent can serve as the immunological target in the development of large bowel inflammation in IL-10(-/-) mice and argue that in nonimmunocompromised hosts IL-10 stimulated in response to intestinal flora is important in preventing IBD.

Partially protective vaccination permits the development of latency in a normally virulent strain of Toxoplasma gondii

The virulent RH strain of Toxoplasma gondii is acutely lethal in mice and fails to establish chronic infection. Vaccination of BALB/c mice with a soluble tachyzoite antigen preparation, STAg, in combination with the immunostimulatory cytokine interleukin-12 results in partial protection against RH lethal challenge. Nevertheless, brain tissue obtained from surviving, vaccinated mice as late as 1 year after RH infection contained latent parasite forms as demonstrated by subinoculation into naive recipients. The tachyzoites arising in the subinoculated animals were genetically indistinguishable from the original RH inoculum. Microscopic examination revealed that the persistent parasite forms present in the brains of vaccinated and challenged mice have a tissue cyst-like morphology and express the bradyzoite antigen BAG-1 but not the tachyzoite-specific antigen SAG-2 but are different from the cysts formed by avirulent T. gondii strains in that the internal parasite stages display ultrastructural features intermediate between tachyzoites and bradyzoites. Moreover, the zoites within the RH tissue cysts are clearly distinct from conventional bradyzoites in their sensitivity to pepsin-HCl digestion. In contrast to the observations made with partially resistant STAg/interleukin-12-vaccinated animals, no latent forms could be detected in brain tissue after RH challenge of mice immunized with a live attenuated tachyzoite vaccine which confers total protection against this parasite isolate. The above findings demonstrate the potential of a virulent T. gondii strain to generate latent parasite stages, a process which may be promoted under conditions of incomplete vaccination.

Paclitaxel (Taxol)-induced killing of Leishmania major in murine macrophages

The antitumor drug paclitaxel (Taxol) has been demonstrated to be a lipopolysaccharide mimetic in murine macrophages. In this study, the capacity of paclitaxel to activate macrophages to become microbicidal for Leishmania major was examined. Paclitaxel and gamma interferon synergized to kill intracellular L. major in Lpsn, but not Lpsd, macrophages by a nitric oxide (NO.)-dependent mechanism.

Ignition of the type 1 response to intracellular infection by dendritic cell-derived interleukin-12

Host resistance to many intracellular pathogens is dependent on the induction of host IFN-gamma. This response in turn is triggered by the critical initiation cytokine, IL-12. Activated macrophages provide a major source of IL-12 during infection yet are unlikely to be the initial cell to produce the cytokine because of their need for IFN-gamma priming and/or other co-stimulatory signals. We have utilized an in vivo approach to identify the primary IL-12 producing cells which respond to the intracellular protozoan Toxoplasma gondii. Our results indicate that in spleen interdigitating dendritic cells (IDC) but not macrophages rapidly synthesize IL-12 after injection of parasite products or live tachyzoites. This response is both IFN-gamma and T lymphocyte independent. The same microbial stimulus results in the migration of IDC precursors into T cell areas and the upregulation of co-stimulatory cell-surface molecules. We postulate that these early dendritic cell activation events represent the "ignition switch" for the subsequent Type 1 cytokine response which leads to control of infection.

IL-12 promotes drug-induced clearance of Mycobacterium avium infection in mice

The intracellular pathogen Mycobacterium avium is a major cause of opportunistic infection in AIDS patients and is difficult to manage using conventional chemotherapeutic approaches. In the current study, we describe a strategy for the treatment of M. avium in T cell-deficient hosts based on the simultaneous administration of antibiotics and the immunomodulatory cytokine IL-12. In contrast to SCID mice, which were partially resistant, animals lacking a functional IL-12 p40 gene were found to be highly susceptible to M. avium infection, suggesting that the cytokine can control bacterial growth even in immunodeficient mice. Indeed, rIL-12 that was injected into infected SCID mice in high doses caused small but significant reductions in splenic pathogen loads. Moreover, a lower dose of IL-12, when combined with the antimycobacterial drugs clarithromycin or rifabutin, induced a decrease in bacterial numbers that was significantly greater than that resulting from the administration of the cytokine or drug alone. A similar synergistic effect of IL-12 and antibiotics was seen when immunocompetent mice were treated with the same regimen. The activity of IL-12 in these experiments was shown to be dependent upon the induction of endogenous IFN-gamma. Nevertheless, IFN-gamma itself, even when given at a higher dose than IL-12, failed to significantly enhance antibiotic clearance of bacteria. Together these findings suggest that IL-12 may be a particularly potent adjunct for chemotherapy of M. avium infection in immunocompromised individuals and may result in more effective control of the pathogen without the need for increased drug dosage.

Failure of P strain mice to respond to vaccination against schistosomiasis correlates with impaired production of IL-12 and up-regulation of Th2 cytokines that inhibit macrophage activation

In contrast to most inbred strains, P mice fail to develop significant resistance to Schistosoma mansoni infection as a result of vaccination with either radiation-attenuated cercariae or schistosome antigens plus Bacillus Calmette Guérin, and this failure correlates with defects in macrophage larvicidal activity. Supernatant fluids from antigen-treated in vitro cultures of splenocytes from vaccinated P mice demonstrate less macrophage stimulatory activity than do supernatants from cells of vaccine-responsive strains such as C57BL/6. This is not due either to diminished production of the macrophage-activating cytokine IFN-gamma by P mice, or to a lesser responsiveness of macrophages from P mice to activation by IFN-gamma. Rather, P splenocytes produce two-to threefold higher amounts of IL-4 and IL-10, cytokines which down-regulate the cytotoxic potential of IFN-gamma-treated macrophages. Thus, the macrophage-activating potential of cytokine preparations from vaccinated P mice can be completely recovered by in vitro treatment with antibodies to IL-4 or IL-10. Moreover, lower levels of IL-12, a cytokine involved in promoting development of Th1 responses, are produced by splenocytes from P mice as compared to C57BL/6 counterparts. These studies indicate that a genetic predisposition toward an impaired production of IL-12 and an increased production of down-regulatory Th2 cytokines correlate with low response to vaccination against S. mansoni.

IL-12 Promotes Drug-Induced Clearance of Mycobacterium avium Infection in Mice

The intracellular pathogen Mycobacterium avium is a major cause of opportunistic infection in AIDS patients and is difficult to manage using conventional chemotherapeutic approaches. In the current study, we describe a strategy for the treatment of M. avium in T cell-deficient hosts based on the simultaneous administration of antibiotics and the immunomodulatory cytokine IL-12. In contrast to SCID mice, which were partially resistant, animals lacking a functional IL-12 p40 gene were found to be highly susceptible to M. avium infection, suggesting that the cytokine can control bacterial growth even in immunodeficient mice. Indeed, rIL-12 that was injected into infected SCID mice in high doses caused small but significant reductions in splenic pathogen loads. Moreover, a lower dose of IL-12, when combined with the antimycobacterial drugs clarithromycin or rifabutin, induced a decrease in bacterial numbers that was significantly greater than that resulting from the administration of the cytokine or drug alone. A similar synergistic effect of IL-12 and antibiotics was seen when immunocompetent mice were treated with the same regimen. The activity of IL-12 in these experiments was shown to be dependent upon the induction of endogenous IFN-γ. Nevertheless, IFN-γ itself, even when given at a higher dose than IL-12, failed to significantly enhance antibiotic clearance of bacteria. Together these findings suggest that IL-12 may be a particularly potent adjunct for chemotherapy of M. avium infection in immunocompromised individuals and may result in more effective control of the pathogen without the need for increased drug dosage.

IL-10 Regulates Liver Pathology in Acute Murine Schistosomiasis mansoni But Is Not Required for Immune Down-Modulation of Chronic Disease

We have used IL-10 gene knockout mice (IL-10T) to examine the role of endogenous IL-10 in the down-modulation of hepatic granuloma formation and lymphocyte responses that occurs in chronic infection with the helminth parasite Schistosoma mansoni. Although IL-10-deficient animals showed 20 to 30% mortality between 8 and 14 wk postinfection, they displayed no alterations in their susceptibility to infection and produced similar numbers of eggs as their wild-type littermates. The IL-10T mice displayed a significant increase in hepatic granuloma size at the acute stage of infection, which was associated with increased IFN-γ, IL-2, IL-1β, and TNF-α mRNA expression in liver and elevated Th1-type cytokine production by lymphoid cells. Despite developing an enhanced Th1-type cytokine response, the IL-10T mice showed no consistent decrease in their Th2-type cytokine profile. Surprisingly, although granulomatous inflammation was enhanced at the acute stage of infection, the livers of IL-10T mice displayed no significant increase in fibrosis and underwent normal immune down-modulation at the chronic stage of infection. Moreover, the down-modulated state could be induced in IL-10T mice by sensitizing the animals to schistosome eggs before infection, further demonstrating that the major down-regulatory mechanism is not dependent upon IL-10. We conclude that while IL-10 plays an important role in controlling acute granulomatous inflammation, it plays no essential role in the process of immune down-modulation in chronic schistosome infection.

IL-10 regulates liver pathology in acute murine Schistosomiasis mansoni but is not required for immune down-modulation of chronic disease

We have used IL-10 gene knockout mice (IL-10T) to examine the role of endogenous IL-10 in the down-modulation of hepatic granuloma formation and lymphocyte responses that occurs in chronic infection with the helminth parasite Schistosoma mansoni. Although IL-10-deficient animals showed 20 to 30% mortality between 8 and 14 wk postinfection, they displayed no alterations in their susceptibility to infection and produced similar numbers of eggs as their wild-type littermates. The IL-10T mice displayed a significant increase in hepatic granuloma size at the acute stage of infection, which was associated with increased IFN-gamma, IL-2, IL-1beta, and TNF-alpha mRNA expression in liver and elevated Th1-type cytokine production by lymphoid cells. Despite developing an enhanced Th1-type cytokine response, the IL-10T mice showed no consistent decrease in their Th2-type cytokine profile. Surprisingly, although granulomatous inflammation was enhanced at the acute stage of infection, the livers of IL-10T mice displayed no significant increase in fibrosis and underwent normal immune down-modulation at the chronic stage of infection. Moreover, the down-modulated state could be induced in IL-10T mice by sensitizing the animals to schistosome eggs before infection, further demonstrating that the major down-regulatory mechanism is not dependent upon IL-10. We conclude that while IL-10 plays an important role in controlling acute granulomatous inflammation, it plays no essential role in the process of immune down-modulation in chronic schistosome infection.