Interleukin-12 modulates the protective immune response in SCID mice infected with Histoplasma capsulatum

Hepatic Histoplasmosis: An Update

Histoplasma capsulatum is the most common cause of endemic mycosis in developing countries. It is a self-limited and asymptomatic disease in immunocompetent individuals but remains a frequent cause of opportunistic infection in patients with compromised immune status. Liver involvement as a part of disseminated histoplasmosis is well known. However, liver infection as a primary manifestation of histoplasmosis without evidence of primary lung involvement is rare. In conclusion, clinicians should be aware of isolated histoplasmosis affecting the hepatobiliary system, and careful evaluation is warranted to confirm the diagnosis. Given the appropriate clinical context, histoplasmosis should be considered in both immunocompetent and immunocompromised patients, regardless of pulmonary symptoms, in non endemic as well as endemic areas.

Granulomatous Hepatitis Secondary to Histoplasmosis in an Immunocompetent Patient

Histoplasma capsulatum is the most common endemic mycosis in the United States and usually occurs in certain geographic areas, such as the Mississippi or Ohio River valleys. Histoplasmosis usually causes a mild disease in the immunocompetent but can progress to disseminated disease in patients with impaired immunity. Granulomatous hepatitis as a manifestation of disseminated histoplasmosis in immunocompetent patients is extremely rare. We report the case of a 62-year-old immunocompetent gentleman with a history of histoplasmosis who presented with abdominal pain, elevated liver enzymes, who was diagnosed with granulomatous hepatitis secondary to histoplasmosis.

Histoplasmosis in an off-trail Hiker receiving ustekinumab: Implications for Preventive and diagnostic strategies for patients receiving anti-IL-12/23 therapy

Ustekinumab, an IL-12/23 inhibitor, is an important agent in treatment of inflammatory bowel disease and psoriasis. Clinical trials have not demonstrated significantly increased infection risk with ustekinumab. We report a case of disseminated histoplasmosis in the setting of ustekinumab and methotrexate following a hike in the Catskill Mountains, a region not commonly associated with Histoplasma encapsulatum. To our knowledge, this is the first reported case of newly acquired histoplasmosis complicating treatment with ustekinumab.

Mechanisms by Which Interleukin-12 Corrects Defective NK Cell Anticryptococcal Activity in HIV-Infected Patients

Cryptococcus neoformans is a pathogenic yeast and a leading cause of life-threatening meningitis in AIDS patients. Natural killer (NK) cells are important immune effector cells that directly recognize and kill C. neoformans via a perforin-dependent cytotoxic mechanism. We previously showed that NK cells from HIV-infected patients have aberrant anticryptococcal killing and that interleukin-12 (IL-12) restores the activity at least partially through restoration of NKp30. However, the mechanisms causing this defect or how IL-12 restores the function was unknown. By examining the sequential steps in NK cell killing of Cryptococcus, we found that NK cells from HIV-infected patients had defective binding of NK cells to C. neoformans. Moreover, those NK cells that bound to C. neoformans failed to polarize perforin-containing granules to the microbial synapse compared to healthy controls, suggesting that binding was insufficient to restore a defect in perforin polarization. We also identified lower expression of intracellular perforin and defective perforin release from NK cells of HIV-infected patients in response to C. neoformans. Importantly, treatment of NK cells from HIV-infected patients with IL-12 reversed the multiple defects in binding, granule polarization, perforin content, and perforin release and restored anticryptococcal activity. Thus, there are multiple defects in the cytolytic machinery of NK cells from HIV-infected patients, which cumulatively result in defective NK cell anticryptococcal activity, and each of these defects can be reversed with IL-12.

The mechanisms by which NK cells bind directly to pathogens and deploy their deadly cytolytic machinery during microbial host defense are only beginning to be elucidated. With the goal of understanding this process, we used NK cells from HIV-infected patients, which were known to have a defect in killing of Cryptococcus neoformans. Taking advantage of previous studies that had shown that IL-12 restored killing, we used the cytokine as a gain-of-function approach to define the relevance of multiple steps in the recognition and cytolytic pathway. We demonstrated that NK cells from HIV-infected patients failed to kill Cryptococcus due to defects in perforin expression, granule polarization, and release of perforin. Additionally, IL-12 restored recognition of C. neoformans through binding of the NK-activating receptor NKp30. These observations identify important mechanisms used by NK cells to kill microbes and determine that defects in NK cells from HIV-infected patients are reversible.

Histoplasmosis presenting as granulomatous hepatitis: case report and review of the literature

Background. Histoplasma capsulatum is the most common endemic mycosis in the United States and is a frequent cause of opportunistic infection in immunodeficient hosts. Histoplasmosis is most often self-limiting and goes unrecognized in the immunocompetent population but can progress to disseminated histoplasmosis in patients with an impaired immune system. Liver involvement as a part of disseminated histoplasmosis which usually originates in the lung is well known. However, extrapulmonary hepatic histoplasmosis as a primary manifestation is extremely rare. Case Presentation. We report a rare case of histoplasmosis that presented as persistent fever and abnormal liver function tests in a 66-year-old female with rheumatoid arthritis, receiving infliximab. Conclusion. Emphasizing histoplasmosis as a major cause of acute granulomatous hepatitis and fever of unknown origin in cell mediated immunodeficient population, this case highlights the need for high index of suspicion and the importance of prompt diagnosis since any delay of treatment can be life threatening in this population.

Usefulness of the murine model to study the immune response against Histoplasma capsulatum infection

The present paper is an overview of the primary events that are associated with the histoplasmosis immune response in the murine model. Valuable data that have been recorded in the scientific literature have contributed to an improved understanding of the clinical course of this systemic mycosis, which is caused by the dimorphic fungus Histoplasma capsulatum. Data must be analyzed carefully, given that misinterpretation could be generated because most of the available information is based on experimental host-parasite interactions that used inappropriate proceedings, i.e., the non-natural route of infection with the parasitic and virulent fungal yeast-phase, which is not the usual infective phase of the etiological agent of this mycosis. Thus, due to their versatility, complexity, and similarities with humans, several murine models have played a fundamental role in exploring the host-parasite interaction during H. capsulatum infection.

Recombinant murine IL-12 promotes a protective Th1/cellular response in Mongolian gerbils infected with Sporothrix schenckii

Sporotrichosis is a cutaneous fungal infection caused by Sporothrix schenckii. It is known to be mainly contained by Th1 responses. As IL-12 is crucial for Th1 response, we investigated if treatment with recombinant murine IL-12 (rmIL-12) promoted Th1 immunity and/or clinical improvement in an experimental sporotrichosis gerbil model. Gerbils were inoculated with S. schenckii in the footpad and treated with rmIL-12. Seven days post infection there was a significant increase in macrophage phagocytosis and oxidative burst, and in delayed-type hypersensitivity (DTH) reaction in rmIL-12 treated gerbils, as well as a ∼10-fold increase of serum IFN-gamma and a decrease of IL-4 and IL-10. Moreover, rmIL-12 substantially decreased (∼70%) S. schenckii burden in liver and spleen and improved the clinical outcome preventing footpad ulcer and tail nodules observed in untreated gerbils. Our study demonstrates that rmIL-12 promotes Th1 immune response against S. schenckii favouring its clearance and preventing clinical symptoms.

Duodenal histoplasmosis presenting with upper gastrointestinal bleeding in an AIDS patient

Gastrointestinal histoplasmosis (GIH) is common in patients with disseminated disease but only rarely comes to clinical attention due to the lack of specific signs and symptoms. We report the unusual case of a 33-year-old Caucasian male with advanced AIDS who presented with upper GI bleeding from diffuse erosions throughout the duodenum. Biopsy of the lesions revealed small bowel mucosa with granulomatous inflammation and macrophages with small intracellular yeasts consistent with disseminated histoplasmosis. The patient demonstrated significant clinical improvement following a two-week course of liposomal amphotericin B. To our knowledge, this is the first case report of duodenal histoplasmosis leading to clinically significant bleeding, manifesting with worsening anemia and melanotic stools. Given our findings, we maintain that GIH should be considered on the differential diagnosis for GI bleeding in AIDS patients at risk, specifically those with advanced immunosuppression (i.e., CD4⁺ cell counts <100 cells/mm³) who reside in endemic areas (Ohio or Mississippi river valleys) and/or have a prior history of histoplasmosis. For diagnostic evaluation, we recommend checking a urine Histoplasma quantitative antigen EIA as well as upper and/or lower endoscopy with biopsy. We recommend treatment with a two-week course of liposomal amphotericin B followed by indefinite itraconazole.

Early activation of the interleukin-23-17 axis in a murine model of oropharyngeal candidiasis

Candida albicans is an oral commensal yeast that causes oropharyngeal candidiasis (OPC) in immunocompromised individuals. The immunological pathways involved in OPC have been revisited after the interleukin-17 (IL-17) pathway was implicated in fungal immunity. We studied immediate (<24 h) and adaptive (3-6 day) IL-12 and IL-23-17 pathway activation in naive p40(-/-) mice, which lack IL-12 and IL-23 and develop severe, chronic OPC upon oral inoculation with C. albicans. Macrophages from p40(-/-) mice were less efficient than C57BL/6J controls at killing C. albicans in vitro but very low numbers in the oral mucosae of infected C57BL/6J mice suggest that they are not critical in vivo, at least in this strain. Migration of macrophages to regional lymph nodes of infected p40(-/-) mice was impaired; however, dendritic cell migration was not affected. Recombinant IL-12 therapy provided only temporary relief from OPC, suggesting that IL-23 is required for full protection. In C57BL/6J mice, but not p40(-/-) mice, messenger RNAs encoding IL-23p19 and IL-17 were induced in the oral mucosa within 24 h of infection (6 ± 0.6 and 12 ± 2.7-fold). By day 6 of infection in C57BL/6J mice, IL-17A messenger RNA level had increased 5.1 ± 1.8 and 83 ± 21-fold in regional lymph nodes and oral tissues respectively. Ablation of p40 was associated with delayed or abrogated induction of IL-17A pathway targets (monocyte chemoattractant protein-1, IL-6 and macrophage inflammatory protein-2), and a lack of organized recruitment of neutrophils to the infected oral mucosa. Overall our data show that the IL-23-17A axis is activated early in the oral mucosae of immunologically naive mice with OPC.

Vaccination with heat shock protein 60 induces a protective immune response against experimental Paracoccidioides brasiliensis pulmonary infection

Paracoccidioides brasiliensis causes a chronic granulomatous mycosis prevalent in Latin America. The successful resolution of infection with this fungus is dependent on the activation of cellular immunity. We previously identified heat shock protein 60 (HSP60) as a target of the humoral response in paracoccidioidomycosis. Herein we expressed the gene encoding HSP60 in Escherichia coli and analyzed the immunological activity of this recombinant antigen. The immunization of BALB/c mice with recombinant protein emulsified in adjuvant stimulated a cellular immune response. Splenocytes from immunized mice proliferated in response to antigen and released interleukin-12 and gamma interferon (IFN-gamma). Vaccination with HSP60 reduced the fungal burden in mice given 10(6) or 10(7) yeasts and protected mice from a lethal challenge. The efficacy of the vaccination was blunted by the neutralization of IFN-gamma. CD4(+) cells were necessary for the efficacy of the vaccination in both the afferent and efferent phases. Thus, we have demonstrated that this immunodominant antigen is a candidate for the development of a vaccine against this fungus.

Animal models: an important tool in mycology

Animal models of fungal infections are, and will remain, a key tool in the advancement of the medical mycology. Many different types of animal models of fungal infection have been developed, with murine models the most frequently used, for studies of pathogenesis, virulence, immunology, diagnosis, and therapy. The ability to control numerous variables in performing the model allows us to mimic human disease states and quantitatively monitor the course of the disease. However, no single model can answer all questions and different animal species or different routes of infection can show somewhat different results. Thus, the choice of which animal model to use must be made carefully, addressing issues of the type of human disease to mimic, the parameters to follow and collection of the appropriate data to answer those questions being asked. This review addresses a variety of uses for animal models in medical mycology. It focuses on the most clinically important diseases affecting humans and cites various examples of the different types of studies that have been performed. Overall, animal models of fungal infection will continue to be valuable tools in addressing questions concerning fungal infections and contribute to our deeper understanding of how these infections occur, progress and can be controlled and eliminated.

Fungal infections in primary immunodeficiencies

Patients with phagocytic, cellular, combined and other primary immunodeficiencies exhibit immune deficits that confer increased susceptibility to fungal infections. A number of yeasts and moulds, most commonly Candida and Aspergillus but also Cryptococcus, Histoplasma, Paecilomyces, Scedosporium, Trichosporon, Penicillium and other, rarely isolated, fungal organisms, have been variably implicated in causing disease in patients with chronic granulomatous disease, severe combined immunodeficiency, chronic mucocutaneous candidiasis, hyper-IgE syndrome, myeloperoxidase deficiency, leukocyte adhesion deficiency, defects in the interferon-gamma/interleukin-12 axis, DiGeorge syndrome, X-linked hyper-IgM syndrome, Wiskott-Aldrich syndrome and common variable immunodeficiency. Differences in the spectrum of fungal pathogens as well as in the incidence and clinical presentation of the infections may be observed among patients, depending upon different immune disorders. Fungal infections in these individuals may occasionally be the presenting clinical manifestation of a primary immunodeficiency and can cause significant morbidity and potentially fatal outcome if misdiagnosed or mistreated. A high degree of suspicion is needed and establishment of diagnosis should actively be pursued using appropriate imaging, mycological and histological studies. A number of antifungal agents introduced over the last fifteen years, such as the lipid formulations of amphotericin B, the second-generation triazoles, and the echinocandins, increase the options for medical management of these infections. Surgery may also be needed in some cases, while the role of adjunctive immunotherapy has not been systematically evaluated. The low incidence of primary immunodeficiencies in the general population complicates single-center prospective or retrospective clinical studies aiming to address diagnostic or therapeutic issues pertaining to fungal infections in these patients.

Human macrophages do not require phagosome acidification to mediate fungistatic/fungicidal activity against Histoplasma capsulatum

Histoplasma capsulatum (Hc) is a facultative intracellular fungus that modulates the intraphagosomal environment to survive within macrophages (Mphi). In the present study, we sought to quantify the intraphagosomal pH under conditions in which Hc yeasts replicated or were killed. Human Mphi that had ingested both viable and heat-killed or fixed yeasts maintained an intraphagosomal pH of approximately 6.4-6.5 over a period of several hours. These results were obtained using a fluorescent ratio technique and by electron microscopy using the 3-(2,4-dinitroanilo)-3'-amino-N-methyldipropylamine reagent. Mphi that had ingested Saccharomyces cerevisae, a nonpathogenic yeast that is rapidly killed and degraded by Mphi, also maintained an intraphagosomal pH of approximately 6.5 over a period of several hours. Stimulation of human Mphi fungicidal activity by coculture with chloroquine or by adherence to type 1 collagen matrices was not reversed by bafilomycin, an inhibitor of the vacuolar ATPase. Human Mphi cultured in the presence of bafilomycin also completely degraded heat-killed Hc yeasts, whereas mouse peritoneal Mphi digestion of yeasts was completely reversed in the presence of bafilomycin. However, bafilomycin did not inhibit mouse Mphi fungistatic activity induced by IFN-gamma. Thus, human Mphi do not require phagosomal acidification to kill and degrade Hc yeasts, whereas mouse Mphi do require acidification for fungicidal but not fungistatic activity.

Progress in vaccination for histoplasmosis and blastomycosis: coping with cellular immunity

Human infection with Histoplasma capsulatum or Blastomyces dermatitidis is sufficiently frequent to warrant exploring the development of vaccines. This review examines the advancements that have been accomplished over the last few years. The availability of molecular tools to create recombinant antigens or mutant strains has produced a small number of useful vaccine candidates. More importantly, the studies summarized herein demonstrate that understanding the host response to a protein or mutant fungus is critical to creating a vaccine that may be useful for the immunocompromised patient.

Apoptosis modulates protective immunity to the pathogenic fungus Histoplasma capsulatum

Pathogen-induced apoptosis of lymphocytes is associated with increased susceptibility to infection. In this study, we determined whether apoptosis influenced host resistance to the fungus Histoplasma capsulatum. The level of apoptotic leukocytes progressively increased in the lungs of naive and immune mice during the course of H. capsulatum infection. T cells constituted the dominant apoptotic population. Apoptosis was diminished in H. capsulatum-infected gld/gld and TNF-alpha-deficient mice; concomitantly, the fungal burden exceeded that of controls. Treatment of naive and H. capsulatum-immune mice with caspase inhibitors decreased apoptosis but markedly enhanced the severity of infection. Administration of a proapoptotic dose of suramin diminished the fungal burden. The increased burden in recipients of a caspase inhibitor was associated with elevations in IL-4 and IL-10 levels. In the absence of either of these cytokines, caspase inhibition suppressed apoptosis but did not increase the fungal burden. Thus, apoptosis is a critical element of protective immunity to H. capsulatum. Production of IL-4 and IL-10 is markedly elevated when apoptosis is inhibited, and the release of these cytokines exacerbates the severity of infection.

Disseminated histoplasmosis in persons with interferon-gamma receptor 1 deficiency

Mutations in the interferon (IFN)-gamma receptor predispose to infection with bacille Calmette-Guérin, nontuberculous mycobacteria, and Salmonella organisms. We identified a patient with recurrent disseminated Histoplasma capsulatum osteomyelitis who had an autosomal dominant form of IFN-gamma receptor 1 deficiency (i.e., a 4-bp deletion at or near base 818). IFN-gamma-mediated immunity is important in the control of histoplasmosis.

Modulation of Infection with Histoplasma capsulatum by Inhibition of Tumor Necrosis Factor–α Activity

Tumor necrosis factor (TNF)-alpha is required for host defenses against a number of pathogenic microbes, including Histoplasma capsulatum. In mice, TNF-alpha is necessary for the generation of a protective immune response to both primary and secondary histoplasmosis. Recent reports indicate that, in humans, treatment with inhibitors of TNF-alpha is associated with disseminated histoplasmosis. Here, I review the mechanisms by which inhibition of TNF-alpha may exacerbate infection with H. capsulatum.

IL-12: the role of p40 versus p75

Interleukin (IL)-12p75 is a heterodimeric cytokine composed of the product of two different genes that specify p35 and p40 subunits. The prevailing view is that IL-12 acts as a proinflammatory cytokine that bridges the innate and adaptive immune responses and skews T-cell reactivity toward a TH1 cytokine pattern. Though the terms IL-12 and IL-12p40 are often used interchangeably, and measurements of the p40 chain are often interpreted as measurements of the intact p75 heterodimer, such interchangeable usage may be incorrect. In the following discussion, I will delineate an alternative hypothesis for the roles of the p40 and p75 proteins, suggesting specifically, that: (1) in vivo, secretion of free p40 precedes that of p75 in response to pathogens; (2) induction of p40 is a T-independent response by antigen presenting cells (APCs) to early host-pathogen interactions; and (3) IL-12p75 is a late product, whose induction requires T-dependent signals. It is made as a result, rather than as a cause, of TH1 differentiation. Thus, it is the p40 protein, either alone or paired with other polypeptides, rather than p75, that acts as an interface between the innate and adaptive immune responses.

Interleukin-12 protects mice against disseminated infection caused by Paracoccidioides brasiliensis but enhances pulmonary inflammation

Paracoccidioides brasiliensis is a facultative, intracellular pathogen causing the most important deep mycosis in Latin America. As the production of IFN-gamma and induction of cell-mediated immunity to P. brasiliensis is of critical importance in host defense, the immunotherapeutic effect of exogenous IL-12 administration was studied in a murine model of susceptibility to pulmonary infection. rIL-12 treatment led to a less disseminated disease, as confirmed by decreased fungal loads in liver and spleen. Administration of rIL-12 did not affect fungal growth in the lungs, although it did induce an augmented pulmonary mononuclear cell inflammation. IL-12 treatment induced an early (week 1) increase in pulmonary IFN-gamma, but decreased cytokine and specific antibody (IgG1 and IgG3) production at week 8 after infection. These results show that IL-12 administration induces a less severe infection, but the high inflammatory response detected in the lungs precludes its possible use as a new therapeutic tool for severe paracoccidioidomycosis.

Perforin is required for primary immunity to Histoplasma capsulatum

Protective immunity against primary and secondary infection by the fungus Histoplasma capsulatum (HC) is multifactorial, requiring cells of the innate and adaptive immune response. Effector mechanisms that could mediate intracellular killing of HC include cytokines such as IFN-gamma and TNF-alpha and/or direct cytolytic activity by T and NK cells. In this regard, although previous work has clearly demonstrated a critical role for IFN-gamma and TNF-alpha in limiting fungal growth in primary HC infection, less is known regarding the role of cytolytic mechanisms. The studies reported here first address the role of perforin in mediating immunity to HC. Remarkably, perforin-deficient knockout (PfKO) mice were shown to have accelerated mortality and increased fungal burden following a lethal or sublethal primary challenge. These data established an essential role for perforin in primary immunity systemic HC infection. Interestingly, depletion of CD8(+) T cells in PfKO mice caused a further increase in fungal burden and accelerated mortality, suggesting a perforin-independent role for CD8(+) T cells. Moreover, adoptive transfer of CD8(+) T cells from PfKO mice into IFN-gamma(-/-) mice caused a reduction in fungal burden following infectious challenge compared with control IFN-gamma(-/-) mice. Together, these data suggest that CD8(+) T cells can mediate immunity to HC through both perforin-dependent and -independent mechanisms.

Efficacy of interferon-gamma and amphotericin B for the treatment of systemic murine histoplasmosis

The number of cases of systemic histoplasmosis has increased substantially in recent years, and improved therapy is needed. We examined the efficacy of immunomodulation with interferon (IFN)-gamma alone or in combination with a suboptimal regimen of amphotericin B for the treatment of primary systemic murine histoplasmosis. In the first study, BALB/c mice were infected with Histoplasma capsulatum G217B and treated with 10(5) U of IFN given every other day either preinfection and postinfection or only postinfection, alone or in combination with amphotericin B. IFN alone given subcutaneously (s.c.) postinfection prolonged survival over untreated controls (P < 0.01), whereas intravenous (i.v.) administration was ineffective. All combination regimens and amphotericin B alone significantly prolonged survival (P < 0.0001). The combination regimens of amphotericin B and IFN i.v. (pre- and postinfection) or IFN s.c. (postinfection) reduced the fungal burden in the liver and spleen; the latter regimen had superior efficacy in the spleen (P < 0.05) to either amphotericin B or IFN alone. After infection with a low-challenge inoculum, IFN given s.c. (pre- and postinfection) alone caused a significant reduction in fungal burden in the spleen (P < 0.001). In an acutely lethal model, combination regimens of IFN s.c. or i.v. and amphotericin B again prolonged survival (P < 0.01-0.001), with amphotericin B plus IFN given s.c. (pre- and postinfection) superior to all regimens (P < 0.05-0.01). This regimen also showed enhanced efficacy in causing the reduction of fungal burden in the spleen (P < 0.05). These results indicate that IFN in combination with AmB shows enhanced efficacy in the treatment of systemic histoplasmosis and support the potential utility of IFN as an adjunctive therapy.

Modulation of host responses to blood-stage malaria by interleukin-12: from therapy to adjuvant activity

This review focuses on the role of interleukin (IL)-12, a proinflammatory cytokine with pleiotropic effects as a potent immunoregulatory molecule and hematopoietic growth factor, in infection with Plasmodium parasites, the causative agents of malaria. IL-12 has been demonstrated to have profound effects on the immune response to blood-stage malaria, to induce protection, and to alleviate malarial anemia. In combination with an anti-malarial drug, IL-12 is effective in an established malaria infection. This cytokine also has potent immune effects as a malaria vaccine adjuvant. However, IL-12 can also mediate pathology during blood-stage malaria.

Immunomodulators in infectious diseases: panoply of possibilites

Infections which caused ravages in the past centuries are again resurgent and newly emerging pathogens capable of human diseases continue to surface. Multidrug antibiotic resistance has turned into a major medical problem. Judicious concepts for combating infections in the 21st century have acquired a new poignancy. Immunomodulators of natural, synthetic, and recombinant origin can stimulate host defense mechanisms for the prophylaxis and treatment of diverse viral, bacterial, parasitic and fungal diseases. Some immunomodulator preparations are already licensed for use in patients and numerous others are being extensively investigated in preclinical and clinical studies. Immunomodulators offer a novel adjunct to established antimicrobial therapies.

Comparison of a new triazole, posaconazole, with itraconazole and amphotericin B for treatment of histoplasmosis following pulmonary challenge in immunocompromised mice

A murine model of intratracheally induced histoplasmosis in immunocompromised B6C3F(1) mice was used to evaluate a new triazole antifungal agent, posaconazole. This compound was previously shown to be comparable to amphotericin B and superior to itraconazole for the treatment of histoplasmosis in immunocompetent mice. The current study used mice that were depleted of T lymphocytes by intraperitoneal injection of anti-CD4 and anti-CD8 monoclonal antibodies beginning 2 days before infection and continuing at 5-day intervals until completion of the study. Groups of B6C3F(1) mice that were depleted of CD4 and CD8 T cells were infected with an inoculum of 10(4) Histoplasma capsulatum yeasts. All mice receiving posaconazole at 1 or 0.1 mg/kg of body weight/day, amphotericin B at 2 mg/kg every other day (qod), or itraconazole at 75 mg/kg/day survived to day 29. Only 60% of mice receiving itraconazole at 10 mg/kg/day and none receiving amphotericin B at 0.2 mg/kg qod survived to that date. Fungal burdens were determined at day 14 of infection, 1 day after discontinuation of therapy. Quantitative colony counts and Histoplasma antigen levels in lung and spleen tissues declined following treatment with amphotericin B at 2 mg/kg qod, posaconazole at 5 and 1 mg/kg/day, and itraconazole at 75 mg/kg/day but not in mice treated with amphotericin B at 0.2 mg/kg qod or itraconazole at 10 mg/kg/day. Posaconazole at 0.1 mg/kg/day reduced fungal colony counts and antigen levels in spleens but not in lungs. This study shows posaconazole activity for the treatment of histoplasmosis in immunosuppressed animals.

Experimental histoplasmosis in mice treated with anti-murine interferon-gamma antibody and in interferon-gamma gene knockout mice

Histoplasma capsulatum is an important fungal pathogen in immunocompromised hosts, including AIDS patients. Experimental evidence suggests interferon-gamma (IFN) plays a role in host defense against H. capsulatum. In these studies we sought to demonstrate the importance of IFN in innate resistance to systemic histoplasmosis. The possible exacerbation of infection in BALB/c mice was assessed by administering 200 microg of hamster anti-IFN antibody prior to infection with H. capsulatum (2 x 10(6) yeasts, i.v.) and by comparing the severity of infection between BALB/c IFN gene knockout mice (GKO) and congenic control animals. In two separate studies, we found that anti-IFN treatment caused a dramatic loss of resistance to lethal infection and resulted in earlier mortality of IFN-depleted animals compared with normal IgG or no treatment (P<0.001). GKO mice were significantly (P<0.001) more susceptible to lethal infection than were control animals, and histological studies corroborated this. These studies clearly demonstrate that IFN is a vital part of the host's innate resistance to systemic infection with H. capsulatum and provide an additional rationale for studying IFN as an immunomodulatory therapeutic for the treatment of this disease.

Immunomodulatory agents for prophylaxis and therapy of infections

The advent of the antibiotic era ushered in a shift towards non-pathogen-specific therapy of infectious diseases. This led to an overt emphasis on targeting microbial pathogens while strategies directed towards enhancing host immunity were neglected. In an effort to decrease sole reliance on antimicrobials, the time has come for a critical reappraisal of nonantibiotic, albeit immune response-enhancing substances. The diverse array of natural, synthetic, and recombinant immunomodulators discussed in this review succinctly demonstrate the potential of these agents to stimulate host defense mechanisms for prophylaxis and treatment of various microbial infections.

Roles of endogenous gamma interferon and macrophage microbicidal mechanisms in host response to chemotherapy in experimental visceral leishmaniasis

In experimental visceral leishmaniasis, in which the tissue macrophage is the target, in vivo responsiveness to conventional chemotherapy (pentavalent antimony [Sb]) requires a T-cell-dependent mechanism. To determine if this mechanism involves gamma interferon (IFN-gamma)-induced activation and/or specific IFN-gamma-regulated macrophage leishmanicidal mechanisms (generation of reactive nitrogen or oxygen intermediates, we treated gene-deficient mice infected with Leishmania donovani. In IFN-gamma gene knockout (GKO) mice, Sb inhibited but did not kill intracellular L. donovani (2% killing versus 76% in controls). Sb was active (>94% killing), however, in both inducible nitric oxide synthase (iNOS) knockout (KO) and respiratory burst (phagocyte oxidase)-deficient chronic granulomatous disease (X-CGD) mice. Sb's efficacy was also maintained in doubly deficient animals (X-CGD mice treated with an iNOS inhibitor). In contrast to Sb, amphotericin B (AmB) induced high-level killing in GKO mice; AmB was also fully active in iNOS KO and X-CGD animals. Although resolution of L. donovani infection requires iNOS, residual visceral infection remained largely suppressed in iNOS KO mice treated with Sb or AmB. These results indicate that endogenous IFN-gamma regulates the leishmanicidal response to Sb and achieves this effect via a pathway unrelated to the macrophage's primary microbicidal mechanisms. The role of IFN-gamma is selective, since it is not a cofactor in the response to AmB. Treatment with either Sb or AmB permits an iNOS-independent mechanism to emerge and control residual intracellular L. donovani infection.

In-vivo treatment with benznidazole enhances phagocytosis, parasite destruction and cytokine release by macrophages during infection with a drug-susceptible but not with a derived drug-resistant Trypansoma cruzi population

To stuck the effect of chemotherapy on parasite-macrophage interaction we used the wild-type Y strain (drug-susceptible) of Trypanosoma cruzi and a drug-resistant parasite population derived from the same strain. Trypomastigotes isolated from untreated infected mice, as well as, 3 h after treatment with BZ were incubated with inflammatory macrophages and used to study phagocytosis, parasite destruction, cytokine release and reactive nitrogen intermediates (RN!) synthesis. Phagocytosis and destruction of the drug-susceptible parasites were significant/v enhanced by drug treatment. These enhancements were accompanied by an increase in cytokines [interleukin (IL)-12 and tumour necrosis factor (TNF)alpha] and RNI release by murine inflammatory macrophages primed with IFN-gamma. In contrast, BZ treatment of mice infected with drug-resistant T. cruzi population showed no effect whatsoever. The synthesis of IFN-gamma and RNI by splenocytes of mice infected with either susceptible and drug-resistant parasite populations, before and after treatment with BZ were also studied. On/v the splenocytes from mice infected with the drug-susceptible parasites treated with BZ produced high levels of IFN-gamma and RNI. Our findings indicate that BZ acts on the drug-susceptible T. cruzi parasites by enhancing the phagocytosis and the production of cytokines and RN!, thus, favouring the destruction of the intracellular parasites by the cellular compartment of the immune system.

Therapy with a combination of low doses of interleukin 12 and chloroquine completely cures blood-stage malaria, prevents severe anemia, and induces immunity to reinfection

The immunoregulatory cytokine interleukin 12 (IL-12) induces host resistance against experimental malaria. In this study, we tested the feasibility of using IL-12 in combination with chloroquine (CQ) to rescue susceptible A/J mice from lethal blood-stage Plasmodium chabaudi AS infection. Combined treatment with low doses of CQ and IL-12 resulted in a >15-fold reduction in the parasite load and 100% survival of A/J mice with established infections. Compared to control mice, which succumbed to severe anemia, CQ-plus-IL-12-treated mice had significantly higher early- and late-stage erythroid-cell progenitors in the bone marrow and spleen, resulting in significantly higher hematocrits, erythrocyte counts, and percentages of reticulocytes. Production of parasite-specific gamma interferon (IFN-gamma) by splenocytes from these mice was upregulated >20-fold relative to controls in parallel with enhanced IFN-gamma mRNA expression. Further, enhanced responsiveness to IL-12 and increased downstream IFN-gamma production in CQ-plus-IL-12-treated mice was evident from increased mRNA expression for the beta1 and beta2 subunits of IL-12 receptor in the splenocytes. Moreover, this combined therapy induced higher levels of anti-malaria antibodies than did CQ alone as well as sterile immunity against reinfection. Because IL-12 can be used at low doses and is effective even in established infections, it may be feasible to use this immunochemotherapeutic approach in human malaria.

Interleukin-12 enhances in vivo parasiticidal effect of benznidazole during acute experimental infection with a naturally drug-resistant strain of Trypanosoma cruzi

The roles of gamma interferon (IFN-gamma) and interleukin-12 (IL-12) in mediating and/or enhancing the in vivo trypanosomicidal activity of the nitroheterocyclic derivative benznidazole (Bz) were evaluated during early stages of experimental Chagas' disease. Our results show that treatment of Trypanosoma cruzi-infected mice with anti-cytokine monoclonal antibodies (MAbs) had no apparent effect when the optimal dose of Bz (100 mg/kg of body weight) was used. In contrast, treatment with anti-IL-12 or anti-IFN-gamma MAbs enhanced the parasitemia and accelerated the mortality of mice treated with a suboptimal dose of Bz (25 mg/kg). Simultaneous treatment with a suboptimal dose of Bz and recombinant IL-12 (rIL-12) enhanced the efficacy of drug treatment in terms of parasitemia and mouse survival. Interestingly, we found that drug-resistant T. cruzi strains were found to be poor inducers of IL-12 both in vitro and in vivo compared to strains of T. cruzi which are susceptible or partially resistant to Bz treatment. These results suggest that early activation of the cellular compartment of the immune system by IL-12 may favor in vivo Bz activity against T. cruzi. In order to test this hypothesis mice infected with the drug-resistant Colombiana strain of T. cruzi were treated with 100 mg of Bz per kg plus different concentrations of rIL-12. By using the results of PCR and serological and parasitological methods as the criteria of a cure, our results indicate that a higher percentage of mice treated with Bz combined with rIL-12 than mice treated with Bz alone are cured.

CD40 ligand is not essential for induction of type 1 cytokine responses or protective immunity after primary or secondary infection with histoplasma capsulatum

The induction of type 1 immune responses (interleukin [IL]-12, interferon [IFN]-gamma) has been shown to be important in mediating protection against many intracellular infections including Histoplasma capsulatum. Costimulatory molecules such as CD40 ligand (CD40L) have been shown to be a central regulator of type 1 responses in vivo. To study the role of CD40L in mediating protection against infection with H. capsulatum, CD40L-deficient (CD40L-/-) and CD40L+/+ mice were infected with H. capsulatum and assessed for various parameters. After a lethal challenge of H. capsulatum, CD40L-/- mice were not substantially different from CD40L+/+ mice in terms of mortality, fungal burden, or production of IFN-gamma, IL-12, nitric oxide, or tumor necrosis factor alpha. Moreover, CD40L-/- mice treated with anti-IFN-gamma or anti-IL-12 at the time of infection had accelerated mortality, providing further evidence that IL-12 and IFN-gamma are produced in vivo in the absence of CD40L. In addition, CD40L-/- mice infected with a sublethal dose of H. capsulatum survived infection, whereas all mice infected with the same dose and treated with anti-IFN-gamma had accelerated mortality, demonstrating that IFN-gamma but not CD40L was essential for primary immunity to H. capsulatum infection. Interestingly, depletion of either CD4+ or CD8+ T cells resulted in accelerated mortality in CD40L-/- mice, suggesting a critical role for these cells in response to infection. Finally, CD40L-/- mice initially infected with a sublethal dose of H. capsulatum were protected from secondary infection with a lethal dose of H. capsulatum, demonstrating that CD40L is not required for the maintenance of memory immunity.

Factors Involved in Regulating Primary and Secondary Immunity to Infection with Histoplasma capsulatum: TNF-α Plays a Critical Role in Maintaining Secondary Immunity in the Absence of IFN-γ

Primary infection to Histoplasma capsulatum often results in a self-limited upper respiratory infection in humans; however, in immunocompromised hosts, disseminated infection can occur through reactivation of a previous infection. Since disseminated histoplasmosis has emerged as a difficult clinical entity to treat in individuals infected with HIV, it was of interest to study the mechanisms involved in maintaining an effective memory immune response. It has been previously shown in a murine model of disseminated histoplasmosis that IL-12, IFN-γ, and TNF-α were important factors in mediating primary protection. To study whether these and other factors were involved in maintaining a protective immune response following secondary infection, normal C57BL/6 mice were first infected with a sublethal dose of H. capsulatum (1 × 105) and then reinfected 3 wk later with a lethal dose of H. capsulatum (6 × 105). Under these conditions, all mice developed an effective immune response with sterilizing immunity. Moreover, normal C57BL/6 mice treated with neutralizing Abs against either IL-12, TNF-α, or IFN-γ, depleted of neutrophils or treated with aminoguanidine at the time of reinfection, maintained an effective immune response. The ability of animals to survive a secondary infection in the absence of IFN-γ was verified by showing that IFN-γ−/− mice previously immunized with H. capsulatum and treated with amphotericin B at the time of primary infection had prolonged survival following reinfection with a normally lethal dose. It was further shown that enhancement of TNF-α production in IFN-γ−/− mice was the major mechanism by which these mice were effective in controlling secondary infection.

Interleukin-12 in infectious diseases

Interleukin-12 (IL-12) is a potent immunoregulatory cytokine that is crucially involved in a wide range of infectious diseases. In several experimental models of bacterial, parasitic, viral, and fungal infection, endogenous IL-12 is required for early control of infection and for generation and perhaps maintenance of acquired protective immunity, directed by T helper type 1 (Th1) cells and mediated by phagocytes. Although the relative roles of IL-12 and gamma interferon in Th1-cell priming may be to a significant extent pathogen dependent, common to most infections is that IL-12 regulates the magnitude of the gamma interferon response at the initiation of infection, thus potentiating natural resistance, favoring Th1-cell development; and inhibiting Th2 responses. Treatment of animals with IL-12, either alone or as a vaccine adjuvant, has been shown to prevent disease by many of the same infectious agents, by stimulating innate resistance or promoting specific reactivity. Although IL-12 may enhance protective memory responses in vaccination or in combination with antimicrobial chemotherapy, it is yet unclear whether exogenous IL-12 can alter established responses in humans. Continued investigation into the possible application of IL-12 therapy to human infections is warranted by the role of the cytokine in inflammation, immunopathology, and autoimmunity.

Interleukin-12 and infectious diseases: a potential novel therapy

Interleukin-12 (IL-12) is emerging as a central component of both innate and acquired immunity. The multiplicity of biological activities associated with this cytokine, particularly the stimulation of cell-mediated immunity, suggests that it may be crucial in the control of extracellular and intracellular infections. In in vitro studies, IL-12 production is initiated rapidly after infection with a variety of viral, parasitic, fungal and bacterial agents. This induction correlates well with the reported resistance or susceptibility of animals to infection with these agents. Other factors may, however, influence responses in vivo, including host genetic make-up, microbial load and the induction of antagonistic cytokine pathways, notably IL-4 and IL-10. In some situations, IL-12 may direct immune responses to inappropriate pathways, and worsen disease, so that careful consideration of the type of required immune response is needed before IL-12 therapy is initiated. IL-12 treatment may also be useful in promoting protective immune responses to vaccines, allowing systemic immunisation with lower doses, or even normally non-immunogenic preparations, of antigen. Finally, IL-12 has been demonstrated to act in concert with standard antimicrobial chemotherapy in viral, parasitic, fungal and bacterial infections, allowing a reduction in the dose of the agent used and providing hope that such combination therapy may more effectively control drug-resistant strains of infectious agents.

The T cell response against fungal infections

A variety of pathological conditions, including impaired immune function, is believed to underlie host susceptibility to fungal infections and to determine both the severity and the characteristic of the associated pathology. Although the redundancy and the interdependence of antifungal responses may not favor the proper dissection and appreciation of individual effector mechanisms, the T helper type 1/type 2 paradigm of acquired immunity to fungi is proving essential for a better understanding of the host response from a regulatory perspective. The recent understanding of the importance of the different T helper cell subsets in fungal infections and the increasing appreciation of the reciprocal regulation between the innate, humoral, and adaptive immune systems in the development of optimal antimicrobial immunity have offered us new clues which may lead to an understanding of T cell dependent immunity to fungi.