Enhanced sensitivity of tumor necrosis factor/lymphotoxin-alpha-deficient mice to Cryptococcus neoformans infection despite increased levels of nitrite/nitrate, interferon-gamma, and interleukin-12

Cryptococcus gattii in Patients with Lymphoid Neoplasms: An Illustration of Evolutive Host-Fungus Interactions

Recent outbreaks of Cryptococcus gattii (CG) infections in North America have sparked renewed interest in the pathogenic potential of CG, and have underscored notable differences with Cryptococcus neoformans in terms of geographic distribution, pathogen virulence, and host susceptibility. While cases of CG are increasingly reported in patients with a wide variety of underlying conditions, only very few have been reported in patients with lymphoid neoplasms. Herein, we report a case of autochthonous CG meningitis in a patient receiving ibrutinib for chronic lymphocytic leukemia in France, and review available data on the clinical epidemiology of CG infections in patients with lymphoid neoplasms. We also summarise recent data on the host responses to CG infection, as well as the potential management pitfalls associated with its treatment in the haematological setting. The clinical epidemiology, clinical presentation, and course of disease during infections caused by CG involve complex interactions between environmental exposure to CG, infecting genotype, pathogen virulence factors, host susceptibility, and host immune responses. Future treatment guidelines should address the challenges associated with the management of antifungal treatments in the onco-haematological setting and the potential drug-drug interactions.

Dectin-2-mediated signaling triggered by the cell wall polysaccharides of Cryptococcus neoformans

Cryptococcus neoformans is rich in polysaccharides of the cell wall and capsule. Dectin-2 recognizes high-mannose polysaccharides and plays a central role in the immune response to fungal pathogens. Previously, we demonstrated Dectin-2 was involved in the activation of dendritic cells upon stimulation with C. neoformans, suggesting the existence of a ligand recognized by Dectin-2. In the present study, we examined the cell wall structures of C. neoformans contributing to the Dectin-2-mediated activation of immune cells. In a NFAT-GFP reporter assay of the reported cells expressing Dectin-2, the lysates, but not the whole yeast cells, of an acapsular strain of C. neoformans (Cap67) delivered Dectin-2-mediated signaling. This activity was detected in the supernatant of β-glucanase-treated Cap67 and more strongly in the semi-purified polysaccharides of this supernatant using ConA-affinity chromatography (ConA-bound fraction), in which a large amount of saccharides, but not protein, were detected. Treatment of this supernatant with periodic acid and the addition of excessive mannose, but not glucose or galactose, strongly inhibited this activity. The ConA-bound fraction of the β-glucanase-treated Cap67 supernatant was bound to Dectin-2-Fc fusion protein in a dose-dependent manner and strongly induced the production of interleukin-12p40 and tumour necrosis factor-α by dendritic cells; this was abrogated under the Dectin-2-deficient condition. Finally, 98 kDa mannoprotein (MP98) derived from C. neoformans showed activation of the reporter cells expressing Dectin-2. These results suggested that a ligand with mannose moieties may exist in the cell walls and play a critical role in the activation of dendritic cells during infection with C. neoformans.

Innate Immunity against Cryptococcus, from Recognition to Elimination

Cryptococcus species, the etiological agents of cryptococcosis, are encapsulated fungal yeasts that predominantly cause disease in immunocompromised individuals, and are responsible for 15% of AIDS-related deaths worldwide. Exposure follows the inhalation of the yeast into the lung alveoli, making it incumbent upon the pattern recognition receptors (PRRs) of pulmonary phagocytes to recognize highly conserved pathogen-associated molecular patterns (PAMPS) of fungi. The main challenges impeding the ability of pulmonary phagocytes to effectively recognize Cryptococcus include the presence of the yeast's large polysaccharide capsule, as well as other cryptococcal virulence factors that mask fungal PAMPs and help Cryptococcus evade detection and subsequent activation of the immune system. This review will highlight key phagocyte cell populations and the arsenal of PRRs present on these cells, such as the Toll-like receptors (TLRs), C-type lectin receptors, NOD-like receptors (NLRs), and soluble receptors. Additionally, we will highlight critical cryptococcal PAMPs involved in the recognition of Cryptococcus. The question remains as to which PRR-ligand interaction is necessary for the recognition, phagocytosis, and subsequent killing of Cryptococcus.

Dissociation of IFN-γ from IL-12 and IL-18 production during endotoxin tolerance

Endotoxin tolerance was induced in mice following one, two or three injections of low amounts of lipopolysaccharide (LPS) before a further LPS injection, and circulating cytokines were analyzed 1.5 h and 3 h after LPS challenge. Three different patterns of cytokine production were obtained. In a first group of cytokines, including tumor necrosis factor (TNF), interleukin-6 (IL-6) and gamma interferon (IFN-γ), the reduction of plasma peak levels was already significantly pronounced after one tolerizing injection of LPS. The second group of cytokines includes the CC chemokine KC, the CXC chemokine monocyte-chemo-attractant protein-1 (MCP-1) and IL-12. The plasma levels of these cytokines were modestly reduced, and the reduction was more pronounced with increasing numbers of tolerizing injections of LPS. The third group of cytokines includes IL-1β and IL-18, the levels of which 3 h after LPS challenge (i.e. at the peak timing) remained essentially similar to those of control mice and after 1.5 h were even enhanced. Altogether, these data illustrate that, in tolerized animals, in vivo regulation of cytokine production differs greatly among different mediators and that immunoparalysis is not a general state. Furthermore, despite the presence of large amounts of IL-12 and IL-18, IFN-γ was essentially suppressed in tolerized animals.

Defect of CARD9 leads to impaired accumulation of gamma interferon-producing memory phenotype T cells in lungs and increased susceptibility to pulmonary infection with Cryptococcus neoformans

Caspase recruitment domain-containing protein 9 (CARD9) is an adaptor molecule signal that is critical for NF-κB activation and is triggered through C-type lectin receptors (CLRs), which are pattern recognition receptors that recognize carbohydrate structures. Previous studies have reported that Cryptococcus neoformans, a fungal pathogen that causes meningoencephalitis in AIDS patients, is recognized through some CLRs, such as mannose receptors or DC-SIGN. However, the role of CARD9 in the host defense against cryptococcal infection remains to be elucidated. In the present study, we analyzed the role of CARD9 in the host defense against pulmonary infection with C. neoformans. CARD9 gene-disrupted (knockout [KO]) mice were highly susceptible to this infection, as shown by the reduced fungal clearance in the infected lungs of CARD9 KO mice, compared to that in wild-type (WT) mice. Gamma interferon (IFN-γ) production was strongly reduced in CARD9 KO mice during the innate-immunity phase of infection. Reduced IFN-γ synthesis was due to impaired accumulation of NK and memory phenotype T cells, which are major sources of IFN-γ innate-immunity-phase production; a reduction in the accumulation of these cells was correlated with reduced CCL4, CCL5, CXCL9, and CXCL10 synthesis. However, differentiation of Th17 cells, but not of Th1 cells, was impaired at the adaptive-immunity phase in CARD9 KO mice compared to WT mice, although there was no significant difference in the infection susceptibility between interleukin 17A (IL-17A) KO and WT mice. These results suggest that CARD9 KO mice are susceptible to C. neoformans infection probably due to the reduced accumulation of IFN-γ-expressing NK and memory phenotype T cells at the early stage of infection.

N-Acetylcysteine prevents baker's-yeast-induced inflammation and fever

OBJECTIVE AND DESIGN

To investigate whether N-acetylcysteine (NAC) alters baker's-yeast-induced fever and inflammation.

MATERIAL OR SUBJECTS

Male Wistar rats (26-28 days old) injected with baker's yeast (135 mg/kg, intraperitoneal) or prostaglandin E(2) (300 ng/100 μL, intrathecal).

TREATMENT

Rats were injected with NAC (500 mg/kg, subcutaneous, or 50 μg/100 μL, intrathecal) 1 h before, or 2 h after, pyrogen injection.

METHODS

Rectal temperature changes induced by baker's yeast, PGE(2) and NAC were followed up over time. Four hours after baker's yeast injection, total leukocytes, protein, interleukin (IL)-1β, tumor necrosis factor (TNF)-α and nonprotein thiol content were assessed in peritoneal lavage and hypothalamus.

RESULTS

Systemic administration of NAC decreased leukocytes, protein, IL-1β and TNF-α levels in peritoneal lavage, and decreased IL-1β levels in the hypothalamus. The central administration of NAC prevented baker's-yeast-induced fever, but did not alter the febrile response elicited by prostaglandin E(2).

CONCLUSION

These results suggest an anti-inflammatory and antipyretic role for NAC in yeast-induced peritonitis.

Cryptococcus neoformans suppresses the activation of bone marrow-derived dendritic cells stimulated with its own DNA, but not with DNA from other fungi

DNA from Cryptococcus neoformans activates bone marrow-derived dendritic cells (BM-DCs) in a TLR9-dependent manner. In this study, we examined the effect of the culture supernatants of C. neoformans on the activation of BM-DCs caused by its own DNA. C. neoformans supernatants suppressed IL-12p40, IL-6 production and CD40 expression by BM-DCs stimulated with its own DNA, but not with CpG-ODN and DNA from Candida albicans, Saccharomyces cerevisiae or Escherichia coli. In a confocal microscopic analysis, C. neoformans DNA was colocalized with LAMP-1, a late endosomal marker, and TLR9. The culture supernatants did not show any apparent suppression of these responses. In a luciferase reporter assay, C. neoformans supernatants inhibited NFκB activation caused by its own DNA. These inhibitory activities were attenuated by treatment with heat or trypsin. These results indicate that C. neoformans secrete certain proteinous molecules that suppress the activation of BM-DCs caused by its own DNA.

Yam storage protein dioscorins from Dioscorea alata and Dioscorea japonica exhibit distinct immunomodulatory activities in mice

The aim of this study was to elucidate the effect of the major storage protein dioscorin isolated from two different yam species, Tainong No. 1 (TN1-dioscorins) and Japanese yam (Dj-dioscorins), on the immune activities of mice. Dj-dioscorins, like TN1-dioscorins, could induce expression of the pro-inflammatory cytokines and stimulate phagocytosis of RAW 264.7. Intraperitoneal injection of the TN1-dioscorins into mice stimulated phagocytosis of bone marrow, spleen, and thymic cells. In contrast, the T and B cells in bone marrow, spleen, and thymus isolated from mice injected with Dj-dioscorins had higher proliferative responses to mitogens. Furthermore, Dj-dioscorins enhanced proliferation of CD4(+), CD8(+), and Tim3(+) (Th1) cells in spleen and CD19(+) cells in both spleen and thymus. Supplement of Dj-dioscorins in the lymphoid cells isolated from Dj-dioscorins primed mice induced cell proliferation of both spleen and thymic cells. These findings indicated that TN1-dioscorins have a higher ability to stimulate the phagocytic activity of the lymphoid cells than Dj-dioscorins, whereas Dj-dioscorins possess more abilities than TN1-dioscorins to enhance the proliferation of the lymphoid cells.

Cryptococcus neoformans inhibits nitric oxide synthesis caused by CpG-oligodeoxynucleotide-stimulated macrophages in a fashion independent of capsular polysaccharides

Cryptococcus neoformans is eradicated by macrophages via production of NO. Unmethylated CpG-ODN protect mice from infection with this fungal pathogen by inducing IFN-gamma. The present study was designed to elucidate the effect of C. neoformans on the synthesis of NO by alveolar macrophages. For this purpose, MH-S, an alveolar macrophage cell line, was stimulated with CpG-ODN in the presence of IFN-gamma. A highly virulent strain of C. neoformans with thick capsule suppressed the production of NO. Capsular polysaccharides were not essential for this suppression, because there was no difference between acapsular mutant (Cap67) and its parent strain. Physical or close interaction of Cap67 with MH-S was necessary, as shown by the loss of such effect when direct contact was interfered by nitrocellulose membrane. Similar effects were observed by disrupted as well as intact Cap67. Whereas the inhibitory effect of intact Cap67 was completely abrogated by heat treatment, disrupted Cap67 did not receive such influence. Finally, disrupted Cap67 did not show any inhibitory effect on the TLR9-mediated activation of NF-kappaB in a luciferase reporter assay with HEK293T cells, although the TLR4-mediated activation was suppressed. These results revealed that C. neoformans suppressed the synthesis of NO by CpG-ODN and IFN-gamma-stimulated macrophages in a fashion independent of capsular polysaccharides, although the precise mechanism remains to be elucidated.

Phenotypic switching in Cryptococcus neoformans contributes to virulence by changing the immunological host response

Cryptococcus neoformans is an encapsulated opportunistic organism that can undergo phenotypic switching. In this process, the parent smooth colony (SM) switches to a more virulent mucoid colony (MC) variant. The host responses mounted against the SM and MC variants differ, and lower tissue interleukin 10 (IL-10) levels are consistently observed in lungs of MC-infected C57BL/6 and BALB/c mice. This suggested different roles of this cytokine in SM and MC infections. The objective of this study was to compare survival rates and characterize the host responses of SM- and MC-infected IL-10-depleted (IL-10(-/-)) mice, which exhibit a Th1-polarized immune response and are considered resistant hosts. As expected, SM-infected IL-10(-/-) mice survived longer than wild-type mice, whereas MC-infected IL-10(-/-) mice did not exhibit a survival benefit. Consistent with this observation, we demonstrated marked differences in the inflammatory responses of SM- and MC-infected IL-10(-/-) and wild-type mice. This included a more Th1-polarized inflammatory response with enhanced recruitment of macrophages and natural killer and CD8 cells in MC- than in SM-infected IL-10(-/-) and wild-type mice. In contrast, both SM-infected IL-10(-/-) and wild-type mice exhibited higher recruitment of CD4 cells, consistent with enhanced survival and differences in recruitment and Th1/Th2 polarization. Lung tissue levels of IL-21, IL-6, IL-4, transforming growth factor beta, IL-12, and gamma interferon were higher in MC-infected IL-10(-/-) and wild-type mice than in SM-infected mice, whereas tumor necrosis factor alpha levels were higher in SM-infected IL-10(-/-) mice. In conclusion, the MC variant elicits an excessive inflammatory response in a Th1-polarized host environment, and therefore, the outcome is negatively affected by the absence of IL-10.

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.

Peroxisome proliferator activated receptor gamma is not necessary for the development of LPS-induced tolerance in macrophages

Peroxisome proliferator activated receptor-gamma (PPARgamma) has been reported to exert anti-inflammatory properties in endotoxic shock and sepsis. One phenomenon that alters the inflammatory response to endotoxin [lipopolysaccharide (LPS)] is endotoxin tolerance, which is caused by previous exposure to endotoxin. Here, we investigate whether changes in endogenous PPARgamma function regulate this phenomenon using three different models of LPS-induced tolerance in macrophages. In a first in vitro model, previous LPS exposure of murine J774.2 macrophages suppressed tumour necrosis factor-alpha (TNF-alpha) release in response to subsequent LPS challenge. Treatment of J774.2 cells with the PPARgamma inhibitor GW9662 did not alter tolerance induction because these cells were still hyporesponsive to the secondary LPS challenge. In a second ex vivo model, primary rat peritoneal macrophages from LPS-primed rats exhibited suppression of thromboxane B2 and TNF-alpha production, while maintaining nitrite production in response to in vitro LPS challenge. Pretreatment of rats with the PPARgamma inhibitor GW9662 in vivo failed to alter the tolerant phenotype of these primary macrophages. In a third ex vivo model, primary peritoneal macrophages with conditional deletion of PPARgamma were harvested from LPS-primed Cre-lox mice (Cre+/+ PPARgamma-/-) and exhibited significant suppression of TNF-alpha production in response to in vitro LPS challenge. Furthermore, both LPS-primed PPARgamma-deficient Cre+/+ PPARgamma-/- mice and wild-type Cre-/- PPARgamma+/+ mice exhibited reduced plasma TNF-alpha levels in response to a high dose of LPS in vivo. These data demonstrate that PPARgamma does not play a role in the LPS-induced tolerant phenotype in macrophages.

Determinants of disease presentation and outcome during cryptococcosis: the CryptoA/D study

BACKGROUND

Cryptococcosis is a life-threatening opportunistic fungal infection in both HIV-positive and -negative patients. Information on clinical presentation and therapeutic guidelines, derived mostly from clinical trials performed before introduction of highly active antiretroviral therapy in patients with cryptococcal meningoencephalitis, is missing data on extrameningeal involvement and infections by serotype D as opposed to serotype A of Cryptococcus neoformans.

METHODS AND FINDINGS

The prospective multicenter study CryptoA/D was designed in France (1997-2001) to analyse the factors influencing clinical presentation and outcome without the bias of inclusion into therapeutic trials. Of the 230 patients enrolled, 177 (77%) were HIV-positive, 50 (22%) were female, and 161 (72.5%) were infected with serotype A. Based on culture results at baseline, cryptococcosis was more severe in men, in HIV-positive patients, and in patients infected with serotype A. Factors independently associated with mycological failure at week 2 independent of HIV status were initial dissemination (OR, 2.4 [95% confidence interval (CI), 1.2-4.9]), high (>1:512) serum antigen titre (OR, 2.6 [1.3-5.4]), and lack of flucytosine during induction therapy (OR, 3.8 [1.9-7.8]). The three-month survival was shorter in patients with abnormal neurology or brain imaging at baseline, and in those with haematological malignancy.

CONCLUSIONS

Thus sex, HIV status, and infecting serotype are major determinants of presentation and outcome during cryptococcosis. We propose a modification of current guidelines for the initial management of cryptococcosis based on systematic fungal burden evaluation.

Role of capsule and interleukin-6 in long-term immune control of Cryptococcus neoformans infection by specifically activated human peripheral blood mononuclear cells

Cryptococcus neoformans is a frequent cause of meningoencephalitis in immunosuppressed individuals. To better understand the mechanisms of a protective immune response to C. neoformans, a long-term in vitro model of human immune control of cryptococcal infection was developed. Peripheral blood mononuclear cells (PBMC) prestimulated with heat-killed C. neoformans significantly restricted the growth of C. neoformans after a subsequent live infection compared to that with unstimulated PBMC. Live infection with encapsulated C. neoformans was controlled for as long as 10 days, while infection with acapsular organisms could sometimes be eradicated. During immune control, fungal cells were both intracellular and extracellular within aggregates of mononuclear phagocytes and lymphocytes. Optimal immune control depended on the presence of both CD4+ and CD8+ T cells. Immune control of cryptococcal growth was more effective following prestimulation with acapsular compared with encapsulated organisms. Prestimulation with acapsular organisms was associated with a significant and prolonged increase in interleukin-6 (IL-6) production compared with prestimulation with encapsulated C. neoformans. Addition of IL-6 and depletion of CD25+ T cells prior to prestimulation and infection with encapsulated organisms resulted in reductions in cryptococcal growth that reached borderline statistical significance. Depletion of CD25+ T cells significantly reduced cryptococcal growth in wells with unstimulated PBMC. The results demonstrate an association between high levels of IL-6 and resistance to infection and, through suppression of IL-6 release, an additional mechanism whereby the cryptococcal capsule subverts a protective immune response. Further work is required to clarify the mechanism of action of IL-6 in this setting and any interaction with regulatory T cells.

In the Absence of Reactive Oxygen Species, T Cells Default to a Th1 Phenotype and Mediate Protection against PulmonaryCryptococcus neoformansInfection

In recent years, the prevalence of invasive fungal infections has increased, attributed mostly to the rising population of immunocompromised individuals. Cryptococcus neoformans has been one of the most devastating, with an estimated 6-8% of AIDS-infected patients succumbing to Cryptococcus-associated meningitis. Reactive oxygen species (ROS) are potent antimicrobial agents but also play a significant role in regulating immune cell phenotype, but cause immunopathology when produced in excess. We now show that mice lacking phagocyte NADPH oxidase have heightened macrophage and Th1 responses and improved pathogen containment within pulmonary granulomatous lesions. Consequently, dissemination of this fungus to the brain is diminished, an effect that is independent of IL-12. Similar results are described using the metalloporphyrin antioxidant manganese(III) tetrakis(N-ethyl pyridinium-2-yl)porphyrin, which also promoted a protective Th1 response and reduced dissemination to the brain. These findings are in sharp contrast to the protective potential of ROS against other fungal pathogens, and highlight the pivotal role that ROS can fulfill in shaping the profile of the host's immune response.

NEUTRALIZATION OF TUMOR NECROSIS FACTOR IN PRECLINICAL MODELS OF SEPSIS

Tumor necrosis factor (TNFalpha), a cardinal early mediator of the innate host inflammatory response, has been an attractive target for therapeutic intervention in human sepsis. However, pooled data from 12 completed randomized controlled trials show only a very modest impact on mortality in a highly heterogeneous population of patients. To gain insight into the preclinical in vivo biology of TNFalpha that might aid in better identifying appropriate patient populations for therapeutic intervention, we undertook a systematic review of published reports of preclinical studies assessing the consequences of neutralization of TNFalpha in models of acute infection or inflammation. We identified 143 reports incorporating 484 unique experimental comparisons in seven different animal species. The effects of neutralization of TNFalpha in these were quite variable. Neutralization of TNFalpha was beneficial in endotoxemia, or after systemic challenge with gram-negative organisms, Staphylococcus aureus, or Group B streptococci. On the other hand, neutralization was detrimental in infections caused by Streptococcus pneumoniae, Candida spp., or intracellular pathogens such as Listeria and Mycobacterium tuberculosis, and in models of pneumonia. Treatment was more efficacious when delivered before infectious challenge, and the therapeutic signal increased as the baseline mortality in the placebo group increased. Evidence of neutralization of TNFalpha bioactivity, and of attenuation of inflammation, was typically accompanied by evidence of impairment of antimicrobial defenses. Multiple specific and nonspecific therapeutic strategies were identified. We conclude that the beneficial effects of TNF in systemic inflammation occur at the cost of impaired antimicrobial defenses, and that a better understanding of the consequences of neutralization of TNFalpha in vivo could aid in better defining optimal patient populations for therapeutic intervention.

CpG oligodeoxynucleotides promote the host protective response against infection with Cryptococcus neoformans through induction of interferon-gamma production by CD4+ T cells

In the present study, we elucidated the effect of synthetic CpG-containing oligodeoxynucleotides (ODN) on pulmonary and disseminated infection caused by Cryptococcus neoformans. CDF-1 mice were inoculated intratracheally with a highly virulent strain of this pathogen, which resulted in massive bacterial growth in the lung, dissemination to the brain and death. Administration of CpG-ODN promoted the clearance of C. neoformans in the lungs, decreased their dissemination to brain and prolonged the survival of infected mice. These effects correlated well with the enhanced production of interleukin (IL)-12 and interferon (IFN)-gamma and attenuated secretion of IL-4 in bronchoalveolar lavage fluids (BALF) and promoted development of Th1 cells, as indicated by the increased production of IFN-gamma by paratracheal lymph node cells upon restimulation with cryptococcal antigens. The IFN-gamma synthesis in BALF was inhibited by depletion of CD8(+) and CD4(+) T cells on days 7 and 14 after infection, respectively, but not by depletion of NK and gammadelta T cells. Consistent with these data, intracellular expression of IFN-gamma was detected predominantly in CD8(+) and CD4(+) T cells in the lung on days 7 and 14, respectively. The protective effect of CpG-ODN, as shown by the prolonged survival, was completely and partially inhibited by depletion of CD4(+) or CD8(+) T cells, respectively, but not by depletion of other cells. Finally, TNF-alpha was markedly induced by CpG-ODN, and the protective effect of this agent was strongly inhibited by neutralizing anti-TNF-alpha MoAb. Our results indicate that CpG-ODN alters the Th1-Th2 cytokine balance and promotes host resistance against infection with C. neoformans.

Distinct roles for IL-4 and IL-10 in regulating T2 immunity during allergic bronchopulmonary mycosis

Pulmonary Cryptococcus neoformans infection of C57BL/6 mice is an established model of an allergic bronchopulmonary mycosis that has also been used to test a number of immunomodulatory agents. Our objective was to determine the role of IL-4 and IL-10 in the development/manifestation of the T2 response to C. neoformans in the lungs and lung-associated lymph nodes. In contrast to wild-type (WT) mice, which develop a chronic infection, pulmonary clearance was significantly greater in IL-4 knockout (KO) and IL-10 KO mice but was not due to an up-regulation of a non-T cell effector mechanism. Pulmonary eosinophilia was absent in both IL-4 KO and IL-10 KO mice compared with WT mice. The production of IL-4, IL-5, and IL-13 by lung leukocytes from IL-4 KO and IL-10 KO mice was lower but IFN-gamma levels remained the same. TNF-alpha and IL-12 production by lung leukocytes was up-regulated in IL-10 KO but not IL-4 KO mice. Overall, IL-4 KO mice did not develop the systemic (lung-associated lymph nodes and serum) or local (lungs) T2 responses characteristic of the allergic bronchopulmonary C. neoformans infection. In contrast, the systemic T2 elements of the response remained unaltered in IL-10 KO mice whereas the T2 response in the lungs failed to develop indicating that the action of IL-10 in T cell regulation was distinct from that of IL-4. Thus, although IL-10 has been reported to down-regulate pulmonary T2 responses to isolated fungal Ags, IL-10 can augment pulmonary T2 responses if they occur in the context of fungal infection.

Different modes of IL-10 and TGF-beta to inhibit cytokine-dependent IFN-gamma production: consequences for reversal of lipopolysaccharide desensitization

LPS hyporesponsiveness is characterized by a diminished production of proinflammatory cytokines which can be caused by pretreatment with either LPS (=LPS desensitization) or the combination of the anti-inflammatory cytokines IL-10 and TGF-beta. However, the resulting hyporesponsive states differ regarding their reversibility by the IFN-gamma-inducing cytokine IL-12. Therefore, we aimed at studying the reasons for this differential IL-12 responsiveness of IFN-gamma-producing cells and its consequences for LPS hyporesponsiveness in more detail. In an in vitro IL-12/IL-18 responsiveness model, we demonstrated that IL-10, if permanently present, does not directly inhibit IL-12/IL-18 responsiveness in T/NK cells but indirectly interferes with IFN-gamma production in the presence of monocytes. In contrast, TGF-beta acted directly on IFN-gamma-producing cells by interfering with IL-12/IL-18 responsiveness. After removal of IL-10 but not of TGF-beta, LPS hyporesponsiveness can be reverted by IL-12/IL-18. Consequently, the addition of recombinant TGF-beta during LPS desensitization rendered PBMCs hyporesponsive to a reversal by IL-12/IL-18. Our data suggest that the persistence of IL-10 and the presence of TGF-beta determine the level of IFN-gamma inhibition and may result in different functional phenotypes of LPS desensitization and LPS hyporesponsiveness in vitro and in vivo.

Cryptococcal infection and Th1-Th2 cytokine balance

Cytokines have been recognized as key factors in determining host resistance to infectious pathogens. In particular, Th1-Th2 cytokine balance in hosts is profoundly associated with the outcome of infection caused by intracellular microbes. In a murine model of pulmonary and disseminated infection with Cryptococcus neoformans, an opportunistic fungal pathogen that frequently leads to fatal meningoencephalitis in severely immunocompromised hosts, expression of cytokine mRNA in the lungs from infected animals revealed Th2-dominant profiles, while administration of IL-12, which rescued mice from fatal infection, converted such balance toward Th1-dominant states in a drastic fashion. Thus, commitment of Th phenotypes critically determines host sensitivity to cryptococcal infection. In this review, we described how Th1-Th2 cytokine balance influences host protective responses to C. neoformans, and we identify the host and pathogen factors that regulate such balance.

Induction of interleukin-12 and gamma interferon requires tumor necrosis factor alpha for protective T1-cell-mediated immunity to pulmonary Cryptococcus neoformans infection

The development of T1-cell-mediated immunity is required to clear a pulmonary Cryptococcus neoformans infection. The objective of these studies was to determine the mechanism by which tumor necrosis factor alpha (TNF-alpha) augments the development of pulmonary T1 immunity to C. neoformans infection. TNF-alpha expression was detected in lavage sample cells at days 2, 3, and 7 following C. neoformans infection. The numbers of CFU in the lung were not different between control and anti-TNF-alpha-treated mice at any time point examined during the afferent phase of the response (days 0 to 7). However, neutralization of TNF-alpha prevented the initiation of pulmonary clearance during the efferent phase of the response (day 14). Administration of anti-TNF-alpha monoclonal antibody (day 0) diminished the lung levels of TNF-alpha, interleukin-12 (IL-12), and gamma interferon (IFN-gamma) induced by C. neoformans at day 7 postinfection. Neutralization of TNF-alpha (day 0) also altered the IFN-gamma/IL-4 ratio in the lung-associated lymph nodes at day 7 following C. neoformans infection. Anti-TNF-alpha-treated mice developed a pulmonary eosinophilia at day 14 postinfection. Consistent with the pulmonary eosinophilia, anti-TNF-alpha-treated mice exhibited elevated serum immunoglobulin E and inhibition of the anticryptococcal delayed-type hypersensitivity response, indicating a shift toward a T2 response. Neutralization of IL-12 also prevented lung leukocyte production of IFN-gamma in response to the infection. These findings demonstrate that afferent-phase TNF-alpha production is essential for the induction of IL-12 and IFN-gamma and neutralization of early TNF-alpha results in a T2 shift of the T1/T2 balance of antifungal immunity.

Mechanisms for induction of immunosuppression during experimental cryptococcosis: role of glucuronoxylomannan

In previous work we have demonstrated that spleen mononuclear (Spm) cells from rats obtained 14 days after infection with Cryptococcus neoformans showed a diminution in proliferative response to Concanavalin A (Con A). In this study we further investigate some characteristics of the Spm cell population involved in the immunosuppressor phenomenon induced by C. neoformans. We observed that unstimulated Spm cells expressing T-cell receptor (TCR+) from infected rats were reduced in number after 96 h of culture. When the Spm cells from infected rats were stimulated with Con A, increased production of IL-10, reduced levels of IL-2, and decreased CD11a surface expression were shown. These immunosuppressor phenomena were also observed when the capsular polysaccharide, glucuronoxylomannan (GXM), was added to cultures of Spm cells from normal rats. However, GXM had a more pronounced effect in reducing the number of cells surviving in culture than that observed during infection and produced an increase in IL-4 production by Con-A-stimulated Spm cells. Addition of anti-IL-10 monoclonal antibody to cultures restored the lymphoproliferation of Spm cells from infected animals, indicating that IL-10 production is a suppressor mechanism of cell-mediated immunity during experimental infection. The results presented here indicate that at least two mechanisms mediate the nonspecific suppression in this model of cryptococcosis: IL-10 production and diminution of the number of T cells. GXM could be involved, since it has a pronounced effect in the reduction of Spm cells in vitro.

Visceral leishmaniasis in mice devoid of tumor necrosis factor and response to treatment

Tumor necrosis factor (TNF)-deficient mice were challenged with Leishmania donovani to characterize TNF in the response of visceral intracellular infection to antileishmanial chemotherapy. In wild-type controls (i) liver infection peaked at week 2 and resolved, (ii) discrete liver granulomas developed at weeks 2 to 4 and involuted, and (iii) leishmanicidal responses to antimony (Sb), amphotericin B (AmB), and miltefosine were intact. In TNF knockout (KO) mice (i) initial liver infection was unrestrained, plateaued, and then declined somewhat by week 6, (ii) an absent early granulomatous reaction abruptly accelerated with striking tissue inflammation, widespread hepatic necrosis, and 100% mortality by week 10, and (iii) while the initial response to AmB and miltefosine was intact, killing induced by Sb therapy was reduced by >50%. Although initial AmB treatment during weeks 2 to 3 killed 98% of liver parasites, 75% of AmB-treated KO mice subsequently relapsed and died by week 12; however, additional maintenance AmB preserved long-term survival. These results for a model of visceral infection indicate that endogenous TNF is required early on to control intracellular L. donovani, support granuloma development, and mediate optimal initial effects of Sb and prevent relapse after ordinarily curative AmB treatment. A compensatory, TNF-independent antileishmanial mechanism developed in TNF KO mice; however, its effect was uncontrolled fatal inflammation. Chemotherapeutic elimination of the parasite stimulus reversed the hyperinflammatory response and preserved survival.

Administration of endotoxin associated with lipopolysaccharide tolerance protects mice against fungal infection

Lipopolysaccharide (LPS) pretreatment of mice resulted in a significantly enhanced survival after disseminated Cryptococcus neoformans infection. The survival was associated with reduced fungal burden in tissues. LPS-pretreated mice had lower levels of cytokines in blood, spleen, and lungs and higher levels in brain. Pentoxifylline abolished the beneficial effect of LPS pretreatment.

Cytokine profiles of AIDS patients are similar to those of mice with disseminated Cryptococcus neoformans infection

Cryptococcosis is an hematogenously disseminated meningoencephalitis during which the relationship between the disease severity and the immune response remains unclear. We thus analyzed, by enzyme-linked immunosorbent assay, proinflammatory (tumor necrosis factor alpha [TNF-alpha] and interleukin-6 [IL-6]) and anti-inflammatory (IL-10) cytokine levels in plasma at the time of diagnosis in 51 AIDS patients with culture-proven cryptococcosis. We used a murine model to determine the correlation between cytokine levels and fungal burden in blood and tissues and the kinetics of the immune response and of the formation of cerebral lesions. In AIDS patients, plasma TNF-alpha and IL-10, but not IL-6, levels were significantly higher in the case of fungemia or disseminated infection than in their absence, whereas the presence of meningitis had no influence on these levels. In mice, none of these cytokines were detected within the first day after inoculation. Later on, TNF-alpha and IL-10, but not IL-6, levels in plasma correlated significantly with the fungal burden in the blood and spleen but not the brain. In the brain, cytokine levels were low compared to those in other compartments, and tissue lesions and a degree of infection similar to those observed in humans were seen, further suggesting the relevance of this experimental model. Thus, AIDS patients with cryptococcosis produce an immune response that reflects the dissemination but not the meningeal involvement. This murine model of disseminated cryptococcosis can be used to investigate the pathophysiology of cryptococcosis and new therapeutic approaches.