B Cells Regulate Neutrophilia during Mycobacterium tuberculosis Infection and BCG Vaccination by Modulating the Interleukin-17 Response

We have previously demonstrated that B cells can shape the immune response to Mycobacterium tuberculosis, including the level of neutrophil infiltration and granulomatous inflammation at the site of infection. The present study examined the mechanisms by which B cells regulate the host neutrophilic response upon exposure to mycobacteria and how neutrophilia may influence vaccine efficacy. To address these questions, a murine aerosol infection tuberculosis (TB) model and an intradermal (ID) ear BCG immunization mouse model, involving both the μMT strain and B cell-depleted C57BL/6 mice, were used. IL (interleukin)-17 neutralization and neutrophil depletion experiments using these systems provide evidence that B cells can regulate neutrophilia by modulating the IL-17 response during M. tuberculosis infection and BCG immunization. Exuberant neutrophilia at the site of immunization in B cell-deficient mice adversely affects dendritic cell (DC) migration to the draining lymph nodes and attenuates the development of the vaccine-induced Th1 response. The results suggest that B cells are required for the development of optimal protective anti-TB immunity upon BCG vaccination by regulating the IL-17/neutrophilic response. Administration of sera derived from M. tuberculosis-infected C57BL/6 wild-type mice reverses the lung neutrophilia phenotype in tuberculous μMT mice. Together, these observations provide insight into the mechanisms by which B cells and humoral immunity modulate vaccine-induced Th1 response and regulate neutrophila during M. tuberculosis infection and BCG immunization.     

Th2 cytokines in susceptibility to tuberculosis

We need to understand what is different about susceptibility to tuberculosis (TB) in developing countries where most TB occurs, and where the current vaccine, Bacillus Calmette et Guérin (BCG) usually fails to protect. The presence of a background mixed IFN-gamma and Th2 response to mycobacterial antigens before infection with M. tuberculosis (Mtb), and the development of a large IL-4 response during progressive TB, are characteristics of individuals in the locations where BCG fails, which are also seen in animal models in the same countries. Recent data suggest that the background Th1 component in developing countries protects from low dose challenge with Mtb in mouse and man, but that following high dose challenge the pre-existing IL-4 component increases and blocks immunity unless the individual's immune system releases IL-4delta2, an antagonist of IL-4, which is raised in the blood of donors with stable latent TB. We outline how IL-4 (and IL-13) can undermine Th1-mediated immunity and drive inappropriate alternative activation of macrophages. The mechanisms of the effects of IL-4 include impaired antimicrobial activity due to reduced TNF-alpha-mediated apoptosis of infected cells, reduced activity of iNOS, increased availability of iron to intracellular Mtb, and increased proliferation of antigen-specific FOXP-3+ regulatory T cells. IL-4 also increases the toxicity of TNF-alpha and drives pulmonary fibrosis, thus enhancing immunopathology. The conclusion is that a vaccine that will work in developing countries might need to do more than enhance the existing Th1 response. In these environments it might be more important to block the Th2 component.     

Dysregulation of proinflammatory and regulatory cytokines in HIV infected persons with active tuberculosis

Proinflammatory and regulatory cytokines have been implicated to play important role in immunopathology of HIV and tuberculosis (TB) infection. Capacity of unstimulated and mitogen-stimulated peripheral blood mononuclear cells (PBMCs) to secrete cytokines (interleukin (IL)-2, interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), IL-4, IL-10 and IL-6) was estimated for 15 HIV-TB coinfected patients, 22 HIV seropositives without TB, 32 HIV negative TB patients, and 36 healthy subjects. Dually infected patients had suppression of both Th1 and Th2 cytokine secretion as evidenced by significantly lower production of IL-2, IFN-gamma and TNF-alpha as well as IL-4 and IL-10. Production of IL-2 and TNF-alpha was significantly decreased only in case of HIV infection. Significantly higher IL-6 secretion was found in unstimulated cultures in dually infected patients. The mitogen induced cytokine secretion was generally lower in HIV-TB coinfected patients indicating profound perturbation of both Th1 and Th2 responses.     

B cells moderate inflammatory progression and enhance bacterial containment upon pulmonary challenge with Mycobacterium tuberculosis

Though much is known about the function of T lymphocytes in the adaptive immune response against Mycobacterium tuberculosis, comparably little is understood regarding the corresponding role of B lymphocytes. Indicating B cells as components of lymphoid neogenesis during pulmonary tuberculosis, we have identified ectopic germinal centers (GCs) in the lungs of infected mice. B cells in these pulmonary lymphoid aggregates express peanut agglutinin and GL7, two markers of GC B cells, as well as CXCR5, and migrate in response to the lymphoid-associated chemokine CXCL13 ex vivo. CXCL13 is negatively regulated by the presence of B cells, as its production is elevated in lungs of B cell-deficient (B cell(-/-)) mice. Upon aerosol with 100 CFU of M. tuberculosis Erdman, B cell(-/-) mice have exacerbated immunopathology corresponding with elevated pulmonary recruitment of neutrophils. Infected B cell(-/-) mice show increased production of IL-10 in the lungs, whereas IFN-gamma, TNF-alpha, and IL-10R remain unchanged from wild type. B cell(-/-) mice have enhanced susceptibility to infection when aerogenically challenged with 300 CFU of M. tuberculosis corresponding with elevated bacterial burden in the lungs but not in the spleen or liver. Adoptive transfer of B cells complements the phenotypes of B cell(-/-) mice, confirming a role for B cells in both modulation of the host response and optimal containment of the tubercle bacillus. As components of ectopic GCs, moderators of inflammatory progression, and enhancers of local immunity against bacterial challenge, B cells may have a greater role in the host defense against M. tuberculosis than previously thought.     

Interleukin-27 inhibits vaccine-enhanced pulmonary disease following respiratory syncytial virus infection by regulating cellular memory responses

Respiratory syncytial virus (RSV) is the most important cause of lower respiratory tract disease in young children. In the 1960s, infants vaccinated with formalin-inactivated RSV developed a more severe disease characterized by excessive inflammatory immunopathology in lungs upon natural RSV infection. The fear of causing the vaccine-enhanced disease (VED) is an important obstacle for development of safe and effective RSV vaccines. The recombinant vaccine candidate G1F/M2 immunization also led to VED. It has been proved that cellular memory induced by RSV vaccines contributed to VED. Interleukin-27 (IL-27) and IL-23 regulate Th1, Th17, and/or Th2 cellular immune responses. In this study, mice coimmunized with pcDNA3-IL-27 and G1F/M2 were fully protected and, importantly, did not develop vaccine-enhanced inflammatory responses and immunopathology in lungs after RSV challenge, which was correlated with moderate Th1-, suppressed Th2-, and Th17-like memory responses activated by RSV. In contrast, G1F/M2- or pcDNA3-IL-23+G1F/M2-immunized mice, in which robust Th2- and Th17-like memory responses were induced, developed enhanced pulmonary inflammation and severe immunopathology. Mice coimmunized with G1F/M2 and the two cytokine plasmids exhibited mild inflammatory responses as well as remarkable Th1-, suppressed Th2-, and Th17-like memory responses. These results suggested that Th1-, Th2-, and Th17-like memory responses and, in particular, excessive Th2- and Th17-like memory responses were closely associated with VED; IL-27 may inhibit VED following respiratory syncytial virus infection by regulating cellular memory responses.     

Healthy individuals that control a latent infection with Mycobacterium tuberculosis express high levels of Th1 cytokines and the IL-4 antagonist IL-4delta2

The majority of healthy individuals exposed to Mycobacterium tuberculosis will not develop disease and identifying what constitutes "protective immunity" is one of the holy grails of M. tuberculosis immunology. It is known that IFN-gamma is essential for protection, but it is also apparent that IFN-gamma levels alone do not explain the immunity/susceptibility dichotomy. The controversy regarding correlates of immunity persists because identifying infected but healthy individuals (those who are immune) has been problematic. We have therefore used recognition of the M. tuberculosis virulence factor early secretory antigenic target 6 to identify healthy, but infected individuals from tuberculosis (TB)-endemic and nonendemic regions (Ethiopia and Denmark) and have compared signals for cytokines expressed directly ex vivo with the pattern found in TB patients. We find that TB patients are characterized by decreased levels of Th1 cytokines and increased levels of IL-10 compared with the healthy infected and noninfected community controls. Interestingly, the healthy infected subjects exhibited a selective increase of message for the IL-4 antagonist, IL-4delta2, compared with both TB patients or noninfected individuals. These data suggest that long-term control of M. tuberculosis infection is associated not just with elevated Th1 responses but also with inhibition of the Th2 response.     

Simian immunodeficiency virus-induced changes in T cell cytokine responses in cynomolgus macaques with latent Mycobacterium tuberculosis infection are associated with timing of reactivation

Understanding the early immunologic events accompanying reactivated tuberculosis (TB) in HIV-infected individuals may yield insight into causes of reactivation and improve treatment modalities. We used the cynomolgus macaque (Macaca fascicularis) model of HIV-Mycobacterium tuberculosis coinfection to investigate the dynamics of multifunctional T cell responses and granuloma T cell phenotypes in reactivated TB. CD4(+) and CD8(+) T cells expressing Th1 cytokines (IFN-γ, IL-2, TNF) and Th2 cytokines (IL-4 and IL-10) were followed from latent M. tuberculosis infection to reactivation after coinfection with a pathogenic SIV. Coinfected animals experienced increased Th1 cytokine responses to M. tuberculosis Ags above the latent-response baseline 3-5 wk post-SIV infection that corresponded with peak plasma viremia. Th2 cytokine expression was not Ag specific, but strong, transient IL-4 expression was noted 4-7 wk post-SIV infection. Animals reactivating <17 wk post-SIV infection had significantly more multifunctional CD4(+) T cells 3-5 wk post-SIV infection and more Th2-polarized and fewer Th0-, Th1-polarized CD8(+) T cells during weeks 1-10 post-SIV infection than animals reactivating >26 wk post-SIV infection. Granuloma T cells included Th0-, Th1-, and Th2-polarized phenotypes but were particularly rich in cytolytic (CD107(+)) T cells. When combined with the changes in peripheral blood T cells, these factors indicate that events during acute HIV infection are likely to include distortions in proinflammatory and anti-inflammatory T cell responses within the granuloma that have significant effects on reactivation of latent TB. Moreover, it appears that mycobacteria-specific multifunctional T cells are better correlates of Ag load (i.e., disease status) than of protection.     

A model to predict cell-mediated immune regulatory mechanisms during human infection with Mycobacterium tuberculosis

A key issue for the study of tuberculosis infection (TB) is to understand why individuals infected with Mycobacterium tuberculosis experience different clinical outcomes. Elaborating the immune mechanisms that determine whether an infected individual will suffer active TB or latent infection can aid in developing treatment and prevention strategies. To better understand the dynamics of M. tuberculosis infection and immunity, we have developed a virtual human model that qualitatively and quantitatively characterizes the cellular and cytokine control network operational during TB infection. Using this model, we identify key regulatory elements in the host response. In particular, factors affecting cell functions, such as macrophage activation and bactericidal capabilities, and effector T cell functions such as cytotoxicity and cytokine production can each be determinative. The model indicates, however, that even if latency is achieved, it may come at the expense of tissue damage if the response is not properly regulated. A balance in Th1 and Th2 immune responses governed by IFN-gamma, IL-10, and IL-4 facilitate this down-regulation. These results are further explored through virtual deletion and depletion experiments.     

Caspase-1 has both proinflammatory and regulatory properties in Helicobacter infections, which are differentially mediated by its substrates IL-1β and IL-18

The proinflammatory cysteine protease caspase-1 is autocatalytically activated upon cytosolic sensing of a variety of pathogen-associated molecular patterns by Nod-like receptors. Active caspase-1 processes pro-IL-1β and pro-IL-18 to generate the bioactive cytokines and to initiate pathogen-specific immune responses. Little information is available on caspase-1 and inflammasome activation during infection with the gastric bacterial pathogen Helicobacter pylori. In this study, we addressed a possible role for caspase-1 and its cytokine substrates in the spontaneous and vaccine-induced control of Helicobacter infection, as well as the development of gastritis and gastric cancer precursor lesions, using a variety of experimental infection, vaccine-induced protection, and gastric disease models. We show that caspase-1 is activated and IL-1β and IL-18 are processed in vitro and in vivo as a consequence of Helicobacter infection. Caspase-1 activation and IL-1 signaling are absolutely required for the efficient control of Helicobacter infection in vaccinated mice. IL-1R(-/-) mice fail to develop protective immunity but are protected against Helicobacter-associated gastritis and gastric preneoplasia as a result of their inability to generate Helicobacter-specific Th1 and Th17 responses. In contrast, IL-18 is dispensable for vaccine-induced protective immunity but essential for preventing excessive T cell-driven immunopathology. IL-18(-/-) animals develop strongly accelerated pathology that is accompanied by unrestricted Th17 responses. In conclusion, we show in this study that the processing and release of a regulatory caspase-1 substrate, IL-18, counteracts the proinflammatory activities of another caspase-1 substrate, IL-1β, thereby balancing control of the infection with the prevention of excessive gastric immunopathology.     

Maintenance of pulmonary Th1 effector function in chronic tuberculosis requires persistent IL-12 production

The mechanisms that prevent reactivation of latent Mycobacterium tuberculosis infection in asymptomatic individuals are poorly understood. Although IL-12 is critical for the induction of IFN-gamma-dependent host control of M. tuberculosis, the requirement for the cytokine in the maintenance of host resistance and pulmonary Th1 effector function has not yet been formally examined. In this study, we reconstituted IL-12p40-deficient mice with IL-12 during the first 4 wk of infection and then assessed the effects of cytokine withdrawal. Although IL-12 administration initially resulted in restricted mycobacterial growth and prolonged survival, the reconstituted animals eventually succumbed to infection. This breakdown in bacterial control was accompanied by a marked reduction in the numbers of IFN-gamma-producing CD4(+) T cells in lungs. Moreover, whereas CD4(+) T cells isolated from chronically infected wild-type mice expanded and transferred long-term protection to M. tuberculosis-challenged RAG(-/-) mice, they failed to do so in IL-12p40-deficient RAG(-/-) recipients and were clearly reduced in frequency within pulmonary granulomas in the latter animals. These studies establish that continuous IL-12 production is necessary for maintenance of the pulmonary Th1 cells required for host control of persistent M. tuberculosis infection and suggest that breakdown of this mechanism could be a contributing factor in reactivated disease.     

Decreased leukocyte accumulation and delayed Bordetella pertussis clearance in IL-6-/- mice

IL-6, a pleiotropic cytokine primarily produced by the innate immune system, has been implicated in the development of acquired immune responses, though its roles are largely undefined and may vary in the context of different diseases. Using a murine model of infection, we established that IL-6 influences the adaptive immune responses against the endemic human respiratory pathogen Bordetella pertussis. IL-6 was induced in the lungs of C57BL/6 mice by B. pertussis. IL-6(-/-) mice showed a protracted infectious course and were less efficiently protected by B. pertussis vaccination than wild-type mice. Abs from IL-6(-/-) mice, though lower in titer, efficiently reduced B. pertussis numbers in IL-6-sufficient mice. Pulmonary leukocyte recruitment and splenic or pulmonary T cell cytokine responses to B. pertussis, including Th1 and Th17 cytokine production, were lower in IL-6(-/-) mice than in wild-type mice. Adoptive transfer of immune wild-type CD4(+) cells ameliorated the defect of IL-6(-/-) mice in the control of B. pertussis numbers. Together, these results reveal the dysregulation of multiple aspects of adaptive immune responses in B. pertussis-infected IL-6(-/-) mice and suggest that IL-6 is involved in regulating Ab generation, pulmonary leukocyte accumulation, and T cell cytokine production in response to B. pertussis as well as the generation of effective vaccine-induced immunity against this pathogen.     

Relationship between single nucleotide polymorphism of interleukin-18 and susceptibility to pulmonary tuberculosis in the Chinese Han population

Interleukin-18 (IL-18) is a multi-functional cytokine capable of inducing either Th1 or Th2 polarization depending on the immunologic milieu. IL-18 may influence the host response to Mycobacterium tuberculosis (M.tb) infection. To investigate the relationship between single nucleotide polymorphisms of the IL-18 and susceptibility to pulmonary tuberculosis in the Chinese Han population, the IL-18 gene was sequenced to detect polymorphisms and to examine the genotype frequencies in 300 patients and 702 healthy controls. DNA sequencing revealed three IL-18 variants: rs1946518, rs5744247, and rs549908. It also revealed that allele A of rs1946518 confers a 1.47-fold increased risk of developing tuberculosis (TB) (P = 0.0001, OR [95%CI] = 1.47 [1.21-1.78]), and that the C allele of rs5744247 confers a 0.77-fold decreased risk of disease (P = 0.01, R [95%CI] = 0.77 [0.632-0.937]). The genotypes rs1946518, rs5744247 and rs549908 were found to be significantly associated with TB. Estimation of the frequencies of haplotypes revealed a potential risk haplotype AGA (P = 0.01, OR [95%CI] = 1.41 [1.15-1.72]) and a protective haplotype CCA (P = 0.01, OR [95%CI] = 0.70 [0.57-0.85]) for TB. The present findings suggest that polymorphisms in the IL-18 gene may affect susceptibility to TB and increase the risk of developing the disease in the Chinese Han population.     

Lack of Toll IL-1R8 exacerbates Th17 cell responses in fungal infection

TLRs contribute to the inflammatory response in fungal infections. Although inflammation is an essential component of the protective response to fungi, its dysregulation may significantly worsen fungal diseases. In this study, we tested the hypothesis that Toll IL-1R8 (TIR8)/single Ig IL-1-related receptor, a member of the IL-1R family acting as a negative regulator of TLR/IL-1R signaling, affects TLR responses in fungal infections. Genetically engineered Tir8(-/-) mice were assessed for inflammatory and adaptive Th cell responses to Candida albicans and Aspergillus fumigatus. Inflammatory pathology and susceptibility to infection were higher in Tir8(-/-) mice and were causally linked to the activation of the Th17 pathway. IL-1R signaling was involved in Th17 cell activation by IL-6 and TGF-beta in that limited inflammatory pathology and relative absence of Th17 cell activation were observed in IL-1RI(-/-) mice. These data demonstrate that TIR8 is required for host resistance to fungal infections and that it functions to negatively regulate IL-1-dependent activation of inflammatory Th17 responses. TIR8 may contribute toward fine-tuning the balance between protective immunity and immunopathology in infection.     

Pleural Tuberculosis in Patients with Early HIV Infection Is Associated with Increased TNF-Alpha Expression and Necrosis in Granulomas

Although granulomas may be an essential host response against persistent antigens, they are also associated with immunopathology. We investigated whether HIV co-infection affects histopathological appearance and cytokine profiles of pleural granulomas in patients with active pleural tuberculosis (TB). Granulomas were investigated in pleural biopsies from HIV positive and negative TB pleuritis patients. Granulomas were characterised as necrotic or non-necrotic, graded histologically and investigated for the mRNA expression of IL-12, IFN-γ, TNF-α and IL-4 by in situ hybridisation. In all TB patients a mixed Th1/Th2 profile was noted. Necrotic granulomas were more evident in HIV positive patients with a clear association between TNF-α and necrosis. This study demonstrates immune dysregulation which may include TNF-α-mediated immunopathology at the site of disease in HIV infected pleural TB patients.     

Antifungal type 1 responses are upregulated in IL-10-deficient mice

C57BL/6 mice are highly resistant to infections caused by Candida albicans and Aspergillus fumigatus. To elucidate the role of IL-10 produced by C57BL/6 mice during these infections, parameters of infection and immunity to it were evaluated in IL-10-deficient and wild-type mice with disseminated or gastrointestinal candidiasis or invasive pulmonary aspergillosis. Unlike parasitic protozoan infection, C. albicans or A. fumigatus infection did not induce significant acute toxicity in IL-10-deficient mice, who, instead, showed reduced fungal burden and fungal-associated inflammatory responses. The increased resistance to infections as compared to wild-type mice was associated with upregulation of innate and acquired antifungal Th1 responses, such as a dramatically higher production of IL-12, nitric oxide (NO) and TNF-alpha as well as IFN-gamma by CD4+ T cells. Pharmacological inhibition of NO production greatly reduced resistance to gastrointestinal candidiasis, thus pointing to the importance of IL-10-dependent NO regulation at mucosal sites in fungal infections. These results are reminiscent of those obtained in genetically susceptible mice, in which IL-10 administration increased, and IL-10 neutralization decreased, susceptibility to C. albicans and A. fumigatus infections. Collectively, these observations indicate that the absence of IL-10 augments innate and acquired antifungal immunity by upregulating type 1 cytokine responses. The resulting protective Th1 responses lead to a prompt reduction of fungal growth, thus preventing tissue destruction and lethal levels of proinflammatory cytokines.     

The IL-27 receptor chain WSX-1 differentially regulates antibacterial immunity and survival during experimental tuberculosis

IL-12 is a potent inducer of IFN-gamma production and promotes a protective cell-mediated immune response after Mycobacterium tuberculosis infection. Recently, the IL-12-related cytokine IL-27 was discovered, and WSX-1 was identified as one component of the IL-27R complex. To determine the functional significance of IL-27/WSX-1 during tuberculosis, we analyzed the course of infection and the immune response in WSX-1-KO mice after aerosol infection with M. tuberculosis. In the absence of WSX-1, an increased production of the proinflammatory cytokines TNF and IL-12p40 resulted in elevated CD4+ T cell activation and IFN-gamma production, which enhanced macrophage effector functions and reduced bacterial loads. This is the first occasion of a selectively gene-deficient mouse strain showing higher levels of protective immunity against M. tuberculosis infection than wild-type mice. However, a concomitantly increased chronic inflammatory response also accelerated death of infected WSX-1-KO mice. In vitro, IL-27 induced STAT3 phosphorylation and inhibited TNF and IL-12 production in activated peritoneal macrophages, indicating a novel feedback mechanism by which IL-27 can modulate excessive inflammation. In conclusion, IL-27 both prevents optimal antimycobacterial protection and limits the pathological sequelae of chronic inflammation.     

Th1-Mediated Immunity against Helicobacter pylori Can Compensate for Lack of Th17 Cells and Can Protect Mice in the Absence of Immunization

Helicobacter pylori (H. pylori) infection can be significantly reduced by immunization in mice. Th17 cells play an essential role in the protective immune response. Th1 immunity has also been demonstrated to play a role in the protective immune response and can compensate in the absence of IL-17. To further address the potential of Th1 immunity, we investigated the efficacy of immunization in mice deficient in IL-23p19, a cytokine that promotes Th17 cell development. We also examined the course of Helicobacter infection in unimmunized mice treated with Th1 promoting cytokine IL-12. C57BL/6, IL-12 p35 KO, and IL-23 p19 KO mice were immunized and challenged with H. pylori. Protective immunity was evaluated by CFU determination and QPCR on gastric biopsies. Gastric and splenic IL-17 and IFNγ levels were determined by PCR or by ELISA. Balb/c mice were infected with H. felis and treated with IL-12 therapy and the resulting gastric bacterial load and inflammatory response were assessed by histologic evaluation. Vaccine induced reductions in bacterial load that were comparable to wild type mice were observed in both IL-12 p35 and IL-23 p19 KO mice. In the absence of IL-23 p19, IL-17 levels remained low but IFNγ levels increased significantly in both immunized challenged and unimmunized/challenged mice. Additionally, treatment of H. felis-infected Balb/c mice with IL-12 resulted in increased gastric inflammation and the eradication of bacteria in most mice. These data suggest that Th1 immunity can compensate for the lack of IL-23 mediated Th17 responses, and that protective Th1 immunity can be induced in the absence of immunization through cytokine therapy of the infected host.     

Genetically resistant mice lacking IL-18 gene develop Th1 response and control cutaneous Leishmania major infection

IL-18 has been shown to play a critical role in the development of a Th1 response and immunity against intracellular pathogens. To determine the role of IL-18 in the development of protective immunity against Leishmania major, we have analyzed the course of cutaneous L. major in IL-18-deficient C57BL/6 mice (IL-18-/-) compared with similarly infected wild-type mice (IL-18+/+). After L. major infection, IL-18-/- mice may develop larger lesions during early phase of infection but eventually will resolve them as efficiently as IL-18+/+ mice. By 2 wk after infection, although Ag-stimulated lymph node cells from L. major-infected IL-18+/+ and IL-18-/- mice produced similar levels of IFN-gamma, those from IL-18-/- mice produced significantly more IL-12 and IL-4. By 10 wk after infection, both IL-18+/+ and IL-18-/- mice had resolved L. major infection. At this time, lymph node cells from both IL-18+/+ and IL-18-/- mice produced IL-12 and IFN-gamma but no IL-4. Furthermore, administration of anti-IFN-gamma Abs to IL-18-/- mice rendered them susceptible to L. major. These results indicate that despite the role IL-18 may play in early control of cutaneous L. major lesion growth, this cytokine is not critical for development of protective Th1 response and resolution of L. major infection.