Differential regulation of the interleukin-12 receptor during the innate immune response to Leishmania major

Resistance Against Leishmania major Infection Depends on Microbiota-Guided Macrophage Activation

Innate immune cells present a dual role during leishmaniasis: they constitute the first line of host defense but are also the main host cells for the parasite. Response against the infection that results in the control of parasite growth and lesion healing depends on activation of macrophages into a classical activated phenotype. We report an essential role for the microbiota in driving macrophage and monocyte-derived macrophage activation towards a resistance phenotype against Leishmania major infection in mice. Both germ-free and dysbiotic mice showed a higher number of myeloid innate cells in lesions and increased number of infected cells, mainly dermal resident and inflammatory macrophages. Despite developing a Th1 immune response characterized by the same levels of IFN-γ production as the conventional mice, germ-free mice presented reduced numbers of iNOS+ macrophages at the peak of infection. Absence or disturbance of host microbiota impaired the capacity of bone marrow-derived macrophage to be activated for Leishmania killing in vitro, even when stimulated by Th1 cytokines. These cells presented reduced expression of inos mRNA, and diminished production of microbicidal molecules, such as ROS, while presenting a permissive activation status, characterized by increased expression of arginase I and il-10 mRNA and higher arginase activity. Colonization of germ-free mice with complete microbiota from conventional mice rescued their ability to control the infection. This study demonstrates the essential role of host microbiota on innate immune response against L. major infection, driving host macrophages to a resistance phenotype.

Potential biomarkers of immune protection in human leishmaniasis

Leishmaniasis is a vector-borne neglected tropical disease endemic in over 100 countries around the world. Available control measures are not always successful, therapeutic options are limited, and there is no vaccine available against human leishmaniasis, although several candidate antigens have been evaluated over the last decades. Plenty of studies have aimed to evaluate the immune response development and a diverse range of host immune factors have been described to be associated with protection or disease progression in leishmaniasis; however, to date, no comprehensive biomarker(s) have been identified as surrogate marker of protection or exacerbation, and lack of enough information remains a barrier for vaccine development. Most of the current understanding of the role of different markers of immune response in leishmaniasis has been collected from experimental animal models. Although the data generated from the animal models are crucial, it might not always be extrapolated to humans. Here, we briefly review the events during Leishmania invasion of host cells and the immune responses induced against Leishmania in animal models and humans and their potential role as a biomarker of protection against human leishmaniasis.

Unraveling the Role of Immune Checkpoints in Leishmaniasis

Leishmaniasis are Neglected Tropical Diseases affecting millions of people every year in at least 98 countries and is one of the major unsolved world health issues. Leishmania is a parasitic protozoa which are transmitted by infected sandflies and in the host they mainly infect macrophages. Immunity elicited against those parasites is complex and immune checkpoints play a key role regulating its function. T cell receptors and their respective ligands, such as PD-1, CTLA-4, CD200, CD40, OX40, HVEM, LIGHT, 2B4 and TIM-3 have been characterized for their role in regulating adaptive immunity against different pathogens. However, the exact role those receptors perform during Leishmania infections remains to be better determined. This article addresses the key role immune checkpoints play during Leishmania infections, the limiting factors and translational implications.

Deficiency of CD40 Reveals an Important Role for LIGHT in Anti-Leishmania Immunity

We previously showed that LIGHT and its receptor herpes virus entry mediator (HVEM) are important for development of optimal CD4(+) Th1 cell immunity and resistance to primary Leishmania major infection in mice. In this study, we further characterized the contributions of this molecule in dendritic cell (DC) maturation, initiation, and maintenance of primary immunity and secondary anti-Leishmania immunity. Flow-cytometric studies showed that CD8α(+) DC subset was mostly affected by HVEM-Ig and lymphotoxin β receptor-Ig treatment. LIGHT signaling is required at both the priming and the maintenance stages of primary anti-Leishmania immunity but is completely dispensable during secondary immunity in wild type mice. However, LIGHT blockade led to impaired IL-12 and IFN-γ responses and loss of resistance in healed CD40-deficient mice after L. major challenge. The protective effect of LIGHT was mediated primarily via its interaction with lymphotoxin β receptor on CD8α(+) DCs. Collectively, our results show that although LIGHT is critical for maintenance of primary Th1 response, it is dispensable during secondary anti-Leishmania immunity in the presence of functional CD40 signaling as seen in wild type mice.

T helper1/t helper2 cells and resistance/susceptibility to leishmania infection: is this paradigm still relevant?

Work in large part on Leishmania major in the 1980s identified two distinct apparently counter-regulatory CD4⁺ T cell populations, T helper (h)1 and Th2, that controlled resistance/susceptibility to infection respectively. However, the generation of IL-4^−/− mice in the 1990s questioned the paramount role of this Th2 archetypal cytokine in the non-healing response to Leishmania infection. The more recent characterization of CD4⁺ T cell regulatory populations and further effector CD4⁺ T helper populations, Th17, Th9, and T follicular (f)h cells as well as the acknowledged plasticity in T helper cell function has further added to the complexity of host pathogen interactions. These interactions are complicated by the multiplicity of cells that respond to CD4⁺ T cell subset signatory cytokines, as well as the diversity of Leishmania species that are often subject to significantly different immune-regulatory controls. In this article we review current knowledge with regard to the role of CD4⁺ T cells and their products during Leishmania infection. In particular we update on our studies using conditional IL-4Rα gene-deficient mice that have allowed dissection of the cell interplay dictating the disease outcomes of the major Leishmania species infecting humans.

Interleukin-12 as an adjuvant for induction of protective antibody responses

Interleukin (IL)-12 is a pivotal cytokine that strongly stimulates Th1-associated cellular immunity. It is now recognized that IL-12 also activates humoral immunity to both T-dependent and T-independent antigens. This has let to considerable interest in exploiting IL-12 as a vaccine adjuvant for protection against various bacterial and viral pathogens, particularly in the lung. Studies examining the efficacy of IL-12-mediated effects on protective antibody response in the mouse model are summarized in this review.

A detrimental role for IgG and FcgammaR in Leishmania mexicana infection

The intracellular protozoan parasite Leishmania causes leishmaniasis, which is the second biggest killer worldwide among parasitic diseases, after malaria. As drug therapy for leishmaniasis is toxic and resistance is growing, a vaccine is an important weapon against this disease. Unfortunately, no effective vaccine exists for any human parasitic infection. Worse yet, nearly all effective vaccines whose mechanisms are known work through the induction of protective antibodies. Leishmania mexicana causes primarily chronic cutaneous disease. Not only are antibodies not effective at killing Leishmania, as it hides inside the parasitophorous vacuole of the host cell, but new research indicates that IgG antibodies may be crucial in suppressing the host immune response by generating an immunosuppressive interleukin-10 response. IL-10 is able to decrease the needed Th1-generated IFN-gamma and downregulates production of nitric oxide, a required effector mechanism of parasite killing. We have been studying the pathways that the host uses to partially control L. mexicana infection, which include STAT4, IFN-gamma, and inducible nitric oxide synthase, but found that the IL-12 pathway is suppressed by IL-10. We are now studying the mechanisms by which IgG, bound to parasites, can induce IL-10 through FcgammaR ligation and how this suppresses a healing immune response. We are examining which IgG isotypes bind to which FcgammaRs and whether macrophages are the necessary source of IL-10 for chronic disease. Elucidation of these mechanisms may help us to design vaccines that will not induce antibody-mediated immunosuppressive IL-10 responses.

Can type-1 responses against intracellular pathogens be T helper 2 cytokine dependent?

While control of intracellular pathogens, such as the protozoan Leishmania, is dependent on the generation of type-1 immune responses, the role of T helper 2 cytokines in the disease process is more controversial. Traditionally these cytokines were perceived as counter-regulating type-1 responses and promoting disease exacerbation. Nevertheless a substantial body of evidence now exists suggesting that the development of effective type-1 immunity can involve the significant involvement of the Th2 cytokines IL-4 and IL-13. This article reviews, using Leishmania species in particular, the circumstances under which these cytokines can promote protective type-1 immunity.

Early infection with Leishmania major restrains pathogenic response to Leishmania amazonensis and parasite growth

Experimental models of infection with Leishmania spp. have provided knowledge of several immunological events involved in the resistance mechanism used by the host to restrain parasite growth. It is well accepted that concomitant immunity exists, and there is some evidence that it would play a major role in long-lasting acquired resistance to infection. In this paper, the resistance to Leishmania amazonensis infection in C57BL/6 mice infected with Leishmania major was investigated. C57BL/6 mice, which spontaneously heal lesions caused by infection with L. major, were infected with L. amazonensis at different times before and after L. major. We demonstrated that C57BL/6 mice previously infected with L. major restrain pathogenic responses induced by L. amazonensis infection and decrease parasite burdens by one order of magnitude. Co-infected mice showed production of IFN-gamma in lesions similar to mice infected solely with L. major, but higher TNF-alpha and nitric oxide synthase (iNOS) mRNA expression was observed. Surprisingly, the restrained pathogenic response was not related to IL-10 production, as evidenced by lower levels of both mRNA, protein expression in lesions from co-infected mice and in co-infections in IL-10(-/-) mice. Examination of the inflammatory infiltrate at the site of infection showed a reduced number of monocytes and lymphocytes in L. amazonensis lesions. Additionally, differential production of the CCL3/MIP-1 alpha and CCL5/RANTES was observed. We suggest that the control of lesion progression caused by L. amazonensis in C57BL/6 mice pre-infected with L. major is related to the induction of a down-regulatory environment at the site of infection with L. amazonensis.

Effects of CXCL10 on dendritic cell and CD4+ T-cell functions during Leishmania amazonensis infection

Leishmania amazonensis can cause progressive disease in most inbred strains of mice. We have previously reported that treatment with CXCL10 activates macrophage (MPhi) effector function(s) in parasite killing and significantly delays lesion development in susceptible C57BL/6 mice via enhanced gamma interferon (IFN-gamma) and interleukin 12 (IL-12) secretion; however, the mechanism underlying this enhanced immunity against L. amazonensis infection remains largely unresolved. In this study, we utilized stationary promastigotes to infect bone marrow-derived dendritic cells (DCs) of C57BL/6 mice and assessed the activation of DC subsets and the capacity of these DC subsets to prime CD4+ T cells in vitro. We found that CXCL10 induced IL-12 p40 production but reduced IL-10 production in uninfected DCs. Yet L. amazonensis-infected DCs produced elevated levels of IL-10 despite CXCL10 treatment. Elimination of endogenous IL-10 led to increased IL-12 p40 production in DCs as well as increased proliferation and IFN-gamma production by in vitro-primed CD4+ T cells. In addition, CXCL10-treated CD4+ T cells became more responsive to IL-12 via increased expression of the IL-12 receptor beta2 chain and produced elevated levels of IFN-gamma. This report indicates the utility of CXCL10 in generating a Th1-favored, proinflammatory response, which is a prerequisite for controlling Leishmania infection.

CXCL10/gamma interferon-inducible protein 10-mediated protection against Leishmania amazonensis infection in mice

Leishmania amazonensis can cause progressive disease in most inbred strains of mice. We have previously shown that L. amazonensis-infected C57BL/6 mice have profound impairments in expression of proinflammatory cytokines and chemokines and in activation of antigen-specific CD4(+) T cells. These impairments are independent of interleukin-4 (IL-4) but partially due to IL-10 production. The precise mechanism of pathogenesis associated with L. amazonensis infection remains largely unresolved. Since chemokines are essential mediators of leukocyte recruitment and effector cell function, we hypothesized that these molecules are important for the initiation of early responses locally and for the eventual control of the infection. In this study, we examined the roles of CXCL10/gamma interferon-inducible protein 10 (IP-10) and CCL2/monocyte chemoattractant protein 1 (MCP-1) in the activation of the macrophage effector function in vitro and their efficacy in ameliorating infection in vivo. Bone marrow-derived macrophages of both BALB/c and C57BL/6 mice were treated with increasing concentrations of recombinant chemokines prior to infection with either stationary-phase promastigotes or tissue-derived amastigotes. We found that treatment with IP-10 or MCP-1 significantly reduced parasite burdens, in a dose-dependent manner, and triggered nitric oxide production. When susceptible C57BL/6 mice were injected locally with IP-10 following L. amazonensis infection, there was a significant delay in lesion development and a reduction in parasite burdens, accompanied by 7- and 3.5-fold increases in gamma interferon and IL-12 secretion, respectively, in restimulated lymph node cells. This study confirms that IP-10 plays a protective role in promoting the reduction of intracellular parasites and thereby opens new avenues for therapeutic control of nonhealing cutaneous leishmaniasis in the New World.

Efficacy of vaccination with a combination of Leishmania amastigote antigens and the lipid A-analogue ONO-4007 for immunoprophylaxis and immunotherapy against Leishmania amazonensis infection in a murine model of New World cutaneous leishmaniasis

Activation of innate immunity using adjuvants that activate Toll-like receptor 4 pathways have great potential for improving the protection induced by parasite vaccines. We investigated protective and therapeutic effects of a vaccine against leishmaniasis containing a combination of an adjuvant synthetic lipid A-analogue, ONO-4007 and Leishmania amazonensis antigens. ONO-4007 was co-injected with soluble and membrane-enriched L. amazonensis-amastigote antigens into BALB/c mice that had either already been infected with 1 x 10(6) L. amazonensis promastigotes (immunotherapy study) or before challenge with the same infectious dose (immunoprophylaxis study). Sixty percent of mice vaccinated before infectious challenge controlled their Leishmania infections - defined by the absence of footpad-swelling and negative Leishmania cultures - compared to 0% of controls, and 40% of mice vaccinated after infection resolved their infections compared to 0% of controls. Protective immunity in both immunoprophylaxis and immunotherapy models was associated with increased protein production of IL-12 and IFN-gamma. These data suggest that vaccination with a combination of ONO-4007 and amastigote antigens of L. amazonensis may be useful for the prevention and treatment of leishmaniasis, and that the protective immunity induced is associated with the production of type-1 cytokines.

Interleukin 10- and Fcgamma receptor-deficient mice resolve Leishmania mexicana lesions

Infection of C57BL/6 (B6) mice with Leishmania mexicana is associated with a minimal immune response and chronic disease. Here we show that B6 interleukin 10-/- (IL-10-/-) mice resolve their lesions and exhibit increased gamma interferon (IFN-gamma), nitric oxide production, and delayed-type hypersensitivity. This enhanced resistance was dependent upon IL-12p40, since treatment of L. mexicana-infected IL-10-/- mice with anti-IL-12p40 monoclonal antibody abrogated healing. Antibody-opsonized L. mexicana induced IL-10 production by B6 macrophages in vitro, implicating antibody binding to Fc receptors as a mechanism involved in IL-10 production in this infection. Furthermore, B6 FcRgamma-/- mice resolve L. mexicana lesions, and lymph node cells from these mice produced less IL-10 and more IFN-gamma than cells from infected wild-type mice. These data demonstrate that removal of IL-10 or FcgammaR leads to resolution of L. mexicana disease and support a model in which ligation of FcgammaR by L. mexicana-bound immunoglobulin G promotes IL-10 production, leading to chronic disease.

Immunopathogenesis of infection with the visceralizing Leishmania species

Human leishmaniasis is a spectral disease that includes asymptomatic self-resolving infection, localized skin lesions, and progressive visceral leishmaniasis. With some overlap, visceral and cutaneous leishmaniasis are usually caused by different species of Leishmania. This review focuses on host responses to infection with the species that cause visceral leishmaniasis, as they contrast with species causing localized cutaneous leishmaniasis. Data from experimental models document significant differences between host responses to organisms causing these diverse syndromes. The visceralizing Leishmania spp. cause localized organ-specific immune responses that are important determinants of disease outcome. Both the Leishmania species causing cutaneous and those causing visceral leishmaniasis require a Type 1 immune response to undergo cure in mouse models. However, during progressive murine infection with the visceralizing Leishmania sp., the Type 1 response is suppressed at least in part by TGF-beta and IL-10 without type 2 cytokine production. This contrasts with the cutaneous species L. major, in which a Type 2 response suppresses type 1 cytokines and leads to murine disease progression. Population and family studies are beginning to elucidate human genetic determinants predisposing to different outcomes of Leishmania infection. These studies should eventually result in a better understanding of the immunopathogenesis and the spectrum of human leishmaniasis.

The role of interleukin-10 in the differential expression of interleukin-12p70 and its beta2 receptor on patients with active or treated paracoccidioidomycosis and healthy infected subjects

Paracoccidioidomycosis patients present an antigen-specific Th1 immunosuppression. To better understand this phenomenon, we evaluated the interleukin (IL)-12 pathway by measuring IL-12p70 production and CD3+ T cell expression of the IL-12 receptor (IL-12R)beta1/beta2 chains, induced with the main fungus antigen (gp43) and a control antigen, from Candida albicans (CMA). We showed that gp43-induced IL-12p70 production and IL-12Rbeta2 expression were significantly decreased in acute and chronic patients as compared to healthy subjects cured from PCM or healthy infected subjects from endemic areas. Interestingly, the healthy infected subjects had higher gp43-induced IL-12p70 production and beta2 expression than the cured subjects. The addition of a neutralizing anti-IL-10 antibody to the cultures increased IL-12p70 levels and beta2 expression in acute and chronic patients to levels observed in cured subjects. Conversely, addition of the cytokine IL-10 strongly inhibited both parameters in the latter group. In conclusion, we have shown that paracoccidioidomycosis-related Th1 immunosuppression is associated with down-modulation of the IL-12 pathway, that IL-10 may participate in this process, and that patients cured from paracoccidioidomycosis may not fully recover their immune responsiveness.

The development of effector and memory T cells in cutaneous leishmaniasis: the implications for vaccine development

Leishmania major infections induce the development of a CD4(+) T-helper 1 (Th1) response that not only controls the primary infection but also results in life-long immunity to reinfection. How that immunity is maintained is unknown, although because of the existence of infection-induced immunity, there has been an assumption that the development of a vaccine against leishmaniasis would be relatively easy. This has turned out not to be the case. One problem has been the finding that a large part of the immunity induced by a primary infection depends upon the presence of persistent parasites. Nevertheless, there are ample situations where immunologic memory persists without the continued presence of antigen, providing the prospect that a non-live vaccine for leishmaniasis can be developed. To do so will require an understanding of the events involved in the development of an effective protective T-cell response and, more importantly, an understanding of how to maintain that response. Here, we review work from our laboratory, describing how Th1 cells develop in L. major-infected mice, the nature of the memory T cells that provide protection to reinfection, and how that information may be utilized in the development of vaccines.

Does the Leishmania major paradigm of pathogenesis and protection hold for New World cutaneous leishmaniases or the visceral disease?

Parasitic protozoa of the genus Leishmania have provided a useful perspective for immunologists in terms of host defense mechanisms critical for the resolution of infection caused by intracellular pathogens. These organisms, which normally reside in a late endosomal, major histocompatibility complex (MHC) class II(+) compartment within host macrophages cells, require CD4(+) T-cell responses for the control of disease. The paradigm for the CD4(+) T-helper 1 (Th1)/Th2 dichotomy is largely based on the curing/non-curing responses, respectively, to Leishmania major infection. However, this genus of parasitic protozoa is evolutionarily diverse, with the cutaneous disease-causing organisms of the Old World (L. major) and New World (Leishmania mexicana/ Leishmania amazonensis) having diverged 40-80 million years ago. Further adaptations to survive within the visceral organs (for Leishmania donovani, Leishmania chagasi, and Leishmania infantum) must have been required. Consequently, significant differences in host-parasite interactions have evolved. Different virulence factors have been identified for distinct Leishmania species, and there are profound differences in the immune mechanisms that mediate susceptibility/resistance to infection and in the pathology associated with disease. These variations not only point to interesting features of the host-pathogen interaction and immunobiology of this genus of parasitic protozoa, but also have important implications for immunotherapy and vaccine development.

CD4+ Th1 cells induced by dendritic cell-based immunotherapy in mice chronically infected with Leishmania amazonensis do not promote healing

The susceptibility of mice to Leishmania amazonensis infection is thought to result from an inability to develop a Th1 response. Our data show that the low levels of gamma interferon (IFN-gamma) produced by the draining lymph node (DLN) cells of chronically infected mice could be enhanced in vitro and in vivo with L. amazonensis antigen-pulsed bone marrow-derived dendritic cells (BM-DC) and the Th1-promoting cytokine interleukin-12 (IL-12). Given intralesionally to chronically infected mice, this treatment induced the upregulation of mRNA levels for IFN-gamma, the transcription factor T-box expressed in T cells, and IL-12 receptor beta 2 in CD4(+) T cells from the DLN and an increase in parasite-specific immunoglobulin G2a in the serum. However, this Th1 response was not associated with healing, and the antigen-specific enhancement of IFN-gamma production remained impaired in the DLN. However, addition of IL-12 to the in vitro recall response was able to recover this defect, suggesting that antigen-presenting cell-derived IL-12 production may be limited in infected mice. This was supported by the fact that L. amazonensis amastigotes limited the production of IL-12p40 from BM-DC in vitro. Altogether, our data indicate that the immune response of mice chronically infected with L. amazonensis can be enhanced towards a Th1 phenotype but that the presence of Th1 CD4(+) T cells does not promote healing. This suggests that the phenotype of the CD4(+) T cells may not always be indicative of protection to L. amazonensis infection. Furthermore, our data support growing evidence that antigen-presenting cell function, such as IL-12 production, may limit the immune response in L. amazonensis-infected mice.

Development and regulation of cell-mediated immunity in experimental leishmaniasis

The development of a Th1 response is critical for controlling many intracellular pathogens. Our laboratory has focused on the role IL-12 plays in initiating such a Th1 response following infection with the obligate intracellular protozoan, Leishmania. Infection of several mouse strains with L. major is associated with IL-12 production and the development of a Th1 response and resistance, although, interestingly, some species of Leishmania (L. mexicana and L. amazonenesis) fail to initiate a Th1 response in the same mouse strains. Consistent with these observations was our finding that IL-12 is an effective adjuvant for the induction of a Th1 response in leishmaniasis (1). Surprisingly, however, in spite of the fact that following resolution of a primary leishmanial infection there is substantial and long-lived resistance to reinfection, an effective prophylactic or therapeutic vaccine for human leishmaniasis does not exist. Our ability to induce a Th1 response in a primate Leishmania vaccine model, but not protection, suggests that long-term resistance to Leishmania requires more than simply initiating a Th1 response (2). Therefore, we recently expanded our studies to investigate how infection-induced resistance to Leishmania operates. We made the unexpected finding that IL-12 is required for L. major-infected mice to remain immune (3). We are now studying how IL-12 participates in maintaining cell-mediated immunity, and more broadly, how immunologic memory works in L. major-healed mice, as well as defining parasite factors that may block the development of cell-mediated immunity.

Cross-talk between CD8+ and CD4+ T cells in experimental cutaneous leishmaniasis: CD8+ T cells are required for optimal IFN-gamma production by CD4+ T cells

Although the importance of CD8(+) T cells for vaccination and immunity to reinfection with Leishmania parasites is well established, their role in primary infections is disputed. In the present study we further characterized the role of CD8(+) T cells in primary L. major infections. We used two groups of L. major infected BALB/c mice: both groups were immunomanipulated to heal and in one group CD8(+) T cells were depleted throughout the course of infection. Our results show that the reversal of healing caused by the absence of CD8(+) T cells did not alter the proliferation of CD4(+) T cells, however, the frequency of CD4(+) T cells expressing IFN-gamma as well as the levels of this cytokine were clearly reduced. These lower levels of IFN-gamma correlated with a higher parasite load. Our results show that transient depletion of CD4(+) T cells allows the establishment of an equilibrium between CD4(+) and CD8(+) T cells and allows CD8(+) T cell activation and effector functions to develop. In addition, our results suggest that cross-talk between CD4(+) and CD8(+) T cells is crucial for the host defence against L. major.

Signaling through the T1/ST2 molecule is not necessary for Th2 differentiation but is important for the regulation of type 1 responses in nonhealing Leishmania major infection

T1/ST2 is a stable cell surface marker selectively expressed on type 2 T helper (Th2) effector cells. Since nonhealing Leishmania major infections in susceptible BALB/c mice have been ascribed to a polarized Th2 response, we used an anti-T1/ST2 monoclonal antibody (MAb) or a T1-Fc fusion protein to investigate the role of CD4+ T1/ST2(+) Th2 cells in experimental leishmaniasis. We show that interfering with T1/ST2 signaling had no effect on lesion development or parasite replication; however, it induced a significantly higher type 1 response and an enhanced capacity of CD4+ T cells to respond to interleukin 12 (IL-12). Surprisingly, even in the presence of an elevated Th1 response, the production of antigen-specific type 2 cytokines was not altered in the group of mice treated with the anti-T1/ST2 MAb or the T1-Fc fusion protein. To characterize further this Th2 response, we assessed the cytokine profile of CD4+ T cells and found that interfering with T1/ST2 signaling did not alter the cytokine profile of CD4+ T1/ST2(+) T cells. These results show that T1/ST2 signaling is not necessary for the differentiation of naive CD4+ T cells into antigen-specific CD4+ T1/ST2(+) Th2 cells. In addition to CD4+ T1/ST2(+) T cells, we detected another subpopulation of CD4+ Th2 cells, negative for the expression of T1/ST2, that could differentiate in vivo in response to L. major infection. Taken together, our results suggest that CD4+ T1/ST2(+) Th2 cells but not CD4+ T1/ST2(-) Th2 cells can downregulate the Th1 response during the course of a nonhealing L. major infection through a mechanism that is independent of IL-4 or IL-10.

CD40-CD40 ligand costimulation is not required for initiation and maintenance of a Th1-type response to Leishmania major infection

Although previous studies demonstrated a requirement for CD40-CD40 ligand (CD40L) interaction in the development of resistance to Leishmania infection, we recently showed that mice lacking the gene for CD40L (CD40L(-/-) mice) can control Leishmania major infection when they are infected with reduced numbers of parasites. In this study, we examine the cytokine pattern in healing versus nonhealing CD40L(-/-) mice and investigated whether CD40 activation is required for resistance to reinfection. We observed that CD4(+) cells in healed CD40L(-/-) mice produce high levels of gamma interferon compared to cells from nonhealing, high-dose-inoculated mice. In addition, we observed a higher frequency of interleukin-12 (IL-12)- producing cells and a reduced number of IL-4-producing cells in mice infected with reduced numbers of parasites. Importantly, we found that healed CD40L(-/-) mice are highly resistant to reinfection with a large parasite inoculum. In addition, by comparing the cytokine patterns at an early and late stage of infection in nonhealing CD40L(-/-) mice, we demonstrated that nonhealing CD40L(-/-) mice produce a weak Th1-type response during the early stage of infection, but this response wanes as a Th2-type response emerges during late stages of infection. Anti-IL-4 antibody treatment, starting either at the beginning of infection or at week 4 postinfection enabled CD40L(-/-) mice to control a high-dose infection. Together, these results show that CD40-CD40L interaction, although important for IL-12 production in high-dose infections, is not required for either the development or maintenance of resistance in mice infected with reduced numbers of parasites.

The TGF-beta response to Leishmania chagasi in the absence of IL-12

Cure of leishmaniasis requires a type 1 immune response characterized by IFN-gamma production. Leishmania major infection leads to a type 2 response suppressing cure of susceptible BALB/c mice, and L. major causes an exacerbated type 2 response in mouse strains with a gene knockout (KO) such that they lack IL-12p40 (IL-12KO mice). In contrast, type 1 responses are inhibited by TGF-beta without Th2 cell expansion in BALB/c mice infected with L. chagasi. We questioned whether the type 2 or the TGF-beta response would dominate during L. chagasi infection of IL-12KO mice. C57BL/6 mice developed self-resolving L. chagasi infection with abundant IFN-gamma. In contrast, L. chagasi disease was exacerbated and IFN-gamma was low in IL-12KO mice. Total TGF-beta was significantly higher in IL-12KO than control C57BL/6 mice, but IL-4 and IL-10 levels were similar. TGF-beta was further augmented in IL-12/IFN-gamma double-KO mice. Thus, in contrast to L. major, the TGF-beta response was exacerbated whereas type 2 cells were not expanded during L. chagasi infection of IL-12KO mice. We conclude that L. chagasi has an inherent propensity to elicit a prominent TGF-beta response that either suppresses, or is suppressed by, a type 1 response. We propose this be termed a "type 3" immune response, which can antagonize a type 1 response.

Mechanism of transforming growth factor beta-induced inhibition of T helper type 1 differentiation

Regulation by transforming growth factor (TGF)-beta plays an important role in immune homeostasis. TGF-beta inhibits T cell functions by blocking both proliferation and differentiation. Here we show that TGF-beta blocks Th1 differentiation by inhibiting the expression of T-bet, the apparent masterregulator of T helper (Th)1 differentiation. Restoration of T-bet expression through retroviral transduction of T-bet into developing Th1 cells abrogated the inhibitory effect of TGF-beta. In addition, we show that, contrary to prior suggestions, downregulation of interleukin 12 receptor beta2 chain is not key to the TGF-beta-mediated effect. Furthermore, we show that the direct inhibitory effect of TGF-beta on T cells is responsible, at least in part, for the inability of BALB/c mice to mount a Leishmania-specific Th1 response and to clear Leishmanial infection.

Administration of plasmids expressing interleukin-4 and interleukin-10 causes BALB/c mice to induce a T helper 2-type response despite the expected T helper 1-type response with a low-dose infection of Leishmania major

BALB/c mice are susceptible to developing an infection with Leishmania major as a result of a fatal T helper 2 (Th2)-type response. However, mice infected with a low dose of parasites are reported to be able to overcome the lesions associated with a T helper 1 (Th1)-type response. To clarify why a difference in the dose of parasites induces a difference in the polarization of the Th phenotype, we first attempted to measure cytokine production. Soon after infection, the mice given high doses of parasites produced elevated levels of both Th1 [interferon-gamma (IFN-gamma)] and Th2 [interleukin (IL)-4 and IL-10] cytokines. However, when assessed at 1 and 2 weeks after infection, no significant difference in the balance of Th1 and Th2 cytokines could be detected between mice infected with low or high doses of L. major. These results support the notion that the Th2 cytokine levels at an early phase of infection could be a key factor for the induction of a Th2 response. In order to assess the efficacy of Th2 cytokines, the mice infected with low doses of L. major were co-administered IL-4 plasmid and IL-10 plasmid. Consequently, the mice (which originally exhibited a Th1 response) showed progressive disease and developed a Th2 response. However, administration of these plasmids at 7 days postinfection could not alter the Th polarization. Furthermore, production of IL-12 from the spleen cells stimulated by L. major was suppressed in the presence of IL-4 and IL-10. These results strongly suggest that the susceptibility to L. major in BALB/c mice depends on the persistence of Th2 cytokine levels at an early phase of infection.

Parasitic adaptive mechanisms in infection by leishmania

Leishmania are a resilient group of intracellular parasites that infect macrophages. The resultant complex of diseases, or leishmaniases, caused by the parasites affect over twelve million people worldwide. Leishmania have developed unique adaptive mechanisms to ensure their survival in the harsh environments faced throughout their life cycle. These parasites must not only contend with the hostile digestive conditions found within the sand fly vector, but they must also avoid destruction by the host immune system while in the bloodstream, before entering the macrophage. To do so, Leishmania express unique lipophosphoglycan (LPG) molecules and the metalloprotease gp63, among other proteins, on their cell surface. To enter the macrophage, Leishmania utilizes a variety of cellular receptors to mediate endocytosis. Once inside the macrophage, Leishmania is protected from phagolysosome degradation by a variety of adaptations to inhibit cellular defense mechanisms. These include the inhibition of phagosome-endosome fusion, hydrolytic enzymes, cell signaling pathways, nitric oxide production, and cytokine production. While other parasites can also infect macrophages, Leishmania is distinctive in that it not only relies on its own defenses to survive and reproduce within the macrophage phagolysosome, but Leishmania also manipulates the host immune response in order to protect itself and to gain entry into the cell. These unique adaptive mechanisms help promote Leishmania survival.

Expression of the bovine high affinity IL-12 receptor beta2

Four fragments of the bovine IL-12 receptor beta2 were sequenced following generation by reverse transcriptase polymerase chain reaction (RT-PCR) amplification of RNA from mitogen-activated bovine peripheral blood mononuclear cells (PBMC). Primers were based on sequences within regions of the human IL-12Rbeta2 gene that displayed high levels of similarity with the mouse IL-12Rbeta2 gene sequence. The amplified bovine IL-12Rbeta2 fragments had 82-87% similarity at the nucleotide level with human IL-12Rbeta2 and 70-88% similarity at the predicted amino acid level. Bovine IL-12Rbeta2 gene expression was induced following culture of PBMC with Concanavalin A (Con A), with immobilized monoclonal antibody to CD3 or with human recombinant IL-12 p70 and correlated with interferon-gamma (IFN-gamma) production. Expression of bovine IL-12Rbeta2 by PBMC was detected by 2h of culture with Con A and sustained for at least 5 days when cultured with rHuIL-12. Expression, however, did not require cellular proliferation since IL-12 did not induce proliferation, although both Con A and anti-CD3 monoclonal antibody did do so. Addition of rHuIL-10 inhibited IFN-gamma production without abrogating bovine IL-12Rbeta2 gene expression.

The development of a Th1-type response and resistance to Leishmania major infection in the absence of CD40-CD40L costimulation

CD40-CD40L interactions have been shown to be essential for the production of IL-12 and IFN-gamma and control of L. major infection. In contrast, C57BL/6 mice deficient in CD28 develop a dominant Th1-type response and heal infection. In this study, we investigate the effects of a deficiency in both CD40L and CD28 molecules on the immune response and the course of L. major infection. We compared infection in mice genetically lacking CD40L (CD40L(-/-)), CD28 (CD28(-/-)), or both (CD40L(-/-)CD28(-/-)), and in C57BL/6 mice, all on a resistant background. Although CD40L(-/-) mice failed to control infection, CD28(-/-) and CD40L(-/-)CD28(-/-) mice, as well as C57BL/6 mice, spontaneously resolved their infections. Healing mice had reduced numbers of lesion parasites compared with nonhealing CD40L(-/-) mice. At wk 9 of infection, we detected similar levels of IL-4, IFN-gamma, IL-12p40, and IL-12Rbeta2 mRNA in draining lymph nodes of healing C57BL/6, CD28(-/-), and CD40L(-/-)CD28(-/-) mice, whereas CD40L(-/-) mice had increased mRNA levels for IL-4 but reduced levels for IFN-gamma, IL-12p40, and IL-12Rbeta2. In a separate experiment, blocking of the CD40-CD40L pathway using Ab to CD40L led to an exacerbation of infection in C57BL/6 mice, but had little or no effect on infection in CD28(-/-) mice. Together, these results demonstrate that in the absence of CD28 costimulation, CD40-CD40L interaction is not required for the development of a protective Th1-type response. The expression of IL-12p40, IL-12Rbeta2, and IFN-gamma in CD40L(-/-)CD28(-/-) mice further suggests the presence of an additional stimulus capable of regulating IL-12 and its receptors in absence of CD40-CD40L interactions.

Inability of IL-12 to down-regulate IgE synthesis due to defective production of IFN-gamma in atopic NC/Nga mice

NC/Nga mice raised in nonsterile circumstances spontaneously suffer from atopic dermatitis-like skin lesions with IgE hyperproduction. We investigated effects of rIL-12 on the IgE production in NC/Nga mice. rIL-12 administration was successful to suppress the increase of IgE levels in BALB/c mice immunized with OVA and aluminum hydroxide, but failed to abrogate that in NC/Nga mice. Both in vivo and in vitro IFN-gamma production induced by rIL-12 was less in NC/Nga mice than in BALB/c mice. Addition of rIFN-gamma to rIL-4 and LPS completely abrogated IgE production by B cells of BALB/c mice, but was insufficient to suppress it by B cells of NC/Nga mice. In splenic cells pretreated with Con A, STAT4 was phosphorylated at the tyrosine residue by addition of rIL-12, which was more weakly inducible in NC/Nga mice than in BALB/c mice. Finally, we examined the preventive ability of rIL-12 on the clinical aspects of atopic dermatitis in NC/Nga mice. rIL-12 administration resulted in exacerbation of development of the skin lesions and IgE production in NC/Nga mice raised in nonsterile circumstances. These results suggest that defective production of IFN-gamma by T cells less sensitive to IL-12 and low responsiveness of B cells to IFN-gamma may contribute to IgE hyperproduction in NC/Nga mice, and that IL-12 may have no ability to improve the clinical aspects of NC/Nga mice.

Cytokine expression by murine DX5+ cells in response to IL-12, IL-18, or the combination of IL-12 and IL-18

In this study we analyzed the response of DX5+ NK and NK T cells to in vitro stimulation with IL-12 or IL-18. Production of IFN-gamma in response to either IL-12 or IL-18 was dependent upon costimulation with either IL-2 or IL-15. DX5+ splenocytes showed a rapid (6 h) and sustained (6-72 h) accumulation of IFN-gamma transcripts followed by a delayed (12-24 h) up-regulation of IL-10 or IL-13 expression in response to IL-2 + IL-12 or IL-2 + IL-18, respectively. Incubation of DX5+ splenocytes with the combination of IL-2 + IL-12 + IL-18 resulted in up-regulation of IFN-gamma and IL-13 transcripts but down-regulation of IL-10 expression. Furthermore, two distinct populations of cells producing differing amounts of IFN-gamma were observed by intracellular staining after stimulation with IL-2 + IL-12 + IL-18. Last, we demonstrate that DX5+ cells respond to IL-18 independently of IL-12, as cells from both wild-type and IL-12Rbeta2KO mice produce IFN-gamma and IL-13 in response to IL-2 + IL-18.

BALB/c mice bearing a transgenic IL-12 receptor beta 2 gene exhibit a nonhealing phenotype to Leishmania major infection despite intact IL-12 signaling

In BALB/c mice infected with Leishmania major, early secretion of IL-4 leads to a Th2-type response and nonhealing. We explored the role of IL-4-induced down-regulation of the IL-12Rbeta2 chain in the establishment of this Th2 response. First, we showed that the draining lymph nodes of resistant C57BL/6 mice infected with L. major were enriched in CD4+/IL-12Rbeta2 chain+ cells producing IFN-gamma. Next, we demonstrated that BALB/c background mice bearing an IL-12Rbeta2-chain transgene manifested a nonhealing phenotype similar to wild-type littermates despite the persistence of their ability to undergo STAT4 activation. Finally, we found that such transgenic mice display more severe infection than wild-type littermates when treated with IL-12 7 days after infection, and under this condition, the mice display increased Leishmania Ag-induced IL-4 secretion. These studies indicate that although CD4+/IL-12Rbeta2 chain+ T cells are important components of the Th1 response, maintenance of IL-12Rbeta2 chain expression is not sufficient to change a Th2 response to a Th1 response in vivo and thus to allow BALB/c mice to heal L. major infection.

Interleukin (IL)-18 promotes the development of chronic gastrointestinal helminth infection by downregulating IL-13

Expulsion of the gastrointestinal nematode Trichuris muris is mediated by a T helper (Th) 2 type response involving interleukin (IL)-4 and IL-13. Here we show that Th1 response-associated susceptibility involves prior activation of IL-18 and caspase-1 followed by IL-12 and interferon (IFN)-gamma in the intestine. IL-18-deficient mice are highly resistant to chronic T. muris infection and in vivo treatment of normal mice with recombinant (r)IL-18 suppresses IL-13 and IL-4 secretion but does not affect IFN-gamma. In vivo treatment of T. muris-infected IFN-gamma-deficient mice with rIL-18 demonstrated that the inhibitory effect of IL-18 on IL-13 secretion is independent of IFN-gamma. Hence, IL-18 does not function as an IFN-gamma-inducing cytokine during chronic T. muris infection but rather as a direct regulator of Th2 cytokines. These results provide the first demonstration of the critical role of IL-18 in regulating Th cell responses during gastrointestinal nematode infection.

Influence of costimulatory molecules on immune response to Leishmania major by human cells in vitro

The importance of CD40, CD80, and CD86 costimulatory molecules in anti-Leishmania immune responses has been established in murine models. A role for these costimulatory molecules in human anti-Leishmania immune responses was investigated in this study. Autologous macrophages and peripheral blood leukocytes (PBL) were prepared from peripheral blood mononuclear cells of Leishmania-naive donors and cultured with or without Leishmania major in various combinations. After 7 days of culture, high levels of CD40 and CD86 were expressed on macrophages in the presence or absence of L. major. When macrophages were cultured for an additional 7 days with PBL, expression of all three costimulatory molecules was detected. When L. major was present in these cultures, the expression of CD80, and to a lesser extent CD40, on macrophages was enhanced. Blockade of CD80, CD86, or both molecules (in the order of greatest effect) in cultures containing macrophages, PBL, and L. major significantly inhibited the production of gamma interferon, interleukin-5 (IL-5), and IL-12. Blockade of CD40-CD154 interactions also significantly inhibited production of these cytokines in response to L. major. Production of IL-10 was unaltered by the blockade of these costimulatory molecules. Thus, these data suggest that CD40, CD80, and CD86 expression and regulation may significantly impact anti-Leishmania immune responses in humans.

Impaired IFN-gamma production in IFN regulatory factor-1 knockout mice during endotoxemia is secondary to a loss of both IL-12 and IL-12 receptor expression

Mice with a targeted mutation in the gene that encodes the transcription factor IFN regulatory factor-1 (IRF-1) were used to assess the contribution of IRF-1 to IL-12-dependent and IL-12-independent pathways of IFN-gamma production. In response to LPS, IRF. 1-/- mice produced less IL-12 p40, IL-12 p35, and IFN-gamma mRNA in the liver than IRF-1+/+ mice. While pulmonary IFN-gamma mRNA levels were also mitigated in IRF-1-/- mice, pulmonary IL-12 p40 and IL-12 p35 mRNA were not dysregulated. Circulating IL-12 p70 and IFN-gamma levels were profoundly attenuated in LPS-challenged IRF-1-/- mice. Further analysis revealed a major deficiency in hepatic IL-12Rbeta1 and IL-12Rbeta2 mRNA expression as well as pulmonary IL-12Rbeta1 mRNA expression in LPS-challenged IRF-1-/- mice. In vitro, IFN-gamma up-regulated IL-12Rbeta1 mRNA in macrophages from IRF-1+/+, but not IRF-1-/-, mice. IFN-gamma-induced IL-12Rbeta2 mRNA expression was also diminished in macrophages from IRF-1-/- mice. In contrast to IRF-1+/+ mice, administration of exogenous IL-12 to IRF-1-/- mice resulted in reduced serum IFN-gamma and hepatic and pulmonary IFN-gamma mRNA, demonstrating that loss of IL-12R results in diminished IL-12 responsiveness. While LPS-challenged IRF-1-/- mice also had reduced IL-15 mRNA levels, serum IL-18 responses were intact. Finally, induction of IRF-1 mRNA by LPS in livers of IFN-gamma knockout mice were markedly attenuated, suggesting a feedback amplification loop. These studies indicate that IRF-1 deficiency disrupts both IL-12-dependent and -independent pathways of IFN-gamma production and that IRF-1 is a critical transcription factor involved in the regulation of not only IL-12, but also IL-12R.

Salmonella infection does not increase expression and activity of the high affinity IL-12 receptor

Expression of high affinity IL-12 receptors is required for IL-12-mediated IFN-gamma production. Activation of this pathway has been shown to be critical in generating optimal cell-mediated immunity. Therefore, increased IL-12 receptor expression might be expected in the host response after infection by an intracellular bacterial pathogen. In the present study, we have made the surprising discovery that infection with Salmonella results in an early reduction of high affinity IL-12 receptor expression and activation. After oral inoculation with Salmonella, the level of mRNA expression encoding IL-12 receptor beta2 (IL-12Rbeta2) subunit was diminished 12 h postinfection in the mesenteric lymph nodes and subsequently in the spleen. Furthermore, decreased IL-12Rbeta2 mRNA expression was observed in CD4+ T lymphocytes isolated from the mesenteric lymph nodes and spleens of infected mice. Attenuated IL-12Rbeta2 mRNA expression correlated with reduced receptor signaling, as demonstrated by reduced IL-12-induced STAT4 phosphorylation in enriched T lymphocytes isolated from the mesenteric lymph nodes and spleens of Salmonella-infected mice. These in vivo results were substantiated with an in vitro model system. In this model system, T lymphocytes cocultured with Salmonella-infected macrophages expressed less IL-12Rbeta2 mRNA. The cocultured T cells were also less responsive to IL-12 as assessed by reduced phosphorylation of STAT4 and limited IFN-gamma secretion. Together, these studies suggest that Salmonella can limit an optimal host immune response by reducing the expression and activity of high affinity IL-12 receptors.

Preformed membrane-associated stores of interleukin (IL)-12 are a previously unrecognized source of bioactive IL-12 that is mobilized within minutes of contact with an intracellular parasite

The prevailing paradigm is that production of the interleukin (IL)-12 p70 heterodimer, a critical T helper cell type 1 (Th1)-inducing cytokine, depends on the induced transcription of the p40 subunit. Concordant with this paradigm, we found that dendritic cells (DCs) produced IL-12 p70 only after at least 2-4 h of stimulation with lipopolysaccharide plus interferon gamma. However, using several complementary experimental approaches, including electron and confocal microscopy, we now show that resting murine and human myeloid cells, including macrophages/DCs and DC-rich tissues, contain a novel source of bioactive IL-12 that is preformed and membrane associated. These preformed, membrane-associated IL-12 p70 stores are released within minutes after in vitro or in vivo contact with Leishmania donovani, an intracellular pathogen. Our findings highlight a novel source of bioactive IL-12 that is readily available for the rapid initiation of Th1 host responses to pathogens such as Leishmania species.

Cytokines and T cells in host defense

Soluble and cell-bound ligands profoundly influence the differentiative fate of lymphocytes during an immune response. Recent advances have been made in understanding the role of cytokine signals and costimulatory signals in the regulation of T cell responses associated with resistance or susceptibility to infection. There has also been recent progress in defining the requirements for the generation and maintenance of immunologic memory, a critical component of adaptive immunity.

IL-4-independent inhibition of IL-12 responsiveness during Leishmania amazonensis infection

Leishmania amazonensis induces a nonhealing infection in C3H mice, whereas infection with Leishmania major is self-healing. We found that C3H mice infected with L. amazonensis exhibited decreased IL-12 production, which could account for the susceptibility to this organism. However, exogenous IL-12 administration failed to induce a healing immune response. The failure of L. amazonensis-infected C3H mice to respond to IL-12 was associated with a specific defect in IL-12 receptor beta2 (IL-12Rbeta2) mRNA expression by CD4+ T cells. Furthermore, decreased IL-12Rbeta2 mRNA expression correlated with a decrease in the IL-12-signaling capacity of the lymph node (LN) cells. IL-4 did not contribute to susceptibility or down-regulation of the IL-12Rbeta2 subunit, because IL-4-/- mice remained susceptible to L. amazonensis infection, even after IL-12 administration, and CD4+ cells from infected IL-4-/- mice also had reduced expression of IL-12Rbeta2 mRNA. These results demonstrate that regulation of the IL-12 receptor, independent of IL-4, is a point of control for the immune response to leishmaniasis. In contrast to experimental L. major infections, where host genetics control susceptibility, these studies demonstrate that the lack of IL-12 responsiveness may be dictated by the pathogen, rather than the host.

Beryllium, an adjuvant that promotes gamma interferon production

Beryllium is associated with a human pulmonary granulomatosis characterized by an accumulation of CD4(+) T cells in the lungs and a heightened specific lymphocyte proliferative response to beryllium (Be) with gamma interferon (IFN-gamma) release (i.e., a T helper 1 [Th1] response). While an animal model of Be sensitization is not currently available, Be has exhibited adjuvant effects in animals. The effects of Be on BALB/c mice immunized with soluble leishmanial antigens (SLA) were investigated to determine if Be had adjuvant activity for IFN-gamma production, an indicator of the Th1 response. In this strain of Leishmania-susceptible BALB/c mice, a Th2 response is normally observed after in vivo SLA sensitization and in vitro restimulation with SLA. If interleukin-12 (IL-12) is given during in vivo sensitization with SLA, markedly increased IFN-gamma production and decreased IL-4 production are detected. We show here that when beryllium sulfate (BeSO(4)) was added during in vivo sensitization of BALB/c mice with SLA and IL-12, significantly increased IFN-gamma production and decreased IL-4 production from lymph node and spleen cells were detected upon in vitro SLA restimulation. No specific responses were observed to Be alone. Lymph node and spleen cells from all mice proliferated strongly and comparably upon in vitro restimulation with SLA and with SLA plus Be; no differences were noted among groups of mice that received different immunization regimens. In vivo, when Be was added to SLA and IL-12 for sensitization of BALB/c mice, more effective control of Leishmania infection was achieved. This finding has implications for understanding not only the development of granulomatous reactions but also the potential for developing Be as a vaccine adjuvant.

In BALB/c mice, IL-4 production during the initial phase of infection with Leishmania major is necessary and sufficient to instruct Th2 cell development resulting in progressive disease

In contrast to intact BALB/c mice, BALB/c mice rendered deficient in Vbeta4+ CD4+ T cells develop a Th1 response to infection with Leishmania major and are resistant. Vbeta4-deficient BALB/c mice are unable to generate the early IL-4 transcription occurring in Vbeta4 Valpha8 CD4+ T cells of BALB/c mice within 1 day of infection. Here we demonstrate that treatment of Vbeta4-deficient BALB/c mice with IL-4 during the first 64 h after infection instructs Th2 cell development and susceptibility to infection. The demonstrated inability of IL-4 to reverse the resistant phenotype of BALB/c mice treated with anti-CD4 mAb the day before infection suggest that these effects of IL-4 require its interaction with CD4+ T cells. In contrast to draining lymph node cells from BALB/c mice, cells from Vbeta4-deficient BALB/c mice remain responsive to IL-12 following infection. Strikingly, administration of IL-4 to Vbeta4-deficient BALB/c mice renders their lymph node cells unresponsive to IL-12 by down-regulating IL-12R beta2-chain expression. This study directly demonstrates that in BALB/c mice IL-4 is necessary and sufficient to initiate the molecular events steering Th2 cell maturation and susceptibility to L. major.

Autocrine Regulation of IL-12 Receptor Expression Is Independent of Secondary IFN-γ Secretion and not Restricted to T and NK Cells

The biological response to IL-12 is mediated through specific binding to a high affinity receptor complex composed of at least two subunits (designated IL-12Rβ1 and IL-12Rβ2) that are expressed on NK cells and activated T cells. The selective loss of IL-12Rβ2 expression during Th2 T cell differentiation suggests that regulation of this receptor component may govern IL-12 responsiveness. In murine assays, down-regulation of IL-12Rβ2 expression can be prevented by treatment with IFN-γ, indicating that receptor expression and hence IL-12 responsiveness may be regulated, at least in part, by the local cytokine milieu. In this study, we report that cellular expression of both IL-12Rβ1 and β2 mRNA is increased in the lymph nodes of naive mice following systemic administration of murine rIL-12 (rmIL-12). Changes in IL-12R mRNA were associated with increased IFN-γ secretion following ex vivo activation of lymph node cells with rmIL-12, indicating the presence of a functional receptor complex. Expression of IL-12R mRNA was not restricted to lymph node T cells, and its autocrine regulation was independent of secondary IFN-γ secretion. Data from fractionated lymph node cells as well as rmIL-12-treated B cell-deficient mice suggest that IL-12-responsive B cells may represent an alternative cellular source for IFN-γ production. However, the strength of the biological response to rmIL-12 is not governed solely by receptor expression, as rmIL-12-induced IFN-γ secretion from cultured lymph node cells is accessory cell dependent and can be partially blocked by inhibition of B7 costimulation.

Leishmania species: models of intracellular parasitism

Leishmania species are obligate intracellular parasites of cells of the macrophage-dendritic cell lineage. Indeed, the ability to survive and multiply within macrophages is a feature of a surprising number of infectious agents of major importance to public health, including Mycobacterium tuberculosis, Mycobacterium leprae, Listeria monocytogenes, Salmonella typhimurium, Toxoplasma gondii and Trypanosoma cruzi. The relationship between such organisms and their host cells is particularly intriguing because, not only are macrophages capable of potent microbicidal activity, but in their antigen-presenting capacity they can orchestrate the developing immune response. Thus, to initiate a successful infection parasites must gain entry into macrophages, and also withstand or circumvent their killing and degradative functions. However, to sustain a chronic infection, parasites must also subvert macrophage-accessory-cell activities and ablate the development of protective immunity. The leishmanias produce a wide spectrum of disease in mice, and as such they have provided excellent models for studying problems associated with intracellular parasitism. In recent years, largely using these organisms, we have made enormous progress in elucidating the mechanisms by which successful intracellular infection occurs. Furthermore, characterization of immunological pathways that are responsible for resistance or susceptibility to Leishmania has given rise to the Th1/Th2 paradigm of cellular/humoral dominance of the immune response.

Cure of progressive murine leishmaniasis: interleukin 4 dominance is abolished by transient CD4(+) T cell depletion and T helper cell type 1-selective cytokine therapy

Progressive infection with Leishmania major in susceptible BALB/c mice is mediated by interleukin (IL)-4-producing T helper cell type 2 (Th2) CD4(+) T cells that, once established, become resistant to Th1-deviating therapies with recombinant (r)IL-12 and/or neutralizing anti-IL-4 antibodies. We sought to restore protective immunity in advanced leishmaniasis by depletion of Th2-biased CD4(+) populations and by cytokine-directed reconstitution of Th1 cellular responses during lymphocyte recovery. Treatment with cytolytic GK1.5 anti-CD4 mAb alone did not reverse disease in 3 wk-infected BALB/c mice, but GK1.5 combined with anti-IL-4 antibody and intralesional rIL-12 cured cutaneous lesions in 80% of mice and established a Th1-polarized cytokine response to L. major antigen protective against reinfection. The curative effects of GK1.5 were not replaced by cytotoxic anti-CD8 monoclonal antibody 2.43 or nondepleting anti-CD4 mAb YTS177, confirming that depletion of CD4(+) cells was specific and essential for therapeutic effect. Finally, combined CD4(+) depletion and IL-4 neutralization were curative, indicating that neither increased parasite burden nor altered accessory cell function independently biased towards Th2 reconstitution in advanced leishmaniasis. Advanced leishmaniasis can be cured by T cell depletion and cytokine-directed recovery of Th1 cellular responses, suggesting novel interventions for other immune-mediated diseases and identifying distinct roles for CD4(+) T cell and non-T cell in the maintenance of Th2 and Th1 phenotypes.

Expression and Contribution of B7-1 (CD80) and B7-2 (CD86) in the Early Immune Response to Leishmania major Infection

CD28 interactions promote T cell responses, and whether B7-1 or B7-2 is utilized may influence Th cell subset development. CD28 blockade by CTLA-4Ig treatment or by targeted gene disruption has yielded different conclusions regarding the role of CD28 in the development of Th1 and Th2 cells following Leishmania major infection. In this study, we demonstrate that B7-mediated costimulation is required for the development of the early immune response following infection of resistant or susceptible mice. In contrast, CD28−/− BALB/c mice infected with L. major produce cytokines comparable to those of infected wild-type mice. Treatment of CD28−/− mice with CTLA-4Ig did not diminish this response, suggesting that a B7-independent pathway(s) contributes to the early immune response in these mice. In conventional BALB/c or C3H mice, B7-2 functions as the dominant costimulatory molecule in the initiation of early T cell activation following L. major infection, leading to IL-4 or IFN-γ production, respectively. The preferential interaction of B7-2 with its ligand(s) in the induction of these responses correlates with its constitutive expression relative to that of B7-1. However, B7-1 can equally mediate costimulation for the production of either IL-4 or IFN-γ when expressed at high levels. Thus, in leishmaniasis, costimulation involving B7-1 or B7-2 can result in the production of either Th1 or Th2 cytokines, rather than a preferential induction of one type of response.

Interleukin-12 is capable of generating an antigen-specific Th1-type response in the presence of an ongoing infection-driven Th2-type response

Previously we demonstrated that recombinant murine interleukin-12 (rmIL-12) administration can promote a primary Th1 response while suppressing the Th2 response in mice primed with 2,4, 6-trinitrophenyl-keyhole limpet hemocyanin (TNP-KLH). The present studies examined the capacity of rmIL-12 to drive a Th1 response to TNP-KLH in the presence of an ongoing Th2-mediated disease. To establish a distinct Th2 response, we used a murine model of leishmaniasis. Susceptible BALB/c mice produce a strong Th2 response when infected with Leishmania major and develop progressive visceral disease. On day 26 postinfection, when leishmaniasis was well established, groups of mice were immunized with TNP-KLH in the presence or absence of exogenous rmIL-12. Even in the presence of overt infection, TNP-KLH-plus-rmIL-12-immunized mice were still capable of generating KLH-specific gamma interferon (IFN-gamma) as well as corresponding TNP-specific immunoglobulin G2a (IgG2a) titers. In addition, the KLH-specific IL-4 was suppressed in infected mice immunized with rmIL-12. However, parasite-specific IL-4 and IgG1 production with a lack of parasite-specific IFN-gamma secretion were maintained in all infected groups of mice including those immunized with rmIL-12. These data show that despite the ongoing infection-driven Th2 response, rmIL-12 was capable of generating an antigen-specific Th1 response to an independent immunogen. Moreover, rmIL-12 administered with TNP-KLH late in infection did not alter the parasite-specific cytokine or antibody responses.

Expression of the IL-12 Receptor β1 and β2 Subunits in Human Tuberculosis

To determine whether the Th1 response in tuberculosis correlated with IL-12R expression, we measured expression of the IL-12Rβ1 and IL-12Rβ2 subunits, as well as IL-12Rβ2 mRNA expression in tuberculosis patients and healthy tuberculin reactors. In tuberculosis patients, IFN-γ production by Mycobacterium tuberculosis-stimulated PBMC was reduced, the percentages of T cells expressing IL-12Rβ1 and IL-12Rβ2 were significantly decreased, and IL-12Rβ2 mRNA expression was also markedly reduced. In contrast, in pleural fluid and lymph nodes at the site of disease in tuberculosis patients, in which IFN-γ production is enhanced, IL-12Rβ2 mRNA expression was also increased. In M. tuberculosis-stimulated peripheral blood T cells from tuberculosis patients, anti-IL-10 and anti-TGF-β enhanced IL-12Rβ1 and IL-12Rβ2 expression, and IFN-γ production. In M. tuberculosis-stimulated peripheral blood T cells from healthy tuberculin reactors, recombinant IL-10 and TGF-β reduced IL-12Rβ1 and IL-12Rβ2 expression, as well as IFN-γ production. In combination with prior studies showing increased production of TGF-β by blood monocytes from tuberculosis patients, this suggests that increased TGF-β production is the underlying abnormality that reduces IL-12Rβ1 and IL-12Rβ2 expression in tuberculosis. Our findings provide evidence that IL-12R expression correlates well with IFN-γ production in human tuberculosis, and that expression of IL-12Rβ1 and IL-12Rβ2 may play a central role in mediating a protective Th1 response.