Leishmania major infection in major histocompatibility complex class II-deficient mice: CD8+ T cells do not mediate a protective immune response

A pyoderma gangrenous-like cutaneous leishmaniasis in a Libyan woman with rheumatoid arthritis: a case report

Background

Several case reports describe diseases presenting with skin ulcerations, which resemble pyoderma gangrenosum especially in immune-compromised patients, often proven on further workup, to have an infective or malignant etiology. However, treatment of pyoderma gangrenosum by systemic steroids or other immunosuppressive agents may worsen the condition.

Case presentation

We report here, a 45 year-old Libyan woman with rheumatoid arthritis on low dose steroids with pyoderma gangrenosum-like skin lesions and positive pathergy. Slit–smear was positive for Leishmania amastigotes and histopathological examination confirmed the diagnosis of cutaneous leishmaniasis. The lesions healed completely by parenteral sodium stibogluconate (Pentostam) 600 mg daily.

Conclusion

We report for the first time, a rare and unusual presentation of pyoderma gangrenosum like-cutaneous leishmaniasis in a patient with rheumatoid arthritis. Atypical cutaneous leishmaniasis should not be ruled out in the differential diagnosis of unresponsive skin diseases, with slit/smear and a skin biopsy is required.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3272-2) contains supplementary material, which is available to authorized users.

American cutaneous leishmaniasis in infancy and childhood

Infant and young child skin diseases are among the most common features of morbidity throughout the tropics. Because the skin is directly exposed to the environment, it is considerably affected by climatic and local conditions such as vectors and microorganisms, as in the case of leishmaniasis. In America the observed magnitude of cutaneous leishmaniasis in children has led to the study of increased risk of exposure of this group due to the possibility of peri- and intradomiciliary transmission. The present review pretends to make a concrete approach all through the broad and main figures of this parasitic disease, including the clinical, physiopathological, epidemiological, diagnostic, and therapeutic aspects, in order to be used as a practical source of reference for pediatricians leading with tropical cutaneous pathology in the region.

Immune responses against protozoan parasites: a focus on the emerging role of Nod-like receptors

Nod-like receptors (NLRs) have gained attention in recent years because of the ability of some family members to assemble into a multimeric protein complex known as the inflammasome. The role of NLRs and the inflammasome in regulating innate immunity against bacterial pathogens has been well studied. However, recent studies show that NLRs and inflammasomes also play a role during infections caused by protozoan parasites, which pose a significant global health burden. Herein, we review the diseases caused by the most common protozoan parasites in the world and discuss the roles of NLRs and inflammasomes in host immunity against these parasites.

Mechanisms of immune evasion in leishmaniasis

Diseases caused by Leishmania present a worldwide problem, and current therapeutic approaches are unable to achieve a sterile cure. Leishmania is able to persist in host cells by evading or exploiting host immune mechanisms. A thorough understanding of these mechanisms could lead to better strategies for effective management of Leishmania infections. Current research has focused on parasite modification of host cell signaling pathways, entry into phagocytic cells, and modulation of cytokine and chemokine profiles that alter immune cell activation and trafficking to sites of infection. Immuno-therapeutic approaches that target these mechanisms of immune evasion by Leishmania offer promising areas for preclinical and clinical research.

Roles of major histocompatibility complex class II in inducing protective immune responses to influenza vaccination

Major histocompatibility complex class II-deficient (MHC-II KO; Aβ(-/-)) mice were used to assess the roles of MHC-II molecules in inducing protective immune responses to vaccination. After vaccination with influenza A/PR8 virus-like particle (VLP) vaccine, in vivo and in vitro vaccine antigen-specific IgG isotype antibodies were not detected in MHC-II KO mice, which is quite different from CD4 T cell-deficient mice that induced vaccine-specific IgG antibodies. The deficiency in MHC-II did not significantly affect the induction of antigen-specific IgM antibody in sera. MHC-II KO mice that were vaccinated with influenza VLP, whole inactivated influenza virus, or live attenuated influenza virus vaccines were not protected against lethal infection with influenza A/PR8 virus. Adoptive transfer of fractionated spleen cells from wild-type mice to MHC-II KO mice indicated that CD43(+) cell populations with MHC-II contributed more significantly to producing vaccine-specific IgG antibodies than CD43(-) B220(+) conventional B cell or CD4 T cell populations, as well as conferring protection against lethal infection. Bone marrow-derived dendritic cells from MHC-II KO mice showed a significant defect in producing interleukin-6 and tumor necrosis factor alpha cytokines. Thus, results indicate that MHC-II molecules play multiple roles in inducing protective immunity to influenza vaccination. Importance: Major histocompatibility complex class II (MHC-II) has been known to activate CD4 T helper immune cells. A deficiency in MHC-II was considered to be equivalent to the lack of CD4 T cells in developing host immune responses to pathogens. However, the roles of MHC-II in inducing protective immune responses to vaccination have not been well understood. In the present study, we demonstrate that MHC-II-deficient mice showed much more significant defects in inducing protective antibody responses to influenza vaccination than CD4 T cell-deficient mice. Further analysis showed that CD43 marker-positive immune cells with MHC-II, as well as an innate immunity-simulating adjuvant, could rescue some defects in inducing protective immune responses in MHC-II-deficient mice. These results have important implications for our understanding of host immunity-inducing mechanisms to vaccination, as well as in developing effective vaccines and adjuvants.

CD4+ T Cells: guardians of the phagosome

CD4(+) T cells are key cells of the adaptive immune system that use T cell antigen receptors to recognize peptides that are generated in endosomes or phagosomes and displayed on the host cell surface bound to major histocompatibility complex molecules. These T cells participate in immune responses that protect hosts from microbes such as Mycobacterium tuberculosis, Cryptococcus neoformans, Leishmania major, and Salmonella enterica, which have evolved to live in the phagosomes of macrophages and dendritic cells. Here, we review studies indicating that CD4(+) T cells control phagosomal infections asymptomatically in most individuals by secreting cytokines that activate the microbicidal activities of infected phagocytes but in a way that inhibits the pathogen but does not eliminate it. Indeed, we make the case that localized, controlled, persistent infection is necessary to maintain large numbers of CD4(+) effector T cells in a state of activation needed to eradicate systemic and more pathogenic forms of the infection. Finally, we posit that current vaccines for phagosomal infections fail because they do not produce this "periodic reminder" form of CD4(+) T cell-mediated immune control.

Dendritic cells in Leishmania major infections: mechanisms of parasite uptake, cell activation and evidence for physiological relevance

Leishmaniasis is one of the most important infectious diseases worldwide; a vaccine is still not available. Infected dendritic cells (DC) are critical for the initiation of protective Th1 immunity against Leishmania major. Phagocytosis of L. major by DC leads to cell activation, IL-12 release and (cross-) presentation of Leishmania antigens by DC. Here, we review the role of Fcγ receptor- and B cell-mediated processes for parasite internalization by DC. In addition, the early events after parasite inoculation that consist of mast cell activation, parasite uptake by skin-resident macrophages (MΦ), followed by neutrophil and monocyte immigration and DC activation are described. All these events contribute significantly to antigen processing in infected DC and influence resulting T cell priming in vivo. A detailed understanding of the role of DC for the development of efficient anti-Leishmania immunity will aid the development of potent anti-parasite drugs and/or vaccines.

CD4+ T cells rely on a cytokine gradient to control intracellular pathogens beyond sites of antigen presentation

Effector T cells are critical for clearance of pathogens from sites of infection. Like cytotoxic CD8(+) T cells, CD4(+) helper T cells have been shown to deliver effector molecules directionally toward the immunological synapse, suggesting that infected cells need to be engaged individually to receive effector signals. In contrast, we show here that CD4(+) T cells stably contacted a minority of infected cells, yet these interactions triggered intracellular defense mechanisms in bystander cells in vivo. By using a functional read-out, we provide evidence that this effector bystander activity extends via a gradient of IFN-γ more than 80 μm beyond the site of antigen presentation, promoting pathogen clearance in the absence of immunological synapse formation. Our results thus demonstrate that CD4(+) T cells can exert their protective activity by engaging a minority of infected cells.

A MATHEMATICAL MODEL OF IMMUNE RESPONSE IN CUTANEOUS LEISHMANIASIS

The TH1/TH2 paradigm has been largely used in the interpretation of several diseases, particularly in leishmaniasis. As far as we know there is no mathematical description of this model related to leishmaniasis. We have extended and modified a previous published set of equations1in order to adapt it to leishmanial disease particularities. The main modifications were: (1) the analysis of logistic and exponential parasite growth curves, (2) the assumption of the TH2 arm of the immune response having a positive action on parasite growth. The set of three simultaneous differential equations describing the TH1 arm, TH2 arm and parasite growth were analyzed for conditions of existence and stability of the solutions.

Stable solutions valid for the logistic and exponential parasite growth models, with its possible clinical correlations, were obtained in the following situations: (1) parasite and TH2 extinction [TH1 cure], (2) parasite extinction and TH1/TH2 co-existence [TH1/TH2 cure], (3) TH1 and parasite co-existence, TH2 extinction [stable TH1 infection], and (4) TH1, TH2 and parasite co-existence [stable TH1/TH2 infection]. TH2 and parasite co-existence associated to TH1 extinction [stable TH2 infection] was obtained only with the logistic growth model. The model also provides an alternative hypothesis for TH1 bias in resistant mice and emphazises the importance of natural immunity for the existence of chronic states.

Immune evasive mechanisms contributing to persistent Leishmania donovani infection

The protozoan parasite Leishmania donovani, a causative agent of visceral leishmaniasis, has evolved several strategies to interfere with the immune system and establish persistent infections that are potentially lethal. In this article, we discuss two mechanisms of immune evasion adopted by the parasite: the induction of immune suppressive IL-10 responses and the generation of poor and functionally impaired CD8(+) T-cell responses.

Vaccination with TAT-antigen fusion protein induces protective, CD8(+) T cell-mediated immunity against Leishmania major

In murine leishmaniasis, healing is mediated by IFN-γ-producing CD4(+) and CD8(+) T cells. Thus, an efficacious vaccine should induce Th1 and Tc1 cells. Dendritic cells (DCs) pulsed with exogenous proteins primarily induce strong CD4-dependent immunity; induction of CD8 responses has proven to be difficult. We evaluated the immunogenicity of fusion proteins comprising the protein transduction domain of HIV-1 TAT and the Leishmania antigen LACK (Leishmania homolog of receptors for activated C kinase), as TAT-fusion proteins facilitate major histocompatibility complex class I-dependent antigen presentation. In vitro, TAT-LACK-pulsed DCs induced stronger proliferation of Leishmania-specific CD8(+) T cells compared with DCs incubated with LACK alone. Vaccination with TAT-LACK-pulsed DCs or fusion proteins plus adjuvant in vivo significantly improved disease outcome in Leishmania major-infected mice and was superior to vaccination with DCs treated with LACK alone. Vaccination with DC+TAT-LACK resulted in stronger proliferation of CD8(+) T cells when compared with immunization with DC+LACK. Upon depletion of CD4(+) or CD8(+) T cells, TAT-LACK-mediated protection was lost. TAT-LACK-pulsed IL-12p40-deficient DCs did not promote protection in vivo. In summary, these data show that TAT-fusion proteins are superior in activating Leishmania-specific Tc1 cells when compared with antigen alone and suggest that IL-12-dependent preferential induction of antigen-specific CD8(+) cells promotes significant protection against this important human pathogen.

Regulation of CD8+ T cell responses to infection with parasitic protozoa

There are over 10,000 species of parasitic protozoa, a subset of which can cause considerable disease in humans. Here we examine in detail the complex immune response generated during infection with a subset of these parasites: Trypanosoma cruzi, Leishmania sp., Toxoplasma gondii, and Plasmodium sp. While these particular species perhaps represent the most studied parasites in terms of understanding how T cells function during infection, it is clear that the lessons learned from this body of work are also relevant to the other protozoa known to induce a CD8(+) T cell response. This review will highlight some of the key studies that established that CD8(+) T cells play a major role in protective immunity to protozoa, the factors that promote the generation as well as maintenance of the CD8(+) T cell response during these infections, and draw attention to some of the gaps in our knowledge. Moreover, the development of new tools, including MHC-Class I tetramer reagents and the use of TCR transgenic mice or genetically modified parasites, has provided a better appreciation of how parasite specific CD8(+) T cell responses are initiated and new insights into their phenotypic plasticity.

CTL responses to Leishmania mexicana gp63-cDNA vaccine in a murine model

Immunity to Leishmania is believed to be strongly dependent upon the activation of Th1 immune responses, although the exact role of cytotoxic T lymphocytes (CTLs) has not yet been determined. The aims of this study were to establish a suitable cytotoxicity assay to measure CTL activity and to compare immunity induced by Leishmania mexicana gp63 cDNA via i.m. injection and gene gun immunization in the BALB/c mouse model. The CTL activity was evaluated by short-term (51)Cr-release cytotoxicity assays against CT26 tumour cells transfected with L. mexicana gp63 cDNA and dendritic cells (DCs) loaded with soluble Leishmania antigen (SLA) as targets. The results clearly demonstrated that higher protection to L. mexicana infection was induced by gene gun DNA-immunization vs. i.m. injection. Cytotoxic T lymphocyte activity of splenocytes was observed in mice immunized either with L. mexicana gp63 cDNA or SLA and long-lived CTL activity was observed in immunized and/or re-challenged mice but not naïve mice infected with the parasite.

Priming of CD8+ and CD4+ T cells in experimental leishmaniasis is initiated by different dendritic cell subtypes

The biological role of Langerin+ dendritic cells (DCs) such as Langerhans cells and a subset of dermal DCs (dDCs) in adaptive immunity against cutaneous pathogens remains enigmatic. Thus, we analyzed the impact of Langerin+ DCs in adaptive T cell-mediated immunity toward Leishmania major parasites in a Lang-DTR mouse model that allows conditional diphtheria toxin (DT)-induced ablation of Langerin+ DCs in vivo. For the first time, infection experiments with DT-treated Lang-DTR mice revealed that proliferation of L. major-specific CD8+ T cells is significantly reduced during the early phase of the immune response following depletion of Langerin+ DCs. Consequently, the total number of activated CD8+ T cells within the draining lymph node and at the site of infection is diminished. Furthermore, we show that the impaired CD8+ T cell response is due to the absence of Langerin+ dDCs and not Langerhans cells. Nevertheless, the CD4+ T cell response is not altered and the infection is cleared as effectively in DT-treated Lang-DTR mice as in control mice. This clearly demonstrates that Langerin+ DCs are, in general, dispensable for an efficient adaptive immune response against L. major parasites. Thus, we propose a novel concept that, in the experimental model of leishmaniasis, priming of CD4+ T cells is mediated by Langerin- dDCs, whereas Langerin+ dDCs are involved in early priming of CD8+ T cells.

What about Th1/Th2 in cutaneous leishmaniasis vaccine discovery?

The T helper cell type 1 (Th1) response is essential to resist leishmaniasis, whereas the Th2 response favors the disease. However, many leishmanial antigens, which stimulate a Th1 immune response during the disease or even after the disease is cured, have been shown to have no protective action. Paradoxically, antigens associated with an early Th2 response have been found to be highly protective if the Th1 response to them is generated before infection. Therefore, finding disease-associated Th2 antigens and inducing a Th1 immune response to them using defined vaccination protocols is an interesting unorthodox alternative approach to the discovery of a leishmania vaccine.

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.

Low dose Leishmania major promotes a transient T helper cell type 2 response that is down-regulated by interferon gamma-producing CD8+ T cells

An unresolved issue in the field of T helper (Th) cell development relates to the findings that low doses of antigen promote Th2 cell development in vitro, whereas several classic in vivo studies suggest the opposite. Here we resolve this paradox by studying the early immune response in mice after infection with different doses of Leishmania major. We found that low parasite doses induced a Th2 response in C57BL/6 (B6) mice, whereas high doses induced a Th1 response. However, the Th2 response in low dose–infected mice was transient and the animals healed. The appearance of a Th1 response after low dose infection was dependent upon the concomitant activation of interferon γ–producing CD8⁺ T cells. In the absence of CD8⁺ T cells, the Th2 response was maintained. However, either neutralization of interleukin (IL)-4 or administration of IL-12 promoted a Th1 response after low dose infection of CD8-deficient mice, indicating that the required role for CD8⁺ T cells was limited to modulation of CD4⁺ T cell responses. Thus, the discrepant results seen between in vivo and in vitro studies on the effects of antigen dose on Th cell differentiation may depend upon whether CD8⁺ T cells participate in the immune response.

Enhanced replication of Leishmania amazonensis amastigotes in gamma interferon-stimulated murine macrophages: implications for the pathogenesis of cutaneous leishmaniasis

During Leishmania major infection in mice, gamma interferon (IFN-gamma) plays an essential role in controlling parasite growth and disease progression. In studies designed to ascertain the role of IFN-gamma in Leishmania amazonensis infection, we were surprised to find that IFN-gamma could promote L. amazonensis amastigote replication in macrophages (Mphis), although it activated Mphis to kill promastigotes. The replication-promoting effect of IFN-gamma on amastigotes was independent of the source and genetic background of Mphis, was apparently not affected by surface opsonization of amastigotes, was not mediated by interleukin-10 or transforming growth factor beta, and was observed at different temperatures. Consistent with the different fates of promastigotes and amastigotes in IFN-gamma-stimulated Mphis, L. amazonensis-specific Th1 transfer helped recipient mice control L. amazonensis infection established by promastigotes but not L. amazonensis infection established by amastigotes. On the other hand, IFN-gamma could stimulate Mphis to limit amastigote replication when it was coupled with lipopolysaccharides but not when it was coupled with tumor necrosis factor alpha. Thus, IFN-gamma may play a bidirectional role at the level of parasite-Mphi interactions; when it is optimally coupled with other factors, it has a protective effect against infection, and in the absence of such synergy it promotes amastigote growth. These results reveal a quite unexpected aspect of the L. amazonensis parasite and have important implications for understanding the pathogenesis of the disease and for developing vaccines and immunotherapies.

The role of CD4(+)CD25(+) regulatory T cells in Leishmania infection

There is growing evidence that regulatory T cells and, in particular, the endogenous CD4(+)CD25(+) T cells (T(reg)) are playing a fundamental role in the infectious process due to the protozoan parasite Leishmania. Endogenous CD4(+)CD25(+) T cells are a population of regulatory T cells, recently described for their capacity to control excessive or misdirected immune response. During human or murine Leishmaniasis, many features characteristic of T(reg )function, such as high levels of IL-10, transforming growth factor-beta (TGF-beta) or immunosuppression, have been extensively described. Recent reports formally involved T(reg) in the control of Leishmania major infection. Such control occurs by modulation of the effector immune response; in susceptible mouse strains, CD4(+)CD25(+) T cells suppress excessive T helper (Th)2 response, while in genetically resistant mouse strains, they control protective Th1 responses, allowing for parasite survival and maintenance of memory response. The mechanisms and consequences of such control in both susceptible or resistant mouse strains are discussed.

The immunology of susceptibility and resistance to Leishmania major in mice

Established models of T-helper-2-cell dominance in BALB/c mice infected with Leishmania major -- involving the early production of interleukin-4 by a small subset of Leishmania-specific CD4+ T cells -- have been refined by accumulating evidence that this response is not sufficient and, under some circumstances, not required to promote susceptibility. In addition, more recent studies in L. major-resistant mice have revealed complexities in the mechanisms responsible for acquired immunity, which necessitate the redesign of vaccines against Leishmania and other pathogens that require sustained cell-mediated immune responses.

CD8+ T cells are required for primary immunity in C57BL/6 mice following low-dose, intradermal challenge with Leishmania major

Standard murine models of cutaneous leishmaniasis, involving s.c. inoculation of large numbers of Leishmania major promastigotes, have not supported an essential role for CD8(+) T cells in the control of primary infection. Recently, a L. major model combining two main features of natural transmission, low parasite dose and inoculation into a dermal site, has been established in resistant C57BL/6 mice. In the present studies, C57BL/6 mice with CD8(+) T cell deficiencies, including CD8(-/-) and CD8-depleted mice, failed to control the growth of L. major following inoculation of 100 metacyclic promastigotes into the ear dermis. The resulting dermal pathology was minor and delayed. Lesion formation in wild-type mice was coincident with the killing of parasites in the inoculation site. Both events were associated with the accumulation of CD8(+) T lymphocytes in the skin and with the capacity of CD8(+) T cells recovered from draining lymph nodes or infected dermis to release IFN-gamma following coculture with infected dendritic cells. Reconstitution of resistance to L. major in RAG(-/-) mice using T cells from naive donors was optimal when both CD4(+) and CD8(+) T cells were transferred. Primed CD8(+) T lymphocytes obtained from C57BL/6 mice during the acute stage of infection were able to mediate both pathology and immunity when transferred alone. The low dose, intradermal challenge model reveals that CD8(+) T cells play an essential role in both pathogenesis of and immunity to primary infection with L. major in the skin.

Leishmania amazonensis-dendritic cell interactions in vitro and the priming of parasite-specific CD4(+) T cells in vivo

The progressive disease following Leishmania amazonensis infection in mice requires functional CD4(+) T cells, which are primed to a disease-promoting phenotype during the infection. To understand how these pathogenic T cells are generated and the role of dendritic cells (DCs) in this process, we use DCs of susceptible BALB/c and resistant C3H/HeJ mice to examine parasite-DC interactions in vitro as well as the effector phenotype of T cells primed by parasite-exposed DCs in vivo. Our results demonstrate that amastigotes and metacyclics efficiently enter and activate DCs of both genetic backgrounds. Infection with amastigotes fails to induce CD40-dependent IL-12 production, but rather potentiates IL-4 production in BALB/c DCs. Upon transfer into syngeneic recipients, amastigote-exposed BALB/c DCs prime parasite-specific Th cells to produce significantly higher levels of IL-4 and IL-10 than their C3H/HeJ counterparts. Transfer studies with IL-4(-/-) DCs indicate that this enhanced Th2 priming seen in BALB/c mice is partially due to the IL-4 production by amastigote-carrying DCs. These results suggest that L. amazonensis amastigotes may condition DCs of a susceptible host to a state that favors activation of pathogenic CD4(+) T cells, and thereby provide a new perspective on the pathogenesis of cutaneous leishmaniasis and protozoan parasite-host interactions in general.

The potency and durability of DNA- and protein-based vaccines against Leishmania major evaluated using low-dose, intradermal challenge

DNA- and protein- based vaccines against cutaneous leishmaniasis due to Leishmania major were evaluated using a challenge model that more closely reproduces the pathology and immunity associated with sand fly-transmitted infection. C57BL/6 mice were vaccinated s.c. with a mixture of plasmid DNAs encoding the Leishmania Ags LACK, LmSTI1, and TSA (AgDNA), or with autoclaved L. major promastigotes (ALM) plus rIL-12, and the mice were challenged by inoculation of 100 metacyclic promastigotes in the ear dermis. When challenged at 2 wk postvaccination, mice receiving AgDNA or ALM/rIL-12 were completely protected against the development of dermal lesions, and both groups had a 100-fold reduction in peak dermal parasite loads compared with controls. When challenged at 12 wk, mice vaccinated with ALM/rIL-12 maintained partial protection against dermal lesions and their parasite loads were no longer significantly reduced, whereas the mice vaccinated with AgDNA remained completely protected and had a 1000-fold reduction in dermal parasite loads. Mice vaccinated with AgDNA also harbored few, if any, parasites in the skin during the chronic phase, and their ability to transmit L. major to vector sand flies was completely abrogated. The durable protection in mice vaccinated with AgDNA was associated with the recruitment of both CD8(+) and CD4(+) T cells to the site of intradermal challenge and with IFN-gamma production by CD8(+) T cells in lymph nodes draining the challenge site. These data suggest that under conditions of natural challenge, DNA vaccination has the capacity to confer complete protection against cutaneous leishmaniasis and to prevent the establishment of infection reservoirs.

Evolution of lesion formation, parasitic load, immune response, and reservoir potential in C57BL/6 mice following high- and low-dose challenge with Leishmania major

A model of cutaneous leishmaniasis using 10(2) Leishmania major metacyclic promastigotes inoculated into the footpads of genetically resistant C57BL/6 mice was studied in order to more accurately reproduce the evolution of lesion formation and the kinetics of parasite growth and immune response as they might occur in naturally exposed reservoirs and in human hosts. In contrast to the more conventional experimental model employing 10(6) metacyclic promastigotes, in which the rapid development of footpad lesions was associated with an increasing number of amastigotes in the site, the low-dose model revealed a remarkably "silent" phase of parasite growth, lasting approximately 6 weeks, during which peak parasitic loads were established in the absence of any overt pathology. Footpad swelling was observed after 6 weeks, coincident with the onset of parasite clearance and with production of high levels of interleukin-12 (IL-12) and gamma interferon (IFN-gamma) in draining lymph nodes. Low-dose challenge of IL-12- and IFN-gamma-depleted or -deficient mice provided strong evidence that the induction or expression of cellular immunity is essentially absent during the first 6 to 8 weeks of intracellular growth, since the concentration of amastigotes in the site was not enhanced compared to that for wild-type animals during this time. By monitoring the ability of infected mice to transmit parasites to vector sand flies, it was observed that following low-dose challenge, footpads without apparent lesions provided an efficient source of parasites for exposed flies and that the low-dose challenge actually extended the duration of parasite transmissibility during the course of infection.

A natural model of Leishmania major infection reveals a prolonged "silent" phase of parasite amplification in the skin before the onset of lesion formation and immunity

A model of Leishmania major infection in C57BL/6 mice has been established that combines two main features of natural transmission: low dose (100 metacyclic promastigotes) and inoculation into a dermal site (the ear dermis). The evolution of the dermal lesion could be dissociated into two distinct phases. The initial "silent" phase, lasting 4-5 wk, favored establishment of the peak load of parasites in the dermis in the absence of lesion formation or any overt histopathologic changes in the site. The second phase corresponds to the development of a lesion associated with an acute infiltration of neutrophils, macrophages, and eosinophils into the dermis and was coincident with the killing of parasites in the site. The onset of immunity/pathology was correlated with the appearance of cells staining for IL-12p40 and IFN-gamma in the epidermal compartment, and an expansion of T cells capable of producing IFN-gamma in the draining lymph node. Parasite growth was not enhanced over the first 4.5 wk in anti-CD4-treated mice, SCID mice, or C57BL/6 mice deficient in IL-12p40, IFN-gamma, CD40 ligand, or inducible NO synthase. These mice all failed to ultimately control infection in the site, but in some cases (anti-CD4 treated, IL-12p40-/-, CD40 ligand-/-, and SCID) high dermal parasite loads were associated with little or no pathology. These results extend to a natural infection model a role for Th1 cells in both acquired resistance and lesion formation, and document the remarkable avoidance of this response during a prolonged phase of parasite amplification in the skin.

Th1/Th2 balance in infection

Cytokines produced by T helper (Th) cells are of critical importance for the outcome of many infectious diseases. Producing the "right" set of cytokines in response to an infectious agent can be a matter of life or death. The Th1/Th2 dichotomy, although an oversimplification has proven useful in the analysis of immune responses to infections. In some infectious diseases, most notably leishmaniasis or infections with gastrointestinal helminths, one Th subset is indispensable for clearing the infection, whereas the opposite Th subset is detrimental. More frequently, both Th1 and Th2 responses are required at different time points to effectively eradicate an infectious agent. The granuloma responses to either Mycobacterium tuberculosis or Schistosoma mansoni provide illustrative examples and are discussed in this review. There is accumulating evidence for frequent coexpression of Th1 and Th2 cytokines during the in vivo immune response to infections. The mechanisms by which infectious agents modulate Th1/Th2 phenotype development are summarized here. Finally, we review here the current evidence for cytokine imbalances induced by infections as pathogenic or protective factors in autoimmunity and allergy.

Immunological response of major histocompatibility complex class II-deficient (Abeta(o)) mice infected by the parasite Schistosoma mansoni

We have characterized the immunological behaviour of major histocompitibility complex (MHC) Class II molecule-deficient (Abeta(o)) mice after infection by Schistosoma mansoni. In Abeta(o) mice, morbidity developed dramatically 7 weeks after infection leading to death, despite the absence of an increase in parasite burden or of eggs trapped in the liver. Histological examination of the liver revealed the absence of a classical granulomatous reaction. Antibodies were produced only against schistosomulum antigens. Specific antibodies against adult worm (SWAP) or egg antigen (SEA) were not detected. Cytokine production (IFN-gamma and IL-4) was absent after in vitro restimulation of splenic cells from infected Abeta(o) mice with parasite antigens. Adoptive transfer of primed splenic cells (total, purified CD4+ or CD8+ T cells) failed to improve survival or to induce a granulomatous reaction in infected Abeta(o) mice. Survival, cellular and humoral responses in CD8+ T-cell-depleted Abeta(o) mice or MHC(o) mice (lacking MHC class I and II molecules) were similar to nondepleted Abeta(o) mice, suggesting that anti-schistosomula antibody production was thymo-independent. Our results demonstrate a high degree of susceptibility of Abeta(o) mice to infection and corroborate the importance of CD4+ T cells in the initiation of the granulomatous response. However, our results do not show evidence for the involvement of CD8+ T cells in response to S. mansoni infection.