A repetitive peptide of Leishmania can activate T helper type 2 cells and enhance disease progression

Efficient Gene Knockout and Knockdown Systems in Neospora caninum Enable Rapid Discovery and Functional Assessment of Novel Proteins

The development of molecular genetics has greatly enhanced the study of the biology and pathology associated with parasites of the phylum Apicomplexa. While the molecular tools are highly developed for the apicomplexan Toxoplasma gondii, the closely related parasite Neospora caninum lacks efficient tools for genetic manipulation. To enable efficient homologous recombination in N. caninum, we targeted the Ku heterodimer DNA repair mechanism in the genomic reference strain, Nc-Liverpool (NcLiv), and show that deletion of Ku80 results in a destabilization and loss of its partner Ku70. Disruption of Ku80 generated parasites in which genes are efficiently epitope tagged and only short homology regions are required for gene knockouts. We used this improved strain to target novel nonessential genes encoding dense granule proteins that are unique to N. caninum or conserved in T. gondii. To expand the utility of this strain for essential genes, we developed the auxin-inducible degron system for N. caninum using parasite-specific promoters. As a proof of concept, we knocked down a novel nuclear factor in both N. caninum and T. gondii and showed that it is essential for survival of both parasites. Together, these efficient knockout and knockdown technologies will enable the field to unravel specific gene functions in N. caninum, which is likely to aid in the identification of targets responsible for the phenotypic differences observed between these two closely related apicomplexan parasites.

IMPORTANCENeospora caninum is a parasite with veterinary relevance, inducing severe disease in dogs and reproductive disorders in ruminants, especially cattle, leading to major losses. The close phylogenetic relationship to Toxoplasma gondii and the lack of pathogenicity in humans drives an interest of the scientific community toward using N. caninum as a model to study the pathogenicity of T. gondii. To enable this comparison, it is important to develop efficient molecular tools for N. caninum, to gain accuracy and save time in genetic manipulation protocols. Here, we have developed base strains and protocols using the genomic reference strain of N. caninum to enable efficient knockout and knockdown assays in this model. We demonstrate that these tools are effective in targeting known and previously unexplored genes. Thus, these tools will greatly improve the study of this protozoan, as well as enhance its ability to serve as a model to understand other apicomplexan parasites.

LmjF.36.3850, a novel hypothetical Leishmania major protein, contributes to the infection

Leishmania is a protozoan parasite that resides in mammalian macrophages and inflicts the disease known as leishmaniasis. Although prevalent in 88 countries, an anti-leishmanial vaccine remains elusive. While comparing the virulent and avirulent L. major transcriptomes by microarray, PCR and functional analyses for identifying a novel virulence-associated gene, we identified LmjF.36.3850, a hypothetical protein significantly less expressed in the avirulent parasite and without any known function. Motif search revealed that LmjF.36.3850 protein shared phosphorylation sites and other structural features with sucrose non-fermenting protein (Snf7) that shuttles virulence factors. LmjF.36.3850 was predicted to bind diacylglycerol (DAG) with energy value similar to PKCα and PKCβ, to which DAG is a cofactor. Indeed, 1-oleoyl-2-acetyl-sn-glycerol (OAG), a DAG analogue, enhanced the phosphorylation of PKCα and PKCβI. We cloned LmjF.36.3850 gene in a mammalian expression vector and primed susceptible BALB/c mice followed by challenge infection. We observed a higher parasite load, comparable antibody response and higher anti-inflammatory cytokines such as IL-4 and IL-10, while expression of major anti-leishmanial cytokine, IFN-γ, remained unchanged in LmjF.36.3850-vaccinated mice. CSA restimulated LN cells from vaccinated mice after challenge infection secreted comparable IL-4 and IL-10 but reduced IFN-γ, as compared to controls. These observations suggest a skewed Th2 response, diminished IFN-γ secreting Th1-T_EM cells and increased central and effector memory subtype of Th2, Th17 and Treg cells in the vaccinated mice. These data indicate that LmjF.36.3850 is a plausible virulence factor that enhances disease-promoting response, possibly by interfering with PKC activation and by eliciting disease-promoting T cells.

Immunological consequences of stress-related proteins – cytosolic tryparedoxin peroxidase and chaperonin TCP20 – identified in splenic amastigotes ofLeishmania donovanias Th1 stimulatory, in experimental visceral leishmaniasis

In earlier studies, proteomic characterization of splenic amastigote fractions from clinical isolates ofLeishmania donovani, exhibiting significant cellular responses in curedLeishmaniasubjects, led to the identification of cytosolic tryparedoxin peroxidase (LdcTryP) and chaperonin-TCP20 (LdTCP20) as Th1-stimulatory proteins. Both the proteins, particularly LdTCP20 for the first time, were successfully cloned, overexpressed, purified and were found to be localized in the cytosol of purified splenic amastigotes. When evaluated against lymphocytes of curedLeishmania-infected hamsters, the purified recombinant proteins (rLdcTryP and rLdTCP20) induced their proliferations as well as nitric oxide production. Similarly, these proteins also generated Th1-type cytokines (IFN-γ/IL-12) from stimulated PBMCs of cured/endemicLeishmaniapatients. Further, vaccination with rLdcTryP elicited noticeable delayed-type hypersensitivity response and offered considerably good prophylactic efficacy (~78% inhibition) againstL. donovanichallenge in hamsters, which was well supported by the increased mRNA expression of Th1 and Th2 cytokines. However, animals vaccinated with rLdTCP20 exhibited comparatively lesser prophylactic efficacy (~55%) with inferior immunological response. The results indicate the potentiality of rLdcTryP protein, between the two, as a suitable anti-leishmanial vaccine. Since, rLdTCP20 is also an important target, for optimization, further attempts towards determination of immunodominant regions for designing fusion peptides may be taken up.

Characterization of glycolytic enzymes--rAldolase and rEnolase of Leishmania donovani, identified as Th1 stimulatory proteins, for their immunogenicity and immunoprophylactic efficacies against experimental visceral leishmaniasis

Th1 immune responses play an important role in controlling Visceral Leishmaniasis (VL) hence, Leishmania proteins stimulating T-cell responses in host, are thought to be good vaccine targets. Search of such antigens eliciting cellular responses in Peripheral blood mononuclear cells (PBMCs) from cured/exposed/Leishmania patients and hamsters led to the identification of two enzymes of glycolytic pathway in the soluble lysate of a clinical isolate of Leishmania donovani--Enolase (LdEno) and aldolase (LdAld) as potential Th1 stimulatory proteins. The present study deals with the molecular and immunological characterizations of LdEno and LdAld. The successfully cloned and purified recombinant proteins displayed strong ability to proliferate lymphocytes of cured hamsters' along with significant nitric-oxide production and generation of Th1-type cytokines (IFN-γ and IL-12) from stimulated PBMCs of cured/endemic VL patients. Assessment of their prophylactic potentials revealed ∼ 90% decrease in parasitic burden in rLdEno vaccinated hamsters against Leishmania challenge, strongly supported by an increase in mRNA expression levels of iNOS, IFN-γ, TNF-α and IL-12 transcripts along with extreme down-regulation of TGF-β, IL-4 and IL-10. However, animals vaccinated with rLdAld showed comparatively lesser prophylactic efficacy (∼ 65%) with inferior immunological response. Further, with a possible implication in vaccine design against VL, identification of potential T-cell epitopes of both the proteins was done using computational approach. Additionally, in-silico 3-D modelling of the proteins was done in order to explore the possibility of exploiting them as potential drug targets. The comparative molecular and immunological characterizations strongly suggest rLdEno as potential vaccine candidate against VL and supports the notion of its being effective T-cell stimulatory protein.

Upregulated expression of B-cell antigen family tandem repeat proteins by Leishmania amastigotes

Proteins with tandem repeat (TR) domains have been found in various protozoan parasites, and they are often targets of B-cell responses. Through systematic analyses of whole proteomes, we recently demonstrated that two trypanosomatid parasites, Leishmania infantum and Trypanosoma cruzi, are rich in antigenic proteins with large TR domains. However, the reason that these proteins are antigenic was unclear. Here, by performing molecular, immunological, and bioinformatic characterizations of Leishmania TR proteins, we found two possible factors affecting the antigenicity of these proteins; one factor is their fundamental composition as TR proteins, and the other is regulation of their expression by parasites. Enzyme-linked immunosorbent assays (ELISAs) using recombinant proteins revealed that the copy number of the repeat affects the affinity of binding between antigens and antibodies, as expected based on thermodynamic binding kinetics. Other than containing TR domains, the TR proteins do not share characteristics, such as sequence similarity or biased cellular location predicted by the presence of a signal sequence(s) and/or a transmembrane domain(s). However, the TR proteome contained a higher percentage of proteins upregulated in amastigotes than the whole proteome, and upregulated expression of a TR protein seemed to affect its antigenicity. These results indicate that Leishmania parasites actively utilize the TR protein family for parasitism in mammalian hosts.

Induction and regulation of CD4+ T cell subsets

It is now generally accepted that CD4+ T cells can be divided into at least two distinct subsets: Th1 and Th2. Th1 cells characteristically secrete interleukin 1 (IL-2) and gamma-interferon (IFN-gamma) whereas Th2 cells produce mainly IL-4, IL-5 and IL-10. Studies from many laboratories have demonstrated that the balance between these two subsets of T cells frequently determines the outcome of infectious and autoimmune diseases. Several factors influence the preferential induction and regulation of Th1 or Th2 cells in vitro and in vivo. Mice were infected with the protozoan parasite Leishmania to show that the gene encoding a major surface glycoprotein of the parasite, delivered orally in a plasmid carried by an auxotrophic Salmonella typhimurium vaccine strain (BRD509), preferentially induced Th1 cells and protective immunity against a challenge infection. The protective effect of the vaccine was augmented by administration of BRD509 carrying the genes encoding IL-2, IFN-gamma or tumour necrosis factor alpha. Cloned mouse Th1 cells specific for malarial antigens have been used to show that nitric oxide (NO) can inhibit the production of IFN-gamma by Th1 cells. Oral delivery of antigen and selective cytokines may preferentially induce CD4+ T cell subsets. Modulation of NO synthesis may further influence this induction and sustain such selective responses leading to effective therapy.

Peptide dendrimers: applications and synthesis

Peptide dendrimers are radial or wedge-like branched macromolecules consisting of a peptidyl branching core and/or covalently attached surface functional units. The multimeric nature of these constructs, the unambiguous composition and ease of production make this type of dendrimer well suited to various biotechnological and biochemical applications. Applications include use as biomedical diagnostic reagents, protein mimetics, anticancer and antiviral agents, vaccines and drug and gene delivery vehicles. This review focuses on the different types of peptide dendrimers currently in use and the synthetic methods commonly employed to generate peptide dendrimers ranging from stepwise solid-phase synthesis to chemoselective and orthogonal ligation.

The multidrug resistance protein 1: a functionally important activation marker for murine Th1 cells

Previously, we described the expression of an energy-dependent pump in resting murine Th2 (but not resting Th1) cells which extruded the fluorescent dye Fluo-3. After stimulation with Ag and APCs, Th1 cells also expressed this pump. Furthermore, expression of the murine multidrug resistance protein 1 (mrp1) correlated with the presence of the pump. In this study, we report that Fluo-3 is indeed transported by murine mrp1 or its human ortholog MRP1, as revealed by transfection of HEK 293 cells with mrp1 or MRP1 cDNA. Like antigenic activation, IL-2 dose-dependently enhanced the Fluo-3-extruding activity in murine Th1 cells. Although TNF-alpha and IL-12 by themselves only weakly enhanced Fluo-3 extrusion, each of them did so in strong synergism with IL-2. An Ab directed against mrp1 was used to quantify the expression of mrp1 protein in T cells at the single-cell level. Like the Fluo-3 pump, mrp1 protein expression was enhanced by IL-2. Immunohistochemical studies using confocal laser microscopy indicated that mrp1 is localized mainly at the plasma membrane. In addition, protein expression of mrp1 was induced in Vbeta8+CD4+ T cells 12 h after in vivo application of Staphylococcal enterotoxin B. Finally, mrp1 was functionally relevant during the activation process of Th1 cells, because T cell activation could be suppressed by exposure of cells to the mrp1 inhibitor MK571. Thus, we present mrp1 as a novel, functionally important activation marker for Th1 cells and short-term in vivo activated CD4+ T cells, whereas its expression seems to be constitutive in Th2 cells.

Leishmania: fine mapping of the Leishmanolysin molecule's conserved core domains involved in binding and internalization

The Leishmanolysin molecule's role in the uptake of Leishmania parasites by the human U937 pro-myelocytic cell line was studied, using synthetic peptides representing the complete Leishmania (Viannia) guyanensis Leishmanolysin protein amino acid sequence. The particular peptides present in two protein's core domains efficiently impaired the internalization of promastigotes from four different Leishmania species and modified the kinetics of the binding of heterologous recombinant Leishmanolysin protein. The functional domains which exhibited this property represent a highly conserved portion of the sequence among different Leishmania species. The peptides' inhibitory activity correlated with their ability to bind molecules present on the surface of the human cell line. One of the two functional core domains identified involves the previously described adhesive sequence (SRYD) and the putative zinc-binding motif (HExxH). The second functional core domain includes a third histidine residue coordinated with zinc which determines the molecule's structural features. These findings indicate that the molecular interactions between Leishmanolysin's conserved domains and the macrophage surface molecules efficiently contribute to the parasite's internalization. Induction of neutralizing immune responses, which impair the early parasite-host interaction described here, may be an important alternative in designing synthetic subunit human leishmaniasis vaccines.

Influence of HLA-DR on the phenotype of CD4+ T lymphocytes specific for an epitope of the 16-kDa alpha-crystallin antigen of Mycobacterium tuberculosis

T helper phenotype may be influenced by cytokine milieu, the differential expression of co-stimulatory molecules, antigen dose, and by differences in affinity at the TCR-peptide-MHC interface. We investigated the latter hypothesis by examining the response of six HLA-DR-restricted CD4+ T cell lines specific for the immunodominant and permissively recognized p91-110 epitope of the 16-kDa alpha-crystallin protein of Mycobacterium tuberculosis. Each line was generated from a sensitized HLA-DR-heterozygous donor and all proliferated when peptide was presented by autologous irradiated peripheral blood mononuclear cells. However, when HLA-DR-matched homozygous Epstein-Barr-virus-transformed B cell lines (L-BCL) were used as peptide-presenting cells there was heterogeneity in the response. The most pronounced proliferative response, and the highest IFN-gamma secretion and cytolytic activity was stimulated by L-BCL expressing molecules (DRB1*0101, *1501 and *0401) with high affinity (IC50 < 10 microM) for the 16p91-110 peptide. By comparison, IL-4 secretion or a lower proliferative response could occur when peptide was presented by alleles of high, or of intermediate (10 microM < IC50 < 100 microM), affinity. These data support the hypothesis that the host MHC can influence CD4+ phenotype and have implications for subunit vaccination against tuberculosis.

Differential regulation of Th1 and Th2 cells by p91-110 and p21-40 peptides of the 16-kD alpha-crystallin antigen of Mycobacterium tuberculosis

Permissively recognized peptides which can activate lymphocytes from subjects with a variety of class II HLA types are interesting diagnostic and vaccine candidates. In this study we generated T helper clones reactive to the permissively recognized p21-40 and p91-110 peptides of the 16-kD heat shock protein of Mycobacterium tuberculosis. All the clones specific for p91-110 secreted interferon-gamma (IFN-gamma) and were of the Th1 phenotype. By contrast, the p21-40 peptide favoured the generation of IL-4-producing clones. Antibody blockade established that the peptide-specific Th clones could either be DR-, DP- or DQ-restricted. Thus, two permissively recognized sequences p21-40 and p91-110 from the same mycobacterial antigen can drive the differentiation of functionally distinct T helper subsets. Attempts to immunize against tuberculosis should bear in mind epitope specificity if a favourable Th subtype response is to be generated.

Influence of both TCR repertoire and severity of the atopic status on the cytokine secretion profile of Parietaria officinalis-specific T cells

Peripheral blood mononuclear cells (PBMC) from both nonatopic and Parietaria officinalis-sensitive donors proliferated in response to the allergen Par o 1 and developed into Par o 1-specific T cell lines and clones, which also showed reactivity for Par o 1-derived peptides. Virtually all Par o 1-specific T cell lines and large numbers of Par o 1-specific T cell clones proliferated in response to two Par o 1 nonapeptides (p92 and p96), which probably contain immunodominant epitopes of the Par o 1 allergen. Both p92- and p96-specific T cell clones showed the ability to produce IFN-gamma, but p92-specific T cell clones produced significantly lower amounts of IL-4 and IL-5 than p96-specific T cell clones, indicating that distinct epitopes, able to elicit functionally different T helper cell responses, may coexist in Par o 1. However, p92-specific T cell clones derived from atopic subjects with high IgE serum levels (high IgE producers) secreted significantly higher amounts of IL-4 and IL-5 than corresponding T cell clones generated from nonatopic subjects or patients with low IgE serum levels (low IgE producers), whereas p96-specific T cell clones secreted high IL-4 and IL-5 concentrations irrespective of whether they derived from high or low IgE producers. The addition of IL-4 and anti-IL-12 mAb to bulk culture significantly up-regulated the development of p92-specific T cells into IL-4-producing cells, whereas the addition of IL-12 and anti-IL-4 mAb shifted the differentiation of p96-specific T cells towards IFN-gamma-producing cells. Taken together, these results suggest that the cytokine profile of allergen-specific T cells is influenced by both the T cell receptor repertoire and the severity of atopic status and can be modulated, at least in vitro, by stimulation with the specific peptide in the presence, or after removal, of appropriate cytokines.

Th1/Th2 cytokine profiles in the nasopharyngeal lymphoid tissues of children with recurrent otitis media

The cytokine profile in adenoidal lymphoid tissue was studied in 22 patients. Lymphocytes from adenoid tissues and peripheral blood were submitted for cytokine assays using an enzyme-linked immunosorbent assay kit for interferon-gamma, interleukin (IL)-2, IL-4, IL-5, IL-6, and IL-10. Adenoidal lymphocytes appear to produce significantly less Th1 cytokines (IL-2, interferon gamma) compared to the patient's peripheral blood lymphocytes, whereas IL-4 and IL-5 (Th2 cytokines) appear to be synthesized to the same extent as, if not slightly more than, in the homologous peripheral blood lymphocytes. Because the relationship between Th1 and Th2 cytokines is extremely important in modulating the immune response, it is advisable to determine the role of the cytokine profiles of T-lymphocytes in the nasopharynx and its relationship to the development of inflammation of the eustachian tube and middle ear.

Development of Th 1- or Th 2-dominated immune responses: what about the polarizing signals?

Type 1 helper T cells and type 2 helper T cells represent two extremely polarized forms of the effector specific immune response, based on a distinctive profile of cytokine production. Type 1- and type 2 helper T cell-dominated immune responses play a different role in both protection and immunopathology. The differentiation of effector phenotypes depends on a complex matrix of interconnecting factors resulting from the evolutionary interplay between vertebrates and microorganisms. These include the physical form of the antigen, as well as the density and affinity of the peptide ligand, the cytokines produced by "natural" immunity cells at the time of antigen presentation, costimulatory signals provided by antigen-presenting cells, and hormones released into the microenvironment. The elucidation of genetic and environmental factors that regulate type 1 or type 2 helper T cell development in response to different antigenic stimulation is the basis for new immunotherapeutic strategies in allergic and autoimmune disorders, as well as for the improvement of vaccines.

Induction of differential T-helper-cell responses in mice infected with variants of the parasitic nematode Trichuris muris

The resistance or susceptibility of mice to infection with the intestinal nematode parasite Trichuris muris is closely correlated with polarization of T helper (Th) cell responses to the type 2 (Th2) or type 1 (Th1) subset. Comparison of infections with three isolates of T. muris (E/K, E/N, and S) in three inbred strains of mice (CBA, C57BL/10, and B10.BR) has shown that host Th response phenotype can be parasite determined. Although the mouse strains used show genetically determined variation in ability to respond to T. muris (CBA > C57BL/10 > B10.BR), the speed of worm expulsion in a given strain depended upon the isolate used for infection (E/K > E/N > S). The two isolates that induced the most effective resistance (E/K and E/N) elicited parasite-specific host antibody responses that were dominated by immunoglobulin G1 (IgG1), and antigen-stimulated T cells from infected mice released interleukin-5 in vitro. With the isolate that induced the least host resistance (S), the dominant antibody response was IgG2a, and T cells released gamma interferon in vitro. These data show clearly that parasite variant-specific factors play a major role in Th subset polarization during infection.

The induction and functions of murine T-helper cell subsets

Through the release of distinct sets of cytokines, Th1 and Th2 cells exert characteristic and often mutually exclusive or antagonistic immune effector functions. In the present report, we document and discuss several findings on the induction mechanisms of these cellular subtypes and present recent findings on their respective functions in vivo. The preferential induction of Th1 or Th2 cytokine patterns in mature CD4+ T cells is generally attributed to the action of cytokines. In addition, there is evidence that prolonged T-cell receptor occupancy may induce the development of the Th2 phenotype. Prolonged occupancy of the T-cell receptor provides enough autocrine interleukin-4 to permit induction of the Th2 phenotype. Both Th1 and Th2 cells may be derived from a single mature CD4+ T cell, providing strong evidence for post-thymic modulation of the T-cell cytokine profile and rendering the possibility of predetermined cytokine patterns in T cells unlikely. CD4+ Th1 cells mediate the tumor necrosis factor- and interferon-gamma-dependent classic delayed type hypersensitivity reaction. We found that Th2 cells were also capable of mediating local inflammatory reactions that depended on their prototypic lymphokine interleukin-4, and, in high tumor necrosis factor-producing mouse strains, upon tumor necrosis factor-alpha. Both Th-cell subsets induced cellular infiltrates that were not distinguishable on histologic grounds. In contrast to the widely accepted belief that only Th1 cells can mediate delayed hypersensitivity reactions, our results demonstrate that T cells with either lymphokine profile can cause tissue inflammation with leukocytic infiltrates.

Immunoglobulin G subclass-specific antileishmanial antibody responses in Indian kala-azar and post-kala-azar dermal leishmaniasis

Antileishmanial antibody responses in the sera of Indian kala-azar (KA) and post-KA dermal leishmaniasis (PKADL) patients were analyzed by enzyme-linked immunosorbent assay (ELISA) and immunoblot experiments using immunoglobulin G (IgG) class- and subclass-specific reagents. All sera showed antileishmanial reactivities in IgG ELISA which followed the order IgG1 > IgG2 > IgG3, with very little IgG4. Immunoblot analysis with IgG class-specific reagents revealed variable patterns of reactivity by KA and PKADL sera, although certain common bands around the 60- to 63-kDa regions were discernible. Sera from antimony-unresponsive KA cases, on the other hand, strongly recognized two bands at around 20 to 22 kDa, in addition to other bands in the high-molecular-mass region. Further analysis showed that the 28-kDa band was preferentially recognized by the IgG2 isotype, while 20- to 22-kDa and 60- to 63-kDa bands were recognized by the IgG1 isotype. Antibodies belonging to the IgG3 isotype reacted to antigens primarily in the region of 14 to 34 kDa and persisted in patients even several months after cure. Immunoblot studies also revealed the presence of a nonspecific band which arose as a result of binding between a 66-kDa leishmanial antigen and streptavidin. Finally, the results presented in this study suggest that certain leishmanial antigens preferentially stimulate the synthesis of a particular IgG subclass(es), depending on the nature of such antigens or their epitopes.

A recombinant Leishmania chagasi antigen that stimulates cellular immune responses in infected mice

Cellular immune mechanisms resulting in gamma interferon production are critical for protection against visceral leishmaniasis. Antigens stimulating T-cell responses are likely present in the intracellular amastigote form of the parasite, since this is the form found in a mammalian host. To identify T-cell antigens of Leishmania chagasi, the parasite causing South American visceral leishmaniasis, we used a double antibody-T-cell technique to screen an amastigote cDNA library. One cDNA selected (Lcr1) encodes an antigen that stimulated proliferation of splenic T lymphocytes from infected mice that were either resistant (C3H.HeJ) or susceptible (BALB/c) to L. chagasi infection. The Lcr1 cDNA contains four highly divergent 201-bp repeats homologous to the 204-bp repeat of a Trypanosoma cruzi flagellar antigen gene. Results are consistent with a single copy of the Lcr1 gene producing an mRNA of > 10 kb and a protein of > 200 kDa. Recombinant Lcr1, cloned adjacent to polyhistidine and purified on a nickel affinity column, stimulated gamma interferon but not interleukin-4 (IL-4), IL-5, or IL-10 secretion by T-cell-enriched splenocytes from either susceptible or resistant mice during L. chagasi infection. Immunization with Lcr1 partially protected BALB/c mice against challenge with L. chagasi, indicating the utility of the double screening approach in selecting relevant T-cell antigens.

Preferential recognition of hepatitis B nucleocapsid antigens by Th1 or Th2 cells is epitope and major histocompatibility complex dependent

Regulatory T-helper (Th) cells have been categorized into two functional subsets, Th1 and Th2 cells, which produce distinct lymphokines. In general, Th1 cells mediate cellular immune responses and Th2 cells mediate humoral immunity. Recent serological studies suggest that the Th1-Th2 balance may be relevant in acute and chronic hepatitis B virus (HBV) infections. The purpose of this study was to determine the potential of the nucleocapsid antigens (Ags) (hepatitis B core and e Ags [HBc/eAg]) of HBV to preferentially elicit either a Th1 or a Th2 dominant response. For this purpose, H-2 congenic B10.S and B10 mice were immunized with HBc/eAg, and Ag-specific T-cell proliferative responses, T-cell helper function, and T-cell cytokine production were analyzed. The results indicated that B10.S mice preferentially develop a Th1-like response whereas B10 mice preferentially develop a Th2-like response after immunization with HBc/eAg. Furthermore, the preferential Th1 and Th2 response patterns were reproduced when 12-residue peptides representing the dominant HBc/eAg-specific T-cell sites for B10.S (peptide 120-131) and B10 (peptide 129-140) mice were used as immunogens. Therefore, the combination of the T-cell site recognized and the major histocompatibility complex restricting element can in large part determine the Th phenotype of the HBc/eAg-specific T-cell response. Other factors that influenced Th phenotype were the presence of exogenous cytokines, Ag structure, and tissue distribution.

Cytokines and T-cell response in malaria

The intracellular protozoan Plasmodium sp induces a complex immune response which sometimes implies serious pathological effects for the host. According to in vitro studies and epidemiological surveys, several effector mechanisms are displayed against plasmodial blood stages and a large interaction between humoral and cell-mediated immunity is presumed to occur among protected individuals. The key role of T cells in the antiplasmodial immune response is now well established, but all the regulatory heterogenous mechanisms are not yet fully known. An increasing body of data shows a dual role during malaria attack for some cytokines released by monocytes and macrophages (TNF, IL-1, IL-6) or by T cells (IFN-gamma, lymphotoxin (LT), IL-4). The importance of some plasmodial proteins in the cytokine-induced pathology and the stimulation of a preferential TH1 or TH2 mediated immune response to achieve protective immunity against Plasmodium sp are discussed.

Cytokine-mediated regulation of chronic intestinal helminth infection

Most inbred strains of mouse infected with the intestinal nematode Trichuris muris are resistant to infection expelling the parasite before adult worms establish. However, a few susceptible strains exist that are incapable of worm expulsion and harbor chronic infections of mature adult worms. Analyses of in vitro cytokine production by cells from the draining lymph node (mesenteric lymph node) have indicated that expulsion phenotype is tightly correlated with the selective expansion of helper T cells (Th) of the Th1 or Th2 cell subset within the mesenteric lymph node, resulting in susceptibility and resistance to T. muris, respectively. We have now confirmed and extended our in vitro observations in a series of experiments involving the in vivo manipulation of host cytokine levels. Depletion of interferon (IFN)-gamma in normally susceptible mice resulted in expulsion of the parasite, representing the first evidence for a role for IFN-gamma in the establishment of chronic helminth infection. Blocking interleukin (IL)-4 function in normally resistant animals prevented the generation of a protective immune response allowing adult stages of the parasite to develop. Conversely the administration of IL-4 to a normally susceptible host facilitated expulsion and indeed enabled established adult worms to be expelled when administered late in infection. In all cases assessment of a variety of in vivo parameters indicative of a Th1- or Th2-type response (parasite-specific immunoglobulin (Ig) G2a and the parasite-specific IgG1, total IgE levels and intestinal mastocytosis, respectively) demonstrated that the in vivo modulation of a Th1- or Th2-specific cytokine allowed the reciprocal Th cell subset to expand and become dominant with dramatic consequences for worm expulsion.

Cytokines in the differentiation of Th1/Th2 CD4+ subsets in leishmaniasis

Leishmania major infect only macrophages in the host, where they reside in endolysosomal compartments into which MHC class II molecules co-localize. Experimental infection in mice has provided a useful model for the differentiation of Th1 CD4+ effector lymphocytes that are required for the generation of IFN-gamma that activates the macrophage to a microbicidal state. Genetically susceptible BALB/c mice aberrantly activate Th2 CD4+ effector cells that are ineffective in arresting infection. Increasing evidence suggests that, rather than discrete parasite antigens or MHC molecules, cytokines mediate the critical decision in the developmental switch to either the Th1 or Th2 effector phenotype.

Encephalitogenic Th1 cells are inhibited by Th2 cells with related peptide specificity: relative roles of interleukin (IL)-4 and IL-10

Cytokines secreted by T-helper type 2 (Th2) cells inhibit the antigen-induced stimulation of type 1 (Th1) helper T cells. To study this form of regulation in an autoimmune disease model, the cytokines secreted by a Th2 clone specific for the encephalitogenic proteolipid protein (PLP) peptide 139-151 were tested for their ability to inhibit proliferation of an encephalitogenic Th1 clone specific for an epitope contained within the same peptide. Cytokines, produced by stimulation of the Th2 clone with CD3-specific monoclonal antibodies (mAbs), inhibited proliferation of the Th1 clone when stimulated by antigen and splenic antigen-presenting cells (APC). Inhibition was, however, not antigen-specific since cytokines released upon stimulation of an unrelated Th2 clone were also inhibitory. Inhibition was found to be caused by effects on either antigen presentation or co-stimulatory activity of the APC and not by direct effects on the Th1 cells. MAbs for the two major regulatory Th2 cytokines were used to identify the inhibitory component secreted by activated Th2 cells. Interleukin-10 (IL-10)-specific mAb abolished the inhibitory effect, while mAb specific for IL-4 had no effect on inhibition. The addition of recombinant IL-4 (rIL-4) and rIL-10 confirmed that inhibition of Th1 proliferation was due to secretion of IL-10 by the Th2 clone and its subsequent effects on APC. The studies described here demonstrate that PLP-specific Th2 cells which recognize peptide 139-151 inhibit encephalitogenic Th1 cells which respond to an epitope on the same peptide. This phenomenon may be important for local, antigen-specific regulation of inflammation in the central nervous system.

Cytokines in leishmaniasis: a complex network of stimulatory and inhibitory interactions

The work of immunologists, cell biologists and parasitologists in the field of leishmaniasis has not only provided important insights into the immunopathogenesis of this disease, but also yielded fundamental contributions to our understanding of basic immunological phenomena and of host-parasite interactions. The ability of recombinant interferon-gamma to induce the microbicidal activity of phagocytes and the opposite effect of inhibitory cytokines was first demonstrated with Leishmania-infected macrophages. The selective development of protective and disease-mediating CD4+ T lymphocytes as well as their differential influence on the course of the disease has been long investigated in the murine Leishmania major model and now represents one of the best examples for the in vivo induction of type 1 versus type 2 T helper lymphocytes. At the same time, this model has also been extensively used for immunization studies and cytokine therapy, which shed light on the functions of cytokines in vivo as well as on the mechanism(s) of disease resistance and susceptibility. In this review we will discuss the present picture of the cytokine network in murine L. major infections.

Heterogeneity in cytokine profiles of Babesia bovis-specific bovine CD4+ T cells clones activated in vitro

The central role of T cells in the immune response against hemoprotozoan parasites, both as helper cells for T cell-dependent antibody production and as effector cells acting on intracellular parasites through the elaboration of cytokines, has prompted an investigation of the bovine cellular immune response against Babesia bovis antigens. CD4+ T helper (Th) cell clones generated from four B. bovis-immune cattle by in vitro stimulation with a soluble or membrane-associated merozoite antigen were characterized for reactivity against various forms of antigen and against different geographical isolates of B. bovis and B. bigemina and analyzed for cytokine production following mitogenic stimulation with concanavalin A. Biological assays to measure interleukin-2 (IL-2), IL-4, gamma interferon (IFN-gamma), and tumor necrosis factor alpha or tumor necrosis factor beta and Northern (RNA) blot analysis to verify the expression of IL-2, IL-4, IFN-gamma, and tumor necrosis factor alpha revealed differential production of cytokines by the Th cell clones. The majority of clones expressed the Th0 pattern of cytokines: IFN-gamma, IL-4, and IL-2. One clone expressed the Th1 profile (IFN-gamma and IL-2 but not IL-4), whereas none of the clones expressed the Th2 profile. All of the Th cell clones examined expressed the low-molecular-weight isoform of the leukocyte common antigen associated with a memory cell phenotype (CD45RO), and all expressed the lymph node homing receptor (L-selectin). These results extend our previous finding of differential cytokine expression by B. bovis-specific Th cell clones and confirm the identity of the specific cytokines produced, showing that a Th0 response is preferentially induced in a panel of 20 CD4+ T cell clones obtained from immune cattle.

Leishmaniases of the New World: current concepts and implications for future research

Recent epidemiologic studies indicate that leishmaniasis in the Americas is far more abundant and of greater public health importance than was previously recognized. The disease in the New World is caused by a number of different parasite species that are capable of producing a wide variety of clinical manifestations. The outcome of leishmanial infection in humans is largely dependent on the immune responsiveness of the host and the virulence of the infecting parasite strain. This article reviews current concepts of the clinical forms, immunology, pathology, laboratory diagnosis, and treatment of the disease as well as aspects of its epidemiology and control. Recommendations for future research on the disease and its control are made.

Chemically modified antigen preferentially elicits induction of Th1-like cytokine synthesis patterns in vivo

Differential activation of CD4+ T cell subsets in vivo leads to the development of qualitatively different effector responses. We identify an approach that allows selective activation of strongly Th1-dominated immune responses to protein antigens. Whereas in vivo administration of ovalbumin (OVA) induces cytokine synthesis that is neither Th1 nor Th2 dominated, administration of glutaraldehyde polymerized, high relative molecular weight OVA (OA-POL) leads to 20-fold increase in the ratio of interferon gamma (IFN-gamma)/IL-4 and IFN-gamma/IL-10 synthesis observed after short-term, antigen-mediated restimulation directly ex vivo. In contrast, concurrent in vivo administration of anti-IFN-gamma mAb and OVA or OA-POL results in marked increases in IL-4 and IL-10, and decreased IFN-gamma production, reflecting a polarization of the response towards a Th2-like pattern of cytokine synthesis. These observations may be useful in clinical settings including hypersensitivity, autoimmune diseases, and vaccine development where the ability to actively select specific patterns of cytokine gene expression would be advantageous.

Patterns of cytokine production by mycobacterium-reactive human T-cell clones

To gain insight into the functional capacity of human T cells in the immune response against Mycobacterium tuberculosis, we evaluated the spectrum of cytokines produced by mycobacterium-reactive human T-cell clones. Nine of 11 T-cell clones bearing alpha beta or gamma delta T-cell receptors produced both Th1 and Th2 cytokines, a pattern resembling that of murine Th0 clones. The most frequent pattern was secretion of gamma interferon, tumor necrosis factor alpha (TNF), and interleukin-10 (IL-10), in combination with IL-2, IL-5, or both. Two clones produced only Th1 cytokines, and none produced exclusively Th2 cytokines. Although IL-4 was not detected in cell culture supernatants, IL-4 mRNA was detected by polymerase chain reaction amplification in two of six clones. There were no differences between the cytokine profiles of alpha beta and gamma delta T cells. A striking finding was the markedly elevated concentrations of TNF in clone supernatants, independent of the other cytokines produced. Supernatants from mycobacterium-stimulated T-cell clones, in combination with granulocyte-macrophage colony-stimulating factor, induced aggregation of bone-marrow-derived macrophages, and this effect was abrogated by antibodies to TNF. The addition of recombinant TNF to granulocyte-macrophage colony-stimulating factor markedly enhanced macrophage aggregation, indicating that TNF produced by T cells may be an important costimulus for the granulomatous host response to mycobacteria. The cytokines produced by T cells may exert immunoregulatory and immunopathologic effects and thus mediate some of the clinical manifestations of tuberculosis.

Cells from healthy non-exposed individuals produce cytokines to selected fractions of Leishmania promastigotes

We are interested in cellular responses to antigens of parasites to which the cell donor has not been previously exposed and how such responses may influence parasite establishment. In order to characterize such responses we have used cells from unexposed healthy donors and analysed the lymphoproliferative response to various Leishmania aethiopica antigen preparations and the cytokines produced in the process. Peripheral blood lymphocytes were stimulated with SDS-PAGE separated L. aethiopica antigen coupled to nitrocellulose particles. Fifteen of the 16 unexposed individuals tested had proliferative responses to either the whole or/and the antigen-bearing nitrocellulose fractions (NC fractions). Although the degree of response to the fractionated antigen varied in individuals, major stimulatory fractions were found in the high molecular weight region of 110-80 kDa (fractions 3-6) and low molecular weight region of 46-18 kDa (fractions 12-16). Substantial amounts of interferon gamma (IFN-gamma) and interleukin 2 (IL-2) were present in the supernatants of cells stimulated with the whole unfractionated antigen. The potential relevance of such responses in resistance to Leishmania infection is discussed.

A synthetic peptide to the E glycoprotein of Murray Valley encephalitis virus defines multiple virus-reactive T- and B-cell epitopes

Synthetic peptides from the envelope glycoprotein sequence of Murray Valley encephalitis (MVE) virus were previously evaluated in various strains of mice for both the induction of antibody and the in vitro proliferation of peptide-primed T-helper (Th) cells. MVE peptide 6 (amino acids 230 to 251) elicited reciprocal Th- and B-cell reactivity with native MVE virus after primary inoculation of C57BL/6 mice. In this study, we prepared overlapping subunit peptides of MVE peptide 6 and evaluated their immunogenicity. Analysis of these peptides delineated at least two B-cell epitopes that induced antibody reactive with MVE and other Japanese encephalitis serocomplex viruses. This antibody at low titer neutralized MVE virus. Genetic restriction of the antibody response to various T-cell elements within peptide 6 was observed in C3H, BALB/c, C57BL/6, and B10 congenic mice. One element demonstrable after primary immunization, located in the carboxy terminus, associated only with major histocompatibility complex class II IAb and IAbiEk glycoproteins. Functional stimulation with the peptides in association with IAkIEk and IAdIEd molecules was observed only after in vivo secondary stimulation. Peptide 6-1 (amino acids 230 to 241) was nonimmunogenic but could be recognized by Th cells from peptide 6-immunized mice. Further association of peptide 6 with the IAkIEk and IAdIEd subregions was demonstrated by the finding that T cells from MVE peptide 6-inoculated C3H and BALB/c mice primed for an antibody response to MVE virus. These results suggest that the peptide 6 sequence, which is relatively conserved among a number of flaviviruses, should be given consideration when synthetic immunogens for vaccine purposes are designed.

Human Th1 and Th2 lymphocytes: their role in the pathophysiology of atopy

In human beings, as in mice, two distinct patterns of cytokine secretion have been defined among CD4+ helper T-cell clones. Human type 1 helper (Th1), but not type 2 helper (Th2), cells produce interleukin-2 (IL-2), gamma-interferon (IFN-gamma), and tumor necrosis factor-beta, whereas Th2, but not Th1, cells secrete IL-4 and IL-5, but not IL-2 or IFN-gamma. Other cytokines, such as IL-3, IL-6, GM-CSF, or TNF-alpha, are produced by both Th1 and Th2 cells. Th0 cells, a third Th subset, show combined production of Th1- and Th2-type cytokines. The different cytokine patterns are associated with different functions. In general, Th2 cells provide an excellent helper function for B-cell antibody production, particularly of the IgE class. On the other hand, Th1 cells are responsible for delayed type hypersensitivity reactions and are cytolytic for autologous antigen-presenting cells, including B cells. Most allergen- or helminth-antigen-specific human CD4+ T-cell clones exhibit a Th2 phenotype, whereas most clones specific for bacterial antigens show a Th1 profile. Allergen-specific Th2 cells seem to play a crucial role in atopy. These cells induce IgE production via IL-4 and favor the proliferation, differentiation, and activation of eosinophils via IL-5. In addition, Th2-derived IL-3 and IL-4 are mast-cell growth factors that act in synergy, at least in vitro. Recent evidence indicates that allergen-specific Th2 cells are selectively enriched in tissues affected by allergic inflammation, such as the bronchial mucosa of subjects with allergic asthma.(ABSTRACT TRUNCATED AT 250 WORDS)

Aberrant interleukin (IL)-4 and IL-5 production in vitro by CD4+ helper T cells from atopic subjects

The cytokine secretion profiles of T cell lines (TCL) specific for purified protein derivative (PPD) or streptokinase (SK), contemporarily derived from nine atopic and nine nonatopic individuals, were compared. Upon stimulation with phorbol myristate acetate (PMA) plus anti-CD3 monoclonal antibody (mAb), all TCL from both atopics and nonatopics produced interleukin (IL)-2 and interferon (IFN)-gamma. The mean IL-2 production by PPD- or SK-specific TCL from both atopics and nonatopics was similar, whereas the mean IFN-gamma production by TCL derived from atopics was significantly lower. In addition, both PPD- and SK-specific TCL from atopics produced detectable amounts of IL-4 and IL-5, whereas the corresponding TCL derived from nonatopics did not. A total number of 107 and 99 PPD-specific CD4+ T cell clones (TCC) were then derived from TCL of 4 atopic and 4 nonatopic donors and assessed for their profile of cytokine production in response to stimulation with either PMA plus anti-CD3 mAb or the specific antigen. Under both these experimental conditions, virtually all PPD-specific TCC from both atopic and nonatopic individuals produced IL-2 and IFN-gamma. In contrast, the great majority of PPD-specific TCC derived from nonatopic individuals did not produce IL-4 and IL-5, whereas high proportions of PPD-specific TCC derived from atopic donors displayed the ability to produce noticeable amounts of IL-4 and IL-5 besides IL-2 and IFN-gamma. These data indicate that CD4+ T cells from atopic individuals are able to produce IL-4 and IL-5 in response to bacterial antigens, such as PPD and SK, that usually evoke responses with a restricted type-1 T helper (Th1)-like cytokine profile in nonatopic individuals. Aberrant IL-4 production by Th cells may represent one of the immune alterations responsible for enhanced IgE antibody production in atopic people.

Thymopentin reduces the susceptibility of aged mice to cutaneous leishmaniasis by modulating CD4 T-cell subsets

BALB/c mice are highly susceptible to Leishmania major infection. The susceptibility increases progressively with the age of the mice. Aged mice produce progressively lower levels of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) but higher levels of IL-4 compared to younger mice. Thymopentin, a pentapeptide with thymopoietin activity, dramatically increases the resistance to Leishmania major infection in aged mice. The thymopentin-treated mice produce enhanced levels of IL-2 and IFN-gamma, but significantly reduced amounts of IL-4. Thus, it appears that the age-related susceptibility to cutaneous leishmaniasis is correlated with the enhancement of Th2 and the reduction of Th1 cell activities. Furthermore, thymic hormone may play an important role in the induction and function of these two subsets of CD4 T cells.

Modulation of cytokine production and response phenotypes in murine trichuriasis

BALB/K mice are usually resistant to infection with the intestinal nematode parasite Trichuris muris and exhibit a Th2 dominated (IL-5, IL-9) response. Conversely in B10.BR mice, which are unable to expel T. muris, Th1 type (IFN-gamma producing) cells predominate. We have manipulated the course of infection in these two strains of mice such that the period of host-parasite contact is extended in the former and curtailed in the latter. Extension of host-parasite contact in BALB/K mice beyond normal (day 21) resulted in the modulation of cytokines produced by in vitro concanavalin A (Con-A) stimulated MLNC away from IL-5 and IL-9 (Th2-type cytokines) in favour of the Th1-type cytokine IFN-gamma. Curtailment of host parasite contact in B10.BR mice to less than 21 days resulted in elevated production of IL-5 and IL-9 by MLNC in the absence of elevated IFN-gamma levels. Thus modulation of expulsion phenotype also modulates cytokine production by T-cells in the MLN draining the site of infection, with a Th2 response being associated with resistance and a Th1 type response with the inability to expel the parasite. Mechanisms by which the modulated cytokine profiles arise are discussed.

Type 1 T helper and type 2 T helper cells: functions, regulation and role in protection and disease

Two very distinct cytokine secretion patterns have been defined among murine CD4+ T cells. Type 1 helper (TH1), but not type 2 helper (TH2), cells produce interleukin (IL)-2, gamma-interferon (IFN-gamma) and tumour necrosis factor-beta, whereas TH2, but not TH1, cells express IL-4, IL-5, IL-6 and IL-10. The different cytokine patterns lead to different functions of the two types of T cell. In general, TH2 cells are excellent helpers for B-cell antibody secretion, particularly IgE responses. On the other hand TH1 cells induce delayed-type hypersensitivity reactions. There is general agreement that the different functional subsets of TH cells arise post-thymically from a common pool of precursors and as a consequence of activation of antigen. However, the factors affecting differentiation of TH precursors into the TH1 or TH2 subsets are still unclear. Mutual cross-regulation between TH1 (via IFN-gamma) and TH2 (via IL-10) has also been reported. Recently, human T cell clones similar to murine TH1 and TH2 cells have been demonstrated. Most allergen- or helminthic antigen-specific CD4+ human T cell clones have a TH2 phenotype, whereas the majority of T-cell clones specific for mycobacterial antigens or antigens responsible for type IV hypersensitivity exhibit a TH1 phenotype. Human TH2 clones provide B-cell help for IgE synthesis, whereas most TH1 clones are cytolytic for antigen-presenting cells, including B lymphocytes. It is highly probable that the selective or preferential activation of CD4+ T-cell subsets secreting defined patterns of cytokines is of major importance in determining the class of immune effector function, thus influencing both protection and immunopathology.

Major histocompatibility complex (MHC) control of CD4 T cell subset activation. II. A single peptide induces either humoral or cell-mediated responses in mice of distinct MHC genotype

CD4 T cells activated in vivo in response to human collagen type IV (hCol IV) resemble either T helper type 1 (Th1) or Th2 cells depending on the major histocompatibility complex (MHC) class II genotype of the responding mice. H-2s mice were shown to selectively activate Th1-like cells, releasing interleukin (IL 2 and interferon-gamma in response to hCol IV, whereas H-2b.d mice were shown to selectively activate Th2-like cells, releasing IL 4 and IL 5 in response to hCol IV. These results suggested that MHC class II regulated the type of effector function observed during an immune response. It was of interest to determine if the functional difference observed between the CD4 T cells of the two strains was due to the presentation of different peptides of the hCol IV molecule by the two MHC class II molecules. The present results demonstrate that a single peptide of the collagen IV molecule will elicit a Th1-like response in H-2s strains and Th2-like responses in H-2b.d strains, as was observed when using the intact hCol IV molecule. Furthermore, the failure to generate Th1-like responses in H-2b.d could be overcome by increasing the dose of this peptide in vitro. Compared to H-2s, the Th1-like response in H-2b required 100 times the amount of peptide to reelicit an equivalent response. These data suggest that a single peptide of hCol IV can control the type of effector response observed.

Polyclonal cell activity of a repeat peptide derived from the sequence of an 85-kilodalton surface protein of Trypanosoma cruzi trypomastigotes

Some in vitro and in vivo biological activities of an octadecapeptide derived from an 85-kDa surface protein of Trypanosoma cruzi trypomastigote were studied. The peptide coupled to a carrier protein induced the proliferative response of lymph node cells from mice immunized with various antigens. Moreover, sera from mice immunized with the coupled peptide were found to contain antibodies against a number of self and nonself antigens: fibronectin, bovine serum albumin, myosin, tetanus toxoid, ovalbumin, keyhole limpet hemocyanin, and DNA. These results are discussed in the context of Chagas' disease immunopathology.

TNF-alpha reverses the disease-exacerbating effect of subcutaneous immunization against murine cutaneous leishmaniasis

Earlier studies have demonstrated that mice injected subcutaneously or intramuscularly with leishmanial antigens develop significantly exacerbated disease compared with unimmunized controls when challenged with the cutaneous protozoan parasites Leishmania major. We report here that this disease enhancement can be prevented, and protective immunity induced, by the incorporation of recombinant tumour necrosis factor (TNF-alpha) in the immunizing inoculum. This effect of TNF-alpha is dose-dependent and is not evident when TNF-alpha and the antigens are injected into separate sites. Furthermore, TNF-alpha injected together with p183, a peptide known to preferentially stimulate Th2 cells and disease exacerbation in H-2d mice, activates spleen and lymph node cells secreting more interferon-gamma (IFN-gamma) and less interleukin-4 (IL-4) and induces a modest but significant degree of resistance against L. major infection in highly susceptible BALB/c mice.

Helper T-cell subsets in mouse leishmaniasis: induction, expansion and effector function

It has been a surprise to find that the two distinct subsets of mouse CD4+ T cells identified from clones cultured in vitro also occur during Leishmania infection. The spectrum of disease encountered during these infections ranges from successful resolution to fatal dissemination and in mice these outcomes are accompanied by expansion of TH1 or TH2 CD4+ cells, respectively. This review focuses on the mechanisms that cause such disparate responses to the parasite.