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More than 20 years ago, immunologists discovered that resistance and susceptibility to experimental infection with the intracellular protozoan Leishmania major was associated with the development of T-helper 1 (Th1)- and Th2-dominated immune responses, respectively. This infectious disease model was later used to identify and assess the role of key factors, such as interleukin-12 (IL-12) and IL-4, in Th1 and Th2 maturation. While infection by Leishmania remains a popular model for immunologists who wish to assess the role of their favorite molecule in T-cell differentiation, other investigators have tried to better understand how Leishmania interact with its insect and mammalian hosts. In this review, we discuss some of these new data with an emphasis on the early events that shape the immune response to Leishmania and on the immune evasion mechanisms that allow this parasite to avoid the development of sterilizing immunity and to secure its transmission to a new host.

Interferon (IFN)-gamma, a cytokine critical for resistance to infection and tumors, is produced by CD4(+) helper T lymphocytes after stimulation by cultured dendritic cells (DCs) that secrete a cofactor, interleukin (IL)-12. We have identified a major IL-12-independent pathway whereby DCs induce IFN-gamma-secreting T helper (Th)1 CD4(+) T cells in vivo. This pathway requires the membrane-associated tumor necrosis family member CD70 and was identified by targeting the LACK antigen from Leishmania major within an antibody to CD205 (DEC-205), an uptake receptor on a subset of DCs. Another major DC subset, targeted with 33D1 anti-DCIR2 antibody, also induced IFN-gamma in vivo but required IL-12, not CD70. Isolated CD205(+) DCs expressed cell surface CD70 when presenting antigen to T cell receptor transgenic T cells, and this distinction was independent of maturation stimuli. CD70 was also essential for CD205(+) DC function in vivo. Detection of the IL-12-independent IFN-gamma pathway was obscured with nontargeted LACK, which was presented by both DC subsets. This in situ analysis points to CD70 as a decision maker for Th1 differentiation by CD205(+) DCs, even in Th2-prone BALB/c animals and potentially in vaccine design. The results indicate that two DC subsets have innate propensities to differentially affect the Th1/Th2 balance in vivo and by distinct mechanisms.

Although non-obese diabetic (NOD) mice spontaneously develop T cell autoimmunity, it is not clear whether this phenomenon results from a defect in tolerance to self-Ag. Furthermore, as autoimmunity has been postulated to result from T cell responses directed toward self-peptides that bind with low affinity to NOD I-A(g7) MHC class II molecules, it is important to determine whether the expression of such peptides induces tolerance. We have constructed NOD transgenic (Tg) mice expressing the Leishmania antigen receptor for C kinase (LACK) Ag in either the thymus or pancreatic beta cells. We identified LACK peptides that were the targets of T cells in LACK-immunized NOD mice while binding to I-A(g7) with low affinity. While CD4(+) T cells from NOD mice secreted IFN-gamma, IL-4, IL-5 and IL-10 in response to LACK, those from LACK-expressing Tg mice secreted reduced levels of cytokines. Experiments using peptide/MHC multimers showed that LACK-expressing Tg mice exhibited self-reactive CD4(+) T cells with impaired proliferation capabilities. Hence, even self-peptides that bind to I-A(g7) with low affinity can induce tolerance in NOD mice. This result is important in light of the commonly held hypothesis that T cells reacting to peptides that bind to MHC with low affinity escape tolerance induction and cause autoimmunity.

It sometimes is difficult to diagnose leishmaniasis in tissue sections or in smears, particularly in unusual sites or if few parasites are present in the lesion. Leishmania species must be differentiated morphologically from a variety of other microorganisms, including Toxoplasma gondii, Histoplasma capsulatum, Trypanosoma cruzi, and Penicillium marneffei. We tested the value of monoclonal antibody p19-11 raised against the Leishmania homologue of receptors for activated C-kinase (LACK) as an immunohistochemical marker for amastigotes of Leishmania infantum. We evaluated a total of 117 paraffin-embedded lesions due to L infantum (92 cases), T gondii (15 cases), H capsulatum (5 cases), T cruzi (3 cases), and P marneffei (2 cases). Amastigotes of Leishmania species were detected in 92 (100%) of the leishmaniasis lesions. There were no false-positive LACK immunoreactions in any of the toxoplasmosis, histoplasmosis, trypanosomiasis, or penicilliosis specimens (0/25). We found the anti-LACK antibody p19-11 to be a highly specific and sensitive paraffin-reactive immunohistochemical marker for the confirmation or identification of Leishmania species in tissue sections.

The major histocompatibility complex (MHC)-dependent presentation of processed tissue-specific self-antigens can contribute to either peripheral (extrathymic) tolerance or the differentiation of autoreactive T cells. Here, we have studied the MHC class II molecule presentation of gastric parietal cell (PC)-specific H(+)/K(+)-ATPase, which induces a destructive autoimmune gastritis in BALB/c mice lacking CD4(+) CD25(+) regulatory T cells. Immunofluorescence microscopy showed physical association of CD11c(+) dendritic cells (DCs) with PCs in the gastric mucosa. H(+)/K(+)-ATPase protein was found within vesicular compartments of a few CD11c(+) DCs only in the draining gastric lymph node (LN) and these antigen-containing DCs increased markedly in number with the onset of tissue destruction in autoimmune animals. Both CD8alpha(hi) and CD8alpha(lo) gastric DCs, but not peripheral or mesenteric DCs, showed evidence of constitutive in vivo processing and presentation of H(+)/K(+)-ATPase. These data provide direct support for a widely held model of local tissue antigen uptake and trafficking by DCs in normal animals and demonstrate that DCs in the draining LN can present a tissue-specific self-antigen under noninflammatory conditions without fully deleting autoreactive T cells or inducing active autoimmunity.

We have previously demonstrated that murine macrophages (Mphi) infected with Leishmania promastigotes, in contrast to Mphi infected with the amastigote stage of these parasites, are able to present the Leishmania antigen LACK (Leishmania homologue of receptors for activated C kinase) to specific, I-Ad-restricted T cell hybrids and to the T cell clone 9.1-2. These T cells react with the LACK (158-173) peptide, which is immunodominant in BALB/c mice. Here, we show that the level of stimulation of the LACK-specific T cell hybridoma OD12 by promastigote-infected Mphi is clearly dependent upon the differentiation state of the internalized parasites. Thus, shortly after infection with log-phase or stationary-phase promastigotes of L. major or of L. amazonensis, Mphi strongly activated OD12. The activity was transient and rapidly lost. However, under the same conditions, activation of OD12 by Mphi infected with metacyclic promastigotes of L. major or of L. amazonensis was barely detectable. At the extreme, Mphi infected with amastigotes were incapable to stimulate OD12. Thus, the presentation of LACK by infected Mphi correlates with the degree of virulence of the phagocytosed parasites, the less virulent being the best for the generation/expression of LACK (158-173)-I-Ad complexes. While the intracellular killing of the parasites appears to be an important condition for the presentation of LACK, it is not the only requisite. The partial or total destruction of intracellular L. amazonensis amastigotes does not allow the presentation of LACK to OD12. A preferential interaction of LACK (158-173) with recycling rather than newly synthesized MHC class II molecules does not explain the transient presentation of LACK by Mphi infected with log-phase or stationary-phase promastigotes because brefeldin A strongly inhibited the presentation of LACK to OD12. Taken together, these results suggest that virulent stages of Leishmania, namely metacyclics and amastigotes, have evolved strategies to avoid or minimize their recognition by CD4+ T lymphocytes.

International audience; More than 20 years ago, immunologists discovered that resistance and susceptibility to experimental infection with the intracellular protozoan Leishmania major was associated with the development of T-helper 1 (Th1)- and Th2-dominated immune responses, respectively. This infectious disease model was later used to identify and assess the role of key factors, such as interleukin-12 (IL-12) and IL-4, in Th1 and Th2 maturation. While infection by Leishmania remains a popular model for immunologists who wish to assess the role of their favorite molecule in T-cell differentiation, other investigators have tried to better understand how Leishmania interact with its insect and mammalian hosts. In this review, we discuss some of these new data with an emphasis on the early events that shape the immune response to Leishmania and on the immune evasion mechanisms that allow this parasite to avoid the development of sterilizing immunity and to secure its transmission to a new host.

Protozoa and bacteria infect various types of phagocytic cells including macrophages, monocytes, dendritic cells and eosinophils. However, it is not clear which of these cells process and present microbial antigens in vivo and in which cellular compartments parasite peptides are loaded onto Major Histocompatibility Complex molecules. To address these issues, we have infected susceptible BALB/c (H-2d) mice with a recombinant Leishmania major parasite expressing a fluorescent tracer. To directly visualize the antigen presenting cells that present parasite-derived peptides to CD4+ T cells, we have generated a monoclonal antibody that reacts to an antigenic peptide derived from the parasite LACK antigen bound to I-Ad Major Histocompatibility Complex class II molecule. Immunogold electron microscopic analysis of in vivo infected cells showed that intracellular I-Ad/LACK complexes were present in the membrane of amastigote-containing phagosomes in dendritic cells, eosinophils and macrophages/monocytes. In both dendritic cells and macrophages, these complexes were also present in smaller vesicles that did not contain amastigote. The presence of I-Ad/LACK complexes at the surface of dendritic cells, but neither on the plasma membrane of macrophages nor eosinophils was independently confirmed by flow cytometry and by incubating sorted phagocytes with highly sensitive LACK-specific hybridomas. Altogether, our results suggest that peptides derived from Leishmania proteins are loaded onto Major Histocompatibility Complex class II molecules in the phagosomes of infected phagocytes. Although these complexes are transported to the cell surface in dendritic cells, therefore allowing the stimulation of parasite-specific CD4+ T cells, this does not occur in other phagocytic cells. To our knowledge, this is the first study in which Major Histocompatibility Complex class II molecules bound to peptides derived from a parasite protein have been visualized within and at the surface of cells that were infected in vivo. © 2010 Muraille et al., SCOPUS: ar.j, info:eu-repo/semantics/published

Antigen binding to the B-cell receptor (BCR) induces multiple signaling cascades that ultimately lead to B lymphocyte activation. In addition, the BCR regulates the key trafficking events that allow the antigen to reach endocytic compartments devoted to antigen processing, i.e., that are enriched for major histocompatibility factor class II (MHC II) and accessory molecules such as H2-DM. Here, we analyze the role in antigen processing and presentation of the tyrosine kinase Syk, which is activated upon BCR engagement. We show that convergence of MHC II- and H2-DM–containing compartments with the vesicles that transport BCR-uptaken antigens is impaired in cells lacking Syk activity. This defect in endocytic trafficking compromises the ability of Syk-deficient cells to form MHC II-peptide complexes from BCR-internalized antigens. Altered endocytic trafficking is associated to a failure of Syk-deficient cells to properly reorganize their actin cytoskeleton in response to BCR engagement. We propose that, by modulating the actin dynamics induced upon BCR stimulation, Syk regulates the positioning and transport of the vesicles that carry the molecules required for antigen processing and presentation.

Class I tetramers have been used to track cytotoxic T cells during bacterial and viral infections. During the past year, the use of such molecules has revealed important information about the role of CD8 + T cells in autoimmune diabetes. Furthermore, class II multimers have been produced and successfully used to stain autoreactive CD4 + T cells from patients with rheumatoid arthritis or Borrelia - burgdorferi -induced Lyme arthritis.

Listeria monocytogeneshas been used as an experimental live vector for the induction of CD8-mediated immune responses in various viral and tumoral experimental models. Susceptibility of BALB/c mice toLeishmania majorinfection has been correlated to the preferential development of Th2 CD4 T cells through an early production of interleukin 4 (IL-4) by a restricted population of CD4 T cells which react to a single parasite antigen, LACK (stands forLeishmaniahomologue of receptors for activated C kinase). Experimental vaccination with LACK can redirect the differentiation of CD4+T cells towards the Th1 pathway if LACK is coadministrated with IL-12. As IL-12 is known to be induced byL. monocytogenes, we have tested the ability of a recombinant attenuatedactAmutantL. monocytogenesstrain expressing LACK to induce the development of LACK-specific Th1 cells in both B10.D2 and BALB/c mice, which are resistant and susceptible toL. major, respectively. After a single injection of LACK-expressingL. monocytogenes, IL-12/p40 transcripts showed a rapid burst, and peaks of gamma interferon (IFN-γ)-secreting LACK-specific Th1 cells were detected around day 5 in the spleens and livers of mice of both strains. These primed IFN-γ-secreting LACK-reactive T cells were not detected ex vivo after day 7 of immunization but could be recruited and detected 15 days later in the draining lymph node after anL. majorfootpad challenge. Although immunization of BALB/c mice with LACK-expressingL. monocytogenesdid not change the course of the infection withL. major, immunized B10.D2 mice exhibited significantly smaller lesions than nonimmunized controls. Thus, our results demonstrate that, in addition of its recognized use for the induction of effector CD8 T cells,L. monocytogenescan also be used as a live recombinant vector to favor the development of potentially protective IFN-γ-secreting Th1 CD4 T lymphocytes.

Parasite-specific CD4+ T cells have been shown to transfer protection against Leishmania major in susceptible BALB/c mice. An epitope-tagged expression library was used to identify the antigen recognized by a protective CD4+ T cell clone. The expression library allowed recombinant proteins made in bacteria to be captured by macrophages for presentation to T cells restricted to major histocompatibility complex class II. A conserved 36-kilodalton member of the tryptophan-aspartic acid repeat family of proteins was identified that was expressed in both stages of the parasite life cycle. A 24-kilodalton portion of this antigen protected susceptible mice when administered as a vaccine with interleukin-12 before infection.

We previously reported that in rat fibroblasts, accumulation of a set of mRNAs ("pIL genes") was modulated as a function of cell growth and transformation, at a posttranscriptional stage, and by a mechanism that depends on a short nucleotide sequence containing an ID repetitive element. In mouse fibroblasts, hybridization with rat pIL probes identified mRNAs with the same pattern of expression, which did not contain ID sequences but contained a different regulatory element, encompassing a repetitive sequence of the B1 family. Expression in mouse cells of a reporter beta-globin gene carrying this element inserted in its 3' noncoding region was growth- and transformation-dependent. The nucleotide sequences of two murine and of three rat pIL cDNAs showed clear similarities in the region immediately adjacent to the ID and B1 repeats. Both the repeat and the flanking sequence were required to confer on beta-globin constructs the pattern of expression characteristic of the pIL genes. The hypothesis is presented that repetitive sequences in the eukaryotic genome might be modular parts of complex regulatory elements ensuring the coordinated expression of various mRNA species.

Cell lines established after transfer into FR3T3 rat fibroblast cells of 'immortalizing' oncogenes (plt gene (large T protein) of polyoma virus, v-myc gene of MC29 virus, rearranged forms of c-myc) exhibited increased rates of sister chromatid exchange (SCE). This was observed neither in cells which expressed one of the oncogenes responsible for the terminal stages of tumorigenic transformation (polyoma virus pmt (middle T protein), mutated ras genes), nor in cell lines carrying oncogenes of both types. Abnormal chromosome numbers were observed in cell lines expressing plt or myc, but not after transformation by pmt or ras oncogenes.

We have analysed nucleotide sequences at the junction between simian virus 40 (SV40) and cellular DNA in the Fisher rat transformed line tsA30-N2. This line contains a single insertion of one complete SV40 genome with a terminal duplication of 267 nucleotides, the recombination sites being located at nucleotides 439 and 705 in the late region of SV40. These two positions are located within short direct repeats in the virus genome. In order to test the significance of such repeats with respect to illegitimate recombination events, we analysed two series of published sequences of SV40 recombination sites: the first one consists of eight SV40 insertion endpoints derived from four SV40-transformed cell lines; the second one consists of 18 junction points from SV40 evolutionary variants. Our analysis demonstrates that in both cases, recombination preferentially takes place near short direct repeats in the virus genome. A model involving a "slipped mispairing" mechanism is proposed in order to account for this finding.

Crosspresentation of self-antigens by antigen-presenting cells is critical for the induction of peripheral tolerance. As apoptosis facilitates the entry of antigens into the crosspresentation pathway, we sought to prevent the development of autoimmune diabetes by inducing pancreatic beta cell apoptosis before disease onset. Accordingly, young nonobese diabetic (NOD) mice injected with a single low dose of streptozotocin (SZ), a drug cytotoxic for beta cells, exhibited impaired T cell responses to islet antigens and were protected from spontaneous diabetes. Furthermore, beta cell apoptosis was necessary for protection since SZ did not protect RIP-CrmA transgenic NOD mice in which beta cells expressed the caspase inhibitor CrmA. Our results support a model in which apoptosis of pancreatic beta cells induces the development of regulatory cells leading to the tolerization of self-reactive T cells and protection from diabetes.

Two distinct forms of the myc oncogene were assayed for their ability to induce, in cultured rat fibroblast cells, the alterations of cellular growth controls observed upon transfer of the gene of polyoma virus encoding only the large T protein (plt). Both of these rearranged myc genes and the plt gene had been previously shown to cooperate with ras oncogenes for transformation of rat embryo fibroblasts (REF) and were thought to induce the same early step ("immortalization") of the tumoral transformation pathway. We now report that these two different oncogenes elicite the same response in the following biological assays: (i) reduction of the requirements in serum factors for growth in culture of cells of the established FR3T3 line; (ii) expression of transformed properties in low serum medium after transfer into FR3T3 cells expressing only the middle T protein of polyoma virus (MTT lines); (iii) conferring on REF cells the ability to grow as clonal colonies after seeding at low cell density; (iv) conferring on REF cells the ability to grow continuously in cell culture. These congruent phenotypes suggest that the activities of the large T and myc proteins result in the induction of the same molecular events. These results also provide simple biological assays and selective systems for oncogenes of the myc class.

Publisher Summary Stepwise progression of tumoral diseases toward increased invasiveness, hormonal independence, and resistance to chemical and physical treatment is an all-too-common phenomenon that has been analyzed in various experimental systems. The stepwise character and apparent irreversibility of these phenotypic changes would be most easily accounted by a sequential accumulation of genetic alterations. Alterations of cellular genes associated with malignancy (oncogenes) are recently identified, first as a result of the natural capacity of retroviruses for gene transduction and subsequently by the transfection of genomic DNA from human tumors into mouse cells. Unlike most retroviruses, oncogenic DNA viruses that have been analyzed so far appear to require more than one gene to establish and maintain neoplastic transformation. This was extensively documented for polyoma viruses and adenoviruses, and more recent observations suggest a similar situation for two herpesviruses— herpes simplex type 2 and Epstein–Barr virus. Experimental evidence summarized in this chapter defines an early stage of the oncogenic transformation process induced by the plt and E1A oncogenes and, at least under some defined conditions of expression, by the rearranged forms of the myc gene.

A group of cellular and viral oncogenes including the plt gene of polyoma virus (large T protein), the E1A genes of adenoviruses and rearranged forms of the c-myc oncogene cooperate with oncogenes of a second class, including the cellular ras oncogenes and the pmt gene of polyoma virus in the oncogenic transformation fat embryo fibroblasts (1). The myc, plt and E1A genes induce by themselves an early stage of transformation (“immortalization”). The properties of cells at this stage are of interest, since they may constitute a useful experimental model for the study of a very early stage of tumor progression. Transfer of the genes did not result in the immediate appearance of a tumorigenic potential in vivo, and, in fact most of the properties of cells grown in vitroremained similar to those of the normal controls. In primary rat embryo fibroblast cultures, the only phenotypes consistently associated with the expression of these genes in cell culture were a change in the hormonal growth, indicated by a decrease in their requirement in serum factors, and the ability of long term growth (2). When rearranged myc genes were transferred into cells of an established line (FR3T3) together with the neor drug-resistance gene, selection in G418 medium produced cell lines which, upon successive generations in culture, exhibited a characteristic high frequency of spontaneous transformation and acquisition of a tumorigenic potential in syngeneic animals. Fluctuation assays indicated a wide distribution of frequencies between parallel sub-cultures, indicative of the random occurrence of secondary genetic events leading to tumorigenesis (3).

Upon expression of the large T protein of polyoma virus from genes encoding only this protein (plt genes: intron-less gene in pPyLT1 plasmid) (Tyndall et al., 1981), hrt mutant NG18 (Schaffhausen et al., 1978)), changes were observed in the behaviour in culture of rodent embryo fibroblast cells in primary cultures (REF) (Rassoulzadegan et al., 1982, 1983): the cells grew in culture for an unlimited number of generations (“immortalization”) and grew in the presence of low concentrations of serum (below 1% newborn calf serum). They maintained, however, the extended morphology, the low saturation density and the anchorage dependency characteristic of the normal fibroblast. Unlike the original REF- cells, these “immortalized” lines could be transformed by transfer of the polyoma virus gene encoding only the middle T protein (pmt gene: plasmid pPyMT1, Treisman et al., 1981), thus demonstrating a two-step process in tumorigenic transformation. A similar two-step mechanism was suggested by Land et al. (1983) for the myc and ras families of cellular oncogenes: expression either of the polyoma plt or of v-myc and rearranged c-myc genes allowed transformation of REF cells with activated forms of the ras genes. These results suggest that both genes exert similar function (s) at an early step of the transformation of REF cells in culture. This class of oncogenes also includes the E1a genes of adenovirus 2, required for transformation by the E1b genes of the same virus and which can also cooperate with a ras oncogene in REF transformation (Van den Elsen et al., 1982, Ruley, 1983).

Activated myc genes are found in a broad variety of spontaneous malignancies in rodents and humans as well: Burkitt Lymphoma (Klein and Klein 1985; Croce et al. 1984; Dalla-Favera et al. 1985; this volume), neuroblastoma (Schwab 1985), small cell lung carcinoma (Nau et al. 1985), mouse plasmacytoma (Klein and Klein 1985; this volume) and rat immunocytoma (Pear, this volume).

More than 50 years ago, Peyton Rous and his colleagues described a tumor progression pathway initiated by infections of rabbits with Shope papilloma virus (Rous and Beard 1935). Experiments performed almost 30 years later by Vogt and Dulbecco (1963) led them to conclude that transformation by polyomavirus is also a multistep process. It is now clear that most, if not all, DNA tumor viruses use more than one oncogene to fully transform rodent embryo fibroblasts in culture. Here, we discuss mechanisms of cooperativity among polyoma virus genes in the transformation of rat embryo fibroblasts (REF).

The state and organization of viral DNA sequences present in independently isolated rat cell lines transformed with SV40 were investigated using restriction-enzyme cleavage of the cellular DNA and blot hybridization with a viral probe. The transformed lines were established under conditions as identical as possible, except for a limited number of variables (multiplicity of infection, physiological state of the cells after infection and procedures used for selecting the transformed derivatives). They were characterized after a limited number of generations in culture. Two distinct types of organization were found: covalently integrated viral genomes were present either as single inserts or as head-to-tail oligomeric structures. The latter was observed among transformants derived from cells maintained after infection under growth-inhibiting conditions (suspension in agarose medium, confluency on a solid substrate). Single inserts were observed only among cell lines isolated after an initial period of active growth. Recurrent patterns of hybridization were observed in independently isolated lines, indicating that the sites of the integrative recombinations were close enough, both in the viral and the cellular sequences, not to be distinguished at the level of sensitivity of the technique (more than ±100 bp). Among cell lines with multiple integration sites, only part of the inserts were found in several instances to be identical to inserts observed in other transformed lines.