Loss of the GP46/M-2 surface membrane glycoprotein gene family in the Leishmania braziliensis complex

Protective Humoral Immune Response Induced by Recombinant Virus-like Particle Vaccine Expressing Leishmania donovani Surface Antigen

To date, Leishmania spp. vaccine studies have mainly focused on cellular immunity induction, which plays a crucial role in host protection. In contrast, vaccine-induced humoral immunity is largely neglected. Virus-like particle (VLP) vaccines generated using the baculovirus expression system are well-known inducers of humoral immunity and would serve as a suitable platform for evaluating humoral immunity-mediated protection against visceral Leishmaniasis. In this study, we investigated the humoral immunity evoked through VLPs expressing the L. donovani promastigote surface antigen (PSA-VLPs) and assessed their contribution to protection in mice. PSA-VLPs vaccines were generated using the baculovirus expression system and used for mouse immunizations. Mice were intramuscularly immunized twice with PSA-VLPs and challenged with L. donovani to confirm vaccine-induced protective immunity. PSA-VLP immunization elicited parasite-specific antibody responses in the sera of mice, which were induced in a dose-dependent manner. B cell, germinal center B cell, and memory B cell responses in the spleen were found to be higher in vaccinated mice compared to unimmunized controls. PSA-VLP immunization diminished the production of pro-inflammatory cytokines IFN-γ and IL-6 in the liver. Overall, the PSA-VLPs conferred protection against L. donovani challenge infection by reducing the total parasite burden within the internal organs. These results suggest that PSA-VLPs induced protective immunity against the L. donovani challenge infection.

Genomes of Endotrypanum monterogeii from Panama and Zelonia costaricensis from Brazil: Expansion of Multigene Families in Leishmaniinae Parasites That Are Close Relatives of Leishmania spp

The Leishmaniinae subfamily of the Trypanosomatidae contains both genus Zelonia (monoxenous) and Endotrypanum (dixenous). They are amongst the nearest known relatives of Leishmania, which comprises many human pathogens widespread in the developing world. These closely related lineages are models for the genomic biology of monoxenous and dixenous parasites. Herein, we used comparative genomics to identify the orthologous groups (OGs) shared among 26 Leishmaniinae species to investigate gene family expansion/contraction and applied two phylogenomic approaches to confirm relationships within the subfamily. The Endotrypanum monterogeii and Zelonia costaricensis genomes were assembled, with sizes of 29.9 Mb and 38.0 Mb and 9.711 and 12.201 predicted protein-coding genes, respectively. The genome of E. monterogeii displayed a higher number of multicopy cell surface protein families, including glycoprotein 63 and glycoprotein 46, compared to Leishmania spp. The genome of Z. costaricensis presents expansions of BT1 and amino acid transporters and proteins containing leucine-rich repeat domains, as well as a loss of ABC-type transporters. In total, 415 and 85 lineage-specific OGs were identified in Z. costaricensis and E. monterogeii. The evolutionary relationships within the subfamily were confirmed using the supermatrix (3384 protein-coding genes) and supertree methods. Overall, this study showed new expansions of multigene families in monoxenous and dixenous parasites of the subfamily Leishmaniinae.

Leishmania species: A narrative review on surface proteins with structural aspects involved in host-pathogen interaction

In tropical and subtropical regions of the world, leishmaniasis is endemic and causes a range of clinical symptoms in people, from severe tegumentary forms (such as cutaneous, mucocutaneous, and diffuse leishmaniasis) to lethal visceral forms. The protozoan parasite of the genus Leishmania causes leishmaniasis, which is still a significant public health issue, according to the World Health Organization 2022. The public's worry about the neglected tropical disease is growing as new foci of the illness arise, which are exacerbated by alterations in behavior, changes in the environment, and an enlarged range of sand fly vectors. Leishmania research has advanced significantly during the past three decades in a few different avenues. Despite several studies on Leishmania, many issues, such as illness control, parasite resistance, parasite clearance, etc., remain unresolved. The key virulence variables that play a role in the pathogenicity-host-pathogen relationship of the parasite are comprehensively discussed in this paper. The important Leishmania virulence factors, such as Kinetoplastid Membrane Protein-11 (KMP-11), Leishmanolysin (GP63), Proteophosphoglycan (PPG), Lipophosphoglycan (LPG), Glycosylinositol Phospholipids (GIPL), and others, have an impact on the pathophysiology of the disease and enable the parasite to spread the infection. Leishmania infection may arise from virulence factors; they are treatable with medications or vaccinations more promptly and might greatly shorten the duration of treatment. Additionally, our research sought to present a modeled structure of a few putative virulence factors that might aid in the development of new chemotherapeutic approaches for the treatment of leishmaniasis. The predicted virulence protein's structure is utilized to design novel drugs, therapeutic targets, and immunizations for considerable advantage from a higher understanding of the host immune response.

The first successful report of the in vitro life cycle of Chinese Leishmania: the in vitro conversion of Leishmania amastigotes has been raised to 94% by testing 216 culture medium compound

Chinese Leishmania isolate MHOM/CN/90/SC10H2 (L. H2), which was obtained from the spinal cords of patients from the Sichuan province of China, is an uncharacterized, pathogenic species closely related to Leishmania tarentolae. The in vitro transformation rate of L. H2 promastigotes into amastigotes has not been studied. This study is the first to successfully define the in vitro life cycle of L. H2 by investigating the percent conversion of L.H2 promastigotes to amastigotes in vitro under 216 different culture conditions. The highest proportion of L. H2 amastigotes observed (94%) was significantly higher than that previously reported. After conversion, the axenic amastigotes remained viable as verified by the levels of stage-specific genes (Gp46, A2 and β-tubulin) detected by RT-PCR. Meanwhile, morphological and protein characterizations of these axenic amastigotes were carried out in order to confirm the successful conversion. Specific antibodies were only able to detect 46 kDa, 52 kDa and 75 kDa proteins in samples isolated from axenic amastigotes. Afterward, these converted axenic amastigotes were transformed into the promastigote form by altering the culture condition. These converted axenic promastigotes still have the ability to infect macrophages, and their morphology changed back to the amastigote form following infection. These findings will assist further investigations into the biological characteristics of the host-parasite relationship and the process of pathogenesis.

Recombinant Forms of Leishmania amazonensis Excreted/Secreted Promastigote Surface Antigen (PSA) Induce Protective Immune Responses in Dogs

Preventive vaccination is a highly promising strategy for interrupting leishmaniasis transmission that can, additionally, contribute to elimination. A vaccine formulation based on naturally excreted secreted (ES) antigens was prepared from L. infantum promastigote culture supernatant. This vaccine achieved successful results in Phase III trials and was licensed and marketed as CaniLeish. We recently showed that newly identified ES promastigote surface antigen (PSA), from both viable promastigotes and axenically-grown amastigotes, represented the major constituent and the highly immunogenic antigen of L. infantum and L. amazonensis ES products. We report here that three immunizations with either the recombinant ES LaPSA-38S (rPSA) or its carboxy terminal part LaPSA-12S (Cter-rPSA), combined with QA-21 as adjuvant, confer high levels of protection in naive L. infantum-infected Beagle dogs, as checked by bone marrow parasite absence in respectively 78.8% and 80% of vaccinated dogs at 6 months post-challenge. The parasite burden in infected vaccinated dogs was significantly reduced compared to placebo group, as measured by q-PCR. Moreover, our results reveal humoral and cellular immune response clear-cut differences between vaccinated and control dogs. An early increase in specific IgG2 antibodies was observed in rPSA/QA-21- and Cter-rPSA/QA-21-immunized dogs only. They were found functionally active in vitro and were highly correlated with vaccine protection. In vaccinated protected dogs, IFN-γ and NO productions, as well as anti-leishmanial macrophage activity, were increased. These data strongly suggest that ES PSA or its carboxy-terminal part, in recombinant forms, induce protection in a canine model of zoonotic visceral leishmaniasis by inducing a Th1-dominant immune response and an appropriate specific antibody response. These data suggest that they could be considered as important active components in vaccine candidates.

Visceral leishmaniasis (VL), a potentially fatal disease caused by L. infantum, represents perfectly the need for a “One Health” approach for disease control, since it affects both humans and dogs, with similar clinical outcome and T-cell mediated immunity commitment. The dog vaccine development is highly required as our present resources for VL treatment and control have a limited effectiveness. It would represent the most convenient and efficient control way to decrease the dog-sandfly-dog transmission cycle, essential for human incidence reduction. The results indicate that recombinant forms of soluble promastigote surface antigen (PSA) are very promising effective vaccine candidates against canine VL. The elicited immune responses effectively reduced parasite load in in vitro pre-infected macrophages and in experimentally infected dogs. Through this approach, we aim to reduce the number of infected animals developing progressive infections thereby positively influencing human public health.

The genome of Leishmania panamensis: insights into genomics of the L. (Viannia) subgenus

Kinetoplastid parasites of the Leishmania genus cause several forms of leishmaniasis. Leishmania species pathogenic to human are separated into two subgenera, Leishmania (Leishmania) and L. (Viannia). Species from the Viannia subgenus cause predominantly cutaneous leishmaniasis in Central and South America, occasionally leading to more severe clinical presentations. Although the genomes of several species of Leishmania have been sequenced to date, only one belongs to this rather different subgenus. Here we explore the unique features of the Viannia subgenus by sequencing and analyzing the genome of L. (Viannia) panamensis. Against a background of conservation in gene content and synteny, we found key differences at the genomic level that may explain the occurrence of molecular processes involving nucleic acid manipulation and differential modification of surface glycoconjugates. These differences may in part explain some phenotypic characteristics of the Viannia parasites, including their increased adaptive capacity and enhanced metastatic ability.

In vitro evaluation of a soluble Leishmania promastigote surface antigen as a potential vaccine candidate against human leishmaniasis

PSA (Promastigote Surface Antigen) belongs to a family of membrane-bound and secreted proteins present in several Leishmania (L.) species. PSA is recognized by human Th1 cells and provides a high degree of protection in vaccinated mice. We evaluated humoral and cellular immune responses induced by a L. amazonensis PSA protein (LaPSA-38S) produced in a L. tarentolae expression system. This was done in individuals cured of cutaneous leishmaniasis due to L. major (CCLm) or L. braziliensis (CCLb) or visceral leishmaniasis due to L. donovani (CVLd) and in healthy individuals. Healthy individuals were subdivided into immune (HHR-Lm and HHR-Li: Healthy High Responders living in an endemic area for L. major or L. infantum infection) or non immune/naive individuals (HLR: Healthy Low Responders), depending on whether they produce high or low levels of IFN-γ in response to Leishmania soluble antigen. Low levels of total IgG antibodies to LaPSA-38S were detected in sera from the studied groups. Interestingly, LaPSA-38S induced specific and significant levels of IFN-γ, granzyme B and IL-10 in CCLm, HHR-Lm and HHR-Li groups, with HHR-Li group producing TNF-α in more. No significant cytokine response was observed in individuals immune to L. braziliensis or L. donovani infection. Phenotypic analysis showed a significant increase in CD4+ T cells producing IFN-γ after LaPSA-38S stimulation, in CCLm. A high positive correlation was observed between the percentage of IFN-γ-producing CD4+ T cells and the released IFN-γ. We showed that the LaPSA-38S protein was able to induce a mixed Th1 and Th2/Treg cytokine response in individuals with immunity to L. major or L. infantum infection indicating that it may be exploited as a vaccine candidate. We also showed, to our knowledge for the first time, the capacity of Leishmania PSA protein to induce granzyme B production in humans with immunity to L. major and L. infantum infection.

Identification and characterization of new Leishmania promastigote surface antigens, LaPSA-38S and LiPSA-50S, as major immunodominant excreted/secreted components of L. amazonensis and L. infantum

We have previously demonstrated that sera from dogs vaccinated with excreted/secreted antigens (ESA) of Leishmania infantum promastigotes (LiESAp) mainly recognized an immunodominant antigen of 54 kDa. An anti-LiESAp-specific IgG2 humoral response was observed and associated to Th1-type response in vaccinated dogs. This response was highly correlated with a long-lasting and strong LiESAp-vaccine protection toward L. infantum experimental infection. In addition, it was also shown that dogs from the vaccinated group developed a selective IgG2 response against an immunodominant antigen of 45 kDa of Leishmania amazonensis ESA promastigotes (LaESAp). In order to identify and characterize these immunodominant antigens, a mouse monoclonal antibody (mAb F5) was produced by immunization against LaESAp. It was found to recognize the major antigenic targets of both LaESAp and LiESAp. Analysis with mAb F5 of L. amazonensis amastigote and promastigote cDNA expression libraries enabled the identification of clones encoding proteins with significant structural homology to the promastigote surface antigens named PSA-2/gp-46. Among them, one clone presented a full-length cDNA and encoded a novel L. amazonensis protein of 38.6 kDa calculated molecular mass (LaPSA-38S) sharing an amino acid sequence consistent with that of the PSA polymorphic family and a N-terminal signal peptide, characteristic of a secreted protein. We then screened a L. infantum promastigote DNA cosmid library using a cDNA probe derived from the LaPSA-38S gene and identified a full-length clone of a novel excreted/secreted protein of L. infantum with a calculated molecular mass of 49.2 kDa and named LiPSA-50S. The fact that a significant immunological reactivity was observed against PSA, suggests that these newly identified proteins could have an important immunoregulatory influence on the immune response. This hypothesis is supported by the fact that (i) these proteins were naturally excreted/secreted by viable Leishmania promastigotes and amastigotes, and (ii) they are selectively recognized by vaccinated and protected dogs.

Development of Leishmania vaccines: predicting the future from past and present experience

Leishmaniasis is a disease that ranges in severity from skin lesions to serious disfigurement and fatal systemic infection. Resistance to infection is associated with a T-helper-1 immune response that activates macrophages to kill the intracellular parasite in a nitric oxide-dependent manner. Conversely, disease progression is generally associated with a T-helper-2 response that activates humoral immunity. Current control is based on chemotherapeutic treatments which are expensive, toxic and associated with high relapse and resistance rates. Vaccination remains the best hope for control of all forms of the disease, and the development of a safe, effective and affordable antileishmanial vaccine is a critical global public-health priority. Extensive evidence from studies in animal models indicates that solid protection can be achieved by immunization with defined subunit vaccines or live-attenuated strains of Leishmania. However, to date, no vaccine is available despite substantial efforts by many laboratories. Major impediments in Leishmania vaccine development include: lack of adequate funding from national and international agencies, problems related to the translation of data from animal models to human disease, and the transition from the laboratory to the field. Furthermore, a thorough understanding of protective immune responses and generation and maintenance of the immunological memory, an important but least-studied aspect of antiparasitic vaccine development, during Leishmania infection is needed. This review focuses on the progress of the search for an effective vaccine against human and canine leishmaniasis.

Proteoliposomes in nanobiotechnology

Proteoliposomes are systems that mimic lipid membranes (liposomes) to which a protein has been incorporated or inserted. During the last decade, these systems have gained prominence as tools for biophysical studies on lipid-protein interactions as well as for their biotechnological applications. Proteoliposomes have a major advantage when compared with natural membrane systems, since they can be obtained with a smaller number of lipidic (and protein) components, facilitating the design and interpretation of certain experiments. However, they have the disadvantage of requiring methodological standardization for incorporation of each specific protein, and the need to verify that the reconstitution procedure has yielded the correct orientation of the protein in the proteoliposome system with recovery of its functional activity. In this review, we chose two proteins under study in our laboratory to exemplify the steps necessary for the standardization of the reconstitution of membrane proteins in liposome systems: (1) alkaline phosphatase, a protein with a glycosylphosphatidylinositol anchor, and (2) Na,K-ATPase, an integral membrane protein. In these examples, we focus on the production of the specific proteoliposomes, as well as on their biochemical and biophysical characterization, with emphasis on studies of lipid-protein interactions. We conclude the chapter by highlighting current prospects of this technology for biotechnological applications, including the construction of nanosensors and of a multi-protein nanovesicular biomimetic to study the processes of initiation of skeletal mineralization.

The development and clinical evaluation of second-generation leishmaniasis vaccines

Infection with Leishmania parasites results in a range of clinical manifestations and outcomes. Control of Leishmania parasite transmission is extremely difficult due to the large number of vectors and potential reservoirs, and none of the current treatments are ideal. Vaccination could be an effective strategy to provide sustained control. In this review, the current global situation with regard to leishmaniasis, the immunology of Leishmania infection and various efforts to identify second generation vaccine candidates are briefly discussed. The variety of clinical trials conducted using the only current second generation vaccine approved for clinical use, LEISH-F1+MPL-SE, are described. Given that epidemiological evidence suggests that reducing the canine reservoir also positively impacts human incidence, efforts at providing a vaccine for leishmaniasis in dogs are highlighted. Finally, potential refinements and surrogate markers that could expedite the introduction of a vaccine that can limit the severity and incidence of leishmaniasis are discussed.

Leishmaniasis

Leishmaniasis is a disease that ranges in severity from skin lesions to serious disfigurement and fatal systemic infection. WHO has classified the disease as emerging and uncontrolled and estimates that the infection results in two million new cases a year. There are 12 million people currently infected worldwide, and leishmaniasis threatens 350 million people in 88 countries. Vaccination remains the best hope for control of all forms of the disease, and the development of a safe, effective and affordable antileishmanial vaccine is a critical global public-health priority. However, to date, no such vaccine is available despite substantial efforts by many laboratories. Main obstacle in vaccine design is the transition from the laboratory to the field and extrapolation of data from animal models to humans. This review discusses recent findings in the antileishmania vaccine field and current difficulties hampering vaccine implementation.

Leishmaniasis Vaccine: Where are We Today?

Leishmaniasis is a disease that ranges in severity from skin lesions to serious disfigurement and fatal systemic infection. WHO has classified the disease as emerging and uncontrolled and estimates that the infection results in two million new cases a year. There are 12 million people currently infected worldwide, and leishmaniasis threatens 350 million people in 88 countries. Current treatment is based on chemotherapy, which relies on a handful of drugs with serious limitations such as high cost, toxicity, difficult route of administration and lack of efficacy in endemic areas. Vaccination remains the best hope for control of all forms of the disease, and the development of a safe, effective and affordable antileishmanial vaccine is a critical global public-health priority. Extensive evidence from studies in animal models indicates that solid protection can be achieved by immunization with defined subunit vaccines or live-attenuated strains of Leishmania. However, to date, no such vaccine is available despite substantial efforts by many laboratories. The major impediment in vaccine design is the translation of data from animal models to human disease, and the transition from the laboratory to the field. Furthermore, a thorough understanding of protective immune responses and generation and maintenance of the immunological memory, the most important and least-studied aspect of antiparasitic vaccine development, during Leishmania infection is needed. This review focuses on recent findings in antileishmania vaccine field and highlights current difficulties facing vaccine development and implementation.

Leishmania (Viannia) panamensis: an in vitro assay using the expression of GFP for screening of antileishmanial drug

Promastigotes of Leishmania (Viannia) panamensis were successfully transfected with p6.5-egfp to express green fluorescent protein. The transfectants remained infective to macrophages, providing an in vitro model for screening antileishmanial drugs. This was demonstrated by flow cytometry of macrophage-associated GFP after exposure of infected cultures to known antileishmanial drugs, i.e. amphotericin B and glucantime. Fluorescence of GFP diminished progressively from infected cells with increasing drug concentrations used in both cases. The availability of this fluorescent assay for infection of macrophages by L. (V.) panamensis facilitates drug discovery program for the Viannia species, which differ significantly from those of the Leishmania subgenus.

Comparisons of mutants lacking the Golgi UDP-galactose or GDP-mannose transporters establish that phosphoglycans are important for promastigote but not amastigote virulence in Leishmania major

Abundant surface Leishmania phosphoglycans (PGs) containing [Gal(beta1,4)Man(alpha1-PO(4))]-derived repeating units are important at several points in the infectious cycle of this protozoan parasite. PG synthesis requires transport of activated nucleotide-sugar precursors from the cytoplasm to the Golgi apparatus. Correspondingly, null mutants of the L. major GDP-mannose transporter LPG2 lack PGs and are severely compromised in macrophage survival and induction of acute pathology in susceptible mice, yet they are able to persist indefinitely and induce protective immunity. However, lpg2(-) L. mexicana amastigotes similarly lacking PGs but otherwise normal in known glycoconjugates remain able to induce acute pathology. To explore this further, we tested the infectivity of a new PG-null L. major mutant, which is inactivated in the two UDP-galactose transporter genes LPG5A and LPG5B. Surprisingly this mutant did not recapitulate the phenotype of L. major lpg2(-), instead resembling the L. major lipophosphoglycan-deficient lpg1(-) mutant. Metacyclic lpg5A(-)/lpg5B(-) promastigotes showed strong defects in the initial steps of macrophage infection and survival. However, after a modest delay, the lpg5A(-)/lpg5B(-) mutant induced lesion pathology in infected mice, which thereafter progressed normally. Amastigotes recovered from these lesions were fully infective in mice and in macrophages despite the continued absence of PGs. This suggests that another LPG2-dependent metabolite is responsible for the L. major amastigote virulence defect, although further studies ruled out cytoplasmic mannans. These data thus resolve the distinct phenotypes seen among lpg2(-) Leishmania species by emphasizing the role of glycoconjugates other than PGs in amastigote virulence, while providing further support for the role of PGs in metacyclic promastigote virulence.

Comparative genomics: from genotype to disease phenotype in the leishmaniases

Recent progress in sequencing the genomes of several Leishmania species, causative agents of cutaneous, mucocutaneous and visceral leishmaniasis, is revealing unusual features of potential relevance to parasite virulence and pathogenesis in the host. While the genomes of Leishmania major, Leishmania braziliensis and Leishmania infantum are highly similar in content and organisation, species-specific genes and mechanisms distinguish one from another. In particular, the presence of retrotransposons and the components of a putative RNA interference machinery in L. braziliensis suggest the potential for both greater diversity and more tractable experimentation in this Leishmania Viannia species.

Leishmaniavaccines: progress and problems

Leishmaniaare protozoan parasites spread by a sandfly insect vector and causing a spectrum of diseases collectively known as leishmaniasis. The disease is a significant health problem in many parts of the world resulting in an estimated 12 million new cases each year. Current treatment is based on chemotherapy, which is difficult to administer, expensive and becoming ineffective due to the emergence of drug resistance. Leishmaniasis is considered one of a few parasitic diseases likely to be controllable by vaccination. The relatively uncomplicated leishmanial life cycle and the fact that recovery from infection renders the host resistant to subsequent infection indicate that a successful vaccine is feasible. Extensive evidence from studies in animal models indicates that solid protection can be achieved by immunisation with protein or DNA vaccines. However, to date no such vaccine is available despite substantial efforts by many laboratories. Advances in our understanding ofLeishmaniapathogenesis and generation of host protective immunity, together with the completedLeishmaniagenome sequence open new avenues for vaccine research. The major remaining challenges are the translation of data from animal models to human disease and the transition from the laboratory to the field. This review focuses on advances in anti-leishmania vaccine development over the recent years and examines current problems hampering vaccine development and implementation.

Molecular trees of trypanosomes incongruent with fossil records of hosts

Molecular trees of trypanosomes have confirmed conventionally accepted genera, but often produce topologies that are incongruent with knowledge of the evolution, systematics, and biogeography of hosts and vectors. These distorted topologies result largely from incorrect assumptions about molecular clocks. A host-based phylogenetic tree could serve as a broad outline against which the reasonability of molecular phylogenies could be evaluated. The host-based tree of trypanosomes presented here supports the " invertebrate first " hypothesis of trypanosome evolution, supports the monophyly of Trypanosomatidae, and indicates the digenetic lifestyle arose three times. An area cladogram of Leishmania supports origination in the Palaearctic during the Palaeocene.

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.

Surface glycoprotein PSA (GP46) expression during short- and long-term culture of Leishmania chagasi

The mRNAs encoding promastigote surface antigen (PSA) of Leishmania chagasi have previously been shown to increase about 30-fold as in vitro cultured parasites progress from logarithmic to stationary phase, growth phases that are, respectively associated with parasites having low and high infectivity to mammals. Experiments reported here establish by western blot analysis that PSA proteins of 44 and 66 kDa also increase about 30-fold as parasite cultures reach stationary phase. Serial passage of parasite cultures resulted in a progressive reduction in PSA protein and RNA abundance to levels less than 3% that of cultures newly-initiated with parasites derived from a parasitized rodent. Loss of PSA mRNA abundance in serially passaged cells was not due to reduced PSA gene transcription rates, as determined by nuclear run-on assays. Neither was the loss associated with a marked decrease in PSA mRNA stability. Analysis of PSA RNA stability in the presence of actinomycin D, an inhibitor of transcription elongation, failed to detect a difference in fully processed cytosolic PSA mRNA stability regardless of the number of times a culture was passaged or the growth phase of the culture. Based on the lack of detectable difference in (cytosolic) mature PSA mRNA stability during promastigote development, the data indirectly suggest that the regulated expression of PSA in cells from low-passage cultures and the loss of PSA expression in high-passage cultures may be mediated by nuclear events that occur after transcription of the PSA genes and before arrival of the mature mRNAs in the cytoplasm.

Vaccination against cutaneous leishmaniasis: current status

The different cutaneous leishmaniases are distinct in their etiology, epidemiology, transmission, and geographical distribution. In most instances cutaneous leishmaniasis is limited to one or a few skin ulcers that develop at the site where the parasites were deposited during the bite of the sandfly vector. Lesions typically heal spontaneously after several months but some lesions can be large and follow a chronic, more severe course. Protective immunity is usually acquired following cutaneous infection with Leishmania spp., so prevention of disease through prophylactic immunization appears to be feasible. Since vaccination with live, virulent parasites is associated with an unacceptable rate of adverse events, attention has turned to the use of killed or attenuated parasite vaccines and defined subunit vaccines. Whole parasite vaccines have the advantage of delivering multiple antigenic epitopes that may be necessary for initiation of a broad-based immune response. Persistent or repeated immune-stimulation by parasite antigens and/or sustained expression of interleukin-12 appear to be critical elements in the development of durable immunity. A number of purified or recombinant antigens, when co-administered with a vaccine adjuvant, appear promising as vaccine candidates against cutaneous leishmaniasis. The sustained expression of recombinant Leishmania antigens by vaccination with DNA is an attractive approach because it mimics the persistent antigenic stimulation of subclinical infection. Effective vaccine-induced immunity must generate an antigen-specific memory T cell population that, upon exposure to the infecting parasite, rapidly produces a type 1 effector T cell response that leads to interferon-gamma-mediated activation of infected macrophages to kill the intracellular parasites. This parasite-directed recall response must be prompt and of sufficient magnitude to overcome the subversive effect that the intracellular infection has on macrophage effector function. It is unlikely that vaccination against cutaneous leishmaniasis would induce sterile immunity, but a small number of parasites are likely to persist subclinically.

Comparison of the post-transcriptional regulation of the mRNAs for the surface proteins PSA (GP46) and MSP (GP63) of Leishmania chagasi

MSP (GP63) and PSA (GP46) are abundant 63- and 46-kDa glycolipid-anchored proteins on the surface of the promastigote form of most Leishmania species. MSP is a zinc metalloprotease that confers resistance to host complement-mediated lysis. PSA contains internal repeats of 24 amino acids, and its function is unknown. The steady state levels of mRNAs for both glycoproteins are regulated post-transcriptionally, resulting in about a 30-fold increase as Leishmania chagasi promastigotes grow in vitro from logarithmic phase to stationary phase. Previous studies showed the 3'-untranslated regions (3'-UTRs) of these mRNAs are essential for this post-transcriptional regulation. These two 3'-UTRs of 1.0 and 1.3 kilobases were cloned immediately downstream of a beta-galactosidase reporter gene in a plasmid, and segments were systematically deleted to examine which portions of the 3'-UTRs contribute to the post-transcriptional regulation. The 92-nucleotide segment of greatest similarity between the two 3'-UTRs was deleted without loss of regulation, but the segments flanking this similarity region have positive regulatory elements essential for the regulation. We propose that similar, but non-identical, molecular mechanisms regulate the parallel expression of these two L. chagasi mRNAs despite their lack of sequence identity. These post-transcriptional mechanisms resemble the mechanism recently suggested for the regulation of mRNAs encoding the dipeptide (EP) and pentapeptide (GPEET) repeat proteins in Trypanosoma brucei that involves interactions between positive and negative regulatory elements in the 3'-UTR.

Leishmaniasis: current status of vaccine development

Leishmaniae are obligatory intracellular protozoa in mononuclear phagocytes. They cause a spectrum of diseases, ranging in severity from spontaneously healing skin lesions to fatal visceral disease. Worldwide, there are 2 million new cases each year and 1/10 of the world's population is at risk of infection. To date, there are no vaccines against leishmaniasis and control measures rely on chemotherapy to alleviate disease and on vector control to reduce transmission. However, a major vaccine development program aimed initially at cutaneous leishmaniasis is under way. Studies in animal models and humans are evaluating the potential of genetically modified live attenuated vaccines, as well as a variety of recombinant antigens or the DNA encoding them. The program also focuses on new adjuvants, including cytokines, and delivery systems to target the T helper type 1 immune responses required for the elimination of this intracellular organism. The availability, in the near future, of the DNA sequences of the human and Leishmania genomes will extend the vaccine program. New vaccine candidates such as parasite virulence factors will be identified. Host susceptibility genes will be mapped to allow the vaccine to be targeted to the population most in need of protection.

Chromosome structure and sequence organization between pathogenic and non-pathogenic Leishmania spp

We have used a chromosome fragmentation strategy based on systematic genomic insertions of the rare cutting yeast I-SceI endonuclease to assess structure and sequence organization of homologous chromosomes between evolutionary divergent pathogenic and non-pathogenic Leishmania species. This method was combined to physical mapping and hybridization studies using a number of specific chromosomal markers as probes. Our studies have concentrated on two different chromosomes of Leishmania major (L. major), L. donovani and L. infantum and of the non-pathogenic species L. tarentolae. Specific chromosome fragmentation events at the level of multiple I-SccI genomic integrations indicated that very similar distances separated internal genomic sequences between homologous chromosomes and that distances from chromosome ends were more variable. The order and orientation of genes along the homologous chromosomes were also conserved between species. With only few exceptions, genome organization between pathogenic and non-pathogenic Leishmania species was found to be highly conserved. Genomic comparison of pathogenic and non-pathogenic species may be useful for depicting regions involved in species-specific related pathologies.

Speculations on the origin and evolution of the genus Leishmania

Recently two hypotheses have been proposed for the evolution of Leishmania involving respectively a Neotropical or Paleartic origin for the species. Here an alternative proposal on the phylogeny of Leishmania based on the major divisions within the genus is presented. In this hypothesis a Neotropic origin is retained for L. (Viannia) and Paraleishmania, a recently described section within the genus Leishmania, while an African origin is proposed for L. (Leishmania) and possibly Sauroleishmania. The current distribution of Leishmania in the Neotropics is explained as the product of multiple introductions of Leishmania parasites into the New World. Problems with organismal identity in Sauroleishmania and the use of molecular sequence data in inferring phylogenies are also discussed.

Further support for a Palaearctic origin of Leishmania

The fossil record and systematics of murid rodents, reservoirs of zoonotic cutaneous leishmaniasis in the Palaearctic, Oriental, African, Nearctic and Neotropical, strongly support a Palaearctic origin of Leishmania. The fossil record and systematics of phlebotomine sand flies reinforce this idea. Interpretations of molecular data that place the origin of Leishmania in the Neotropical are inconsistent with the natural histories of reservoirs and vectors. The evolutionary pattern of New World rats (Sigmodontinae) indicates that they may be the most important reservoirs of zoonotic cutaneous leishmaniasis throughout their range.

Leucine rich repeats are the main epitopes in Leishmania infantum PSA during canine and human visceral leishmaniasis

The PSA protein is one of the major antigens of the surface of the Leishmania infantum parasite membrane. We describe the immune humoral response against the PSA in dogs and human patients with visceral leishmaniasis caused by L. infantum. The immunodominant region of the PSA was determined by subcloning, expression and purification of three fragments covering the complete protein. The analysis revealed that the antibodies are mostly directed against the central region, which is formed exclusively by leucine rich repeats. This region is recognized by 100% of the sera from the infected dogs and 40% of the human sera. These percentages are significantly higher than those observed when the complete protein was used as antigen. The analysis of the isotype of the G immunoglobulins raised against the immunodominant determinants of the PSA indicates that both IgG1 and IgG2 classes are produced during natural infections but that the IgG2 predominates over that of the IgG1.

Palaearctic origin of Leishmania

The hypothesis of a Palaearctic origin of Leishmania in the early Cenozoic, dispersal to the Nearctic in the late Eocene and to the Neotropical in the Pliocene is presented. It is further hypothesized that murid rodents and their immediate ancestors have been important mammalian reservoirs since the origination of Leishmania. Biochemical, molecular, biogeographical, entomological, mammalalogical and ecological support for these hypotheses are reviewed.

Species-specificity in endoplasmic reticulum signal peptide utilization revealed by proteins from Trypanosoma brucei and Leishmania

N-Terminal signal peptides direct secretory and most membrane proteins into the exocytic pathway at the endoplasmic reticulum. Signal sequences can function across kingdoms. However, our attempts at translocating variant surface glycoprotein (VSG) 117, VSG MVAT7, VSG 221 and BiP from Trypanosoma brucei and gp63 from Leishmania chagasi into canine pancreas microsomes failed. On replacing the signal peptide of VSG 117 with that from yeast prepro-alpha-mating factor (ppalphaMF) the chimaeric protein was imported, indicating that the signal sequence of VSG 117 was incompatible with the protein-import machinery of mammalian microsomes. Replacement of the gp63-h-region with a hybrid composed of the N-terminal nine residues from the h-region of gp67 from Autographa californica nuclear polyhedrosis virus and the C-terminal 10 residues from the h-region of gp63 from L. major produced a functional signal peptide. Thus, the h-region of kinetoplastid signal peptides appears to be the subdomain that is non-functional at the mammalian translocon. The calculated biophysical properties and computed discriminant scores (predictive of importability of signal peptides into mammalian microsomes) of the kinetoplastid signal sequences nevertheless are similar to those of ppalphaMF and Escherichia coli beta-lactamase both of which were imported. These signal peptides are the first collection from one biological family that have been found to fail to function across a species barrier. They indicate that signal peptides are not as universally interchangeable as previously believed. Intriguingly, endoplasmic reticulum signal peptides from Leishmania and Crithidia fasciculata are reminiscent of signal peptides from Gram-positive bacteria.

The Leishmania promastigote surface antigen-2 (PSA-2) is specifically recognised by Th1 cells in humans with naturally acquired immunity to L. major

The promastigote surface antigen-2 (PSA-2) is a Leishmania parasite antigen, which can induce Th1-mediated protection against murine leishmaniasis when used as a vaccine. To evaluate PSA-2 as a human vaccine candidate the specific T-cell response to PSA-2 was characterised in individuals immune to cutaneous leishmaniasis. Peripheral blood mononuclear cells from Sudanese individuals with a past history of self-healing cutaneous leishmaniasis proliferated vigorously in response to PSA-2 isolated from Leishmania major, whereas the antigen did not activate cells from presumably unexposed Danes. Peripheral blood mononuclear cells from individuals with previous L. major infection had varying proliferative responses to PSA-2 derived from L. donovani promastigotes. Peripheral blood mononuclear cells activated by PSA-2 from L. major produced high amounts of interferon-gamma and tumour necrosis factor-beta, and little interleukin-4, thereby showing a Th1 cytokine pattern. Parallel cultures showed clear Th1 and Th2 response patterns to purified protein derivative of tuberculin or tetanus toxoid, respectively. Flow cytometric analysis revealed that PSA-2 induced blastogenesis in the CD3 positive population and that these cells were the major source of interferon-gamma. The results show that Th1-like cells recognising PSA-2 are expanded during infection by L. major and that they maintain their Th1-like cytokine profile upon reactivation in vitro. Since immunity to cutaneous leishmaniasis is mediated by antigen-specific Th1-like cells, PSA-2 might be considered a vaccine candidate for human leishmaniasis.

Differential sensitivity of New World Leishmania spp. promastigotes to complement-mediated lysis: correlation with the expression of three parasite polypeptides

American tegumentary leishmaniasis is caused by Leishmania of the subgenera Leishmania and Viannia. In this paper, we demonstrate that promastigotes of these two subgenera display distinct characteristic patterns of complement sensitivity during growth in vitro. Using fresh normal human serum in lytic assays, we show that while promastigotes of two species of the subgenus Leishmania differentiate into forms that are more resistant to the lytic action of complement, promastigotes of three species of the subgenus Viannia remain sensitive to complement mediated lysis during all stages of their growth in vitro. Complement resistance of the subgenus Leishmania is temporary, reaching its peak at the beginning of the stationary phase of growth, and decreasing thereafter. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) we detected in L. amazonensis (subgenus Leishmania), but not in L. guyanensis (subgenus Viannia), three polypeptides whose expression parallels the resistance of promastigotes to complement-mediated lysis.

T cell responses to crude and defined leishmanial antigens in patients from the lower Amazon region of Brazil infected with different species of Leishmania of the subgenera Leishmania and Viannia

Amazonian localized cutaneous leishmaniasis (LCL) is caused by parasites of the subgenera Leishmania and Viannia. Respectively, these parasites may cause diffuse cutaneous leishmaniasis (DCL) and mucocutaneous leishmaniasis (MCL). This, together with differing skin test responses, suggests some species-specificity in cell mediated immunity. In this study, T cell responses (proliferative and interferon-gamma) to crude and defined antigens were examined in paired samples pre and post chemotherapy. Untreated L. (L.) amazonensis LCL patients showed lower responses to crude leishmanial antigens than the L. (V.) spp. group. L. (V.) braziliensis antigen was a more potent stimulator of T cell responses than L. (L.) amazonensis antigen in all patient groups. Few positive responses were seen to the L. (L.) amazonensis glycoprotein GP46. A substantial proportion of LCL patients did respond to the L. (L.) pifanoi amastigote antigens A2, and the surface membrane glycoprotein P8. DCL patients were poor responders to all leishmanial antigens, except GP46. In contrast, MCL patients were good responders to all antigens except GP46 and A2. A significant rise in the response to P8 and A2 antigen was seen post treatment across all LCL and MCL patients, indicating that these antigens might provide suitable vaccine candidates.

Recombinant antigens for specific and sensitive serodiagnosis of Latin American tegumentary leishmaniasis

The diagnostic potential of recombinant leishmanial antigens for Latin American tegumentary leishmaniasis (LATL) was examined. Two Leishmania (Viannia) peruviana recombinant proteins, T26-U2 and T26-U4, were assessed by their reactivity to detect specific anti-leishmanial antibodies. Seventy-eight individual sera from persons with LATL, 39 from those with other diseases, and 10 negative control sera were tested by Western blotting and enzyme-linked immunosorbent assay. The sensitivity of the test using T26-U2 plus T26-U4 was similar to that obtained with whole parasite extract (92%). However, the specificity obtained using both recombinant antigens (87%) was higher than that of the whole parasite extract (65%). All tests using recombinant proteins (T26-U2, T26-U2 plus T26-U4 or T26-U4) had a higher positive predictive value (89%, 92% and 98%, respectively) than the value obtained using total parasites (81%). Eleven Colombian sera were also tested, and the results indicated that T26-U2 plus T26-U4 could be used successfully in Peru and in other Latin American countries.

Glycoprotein 46 mRNA abundance is post-transcriptionally regulated during development of Leishmania chagasi promastigotes to an infectious form

GP46 is an abundant glycoprotein of 46 kDa on the surface of the promastigote form of most Leishmania species. We show that the steady state level of GP46 mRNA increases >>30-fold as Leishmania chagasi promastigotes develop in vitro from a less infectious form during logarithmic growth to a highly infectious form in the stationary phase of cultivation. Nuclear run-on experiments demonstrate that this increase in GP46 mRNA abundance is regulated post-transcriptionally. Plasmids containing the 3'-untranslated regions (UTRs) and downstream intergenic regions (IRs) of two different GP46 genes fused immediately downstream of the beta-galactosidase coding region were transfected into L. chagasi, and beta-galactosidase activity and mRNA levels were examined. The presence of the 3'-UTR + IR of one GP46 gene (gp46A) resulted in a steady increase in beta-galactosidase activity and mRNA level as the transfected promastigotes developed from logarithmic to stationary phase. This differential effect parallels that of the 3'-UTRs + IRs of a family of genes for an unrelated Leishmania surface glycoprotein, GP63. Thus, post-transcriptional regulation of the genes for two different surface glycoproteins of Leishmania occurs via a similar mechanism.

Conservation of low-copy gene loci in Old World leishmanias identifies mechanisms of parasite evolution and diagnostic markers

Genome plasticity has been hypothesized to be a driving force behind parasite speciation. We have evaluated divergence in single and low-copy genes in terms of locus organization, chromosomal localization and gene expression in Leishmania infantum, L. major, L. tropica and three widely divergent geographic isolates of L. donovani. Seventeen genes of low to moderate copy number (1-4 copies/haploid genome) were analyzed to identify restriction fragment length polymorphisms (RFLPs) providing heritable markers distinguishing Old World (OW) leishmanias. These RFLP markers were conserved in parasite isolates from primary infections demonstrating their utility as diagnostic tools. The species designations established by RFLP analysis of field isolates was confirmed by use of monoclonal antibodies. All 17 genes were present in each OW leishmania analyzed except LSIP (A45), which was absent from L. infantum. The 17 genes were found to be distributed among 9 distinct chromosomes. However, in spite of variations in chromosome karyotypes among the various OW leishmanias, individual gene probes localized to a similar sized chromosome from each isolate. These observations coupled with a molecular tree derived from RFLP data suggest that the OW leishmanias comprise a monophyletic lineage, with species associated with cutaneous disease exhibiting the greatest level of divergence. Data from this study supports previous observations that species causing cutaneous and visceral disease have diverged primarily by nucleotide substitutions. Such nucleotide divergence may not only lead to changes in protein function and antigenicity, but may also alter gene regulation programs as exemplified by the finding that the LdI-9-5 and LdE-6-1 genes were expressed only in visceralizing leishmanias.

Identification of differentially expressed Leishmania donovani genes using arbitrarily primed polymerase chain reactions

Arbitrarily primed polymerase chain reactions (AP-PCR) were used to amplify polymorphic DNA fragments from the genomes of a variety of geographic isolates of Leishmania donovani (Ld). From the latter, five polymorphic DNA fragments were cloned and sequence analysis identified 15 unique clones. Northern blot analysis showed that 13 of the 15 clones hybridized to transcribed RNAs isolated from Ld. Eight of these 13 AP-PCR clones specifically hybridized to Ld RNAs that were differentially expressed in promastigote and 'amastigote' cells. Comparative Northern analysis of four differentially expressed AP-PCR clones indicated that two clones, LdS-14-14 and LdI-9-7, were expressed in Ld and several other Leishmania species. However, RNAs corresponding to two other AP-PCR clones, LdE-6-1 and LdI-9-5, were detected only in members of the Ld complex, and not in L. major (Lm) or L. tropica (Lt). Comparative Southern blot analysis of the LdS-14-14 locus revealed numerous restriction-fragment length polymorphisms (RFLP) distinguishing Lm and Lt from the Ld isolates and L. infantum. However, the LdS-14-14 loci were mapped to similar-sized chromosomes observed among all Old World Leishmania species tested, indicating that localized nucleotide divergence, not chromosomal rearrangement, was responsible for altered Southern blot patterns. These results demonstrate that AP-PCR is a very useful method for identifying expressed gene sequences in organisms of relatively low-complexity genomes. Interestingly, the majority of these sequences identified in this study correspond to differentially expressed genes.(ABSTRACT TRUNCATED AT 250 WORDS)

The Leishmania promastigote surface antigen 2 complex is differentially expressed during the parasite life cycle

The promastigote surface antigen 2 (PSA-2) complex comprises a family of antigenically similar polypeptides of M(r) 96,000, 80,000 and 50,000, anchored to the membrane with glycosylphosphatidylinositol. Although PSA-2 was initially detected only in promastigotes, Northern blot analysis indicated that mRNA transcripts are also present in amastigotes. Unlike the situation in promastigotes, where at least four major transcripts (2.6-5.3 kb) were detected, only one major (2.6 kb) and two minor transcripts were present in amastigotes. A cDNA clone encoding a member of the PSA-2 family expressed in amastigotes was isolated using DNA probes. The predicted protein sequence of M(r) 40,000 is distinct from promastigote sequences, but shows significant similarity to previously described members of the family from L major and L amazonensis. Antibodies to the carboxyl terminal sequence conserved in all L major PSA-2 studied to date, as well as antibodies affinity purified on the amastigote cDNA-derived polypeptide recognized a major M(r) 50,000 amastigote polypeptide. Immuno-electron microscopy localized both promastigote and amastigote PSA-2 to the cell surface. The expression of PSA-2 polypeptides during the transformation of amastigotes into promastigotes was ordered in a time-dependent manner, with the promastigote M(r) 80000 polypeptide appearing first, followed by the M(r) 96000 polypeptide. In contrast to the glycosylphosphatidylinositol anchor of promastigote PSA-2, which could be hydrolysed by phosphatidylinositol-specific phospholipase C, the amastigote form was resistant to this enzyme.

Protective vaccination with promastigote surface antigen 2 from Leishmania major is mediated by a TH1 type of immune response

Leishmania major promastigote surface antigen-2 complex (PSA-2) comprises a family of three similar but distinct polypeptides. The three PSA-2 polypeptides were purified from cultured promastigotes by a combination of detergent phase separation and monoclonal antibody affinity chromatography. Intraperitoneal vaccination of C3H/He mice with PSA-2 with Corynebacterium parvum as an adjuvant resulted in complete protection from lesion development after challenge infection with virulent L. major. Significant protection was also obtained in the genetically susceptible BALB/cH-2k and BALB/c mice. One of the PSA-2 genes was cloned and expressed in both Escherichia coli and Leishmania mexicana promastigotes. Vaccination with the recombinant PSA-2 purified from E. coli did not confer protection, in contrast to the L. mexicana-derived recombinant PSA-2, which provided excellent protection. CD4+ T cells isolated from the spleens of vaccinated mice produced large amounts of gamma interferon but no detectable interleukin 4 upon stimulation with PSA-2 in vitro. Limiting dilution analysis showed a marked increase in the precursor frequency of PSA-2-specific gamma interferon-secreting CD4+ T cells. No substantial change in precursor frequency was observed for interleukin 4-secreting T cells in the vaccinated mice. A CD4+ PSA-2 specific T-cell line generated from splenocytes of a vaccinated mouse produces a cytokine pattern consistent with a TH1 phenotype. Intravenous injection of this line into naive mice reduced significantly the parasite burden upon challenge infection. Taken together, the data suggest that vaccination with PSA-2 induces a TH1 type of immune response which protects mice from L. major infection. Moreover, a single recombinant PSA-2 polypeptide derived from a genomic clone can also vaccinate, provided that the structural form of the antigen is near native.

Discrimination amongst Leishmania by polymerase chain reaction and hybridization with small subunit ribosomal DNA derived oligonucleotides

A method for discriminating among Leishmania is described, based upon small subunit ribosomal DNA sequence differences. The method was to amplify the entire 2.2 kb small subunit rDNA by polymerase chain reaction using conserved primers specific for the 5' and 3' termini of the small subunit ribosomal RNA, and then hybridize the product dotted onto nylon membranes with labeled oligonucleotides. The design of the hybridization probes was based upon complete small subunit rDNA sequences from L. amazonensis, L. major and L. guyanensis and partial sequences of L. mexicana, L. braziliensis, L. tropica and L. chagasi. A high degree of sequence similarity (> 99%) among species was found. However, sufficient sequence divergence occurred to permit the design of internal oligonucleotide probes specific for species complexes. This procedure successfully discriminated amongst a wide range of Leishmania isolates. The method detected as few as 10 cultured organisms and detected parasites in tissue samples from experimentally infected animals. Non-radioactive labeling showed the same specificity and sensitivity as radioactive probes.

Recombinant vaccinia viruses expressing GP46/M-2 protect against Leishmania infection

Leishmania is a genus of parasitic protozoa capable of causing a spectrum of human diseases. The GP46/M-2 membrane glycoprotein has been demonstrated in a murine model system to elicit a protective immune response against infection with Leishmania amazonensis; in highly susceptible BALB/c mice, immunization leads to significant protection against infection. In the present study, for induction of long-term immunological effects, two recombinant vaccinia viruses, derived from the wild type and attenuated variant 48-7 and expressing the GP46/M-2 protein, were constructed; to ensure safety, we used the attenuated vaccinia virus mutant (48-7) as a live vector. Susceptible BALB/c mice immunized with either GP46/M-2-recombinant vaccinia virus were significantly protected against infection with L. amazonensis; 45 to 76% of the animals were completely protected (sterile) against a challenge inoculum of 10(3) infective organisms. The protectively immunized animals demonstrated T- and B-cell-dependent immunological responses; both lymphokine responses as well as antibody responses and long-term memory are indicative of T-cell activation. This first report of the use of a recombinant vaccinia virus to induce protection against a Leishmania infection indicates that recombinant vaccinia viruses should be of value in the design of a safe and effective vaccine against this parasitic disease.

Molecular variation in Leishmania

The surface coat of the protozoan parasite Leishmania affords remarkable protection in the harsh environments encountered within the insect vectors and vertebrate hosts. It also provides specificity for the interaction of these parasites with the cells in the sandfly gut and with the human macrophage. Surprisingly few molecules have been identified on the Leishmania surface. The major surface molecules of both promastigotes and amastigotes are the glycoconjugates lipophosphoglycan and a glycoprotein of approximately 63 kDa. These major surface molecules vary structurally between Leishmania species and throughout the life-cycle of the parasite. In addition to these major glycoconjugates, Leishmania produce a number of less abundant surface molecules, including a family of glycosyl-inositol phospholipids, the Promastigote Surface Antigen-2 complex of glycoproteins and a glycoprotein of M(r) 46,000. These molecules share the common feature of attachment to the plasma membrane via glycosylphosphatidylinositol lipid anchors. Leishmania also release molecules from their surface in a species specific manner. In this review we will examine the molecular variation of these molecules and their biological importance. We will also discuss the potential of these molecules as targets for chemotherapy and as candidate vaccines.