Metacyclogenesis of Leishmania (Viannia) braziliensis in vitro: evidence that lentil lectin is a marker of complement resistance and enhanced infectivity

Stimulation of metacyclogenesis in Leishmania (Mundinia) orientalis for mass production of metacyclic promastigotes

Leishmania (Mundinia) orientalis is a human pathogen causing leishmaniasis and studies on the properties of metacyclic promastigotes, the parasite’s infective stage, are required for a better understanding of its transmission and infection. However, information on cultivation for mass production of L. orientalis metacyclic promastigotes and factors that stimulate their metacyclogenesis is limited. Therefore, the objective of this study was to develop a suitable methodology for generating promastigote cultures containing a high proportion and number of L. orientalis metacyclic promastigotes. Various media, i.e., Schneider’s insect medium, Medium 199 and Grace’s insect medium, supplemented with various quantities of dithiothreitol, Basal Medium Eagle vitamins, pooled human urine, and fetal bovine serum, were optimized for metacyclogenesis. The results revealed that the optimum culture medium and conditions of those tested were Schneider’s insect medium supplemented with 100 μM dithiothreitol, 1% (v/v) Basal Medium Eagle vitamins, 2% (v/v) pooled human urine, and 10% (v/v) fetal bovine serum, pH 5.0 at 26°C. We also demonstrated that L. orientalis metacyclic promastigotes could be purified and enriched by negative selection using peanut lectin. Under these culture conditions, the highest yield of metacyclic promastigotes was obtained with a significantly higher percentage of parasite survival, resistance to complement-mediated lysis, and infection index in THP-1 macrophage cells compared to parasites cultured without media supplements at neutral pH. This is the first report providing a reliable method for mass production of L. orientalis metacyclic promastigotes for in vivo infections and other experimental studies of this emerging parasite in the future.

Healed Lesions of Human Cutaneous Leishmaniasis Caused By Leishmania major Do Not Shelter Persistent Residual Parasites

In human cutaneous leishmaniasis (HCL) caused by Leishmania (L.) major, the cutaneous lesions heal spontaneously and induce a Th1-type immunity that confers solid protection against reinfection. The same holds true for the experimental leishmaniasis induced by L. major in C57BL/6 mice where residual parasites persist after spontaneous clinical cure and induce sustainable memory immune responses and resistance to reinfection. Whether residual parasites also persist in scars of cured HCL caused by L. major is still unknown. Cutaneous scars from 53 volunteers with healed HCL caused by L. major were biopsied and the tissue sample homogenates were analyzed for residual parasites by four methods: i) microscope detection of amastigotes, ii) parasite culture by inoculation on biphasic medium, iii) inoculation of tissue exctracts to the footpad of BALB/c mice, an inbred strain highly susceptible to L. major, and iv) amplification of parasite kDNA by a highly sensitive real-time PCR (RT-PCR). Our results show that the scars of healed lesions of HCL caused by L. major do not contain detectable residual parasites, suggesting that this form likely induces a sterile cure at least within the scars. This feature contrasts with other Leishmania species causing chronic, diffuse, or recidivating forms of leishmaniasis where parasites do persist in healed lesions. The possibility that alternative mechanisms to parasite persistence are needed to boost and maintain long-term immunity to L. major, should be taken into consideration in vaccine development against L. major infection.

Intra-Specific Diversity of Leishmania major Isolates: A Key Determinant of Tunisian Zoonotic Cutaneous Leishmaniasis Clinical Polymorphism

The clinical expression of zoonotic cutaneous leishmaniasis (ZCL) caused by Leishmania (L.) major parasites has a broad spectrum ranging from asymptomatic infection to self-limited cutaneous sores or severe disease. In concert with the host immune responses, the vector variability and the number of bites, genetic variation between L. major isolates might impact on the clinical output of the disease. We investigated herein the intra-specific variability of L. major field isolates independently of host or vector factors and then tried to correlate parasite variability to ZCL severity in corresponding patients. Several assays were applied, i.e., in vivo pathogenicity of promastigotes in a BALB/c mice model, resistance/sensibility to complement lysis, in vitro growth kinetics, and expression of different lectins on the promastigote surface. Combining all these parameters allowed us to conclude that the resistance to complement lysis and PNA/Jacalin lectins binding to parasite surfaces are important markers of parasite virulence. These factors correlate significantly with clinic polymorphism of ZCL and modestly with genetic micro-heterogeneity, a characteristic we previously revealed with a MLMT profile.

Metacyclogenesis of Leishmania (Viannia) guyanensis: a comprehensive study of the main transformation features in axenic culture and purification of metacyclic promastigotes by negative selection with Bauhinia purpurea lectin

Leishmania (Viannia) guyanensis is one species that causes cutaneous leishmaniasis in the New World. The incidence of infections with this parasite is probably underestimated and few studies exist on this species, despite its epidemiological importance. In particular, there are no studies concerning L. guyanensis metacyclogenesis and no technique for obtaining metacyclic promastigotes for this species is presently available. Here, we have studied L. guyanensis metacyclogenesis in axenic culture, describing the main changes that occur during this process, namely, in morphology and size, sensitivity to complement-mediated lysis, surface carbohydrates and infectivity to macrophages. We have shown that metacyclogenesis in L. guyanensis promastigotes is basically complete on the 4th day of culture, as determined by decreased body size, increased flagellum length, resistance to complement-mediated lysis and infectivity. We have also found that only a fraction of the parasites is agglutinated by Bauhinia purpurea lectin. The non-agglutinated parasites, which also peaked on the 4th day of culture, had all morphological traits typical of the metacyclic stage. This is the first report describing metacyclogenesis in L. guyanensis axenic promastigotes and a simple and efficient method for the purification of metacyclic forms. Furthermore, a model of human macrophage infection with L. guyanensis was established.

Elucidating in vitro and in vivo phenotypic behaviour of L. infantum/L. major natural hybrids

The clinical manifestation and course of Leishmania infections depend on factors such as species, virulence and host-immunity. Although trypanosomatids are considered to have clonal propagation, genetic hybridization has produced successful natural hybrid lineages. Hybrids displaying strong selective advantages may have an impact on pathogenesis and the eco-epidemiology of leishmaniasis. Thus, characterization of phenotypic properties of Leishmania hybrids could bring significant insight into the biology, infectivity, pathogenicity and transmission dynamics of these atypical strains. The present study focuses on phenotypic features and survival capacity of Leishmania infantum/Leishmania major hybrid isolates as compared with representative putative parental species, L. infantum and L. major. In vitro assays (growth kinetics, susceptibility to different conditions) and in vivo infection (parasite detection and histopathological alterations) showed that hybrids present higher growth capacity and decreased susceptibility to reactive oxygen species. Furthermore, evaluation of infected spleen tissue suggests that hybrids induce a stronger immune reaction than their putative parents, leading to the development of white pulp hyperplasia in B-lymphocyte compartments. Overall, these hybrids have shown high plasticity in terms of their general behaviour within the different phenotypic parameters, suggesting that they might have acquired genetic features conferring different mechanisms to evade host cells.

In vitro metacyclogenesis of Leishmania (Viannia) braziliensis and Leishmania (Leishmania) amazonensis clinical field isolates, as evaluated by morphology, complement resistance, and infectivity to human macrophages

This study was designed to assess in vitro metacyclogenesis of Leishmania (Viannia) braziliensis and Leishmania (Leishmania) amazonensis clinical field isolates obtained from patient lesions (L. braziliensis IMG3 and PPS6m; L. amazonensis MAB6). Metacyclogenesis was evaluated by different criteria, namely, promastigote size (morphometric analysis and flow cytometry), surface modifications (loss of lectin or monoclonal antibody (mAb) binding, complement resistance), and infectivity to human macrophages. Growth curves were similar for all parasites evaluated. The various features analyzed were expressed in a high percentage of promastigotes at 6th and 10th days of culture and a low percentage at the 2nd day. However, in most isolates, these features, considered as markers of metacyclogenesis, seemed to develop with different time courses, since the percentages of metacyclic forms detected with each technique were usually different. Parasites from 6th or 10th day and those negatively selected with lectin or mAb similarly infected human macrophages. From all isolates analyzed, L. amazonensis PH8 and MAB6 showed the highest and the lowest levels of susceptibility, respectively, to leishmanicidal activity of IFN-γ/LPS-activated macrophages. Our results showed that by using different techniques to evaluate different aspects of metacyclogenesis (morphological and biochemical modifications) different percentages of metacyclic promastigotes can be detected in each isolate culture.

In vitro and in vivo behaviour of sympatric Leishmania (V.) braziliensis, L. (V.) peruviana and their hybrids

Leishmania (Viannia) braziliensis is the main cause of highly disfiguring mucocutaneous leishmaniasis (MCL) in South America. The related species L. (V.) peruviana has only been identified in simple cutaneous lesions (CL). Hybrids between L. braziliensis and L. peruviana have been reported although genetic exchange in Leishmania is considered to be rare. Here we compared growth in vitro, adaptive capacity under thermal and oxidative stress and behaviour in a hamster model, of L. braziliensis, L. peruviana, and their putative hybrids. At 24°C, the optimal temperature for in vitro growth, L. braziliensis had the highest growth rate. In in vitro studies hybrid clones presented heterogeneous phenotypes, from slower growth rates, similar to L. peruviana, to higher growth rates, as observed in L. braziliensis. Hamsters infected with hybrid strains, presented the highest parasite densities and aggressive relapses at a later stage of infection. Hybrids generally presented higher plasticity and phenotypic diversity than the putative parental species, with potential eco-epidemiological implications, including an impact on the success of disease control.

Sand fly-Leishmania interactions: long relationships are not necessarily easy

Sand fly and Leishmania are one of the best studied vector-parasite models. Much is known about the development of these parasites within the sand fly, and how transmission to a suitable vertebrate host takes place. Various molecules secreted by the vector assist the establishment of the infection in a vertebrate, and changes to the vector are promoted by the parasites in order to facilitate or enhance transmission. Despite a generally accepted view that sand flies and Leishmania are also one of the oldest vector-pathogen pairs known, such long history has not been translated into a harmonic relationship. Leishmania are faced with many barriers to the establishment of a successful infection within the sand fly vector, and specific associations have been developed which are thought to represent aspects of a co-evolution between the parasite and its vectors. In this review, we highlight the journey taken by Leishmania during its development within the vector, and describe the issues associated with the natural barriers encountered by the parasite. Recent data revealed sexual replication of Leishmania within the sand fly, but it is yet unknown if such reproduction affects disease outcome. New approaches targeting sand fly molecules to prevent parasite transmission are being sought, and various techniques related to genetic manipulation of sand flies are being utilized.

Evaluation of an in vitro and in vivo model for experimental infection with Leishmania (Viannia) braziliensis and L. (V.) peruviana

Leishmania (Viannia) braziliensis and L. (V.) peruviana are two parasite species characterized by a very different pathogenicity in humans despite a high genetic similarity. We hypothesized previously that L. (V.) peruviana would descend from L. (V.) braziliensis and would have acquired its ‘peruviana’ character during the southward colonization and adaptation of the transmission cycle in the Peruvian Andes. In order to have a first appreciation of the differences in virulence between both species, we evaluated an in vitro and in vivo model for experimental infection. A procedure was adapted to enrich culture forms in infective stages and the purified metacyclics were used to infect macrophage cell lines and golden hamsters. The models were tested with 2 representative strains of L. (V.) braziliensis from cutaneous and mucosal origin respectively and 2 representative strains of L. (V.) peruviana from Northern and Southern Peru respectively. Our models were reproducible and sensitive enough to detect phenotypic differences among strains. We showed in vitro as well as in vivo that the L. (V.) braziliensis was more infective than L. (V.) peruviana. Furthermore, we found that in vitro infectivity patterns of the 4 strains analysed, were in agreement with the geographical structuring of parasite populations demonstrated in our previous studies. Further work is needed to confirm our results with more strains of different geographical origin and their specific clinical outcome. However, our data open new perspectives for understanding the process of speciation in Leishmania and its implications in terms of pathogenicity.

Putative markers of infective life stages in Leishmania (Viannia) braziliensis

Gene expression is known to vary significantly during the Leishmania life-cycle. Its monitoring might allow identification of molecular changes associated with the infective stages (metacyclics and amastigotes) and contribute to the understanding of the complex host-parasite relationships. So far, very few studies have been done on Leishmania (Viannia) braziliensis, one of the most pathogenic species. Such studies require, first of all, reference molecular markers. In the present work, we applied differential display analysis (DD analysis) in order to identify transcripts that might be (i) candidate markers of metacyclics and intracellular amastigotes of L. (V.) braziliensis or (ii) potential controls, i.e. constitutively expressed. In total, 48 DNA fragments gave reliable sequencing data, 29 of them being potential markers of infective stages and 12 potential controls. Eight sequences could be identified with reported genes. Validation of the results of DD analysis was done for 4 genes (2 differentially expressed and 2 controls) by quantitative real-time PCR. The infective insect stage-specific protein (meta 1) was more expressed in metacyclic-enriched preparations. The oligopeptidase b showed a higher expression in amastigotes. Two genes, glucose-6-phosphate dehydrogenase and a serine/threonine protein kinase, were found to be similarly expressed in the different biological samples.

Leishmania chagasi: homogenous metacyclic promastigotes isolated by buoyant density are highly virulent in a mouse model

Homogenous metacyclic promastigotes of Leishmania chagasi were isolated by buoyant density from in vitro heterogeneous cultures and used for biochemical characterization of isoforms of the major surface protease (MSP). Compared to stationary phase promastigotes, metacyclic cells had three times more MSP, produced 3-fold higher parasite loads in a mouse model in vivo, and were more resistant to complement-mediated lysis in vitro. These metacyclic L. chagasi expressed both the virulence-associated 59-kDa, and the constitutively expressed 63-kDa, isoforms of MSP.

Leishmania (Viannia) braziliensis metacyclic promastigotes purified using Bauhinia purpurea lectin are complement resistant and highly infective for macrophages in vitro and hamsters in vivo

In this study we characterised metacyclogenesis in axenic culture of Leishmania (Viannia) braziliensis, the causative agent of mucocutaneous leishmaniasis in the New World. Metacyclogenesis of other species of Leishmania has been shown by morphological changes as well as molecular modifications in the lipophosphoglycan, the major cell surface glycoconjugate of the promastigotes. In order to obtain metacyclic forms of L. braziliensis we tested a panel of different lectins. Our results showed that Bauhinia purpurea lectin facilitated the purification of metacyclic promastigotes from stationary-phase culture by negative selection. The B. purpurea non-agglutinated promastigotes had a slender short cell body and long flagella, typical of metacyclic morphology. The ultrastructural analysis showed that B. purpurea non-agglutinated promastigotes have a dense and thicker glycocalyx. They are resistant to complement lysis, and highly infective for macrophage in vitro and hamsters in vivo. Contrary to procyclic promastigotes, B. purpurea non-agglutinated forms were poorly recognised by sand fly gut epithelial cells. These results suggest that the B. purpurea non-agglutinated promastigotes are the metacyclic forms of L. braziliensis.

A lipophosphoglycan-independent method for isolation of infective Leishmania metacyclic promastigotes by density gradient centrifugation

At the end of their growth in the sand fly, Leishmania parasites differentiate into the infective metacyclic promastigote stage, which is transmitted to the mammalian host. Thus, in experimental studies of parasite infectivity toward animals or macrophages, the use of purified metacyclics is generally preferred. While metacyclics of several Leishmania species can be efficiently purified with the aid of lectins or monoclonal antibodies, which differentially exploit stage-specific differences in the structure of the abundant surface glycolipid lipophosphoglycan (LPG), such reagents are unavailable for most species and they are unsuitable for studies involving LPG-deficient mutants. Here we describe a simple density gradient centrifugation method, which allows the rapid purification of infective metacyclic parasites from both wild-type and LPG-deficient Leishmania major. The purified metacyclic promastigotes are authentic, as judged by criteria such as their morphology, expression of the metacyclic-specific gene SHERP, and ability to invade and replicate within macrophages in vitro. Preliminary studies suggest that this method is applicable to other Leishmania species including L. donovani.

Dissemination of Leishmania (Viannia) braziliensis

Destructive human mucocutaneous leishmaniasis may appear many years after the primary cutaneous infection with Leishmania (Viannia) braziliensis. Hamsters (Mesocricetus auratus) were infected with metacyclic L. braziliensis promastigotes. It was found that secondary metastatic visceral lesions could arise from a primary cutaneous lesion, or secondary cutaneous lesions from a primary visceral lesion. Parasites in the viscera were shown to be viable, multiplying and capable of metastasis to either secondary visceral or cutaneous sites. The finding of an early metastasis in the wall of a small cutaneous vessel indicates that dissemination can occur by the haematogenous route. Slow growing organisms in viscera may thus be a source for late metastasis to mucocutaneous sites or for systemic relapse after immunosuppression.