Immune evasion strategies of trypanosomes: a review

Interacting phenotypes and the coevolutionary process: Interspecific indirect genetic effects alter coevolutionary dynamics

Coevolution occurs when species interact to influence one another's fitness, resulting in reciprocal evolutionary change. In many coevolving lineages, trait expression in one species is modified by the genotypes and phenotypes of the other, forming feedback loops reminiscent of models of intraspecific social evolution. Here, we adapt the theory of within-species social evolution, characterized by indirect genetic effects and social selection imposed by interacting individuals, to the case of interspecific interactions. In a trait-based model, we derive general expressions for multivariate evolutionary change in two species and the expected between-species covariance in evolutionary change when selection varies across space. We show that reciprocal interspecific indirect genetic effects can dominate the coevolutionary process and drive patterns of correlated evolution beyond what is expected from direct selection alone. In extreme cases, interspecific indirect genetic effects can lead to coevolution when selection does not covary between species or even when one species lacks genetic variance. Moreover, our model indicates that interspecific indirect genetic effects may interact in complex ways with cross-species selection to determine the course of coevolution. Importantly, our model makes empirically testable predictions for how different forms of reciprocal interactions contribute to the coevolutionary process.

Trypanosoma cruzi trans-Sialidase as a Potential Vaccine Target Against Chagas Disease

Chagas’ disease is caused by the protozoan Trypanosoma cruzi, described in the early 20^th century by the Brazilian physician Dr. Carlos Chagas. There was a great amount of research devoted to diagnosis, treatment and prevention of the disease. One of the most important discoveries made since then, impacting the understanding of how the parasite interacts with the host’s immune system, was the description of trans-sialidase. It is an unique enzyme, capable of masking the parasite’s presence from the host, while at the same time dampening the activation of CD8+ T cells, the most important components of the immune response. Since the description of Chagas’ disease in 1909, extensive research has identified important events in the disease in order to understand the biochemical mechanism that modulates T. cruzi-host cell interactions and the ability of the parasite to ensure its survival. The importance of the trans-sialidase enzyme brought life to many studies for the design of diagnostic tests, drugs and vaccines. While many groups have been prolific, such efforts have encountered problems, among them: the fact that while T. cruzi have many genes that are unique to the parasite, it relies on multiple copies of them and the difficulty in providing epitopes that result in effective and robust immune responses. In this review, we aim to convey the importance of trans-sialidase as well as to provide a history, including the initial failures and the most promising successes in the chasing of a working vaccine for a disease that is endemic in many tropical countries, including Brazil.

Prevalence and Epitope Recognition of Anti-Trypanosoma cruzi Antibodies in Two Procyonid Species: Implications for Host Resistance

More than 180 mammalian species have been found naturally infected with Trypanosoma cruzi. Many of them play an important role in the maintenance of this parasite. In particular, new studies have appeared which indicate that some species of Procyonidae family may play a role as T. cruzi hosts, however, more data are needed to evaluate their long-term physiological response to parasite infection, especially for specific antibodies. In this study, antibodies to T. cruzi were detected and prevalence and epitope recognition were assessed by ELISA (using discrete typing unit (DTU) I as antigen) and WB (using DTU I and DTU II as antigens) and sera from two procyonid species obtained through five-year follow-up of two semicaptive populations living in the same habitat. Marked heterogeneity in antigens recognition between species and differences in seroprevalence (p = 0.0002) between white-nosed coatis (Nasua narica), 51.8% (115/222), and common raccoons (Procyon lotor), 28.3% (23/81), were found. Antigens with high molecular weight when DTU-I was used were the most recognized, while a greater antigen diversity recognition was observed with DTU-II; for white-nosed coatis, low-molecular-weight antigens were mainly recognized, while for common raccoons proteins with molecular weights greater than 80 kDa were recognized most. These divergent humoral immune responses could be related to an alleged pattern of recognition receptors and major histocompatibility complex molecules difference in the procyonids species.

DNA lesions and repair in trypanosomatids infection

Pathological processes such as bacterial, viral and parasitic infections can generate a plethora of responses such as, but not restricted to, oxidative stress that can be harmful to the host and the pathogen. This stress occurs when there is an imbalance between reactive oxygen species produced and antioxidant factors produced in response to the infection. This imbalance can lead to DNA lesions in both infected cells as well as in the pathogen. The effects of the host response on the parasite lead to several kinds of DNA damage, causing alterations in the parasite's metabolism; the reaction and sensitivity of the parasite to these responses are related to the DNA metabolism and life cycle of each parasite. The present review will discuss the survival strategies developed by host cells and Trypanosoma cruzi, focusing on the DNA repair mechanisms of these organisms throughout infection including the relationship between DNA damage, stress response features, and the unique characteristics of these diseases.

Trypanosoma carassii infection in goldfish (Carassius auratus L.): changes in the expression of erythropoiesis and anemia regulatory genes

Trypanosoma carassii is a flagellated bloodstream parasite of cyprinid fish with pathogenesis manifesting primarily as anemia in experimentally infected fish. This anemia is characterized by decreases in the number of circulating red blood cells (RBCs) during peak parasitemia. We examined changes in the key blood metrics and expression of genes known to be important in the regulation of erythropoiesis. Increasing parasitemia was strongly correlated with an overall decrease in the total number of circulating RBCs. Gene expression of key erythropoiesis regulators (EPO, EPOR, GATA1, Lmo2, and HIFα) and proinflammatory cytokines (IFNγ and TNFα) were measured and their expressions differed from those in fish made anemic by injections of phenylhydrazine (PHZ). Significant upregulation of pro-erythropoietic genes was observed in PHZ-induced anemia, but not during peak parasitic infection. Previously, we reported on functional characterization of goldfish erythropoietin (rgEPO) and its ability to induce survival and differentiation of erythroid progenitor cells in vitro. Treatment of goldfish during the infection with rgEPO reduced the severity of anemia but failed to fully prevent the onset of the anemic state in infected fish. Proinflammatory cytokines have been implicated in the suppression of erythropoiesis during trypanosomiasis, specifically the cytokines TNFα, IFNγ, and IL-1β. Analysis of key proinflammatory cytokines revealed that mRNA levels of IFNγ and TNFα were upregulated in response to infection, but only TNFα increased in response to PHZ treatment. Synergistic activity of the proinflammatory cytokines may be required to sustain prolonged anemia. These findings provide insight into the relationship between T. carassii and host anemia and suggest that T. carassii may directly or indirectly suppress host erythropoiesis.

Rational design of novel irreversible inhibitors for human arginase

Parasites have developed a variety of strategies for invading hosts and escaping their immune response. A common mechanism by which parasites escape nitric oxide (NO) toxicity is the activation of host arginase. This activation leads to a depletion of l-arginine, which is the substrate for NO synthase, resulting in lower levels of NO and increased production of polyamines that are necessary for parasite growth and differentiation. For this reason, small molecule inhibitors for arginase show promise as new anti-parasitic chemotherapeutics. However, few arginase inhibitors have been reported. Here, we describe the discovery of novel irreversible arginase inhibitors, and their characterization using biochemical, kinetic, and structural studies. Importantly, we determined the site on human arginase that is labeled by one of the small molecule inhibitors. The tandem mass spectra data show that the inhibitor occupies the enzyme active site and forms a covalent bond with Thr135 of arginase. These findings pave the way for the development of more potent and selective irreversible arginase inhibitors.

Natural selection on MHC IIβ in parapatric lake and stream stickleback: Balancing, divergent, both or neither?

Major histocompatibility complex (MHC) genes encode proteins that play a central role in vertebrates' adaptive immunity to parasites. MHC loci are among the most polymorphic in vertebrates' genomes, inspiring many studies to identify evolutionary processes driving MHC polymorphism within populations and divergence between populations. Leading hypotheses include balancing selection favouring rare alleles within populations, and spatially divergent selection. These hypotheses do not always produce diagnosably distinct predictions, causing many studies of MHC to yield inconsistent or ambiguous results. We suggest a novel strategy to distinguish balancing vs. divergent selection on MHC, taking advantage of natural admixture between parapatric populations. With divergent selection, individuals with immigrant alleles will be more infected and less fit because they are susceptible to novel parasites in their new habitat. With balancing selection, individuals with locally rare immigrant alleles will be more fit (less infected). We tested these contrasting predictions using three-spine stickleback from three replicate pairs of parapatric lake and stream habitats. We found numerous positive and negative associations between particular MHC IIβ alleles and particular parasite taxa. A few allele-parasite comparisons supported balancing selection, and others supported divergent selection between habitats. But, there was no overall tendency for fish with immigrant MHC alleles to be more or less heavily infected. Instead, locally rare MHC alleles (not necessarily immigrants) were associated with heavier infections. Our results illustrate the complex relationship between MHC IIβ allelic variation and spatially varying multispecies parasite communities: different hypotheses may be concurrently true for different allele-parasite combinations.

Evasion and Immuno-Endocrine Regulation in Parasite Infection: Two Sides of the Same Coin in Chagas Disease?

Chagas disease is a serious illness caused by the protozoan parasite Trypanosoma cruzi. Nearly 30% of chronically infected people develop cardiac, digestive, or mixed alterations, suggesting a broad range of host-parasite interactions that finally impact upon chronic disease outcome. The ability of T. cruzi to persist and cause pathology seems to depend on diverse factors like T. cruzi strains, the infective load and the route of infection, presence of virulence factors, the parasite capacity to avoid protective immune response, the strength and type of host defense mechanisms and the genetic background of the host. The host-parasite interaction is subject to a constant neuro-endocrine regulation that is thought to influence the adaptive immune system, and as the infection proceeds it can lead to a broad range of outcomes, ranging from pathogen elimination to its continued persistence in the host. In this context, T. cruzi evasion strategies and host defense mechanisms can be envisioned as two sides of the same coin, influencing parasite persistence and different outcomes observed in Chagas disease. Understanding how T. cruzi evade host's innate and adaptive immune response will provide important clues to better dissect mechanisms underlying the pathophysiology of Chagas disease.

Trade-offs in antibody repertoires to complex antigens

Pathogens vary in their antigenic complexity. While some pathogens such as measles present a few relatively invariant targets to the immune system, others such as malaria display considerable antigenic diversity. How the immune response copes in the presence of multiple antigens, and whether a trade-off exists between the breadth and efficacy of antibody (Ab)-mediated immune responses, are unsolved problems. We present a theoretical model of affinity maturation of B-cell receptors (BCRs) during a primary infection and examine how variation in the number of accessible antigenic sites alters the Ab repertoire. Naive B cells with randomly generated receptor sequences initiate the germinal centre (GC) reaction. The binding affinity of a BCR to an antigen is quantified via a genotype-phenotype map, based on a random energy landscape, that combines local and distant interactions between residues. In the presence of numerous antigens or epitopes, B-cell clones with different specificities compete for stimulation during rounds of mutation within GCs. We find that the availability of many epitopes reduces the affinity and relative breadth of the Ab repertoire. Despite the stochasticity of somatic hypermutation, patterns of immunodominance are strongly shaped by chance selection of naive B cells with specificities for particular epitopes. Our model provides a mechanistic basis for the diversity of Ab repertoires and the evolutionary advantage of antigenically complex pathogens.

Role of Trypanosoma cruzi Trans-sialidase on the Escape from Host Immune Surveillance

Chagas disease is caused by the flagellate protozoan Trypanosoma cruzi, affecting millions of people throughout Latin America. The parasite dampens host immune response causing modifications in diverse lymphoid compartments, including the thymus. T. cruzi trans-sialidase (TS) seems to play a fundamental role in such immunopathological events. This unusual enzyme catalyses the transference of sialic acid molecules from host glycoconjugates to acceptor molecules placed on the parasite surface. TS activity mediates several biological effects leading to the subversion of host immune system, hence favoring both parasite survival and the establishment of chronic infection. This review summarizes current findings on the roles of TS in the immune response during T. cruzi infection.

Haematological and immunological characteristics of eastern hellbenders (Cryptobranchus alleganiensis alleganiensis) infected and co-infected with endo- and ectoparasites

Disease is among the leading causes of the global decline in amphibian populations. In North America, parasites and pathogens are among the factors implicated in precipitous population declines of the giant hellbender salamander (Cryptobranchus alleganiensis), but the incidence of infections and the responses of hellbenders to infections remain poorly studied. Here, we document the prevalence of leech and trypanosome infections in a wild population of eastern hellbenders (Cryptobranchus alleganiensis alleganiensis) and describe haematological and immunological characteristics of hellbenders harbouring these infections. We hypothesized that hellbenders parasitized by trypanosomes would be anaemic, that individuals infected with either or both parasites would exhibit shifts in white blood cell counts and that hellbenders infected with leeches would exhibit altered plasma bactericidal capacity. We found that 24 and 68% of hellbenders in our sample population were infected with leeches and trypanosomes, respectively, and 20% were co-infected with both parasites. We found no evidence suggestive of anaemia among infected individuals. However, hellbenders infected with either or both parasites exhibited marked shifts in circulating white blood cells that were consistent with predictable responses to parasitic infection. Additionally, we found that hellbenders harbouring leeches had much higher plasma bactericidal capacity than individuals without leeches, and we offer multiple potential mechanistic explanations for this observation. We also found evidence that cellular and serological immune responses to parasites were less robust in juvenile than adult hellbenders. This finding warrants further investigation in light of the demographic characteristics, specifically the scarcity of juvenile age classes, of hellbender populations where disease is a possible contributor to declines. Finally, we describe two methodological advances that will improve future studies seeking to diagnose trypanosome infections and to test the bactericidal capacity of hellbenders and perhaps other amphibians. Our study provides fundamental insights into how hellbenders respond physiologically to endo- and ectoparasites, which could ultimately prove useful for their conservation.

Immune Evasion Strategies of Trypanosoma cruzi

Microbes have evolved a diverse range of strategies to subvert the host immune system. The protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease, provides a good example of such adaptations. This parasite targets a broad spectrum of host tissues including both peripheral and central lymphoid tissues. Rapid colonization of the host gives rise to a systemic acute response which the parasite must overcome. The parasite in fact undermines both innate and adaptive immunity. It interferes with the antigen presenting function of dendritic cells via an action on host sialic acid-binding Ig-like lectin receptors. These receptors also induce suppression of CD4(+) T cells responses, and we presented evidence that the sialylation of parasite-derived mucins is required for the inhibitory effects on CD4 T cells. In this review we highlight the major mechanisms used by Trypanosoma cruzi to overcome host immunity and discuss the role of parasite colonization of the central thymic lymphoid tissue in chronic disease.

Evidence of ectoparasite-induced endocrine disruption in an imperiled giant salamander, the eastern hellbender (Cryptobranchus alleganiensis)

Parasitic leeches and trypanosomes release chemical signals into their hosts to evade immuno-detection, but it is unknown whether these compounds manipulate host behavior or endocrine physiology. We determined whether parasitic infections with leeches and/or trypanosomes affected the immune and stress response of an imperiled giant species of amphibian, the eastern hellbender (Cryptobranchus alleganiensis Daudin). We monitored corticosterone and white blood cell counts in response to restraint and injection with adrenocorticotropin hormone (ACTH) or saline for up to 50 h. The presence of leeches dampened hellbender corticosterone responses to restraint and reduced diel patterns of plasma corticosterone. Injection with ACTH restored the normal inter-renal responses of hellbenders, suggesting that leeches, possibly through neurotransmitters in leech saliva, cause down-regulation of corticosterone release at the level of the pituitary or hypothalamus. Infection with leeches also increased the relative abundance of eosinophils, white blood cells often recruited into circulation in response to parasitic infection. Lastly, neutrophil to lymphocyte (N:L) ratios increased in all animals after 24 h of capture and remained elevated for up to 50 h, but these temporal dynamics did not differ with parasite infection. Trypanosome infection did not affect any aspect of hellbender physiology that we measured. Our findings reveal a previously undocumented host-parasite dynamic. While the functional significance to the parasite is unclear, the physiological and behavioral implications for the host are great, given the important role of glucocorticoids in regulating physiology and behavior.

Aflagellar epimastigote forms are found in axenic culture of Trypanosoma caninum

Representatives of the genus Trypanosoma have been traditionally found in epimastigote, espheromastigote and trypomastigote flagellated forms in axenic cultures. Trypanosoma caninum is a trypanosomatid that has recently been reported infecting dogs in endemic areas of canine leishmaniasis in Brazil. It presents specific biological characteristics and it is found exclusively on healthy skin. Here, we describe the evolutive forms of this parasite showing not only the forms commonly found in culture, but also epimastigote forms with no free flagellum. The study was conducted using scanning and transmission electron microscopy and, we demonstrate that typical flagellated epimastigotes originate from forms without flagellum, although the latter may remain without differentiation in the culture. Two hypotheses are considered and discussed in this paper: (i) the aflagellated epimastigotes are a typical developmental forms of T. caninum and (ii) the emergence of these aflagellated forms could be resultant from a disturbed process during cell division caused by interfering specific proteins, which leads to inability to form and regulate the flagellum length. In any case, considering that T. caninum is a parasite that is still little studied, the information brought by our study adds data which may be useful to clarify aspects on the cell cycle of this intriguing parasite that has been found in different regions of Brazil.

Hemomucin, an O-glycosylated protein on embryos of the wasp Macrocentrus cingulum that protects it against encapsulation by hemocytes of the host Ostrinia furnacalis

It is unclear how endoparasites passively evade their host's immune reactions in most parasite-host systems. Hemomucin from the parasitoid wasp Macrocentrus cingulum (McHEM) is a 97-kDa transmembrane protein containing 51 potential O-glycosylation sites that can be specifically recognized by Arachis hypogaea lectin. Mchem mRNA is highly expressed in M. cingulum eggs, morulae and secondary embryos, and McHEM protein is mainly located on the extraembryonic membrane of embryos. When secondary embryos of M. cingulum were transplanted into naïve larvae of their host, Ostrinia furnacalis, the embryos proliferated to generate dozens of embryos. However, more than 90% of these embryos were encapsulated by host hemocytes after blocking with anti-McHEM serum. Similarly, following knockdown of Mchem expression using double-stranded RNA encoding Mchem (dshem), many more embryos were encapsulated by host hemocytes after transplantation compared to controls (p < 0.01). Furthermore, approximately 70% of the embryos were encapsulated by host hemocytes following digestion with O-glycosidase, which specifically digests β-gal (1→3) linkages between GalNAc and Ser/Thr of proteins. Western blotting results showed that O-glycosidase digested McHEM into a smaller product. These results indicate that McHEM may protect embryos from being encapsulated by their host and that the McHEM sugar chains play an important role.

Monovinyl sulfone β-cyclodextrin. A flexible drug carrier system

Cyclodextrins have been conjugated to target various receptors and have also been functionalized with carbohydrates for targeting specific organs. However, this approach is based on a rigid design that implies the ad hoc synthesis of each cyclodextrin-targeting agent conjugate. We hypothesized that: 1)a modular design that decouples the carrier function from the targeting function leads to a flexible system, 2) combining the reactivity of the vinyl sulfone group toward biomolecules that act as targeting agents with the ability of cyclodextrin to form complexes with a wide range of drugs may yield a versatile system that allows the targeting of different organs with different drugs, and 3) the higher reactivity of histidine residues toward the vinyl sulfone group can be exploited to couple the cyclodextrin to the targeting system with a degree of regioselectivity. As a proof of concept, we synthesized a monovinyl sulfone β-cyclodextrin (module responsible for the payload), which, after coupling to recombinant antibody fragments raised against Trypanosoma brucei (module responsible for targeting) and loading with nitrofurazone (module responsible for therapeutic action) resulted in an effective delivery system that targets the surface of the parasites and shows trypanocidal activity.

Bisphosphonates and osteonecrosis of the jaw: moving from the bedside to the bench

Osteonecrosis of the jaw (ONJ) has received significant attention as a potential side effect of bisphosphonate treatment. The limited understanding of the underlying pathophysiology of the condition emphasizes the need to transition ONJ research from the bedside to the bench, supplementing ongoing clinical research with animal/basic science studies. The goal of this review is to briefly highlight the most commonly proposed mechanisms for ONJ and then summarize our laboratory's recent efforts to begin transitioning ONJ research to an animal model. Remodeling suppression, disrupted angiogenesis and infection have all been proposed to connect bisphosphonates to ONJ, although most supportive data for each of these are either indirect or nonexistent. Our laboratory has begun studying the dog as a potential model of ONJ. We have shown regions of necrotic bone matrix within the mandible of dogs treated with oral or intravenous bisphosphonate. We hypothesize these regions are the result of remodeling suppression, and if combined with additional factors such as dental intervention or infection, would result in manifestation of exposed oral lesions, the clinical definition of ONJ. Although these findings suggest the dog may be a viable animal model to study ONJ, many questions remain unanswered. No matter what animal model is found to mimic the clinical presentation of ONJ, once established it will allow significant progress toward understanding the specific role of bisphosphonates in the pathophysiology of ONJ and if/how the entity of ONJ can best be treated and prevented.