Studies on the virulence and attenuation of Trypanosoma cruzi using immunodeficient animals

Disruption of the inositol phosphorylceramide synthase gene affects Trypanosoma cruzi differentiation and infection capacity

Sphingolipids (SLs) are essential components of all eukaryotic cellular membranes. In fungi, plants and many protozoa, the primary SL is inositol-phosphorylceramide (IPC). Trypanosoma cruzi is a protozoan parasite that causes Chagas disease (CD), a chronic illness for which no vaccines or effective treatments are available. IPC synthase (IPCS) has been considered an ideal target enzyme for drug development because phosphoinositol-containing SL is absent in mammalian cells and the enzyme activity has been described in all parasite forms of T. cruzi. Furthermore, IPCS is an integral membrane protein conserved amongst other kinetoplastids, including Leishmania major, for which specific inhibitors have been identified. Using a CRISPR-Cas9 protocol, we generated T. cruzi knockout (KO) mutants in which both alleles of the IPCS gene were disrupted. We demonstrated that the lack of IPCS activity does not affect epimastigote proliferation or its susceptibility to compounds that have been identified as inhibitors of the L. major IPCS. However, disruption of the T. cruzi IPCS gene negatively affected epimastigote differentiation into metacyclic trypomastigotes as well as proliferation of intracellular amastigotes and differentiation of amastigotes into tissue culture-derived trypomastigotes. In accordance with previous studies suggesting that IPC is a membrane component essential for parasite survival in the mammalian host, we showed that T. cruzi IPCS null mutants are unable to establish an infection in vivo, even in immune deficient mice.

Overexpression of TcNTPDase-1 Gene Increases Infectivity in Mice Infected with Trypanosoma cruzi

Ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) are enzymes located on the surface of the T. cruzi plasma membrane, which hydrolyze a wide range of tri-/-diphosphate nucleosides. In this work, we used previously developed genetically modified strains of Trypanosoma cruzi (T. cruzi), hemi-knockout (KO +/−) and overexpressing (OE) the TcNTPDase-1 gene to evaluate the parasite infectivity profile in a mouse model of acute infection (n = 6 mice per group). Our results showed significantly higher parasitemia and mortality, and lower weight in animals infected with parasites OE TcNTPDase-1, as compared to the infection with the wild type (WT) parasites. On the other hand, animals infected with (KO +/−) parasites showed no mortality during the 30-day trial and mouse weight was more similar to the non-infected (NI) animals. In addition, they had low parasitemia (45.7 times lower) when compared with parasites overexpressing TcNTPDase-1 from the hemi-knockout (OE KO +/−) group. The hearts of animals infected with the OE KO +/− and OE parasites showed significantly larger regions of cardiac inflammation than those infected with the WT parasites (p < 0.001). Only animals infected with KO +/− did not show individual electrocardiographic changes during the period of experimentation. Together, our results expand the knowledge on the role of NTPDases in T. cruzi infectivity, reenforcing the potential of this enzyme as a chemotherapy target to treat Chagas disease (CD).

Disruption of Active Trans-Sialidase Genes Impairs Egress from Mammalian Host Cells and Generates Highly Attenuated Trypanosoma cruzi Parasites

Trans-sialidases (TS) are unusual enzymes present on the surface of Trypanosoma cruzi, the causative agent of Chagas disease. Encoded by the largest gene family in the T. cruzi genome, only few members of the TS family have catalytic activity. Active trans-sialidases (aTS) are responsible for transferring sialic acid from host glycoconjugates to mucins, also present on the parasite surface. The existence of several copies of TS genes has impaired the use of reverse genetics to study this highly polymorphic gene family. Using CRISPR-Cas9, we generated aTS knockout cell lines displaying undetectable levels of TS activity, as shown by sialylation assays and labeling with antibodies that recognize sialic acid-containing mucins. In vitro infection assays showed that disruption of aTS genes does not affect the parasite's capacity to invade cells or to escape from the parasitophorous vacuole but resulted in impaired differentiation of amastigotes into trypomastigotes and parasite egress from the cell. When inoculated into mice, aTS mutants were unable to establish infection even in the highly susceptible gamma interferon (IFN-γ) knockout mice. Mice immunized with aTS mutants were fully protected against a challenge infection with the virulent T. cruzi Y strain. Altogether, our results confirmed the role of aTS as a T. cruzi virulence factor and indicated that aTS play a major role during the late stages of intracellular development and parasite egress. Notably, mutants lacking TS activity are completely avirulent in animal models of infection and may be used as a live attenuated vaccine against Chagas disease. IMPORTANCE Trypanosoma cruzi is the causative agent of Chagas disease, a neglected tropical disease that affects approximately 6 to 8 million people and for which there is no effective treatment or vaccine. The parasite expresses a family of surface proteins, named trans-sialidases, responsible for transferring sialic acid from host glycoconjugates to parasite mucins. Although recognized as a main virulence factor, the multiple roles of these proteins during infection have not yet been fully characterized, mainly because the presence of several copies of aTS genes has impaired their study using reverse genetics. By applying CRISPR-Cas9, we generated aTS knockout parasites and showed that, although aTS parasite mutants were able to infect cells in vitro, they have an impaired capacity to egress from the infected cell. Importantly, aTS mutants lost the ability to cause infection in vivo but provided full protection against a challenge infection with a virulent strain.

Macrophages Promote Oxidative Metabolism To Drive Nitric Oxide Generation in Response to Trypanosoma cruzi

Trypanosoma cruzi is the causative agent of chronic chagasic cardiomyopathy. Why macrophages (mφs), the early responders to infection, fail to achieve parasite clearance is not known. Mouse (RAW 264.7) and human (THP-1 and primary) mφs were infected for 3 h and 18 h with T. cruzi TcI isolates, SylvioX10/4 (SYL, virulent) and TCC (nonpathogenic), which represent mφ stimulation and infection states, respectively. Mφs incubated with lipopolysaccharide and gamma interferon (LPS/IFN-γ) and with interleukin-4 (IL-4) were used as controls. We monitored the cytokine profile (using enzyme-linked immunosorbent assay [ELISA]), reactive oxygen species (ROS; fluorescent probes), nitric oxide (·NO; Griess assay), and metabolic state using a custom-designed mitoxosome array and Seahorse XF24 Analyzer. LPS/IFN-γ treatment of mφs elicited a potent increase in production of tumor necrosis alpha (TNF-α) at 3 h and of ROS and ·NO by 18 h. Upon SYL infection, murine mφs elicited an inflammatory cytokine profile (TNF-α ≫ TGF-β + IL-10) and low levels of ·NO and ROS production. LPS/IFN-γ treatment resulted in the inhibition of oxidative metabolism at the gene expression and functional levels and a switch to the glycolytic pathway in mφs, while IL-4-treated mφs utilized oxidative metabolism to meet energy demands. SYL infection resulted in an intermediate functional metabolic state with increased mitoxosome gene expression and glycolysis, and IFN-γ addition shut down the oxidative metabolism in SYL-infected mφs. Further, TCC- and SYL-stimulated mφs exhibited similar levels of cell proliferation and production of TNF-α and ROS, while TCC-stimulated mφs exhibited up to 2-fold-higher levels of oxidative metabolism and ·NO production than SYL-infected mφs. Inhibiting ATP-coupled O₂ consumption suppressed the ·NO generation in SYL-infected mφs. Mitochondrial oxygen consumption constitutes a mechanism for stimulating ·NO production in mφs during T. cruzi infection. Enhancing the oxidative metabolism provides an opportunity for increased ·NO production and pathogen clearance by mφs to limit disease progression.

A monoallelic deletion of the TcCRT gene increases the attenuation of a cultured Trypanosoma cruzi strain, protecting against an in vivo virulent challenge

Trypanosoma cruzi calreticulin (TcCRT) is a virulence factor that binds complement C1, thus inhibiting the activation of the classical complement pathway and generating pro-phagocytic signals that increase parasite infectivity. In a previous work, we characterized a clonal cell line lacking one TcCRT allele (TcCRT+/−) and another overexpressing it (TcCRT+), both derived from the attenuated TCC T. cruzi strain. The TcCRT+/− mutant was highly susceptible to killing by the complement machinery and presented a remarkable reduced propagation and differentiation rate both in vitro and in vivo. In this report, we have extended these studies to assess, in a mouse model of disease, the virulence, immunogenicity and safety of the mutant as an experimental vaccine. Balb/c mice were inoculated with TcCRT+/− parasites and followed-up during a 6-month period. Mutant parasites were not detected by sensitive techniques, even after mice immune suppression. Total anti-T. cruzi IgG levels were undetectable in TcCRT+/− inoculated mice and the genetic alteration was stable after long-term infection and it did not revert back to wild type form. Most importantly, immunization with TcCRT+/− parasites induces a highly protective response after challenge with a virulent T. cruzi strain, as evidenced by lower parasite density, mortality, spleen index and tissue inflammatory response. TcCRT+/− clones are restricted in two important properties conferred by TcCRT and indirectly by C1q: their ability to evade the host immune response and their virulence. Therefore, deletion of one copy of the TcCRT gene in the attenuated TCC strain generated a safe and irreversibly gene-deleted live attenuated parasite with high immunoprotective properties. Our results also contribute to endorse the important role of TcCRT as a T. cruzi virulence factor.

Trypanosoma cruzi is a protozoan parasite which infects 9 million people in Latin America. Currently there is no vaccine to prevent this disease. Therefore, different approaches or alternatives are urgently needed to identify new protective immunogens. Live vaccines are likely to be most effective in inducing protection; however, safety issues associated with their use have been raised. Hence, we genetically manipulated an attenuated strain of T. cruzi as a safety device to rule out the possibility of reversion to the virulent phenotype. The genetically modified parasites were highly susceptible to killing by the complement machinery and presented a reduced propagation and differentiation rate. We have extended these studies to assess, the virulence, immunogenicity and safety of the mutant as an experimental vaccine. Accordingly, we show that genetically modified parasites present attenuated virulence in mice. The genetic alteration was stable and, after long term infection, it did not revert back to wild type form. Furthermore, after challenge with a virulent T. cruzi strain, mutant immunization induces a highly protective response evidenced by significantly lowered parasite density, mortality, spleen weight index and tissue inflammatory response. Our study provides new insights into the host-pathogen interactions and into the use and evaluation of irreversibly gene-deleted live attenuated parasites to protect against Chagas disease.

Effect of serial subculturing on the genetic composition and cytotoxic activity of Mycobacterium tuberculosis

Continuous subculture has been observed to produce changes in the virulence of micro-organisms, e.g. rabies virus, poliovirus and Mycobacterium bovis BCG. The latter has been used as a vaccine for tuberculosis for the last 100 years; however, in some instances its efficacy has been observed to be very low. In order to determine whether similar changes can be produced in Mycobacterium tuberculosis, we selected four isolates, M. tuberculosis H37Rv, a Beijing strain (DR-689), and two more isolates with deletion of the phospholipase C locus (plcA-plcB-plcC ), and subjected them to serial culturing on Middlebrook 7H9 medium, with or without ox bile. After 100 passages, we performed RFLP-IS6110 analysis to determine whether genomic changes were produced. We also checked their genomic composition by microarray analysis. Changes in virulence were studied by measuring the cytotoxic effect of parental and subcultured isolates on a THP-1 macrophage monolayer. The most visible change was the change of position of an IS6110 band of approximately 1400 bp to approximately 1600 bp in the Beijing isolate subcultured in the ox bile medium. Analysis by microarray and PCR confirmation did not reveal any genomic changes. Cytotoxic activity was decreased in the isolates at levels close to that of BCG, and more consistently in those subcultured in the presence of ox bile.

Probing Trypanosoma cruzi biology with DNA microarrays

The application of genome-scale approaches to study Trypanosoma cruzi-host interactions at different stages of the infective process is becoming possible with sequencing and assembly of the T. cruzi genome nearing completion and sequence information available for both human and mouse genomes. Investigators have recently begun to exploit DNA microarray technology to analyze host transcriptional responses to T. cruzi infection and dissect developmental processes in the complex T. cruzi life-cycle. Collectively, information generated from these and future studies will provide valuable insights into the molecular requirements for establishment of T. cruzi infection in the host and highlight the molecular events coinciding with disease progression. While the field is in its infancy, the availability of genomic information and increased accessibility to relatively high-throughput technologies represents a significant advancement toward identification of novel drug targets and vaccine candidates for the treatment and prevention of Chagas' disease.

Trypanosoma cruzi: experimental parasitism of bone and cartilage

Trypanosoma cruzi causes Chagas' disease, a systemic infection that affects cells of meso-, endo-, and ectodermic origin. However, as far as we know, the presence of T. cruzi stages in bone has not been reported previously, and it has scarcely been investigated in cartilage. We inoculated 7- and 20-day-old (8 and 15 g) NMRI albino mice i.p. with metacyclic trypomastigotes from Rhodnius prolixus used for xenodiagnosis of mice previously infected with mammalian, human, and triatomines isolates, characterized by randomly amplified polymorphic DNA as zymodeme 1 (equivalent to T. cruzi I). Tissular parasitism (quantified according to the number of pseudocysts/50 fields 400x) showed amastigotes, intermediate forms, or trypomastigotes in sternum chondroblasts, osteoblasts, macrophages, and fibroblasts; chondrocyte and osteocyte invasion was rare. All isolates parasitized bone marrow macrophages, with few amastigotes. We observed marked associated myotropism, with or without inflammatory infiltration; there were small numbers of intensely parasitized mononuclear cells in perichondrium and periosteum. We discuss the results in relation to the marked differences of the T. cruzi tropism toward the different types of sternum cells, and, additionally, we outline the possibility of transmitting parasitized bone marrow through transplants. The fact of finding parasite stages in sternum bone and cartilage may be considered important due to the studies on Chagas' disease paleoparasitology that are based on histological and molecular analysis.

Relationship between long-term resistance to Trypanosoma cruzi and latent infection, examined by antibody production and polymerase chain reaction in mice

Trypanosoma cruzi infections persist for the lifetime of humans and laboratory animals as either latent or pathogenic parasitism. Mice inoculated with a nonpathogenic, attenuated strain (TCC) display resistance against virulent challenge, with a strong control of parasitemia and protection against tissue lesions for more than 12 mo. Three main approaches were used to test whether protection by TCC inocula is based on a latent infection or on a "sterile" immunological memory: curative Benznidazole (Bzl) treatment, serological reactions, and detection of infection by polymerase chain reaction (PCR). If resistance is maintained in the absence of infection, it should not be reduced by Bzl treatment and TCC-inoculated animals should not maintain long-term serological or PCR reactivity. The Bzl treatment after TCC inoculations did not reduce, after periods of up to 420 days, TCC-induced resistance to challenge. But TCC inocula given during Bzl treatment conferred short-term, but not long-term. protection. Maintenance of high antibody levels and protection were better in the virulent Tulahuen (TUL) strain than in the attenuated TCC strain infections, and trypomastigote inocula of either strain were better inducers of antibodies and resistance than epimastigotes. PCR detection of T. cruzi DNA was positive in almost all TUL strain-inoculated animals and negative in immunocompetent animals inoculated with TCC epimastigotes, although high numbers of TCC trypomastigotes produced persistent PCR signals of infection in newborn BALB mice. Thus, 2 polar models were developed, where latent infection by TCC was either demonstrated or excluded. In both, resistance to virulent challenge was maintained during long periods. But late declination of antibody titers (>200 days) and resistance to challenge (>350 days) was observed in animals displaying clearance of all signals of infection.

Enzymatic activity, protein expression, and gene sequence of cruzipain in virulent and attenuated Trypanosoma cruzi strains

Protein expression, characterized in Western blots and gelatinolytic activity, of cruzipain (Cr), the major Trypanosoma cruzi cysteine proteinase, was compared among 3 attenuated T. cruzi strains (TUL 0, TCC, and Y null) and their virulent counterparts (TUL 2, Tulahuen, and Y). All attenuated strains displayed a weaker gelatinolytic activity as compared with their virulent counterparts. The electrophoretic mobility and immunological reactivity revealed quantitative and qualitative differences, with the attenuated parasites showing bands of less density in all strains and lower mobility in 2 of them, as compared with the virulent strains. Sequence analysis of 1 Cr gene in the Tulahuen and TCC strains indicated 37/1404 base pair substitutions, corresponding to 20 amino acid changes in the attenuated strain. A similar comparative analysis of 1 Cr gene between Y and Y null strains showed 13/1404 base pair substitutions, corresponding to 8 amino acid changes in the attenuated strain. Although enough variability exists in the Cr gene to allow for less- or nonfunctional isoforms of the protein, further clones should be analyzed to establish whether attenuation is regularly associated with specific sequence changes of this enzyme.