Re-circulation of lymphocytes mediated by sphingosine-1-phosphate receptor-1 contributes to resistance against experimental infection with the protozoan parasite Trypanosoma cruzi

Recent Advances in the Development of Adenovirus-Vectored Vaccines for Parasitic Infections

Vaccines against parasites have lagged centuries behind those against viral and bacterial infections, despite the devastating morbidity and widespread effects of parasitic diseases across the globe. One of the greatest hurdles to parasite vaccine development has been the lack of vaccine strategies able to elicit the complex and multifaceted immune responses needed to abrogate parasitic persistence. Viral vectors, especially adenovirus (AdV) vectors, have emerged as a potential solution for complex disease targets, including HIV, tuberculosis, and parasitic diseases, to name a few. AdVs are highly immunogenic and are uniquely able to drive CD8+ T cell responses, which are known to be correlates of immunity in infections with most protozoan and some helminthic parasites. This review presents recent developments in AdV-vectored vaccines targeting five major human parasitic diseases: malaria, Chagas disease, schistosomiasis, leishmaniasis, and toxoplasmosis. Many AdV-vectored vaccines have been developed for these diseases, utilizing a wide variety of vectors, antigens, and modes of delivery. AdV-vectored vaccines are a promising approach for the historically challenging target of human parasitic diseases.

Host-parasite dynamics in Chagas disease from systemic to hyper-local scales

Trypanosoma cruzi is a remarkably versatile parasite. It can parasitize almost any nucleated cell type and naturally infects hundreds of mammal species across much of the Americas. In humans, it is the cause of Chagas disease, a set of mainly chronic conditions predominantly affecting the heart and gastrointestinal tract, which can progress to become life threatening. Yet around two thirds of infected people are long-term asymptomatic carriers. Clinical outcomes depend on many factors, but the central determinant is the nature of the host-parasite interactions that play out over the years of chronic infection in diverse tissue environments. In this review, we aim to integrate recent developments in the understanding of the spatial and temporal dynamics of T. cruzi infections with established and emerging concepts in host immune responses in the corresponding phases and tissues.

Host Genetics Background Influence in the Intragastric Trypanosoma cruzi Infection

Background

Considering the complexity of the factors involved in the immunopathology of Chagas disease, which influence the Chagas' disease pathogenesis, anti-T. cruzi immune response, and chemotherapy outcome, further studies are needed to improve our understanding about these relationships. On this way, in this article we analyzed the host genetic influence on hematological, histopathological and immunological aspects after T. cruzi infection.

Methods

BALB/c and A mice were intragastrically infected with T. cruzi SC2005 strain, isolated from a patient of an outbreak of Chagas disease. Parameters such as parasite load, survival rates, cytokines production, macrophages, T and B cell frequencies, and histopathology analysis were carried out.

Results

BALB/c mice presented higher parasitemia and mortality rates than A mice. Both mouse lineages exhibited hematological alterations suggestive of microcytic hypochromic anemia and histopathological alterations in stomach, heart and liver. The increase of CD8+ T cells, in heart, liver and blood, and the increase of CD19+ B cells, in liver, associated with a high level of proinflammatory cytokines (IL-6, TNF-α, IFN-γ), confer a resistance profile to the host. Although BALB/c animals exhibited the same findings observed in A mice, the response to infection occurred later, after a considerable parasitemia increase. By developing an early response to the infection, A mice were found to be less susceptible to T. cruzi SC2005 infection.

Conclusions

Host genetics background shaping the response to infection. The early development of a cytotoxic cellular response profile with the production of proinflammatory cytokines is important to lead a less severe manifestation of Chagas disease.

CXCR3 chemokine receptor contributes to specific CD8+ T cell activation by pDC during infection with intracellular pathogens

Chemokine receptor type 3 (CXCR3) plays an important role in CD8⁺ T cells migration during intracellular infections, such as Trypanosoma cruzi. In addition to chemotaxis, CXCR3 receptor has been described as important to the interaction between antigen-presenting cells and effector cells. We hypothesized that CXCR3 is fundamental to T. cruzi-specific CD8⁺ T cell activation, migration and effector function. Anti-CXCR3 neutralizing antibody administration to acutely T. cruzi-infected mice decreased the number of specific CD8⁺ T cells in the spleen, and those cells had impaired in activation and cytokine production but unaltered proliferative response. In addition, anti-CXCR3-treated mice showed decreased frequency of CD8⁺ T cells in the heart and numbers of plasmacytoid dendritic cells in spleen and lymph node. As CD8⁺ T cells interacted with plasmacytoid dendritic cells during infection by T. cruzi, we suggest that anti-CXCR3 treatment lowers the quantity of plasmacytoid dendritic cells, which may contribute to impair the prime of CD8⁺ T cells. Understanding which molecules and mechanisms guide CD8⁺ T cell activation and migration might be a key to vaccine development against Chagas disease as those cells play an important role in T. cruzi infection control.

Inflammatory chemokine receptors such as CXCR3 play an important role in T lymphocytes migration into an infected tissue during Th1 response. Recently, the role of CXCR3 as a co-stimulatory molecule was demonstrated, and T lymphocytes from CXCR3 deficient mice had impaired effector function. CXCR3 receptor was highly expressed on specific CD8⁺ T cells after challenge with T. cruzi, and the hypothesis of that molecule is important for CD8⁺ T cells activation, migration and functionality was raised. We used the anti-CXCR3 neutralizing antibody approach and demonstrated that C57BL/6 treated mice died very quickly due to T. cruzi infection, and specific CD8⁺ T cells had decreased effector phenotyping, cytokine production, and cytotoxicity. In addition, anti-CXCR3 treatment decreased the number of dendritic plasmacytoid cells in the lymphoid tissues. The lower quantity of dendritic plasmacytoid cells in those tissues might contribute to the decrease in CD8⁺ T cells activation. Overall, CXCR3 molecule seems to be an important molecule to be explored during vaccine against Chagas disease strategies.

LFA-1 Mediates Cytotoxicity and Tissue Migration of Specific CD8+ T Cells after Heterologous Prime-Boost Vaccination against Trypanosoma cruzi Infection

Integrins mediate the lymphocyte migration into an infected tissue, and these cells are essential for controlling the multiplication of many intracellular parasites such as Trypanosoma cruzi, the causative agent of Chagas disease. Here, we explore LFA-1 and VLA-4 roles in the migration of specific CD8⁺ T cells generated by heterologous prime-boost immunization during experimental infection with T. cruzi. To this end, vaccinated mice were treated with monoclonal anti-LFA-1 and/or anti-VLA-4 to block these molecules. After anti-LFA-1, but not anti-VLA-4 treatment, all vaccinated mice displayed increased blood and tissue parasitemia, and quickly succumbed to infection. In addition, there was an accumulation of specific CD8⁺ T cells in the spleen and lymph nodes and a decrease in the number of those cells, especially in the heart, suggesting that LFA-1 is important for the output of specific CD8⁺ T cells from secondary lymphoid organs into infected organs such as the heart. The treatment did not alter CD8⁺ T cell effector functions such as the production of pro-inflammatory cytokines and granzyme B, and maintained the proliferative capacity after treatment. However, the specific CD8⁺ T cell direct cytotoxicity was impaired after LFA-1 blockade. Also, these cells expressed higher levels of Fas/CD95 on the surface, suggesting that they are susceptible to programmed cell death by the extrinsic pathway. We conclude that LFA-1 plays an important role in the migration of specific CD8⁺ T cells and in the direct cytotoxicity of these cells.

Sphingosine 1-phosphate signaling impacts lymphocyte migration, inflammation and infection

Sphingosine 1-phosphate (S1P) is a sphingosine containing lipid intermediate obtained from ceramide. S1P is known to be an important signaling molecule and plays multiple roles in the context of immunity. This lysophospholipid binds and activates G-protein-coupled receptors (GPCRs) known as S1P receptors 1-5 (S1P1-5). Once activated, these GPCRs mediate signaling that can lead to alterations in cell proliferation, survival or migration, and can also have other effects such as promoting angiogenesis. In this review, we will present evidence demonstrating a role for S1P in lymphocyte migration, inflammation and infection, as well as in cancer. The therapeutic potential of targeting S1P receptors, kinases and lyase will also be discussed.

The Combined Deficiency of Immunoproteasome Subunits Affects Both the Magnitude and Quality of Pathogen- and Genetic Vaccination-Induced CD8+ T Cell Responses to the Human Protozoan Parasite Trypanosoma cruzi

The β1i, β2i and β5i immunoproteasome subunits have an important role in defining the repertoire of MHC class I-restricted epitopes. However, the impact of combined deficiency of the three immunoproteasome subunits in the development of protective immunity to intracellular pathogens has not been investigated. Here, we demonstrate that immunoproteasomes play a key role in host resistance and genetic vaccination-induced protection against the human pathogen Trypanosoma cruzi (the causative agent of Chagas disease), immunity to which is dependent on CD8⁺ T cells and IFN-γ (the classical immunoproteasome inducer). We observed that infection with T. cruzi triggers the transcription of immunoproteasome genes, both in mice and humans. Importantly, genetically vaccinated or T. cruzi-infected β1i, β2i and β5i triple knockout (TKO) mice presented significantly lower frequencies and numbers of splenic CD8⁺ effector T cells (CD8⁺CD44^highCD62L^low) specific for the previously characterized immunodominant (VNHRFTLV) H-2K^b-restricted T. cruzi epitope. Not only the quantity, but also the quality of parasite-specific CD8⁺ T cell responses was altered in TKO mice. Hence, the frequency of double-positive (IFN-γ⁺/TNF⁺) or single-positive (IFN-γ⁺) cells specific for the H-2K^b-restricted immunodominant as well as subdominant T. cruzi epitopes were higher in WT mice, whereas TNF single-positive cells prevailed among CD8⁺ T cells from TKO mice. Contrasting with their WT counterparts, TKO animals were also lethally susceptible to T. cruzi challenge, even after an otherwise protective vaccination with DNA and adenoviral vectors. We conclude that the immunoproteasome subunits are key determinants in host resistance to T. cruzi infection by influencing both the magnitude and quality of CD8⁺ T cell responses.

CD8⁺ t lymphocytes are cells of the immune system that mediate control of intracellular infections by viruses, prokaryote as well as eukaryote pathogens. To confer protection, these lymphocytes need to be elicited by pathogen peptides that are presented in association with MHC class I molecules. The degradation of self and microbial proteins by catalytic domains of the cytosolic proteasome β1, β2 and β5 subunits is intimately linked to the generation of MHC class I-restricted epitopes, which in turn are important determinants of the kinetics, specificity and efficiency of CD8⁺ T cell-mediated immunity. Importantly, inflammatory stimuli trigger the expression of the inducible alternative β1i, β2i and β5i subunits that form the immunoproteasomes. The qualitative and quantitative importance of immunoproteasomes in generating CD8⁺ T cell epitopes has recently been demonstrated in mice that are simultaneously devoid of the β1i, β2i and β5i subunits. In this study, we explored the role of immunoproteasomes in host resistance to Trypanosoma cruzi, a protozoan parasite that causes Chagas disease. We found that β1i, β2i and β5i triply deficient mice have an impaired response of CD8⁺ T cells and are highly susceptible to primary infection with T. cruzi. We also demonstrated that host resistance induced by a genetic vaccine able to protect normal mice from T. cruzi challenge fails to do so in the immunoproteasome-deficient mice. Our study provides strong evidences that β1i, β2i and β5i immunoproteasome subunits are important determinants of both the magnitude and quality of CD8⁺ T cell responses as well as immune-mediated host resistance to a human pathogen.

Successive site translocating inoculation potentiates DNA/recombinant vaccinia vaccination

DNA vaccines have advantages over traditional vaccine modalities; however the relatively low immunogenicity restrains its translation into clinical use. Further optimizations are needed to get the immunogenicity of DNA vaccine closer to the level required for human use. Here we show that intramuscularly inoculating into a different limb each time significantly improves the immunogenicities of both DNA and recombinant vaccinia vaccines during multiple vaccinations, compared to repeated vaccination on the same limb. We term this strategy successive site translocating inoculation (SSTI). SSTI could work in synergy with genetic adjuvant and DNA prime-recombinant vaccinia boost regimen. By comparing in vivo antigen expression, we found that SSTI avoided the specific inhibition of in vivo antigen expression, which was observed in the limbs being repeatedly inoculated. Employing in vivo T cell depletion and passive IgG transfer, we delineated that the inhibition was not mediated by CD8⁺ T cells but by specific antibodies. Finally, by using C3^−/− mouse model and in vivo NK cells depletion, we identified that specific antibodies negatively regulated the in vivo antigen expression primarily in a complement depended way.

Sphingosine-1-phosphate signaling: unraveling its role as a drug target against infectious diseases

Sphingosine-1-phosphate (S1P) signaling is reported in variety of cell types, including immune, endothelial and cancerous cells. It is emerging as a crucial regulator of cellular processes, such as apoptosis, cell proliferation, migration, differentiation and so on. This signaling pathway is initiated by the intracellular production and secretion of S1P through a cascade of enzymatic reactions. Binding of S1P to different S1P receptors (S1PRs) activates different downstream signaling pathways that regulate the cellular functions differentially depending upon the cell type. An accumulating body of evidence suggests that S1P metabolism and signaling is often impaired during infectious diseases; thus, its manipulation might be helpful in the treatment of such diseases. In this review, we summarize recent advances in our understanding of the S1P signaling pathway and its candidature as a novel drug target against infectious diseases.

Early double-negative thymocyte export in Trypanosoma cruzi infection is restricted by sphingosine receptors and associated with human chagas disease

The protozoan parasite Trypanosoma cruzi is able to target the thymus and induce alterations of the thymic microenvironmental and lymphoid compartments. Acute infection results in severe atrophy of the organ and early release of immature thymocytes into the periphery. To date, the pathophysiological effects of thymic changes promoted by parasite-inducing premature release of thymocytes to the periphery has remained elusive. Herein, we show that sphingosine-1-phosphate (S1P), a potent mediator of T cell chemotaxis, plays a role in the exit of immature double-negative thymocytes in experimental Chagas disease. In thymuses from T. cruzi-infected mice we detected reduced transcription of the S1P kinase 1 and 2 genes related to S1P biosynthesis, together with increased transcription of the SGPL1 sphingosine-1-lyase gene, whose product inactivates S1P. These changes were associated with reduced intrathymic levels of S1P kinase activity. Interestingly, double-negative thymocytes from infected animals expressed high levels of the S1P receptor during infection, and migrated to lower levels of S1P. Moreover, during T. cruzi infection, this thymocyte subset expresses high levels of IL-17 and TNF-α cytokines upon polyclonal stimulation. In vivo treatment with the S1P receptor antagonist FTY720 resulted in recovery the numbers of double-negative thymocytes in infected thymuses to physiological levels. Finally, we showed increased numbers of double-negative T cells in the peripheral blood in severe cardiac forms of human Chagas disease.

The formation of mature lineage-committed T cells requires the specialized environment of the thymus, a central organ of the immune system supporting the development of self-tolerant T cells. Key events of intrathymic T-cell development include lineage commitment, selection events and thymic emigration. This organ undergoes physiological involution during aging. However, acute thymic atrophy can occur in the presence autoimmune diseases, malignant tumors and infections caused by intracellular pathogens. The present study shows that the protozoan parasite Trypanosoma cruzi changes the thymic microenvironmental and lymphoid compartments, resulting in premature release of very immature CD4⁻CD8⁻ double-negative thymocytes, TCRneg/low, which bear a pro-inflammatory activation profile. Strikingly, we also found elevated levels of these undifferentiated T lymphocytes in the peripheral blood of patients in severe cardiac forms of chronic Chagas disease. Importantly, we provided evidence that migration of CD4⁻CD8⁻ T cells from infected mouse thymus is due to sphingosine-1-phosphate receptor-1-dependent chemotaxis. These findings point to an important role for bioactive signaling sphingolipids in the thymic escape of immature thymocytes to the periphery in Chagas disease.

CD8(+) T cell-mediated immunity during Trypanosoma cruzi infection: a path for vaccine development?

MHC-restricted CD8⁺ T cells are important during infection with the intracellular protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease. Experimental studies performed in the past 25 years have elucidated a number of features related to the immune response mediated by these T cells, which are important for establishing the parasite/host equilibrium leading to chronic infection. CD8⁺ T cells are specific for highly immunodominant antigens expressed by members of the trans-sialidase family. After infection, their activation is delayed, and the cells display a high proliferative activity associated with high apoptotic rates. Although they participate in parasite control and elimination, they are unable to clear the infection due to their low fitness, allowing the parasite to establish the chronic phase when these cells then play an active role in the induction of heart immunopathology. Vaccination with a number of subunit recombinant vaccines aimed at eliciting specific CD8⁺ T cells can reverse this path, thereby generating a productive immune response that will lead to the control of infection, reduction of symptoms, and reduction of disease transmission. Due to these attributes, activation of CD8⁺ T lymphocytes may constitute a path for the development of a veterinarian or human vaccine.

Genetic vaccination against experimental infection with myotropic parasite strains of Trypanosoma cruzi

In earlier studies, we reported that a heterologous prime-boost regimen using recombinant plasmid DNA followed by replication-defective adenovirus vector, both containing Trypanosoma cruzi genes encoding trans-sialidase (TS) and amastigote surface protein (ASP) 2, provided protective immunity against experimental infection with a reticulotropic strain of this human protozoan parasite. Herein, we tested the outcome of genetic vaccination of F1 (CB10XBALB/c) mice challenged with myotropic parasite strains (Brazil and Colombian). Initially, we determined that the coadministration during priming of a DNA plasmid containing the murine IL-12 gene improved the immune response and was essential for protective immunity elicited by the heterologous prime-boost regimen in susceptible male mice against acute lethal infections with these parasites. The prophylactic or therapeutic vaccination of resistant female mice led to a drastic reduction in the number of inflammatory infiltrates in cardiac and skeletal muscles during the chronic phase of infection with either strain. Analysis of the electrocardiographic parameters showed that prophylactic vaccination reduced the frequencies of sinus arrhythmia and atrioventricular block. Our results confirmed that prophylactic vaccination using the TS and ASP-2 genes benefits the host against acute and chronic pathologies caused by T. cruzi and should be further evaluated for the development of a veterinary or human vaccine against Chagas disease.

Adenovirus vector-induced CD8⁺ T effector memory cell differentiation and recirculation, but not proliferation, are important for protective immunity against experimental Trypanosoma cruzi Infection

Heterologous prime-boost vaccination using plasmid DNA followed by replication-defective adenovirus vector generates a large number of specific CD8⁺ T effector memory (TEM) cells that provide long-term immunity against a variety of pathogens. In the present study, we initially characterized the frequency, phenotype, and function of these T cells in vaccinated mice that were subjected to infectious challenge with the human protozoan parasite Trypanosoma cruzi. We observed that the frequency of the specific CD8⁺ T cells in the spleens of the vaccinated mice increased after challenge. Specific TEM cells differentiated into cells with a KLRG1(High) CD27(Low) CD43(Low) CD183(Low)T-bet(High) Eomes(Low) phenotype and capable to produce simultaneously the antiparasitic mediators IFNγ and TNF. Using the gzmBCreERT2/ROSA26EYFP transgenic mouse line, in which the cells that express Granzyme B after immunization, are indelibly labeled with enhanced yellow fluorescent protein, we confirmed that CD8⁺ T cells present after challenge were indeed TEM cells that had been induced by vaccination. Subsequently, we observed that the in vivo increase in the frequency of the specific CD8⁺ T cells was not because of an anamnestic immune response. Most importantly, after challenge, the increase in the frequency of specific cells and the protective immunity they mediate were insensitive to treatment with the cytostatic toxic agent hydroxyurea. We have previously described that the administration of the drug FTY720, which reduces lymphocyte recirculation, severely impairs protective immunity, and our evidence supports the model that when large amounts of antigen-experienced CD8⁺ TEM cells are present after heterologous prime-boost vaccination, differentiation, and recirculation, rather than proliferation, are key for the resultant protective immunity.

Vaccination using recombinants influenza and adenoviruses encoding amastigote surface protein-2 are highly effective on protection against Trypanosoma cruzi infection

In the present study we evaluated the protection raised by immunization with recombinant influenza viruses carrying sequences coding for polypeptides corresponding to medial and carboxi-terminal moieties of Trypanosoma cruzi ´s amastigote surface protein 2 (ASP2). Those viruses were used in sequential immunization with recombinant adenovirus (heterologous prime-boost immunization protocol) encoding the complete sequence of ASP2 (Ad-ASP2) in two mouse strains (C57BL/6 and C3H/He). The CD8 effector response elicited by this protocol was comparable to that observed in mice immunized twice with Ad-ASP2 and more robust than that observed in mice that were immunized once with Ad-ASP2. Whereas a single immunization with Ad-ASP2 sufficed to completely protect C57BL/6 mice, a higher survival rate was observed in C3H/He mice that were primed with recombinant influenza virus and boosted with Ad-ASP2 after being challenged with T. cruzi. Analyzing the phenotype of CD8+ T cells obtained from spleen of vaccinated C3H/He mice we observed that heterologous prime-boost immunization protocol elicited more CD8+ T cells specific for the immunodominant epitope as well as a higher number of CD8+ T cells producing TNF-α and IFN-γ and a higher mobilization of surface marker CD107a. Taken together, our results suggest that immunodominant subpopulations of CD8+ T elicited after immunization could be directly related to degree of protection achieved by different immunization protocols using different viral vectors. Overall, these results demonstrated the usefulness of recombinant influenza viruses in immunization protocols against Chagas Disease.

Skin-draining lymph node priming is sufficient to induce sterile immunity against pre-erythrocytic malaria

The Plasmodium-infected hepatocyte has been considered necessary to prime the immune responses leading to sterile protection after vaccination with attenuated sporozoites. However, it has recently been demonstrated that priming also occurs in the skin. We wished to establish if sterile protection could be obtained in the absence of priming by infected hepatocytes. To this end, we developed a subcutaneous (s.c.) immunization protocol where few, possibly none, of the immunizing irradiated Plasmodium yoelii sporozoites infect hepatocytes, and also used CD81-deficient mice non-permissive to productive hepatocyte infections. We then compared and contrasted the patterns of priming with those obtained by intradermal immunization, where priming occurs in the liver. Using sterile immunity as a primary read-out, we exploited an inhibitor of T-cell migration, transgenic mice with conditional depletion of dendritic cells and adoptive transfers of draining lymph node-derived T cells, to provide evidence that responses leading to sterile immunity can be primed in the skin-draining lymph nodes with little, if any, contribution from the infected hepatocyte.

Relevance of long-lived CD8(+) T effector memory cells for protective immunity elicited by heterologous prime-boost vaccination

Owing to the importance of major histocompatibility complex class Ia-restricted CD8(+) T cells for host survival following viral, bacterial, fungal, or parasitic infection, it has become largely accepted that these cells should be considered in the design of a new generation of vaccines. For the past 20 years, solid evidence has been provided that the heterologous prime-boost regimen achieves the best results in terms of induction of long-lived protective CD8(+) T cells against a variety of experimental infections. Although this regimen has often been used experimentally, as is the case for many vaccines, the mechanism behind the efficacy of this vaccination regimen is still largely unknown. The main purpose of this review is to examine the characteristics of the protective CD8(+) T cells generated by this vaccination regimen. Part of its efficacy certainly relies on the generation and maintenance of large numbers of specific lymphocytes. Other specific characteristics may also be important, and studies on this direction have only recently been initiated. So far, the characterization of these protective, long-lived T cell populations suggests that there is a high frequency of polyfunctional T cells; these cells cover a large breadth and display a T effector memory (TEM) phenotype. These TEM cells are capable of proliferating after an infectious challenge and are highly refractory to apoptosis due to a control of the expression of pro-apoptotic receptors such as CD95. Also, they do not undergo significant long-term immunological erosion. Understanding the mechanisms that control the generation and maintenance of the protective activity of these long-lived TEM cells will certainly provide important insights into the physiology of CD8(+) T cells and pave the way for the design of new or improved vaccines.