Targeting Myeloid-Derived Suppressor Cells to Enhance a Trans-Sialidase-Based Vaccine Against Trypanosoma cruzi

Can interventions targeting MDSCs improve the outcome of vaccination in vulnerable populations?

Preventive vaccination is a crucial strategy for controlling and preventing infectious diseases, offering both effectiveness and cost-efficiency. However, despite the widespread success of vaccination programs, there are still certain population groups who struggle to mount adequate responses to immunization. These at-risk groups include but are not restricted to the elderly, overweight individuals, individuals with chronic infections and cancer patients. All of these groups are characterized by persistent chronic inflammation. Recent studies have demonstrated that one of the key players in immune regulation and the promotion of chronic inflammation are myeloid-derived suppressor cells (MDSCs). These cells possess a wide range of immunosuppressive mechanisms and are able to dampen immune responses in both antigen-specific and antigen-nonspecific manner, thus contributing to the establishment and maintenance of an inflammatory environment. Given their pivotal role in immune modulation, there is growing interest in understanding how MDSCs may influence the efficacy of vaccines, particularly in vulnerable populations. In this narrative review, we discuss whether MDSCs are able to regulate vaccine-induced immunity and whether their suppression can potentially enhance vaccine efficacy in vulnerable populations.

Control of myeloid-derived suppressor cell dynamics potentiates vaccine protection in multiple mouse models of Trypanosoma cruzi infection

To date, there is no licensed vaccine against the protozoan parasite Trypanosoma cruzi (T. cruzi), the etiological agent of Chagas Disease. T. cruzi has evolved numerous mechanisms to evade and manipulate the host immune system. Among the subversive strategies employed by the parasite, marked increases in CD11b+ Gr-1+ myeloid-derived suppressor cells (MDSCs) in several organs have been described. We have reported that CD11b+ Gr-1+ cells are involved not only during infection but also after immunization with a trans-sialidase fragment (TSf) adjuvanted with a cage-like particle adjuvant (ISPA). Thus, the aim of this work was to gain control over the involvement of MDSCs during immunization to potentiate a vaccine candidate with protective capacity in multiple mouse models of T. cruzi infection. Here, we show that the Gr-1+ cells that increase during TSf-ISPA immunization have suppressive capacity over bone marrow-derived dendritic cells and CD4+ lymphocytes. Protocols using one or two doses of 5-fluorouracil (5FU) were employed to deplete and control MDSC dynamics during immunization. The protocol based on two doses of 5FU (double 5FU TSf-ISPA) was more successful in controlling MDSCs during immunization and triggered a higher immune effector response, as evidenced by increased numbers of CD4+, CD4+CD44+, CD8+, CD8+CD44+, CD11c+, and CD11c+CD8α+ cells in the spleen and lymph nodes of double 5FU TSf-ISPA mice as compared to 5FU-TSf-ISPA mice. In line with these results, the protective capacity of the double 5FU TSf-ISPA protocol was higher compared to the 5FU-TSf-ISPA protocol against high lethal doses of intraperitoneal infection with the Tulahuen T. cruzi strain. When cross-protective capacity was analyzed, the optimized protocol based on double 5FU TSf-ISPA conferred protection in several preclinical models using different discrete typing units (DTU VI and DTU I), different mouse strains (BALB/c and C57BL/6), different parasite doses (1000 to 20000), and routes of administration (intraperitoneal and intradermal). Developing vaccines that are currently lacking may require new strategies to further potentiate vaccine candidates. Results reported herein provide evidence that rational control of cells from the regulatory arm of the immune system could enhance a vaccine candidate with cross-protective capacity in multiple mouse models of T. cruzi infection.

Simple protocol for measuring CD11b+ GR-1+ (Ly6C+/Ly6G+) myeloid cells from a minimum volume of mouse peripheral blood

Myeloid-derived suppressor cells (MDSCs) comprise a heterogeneous population of myeloid origin and immature state, whose hallmark is the capacity to suppress T cells and other immune populations. In mice, the first approach to identify MDSCs relies in the measurement of their phenotypical markers: CD11b and GR-1. In addition, two main subtypes of MDSCs have been defined based on the expression of the following markers: CD11b+ Ly6G- Ly6C+ (monocytic-MDSCs, M-MDSCs) and CD11b+ Ly6G+ Ly6C+/low (polymorphonuclear-MDSCs, PMN-MDSCs). Since CD11b+ GR-1+ (Ly6C+/Ly6G+) MDSCs can increase significantly in peripheral blood during numerous acute or chronic processes, measuring alterations in the phenotypic markers CD11b and GR-1 could be important as a first step before assessing the suppressive function of the cells. In many cases it could be necessary to measure CD11b+ Gr-1+ cells from a minimum volume of peripheral blood cells without greatly affecting animal viability, since this approach would allow for further studies to be conducted on subsequent days, such as measuring parameters of the immune response or even survival in the context of the pathology under study. The following protocol describes a simple and optimized protocol for measuring the presence of CD11b+ GR-1+ (Ly6C+/Ly6G+) myeloid cells using 2+ channel flow cytometry, from a minimum volume of mouse peripheral blood obtained by facial vein puncture.

Here, There, and Everywhere: Myeloid-Derived Suppressor Cells in Immunology

Myeloid-derived suppressor cells (MDSCs) were initially identified in humans and mice with cancer where they profoundly suppress T cell- and NK cell-mediated antitumor immunity. Inflammation is a central feature of many pathologies and normal physiological conditions and is the dominant driving force for the accumulation and function of MDSCs. Therefore, MDSCs are present in conditions where inflammation is present. Although MDSCs are detrimental in cancer and conditions where cellular immunity is desirable, they are beneficial in settings where cellular immunity is hyperactive. Because MDSCs can be generated ex vivo, they are being exploited as therapeutic agents to reduce damaging cellular immunity. In this review, we discuss the detrimental and beneficial roles of MDSCs in disease settings such as bacterial, viral, and parasitic infections, sepsis, obesity, trauma, stress, autoimmunity, transplantation and graft-versus-host disease, and normal physiological settings, including pregnancy and neonates as well as aging. The impact of MDSCs on vaccination is also discussed.

Role of myeloid-derived suppressor cells during Trypanosoma cruzi infection

Chagas disease (CD), caused by the protozoan parasite Trypanosoma cruzi, is the third largest parasitic disease burden globally. Currently, more than 6 million people are infected, mainly in Latin America, but international migration has turned CD into an emerging health problem in many nonendemic countries. Despite intense research, a vaccine is still not available. A complex parasite life cycle, together with numerous immune system manipulation strategies, may account for the lack of a prophylactic or therapeutic vaccine. There is substantial experimental evidence supporting that T. cruzi acute infection generates a strong immunosuppression state that involves numerous immune populations with regulatory/suppressive capacity. Myeloid-derived suppressor cells (MDSCs), Foxp3+ regulatory T cells (Tregs), regulatory dendritic cells and B regulatory cells are some of the regulatory populations that have been involved in the acute immune response elicited by the parasite. The fact that, during acute infection, MDSCs increase notably in several organs, such as spleen, liver and heart, together with the observation that depletion of those cells can decrease mouse survival to 0%, strongly suggests that MDSCs play a major role during acute T. cruzi infection. Accumulating evidence gained in different settings supports the capacity of MDSCs to interact with cells from both the effector and the regulatory arms of the immune system, shaping the outcome of the response in a very wide range of scenarios that include pathological and physiological processes. In this sense, the aim of the present review is to describe the main knowledge about MDSCs acquired so far, including several crosstalk with other immune populations, which could be useful to gain insight into their role during T. cruzi infection.

Myeloid-derived suppressor cells and vaccination against pathogens

It is widely accepted that the immune system includes molecular and cellular components that play a role in regulating and suppressing the effector immune response in almost any process in which the immune system is involved. Myeloid-derived suppressor cells (MDSCs) are described as a heterogeneous population of myeloid origin, immature state, with a strong capacity to suppress T cells and other immune populations. Although the initial characterization of these cells was strongly associated with pathological conditions such as cancer and then with chronic and acute infections, extensive evidence supports that MDSCs are also involved in physiological/non-pathological settings, including pregnancy, neonatal period, aging, and vaccination. Vaccination is one of the greatest public health achievements and has reduced mortality and morbidity caused by many pathogens. The primary goal of prophylactic vaccination is to induce protection against a potential pathogen by mimicking, at least in a part, the events that take place during its natural interaction with the host. This strategy allows the immune system to prepare humoral and cellular effector components to cope with the real infection. This approach has been successful in developing vaccines against many pathogens. However, when the infectious agents can evade and subvert the host immune system, inducing cells with regulatory/suppressive capacity, the development of vaccines may not be straightforward. Notably, there is a long list of complex pathogens that can expand MDSCs, for which a vaccine is still not available. Moreover, vaccination against numerous bacteria, viruses, parasites, and fungi has also been shown to cause MDSC expansion. Increases are not due to a particular adjuvant or immunization route; indeed, numerous adjuvants and immunization routes have been reported to cause an accumulation of this immunosuppressive population. Most of the reports describe that, according to their suppressive nature, MDSCs may limit vaccine efficacy. Taking into account the accumulated evidence supporting the involvement of MDSCs in vaccination, this review aims to compile the studies that highlight the role of MDSCs during the assessment of vaccines against pathogens.

Development of low-cost cage-like particles to formulate veterinary vaccines

Low-cost adjuvants are urgently needed for the development of veterinary vaccines able to trigger strong immune responses. In this work, we describe a method to obtain a low-cost cage-like particles (ISCOMATRIX-like) adjuvant useful to formulate veterinary vaccines candidates. The main components to form the particles are lipids and saponins, which were obtained from egg yolk by ethanolic extraction and by dialyzing a non-refined saponins extract, respectively. Lipids were fully characterized by thin layer chromatography (TLC) and gas-chromatography (GC) and enzymatic methods, and saponins were characterized by TLC, HPLC and MALDI-TOF. Cage-like particles were prepared with these components or with commercial inputs. Both particles and the traditional Alum used in veterinary vaccines were compared by immunizing mice with Ovalbumin (OVA) formulated with these adjuvants and assessing IgG1, IgG2a anti OVA antibodies and specific Delayed-type Hypersensitivity (DTH). In the yolk extract, a mixture of phospholipids, cholesterol and minor components of the extract (e.g. lyso-phospholipids) with suitable proportions to generate cage-like particles was obtained. Also, semi-purified saponins with similar features to those of the QuilA® were obtained. Cage-like particles prepared with these components have 40-50 nm and triggers similar levels of Anti-OVA IgG1 and DTH than with commercial inputs but higher specific-IgG2a. Both adjuvants largely increased the levels of IgG1, IgG2a and DTH in relation to the formulation with Alum. The methods described to extract lipids from egg yolk and saponins from non-refined extract allowed us to obtain an inexpensive and highly effective adjuvant.

Assessment of a combined treatment with a therapeutic vaccine and benznidazole for the Trypanosoma cruzi chronic infection

The difficulties encountered in achieving treatments for chronic Chagas disease have promoted the investigation of new therapeutic strategies. In this study, we used two murine models of Trypanosoma cruzi chronic infection to determine the usefulness of applying a therapeutic vaccine alone or followed by benznidazole (Bz) chemotherapy. A vaccine formulation based on an N-terminal fragment of Trans-sialidase (TS) and Immunostimulant Particle Adjuvant (ISPA) - TSNt-ISPA was obtained. Firstly, the immunogenicity and protective capacity of TSNt-ISPA was demonstrated as a prophylactic formulation in an acute model of infection. Later, the formulation was assessed as a therapeutic vaccine alone or combined with (Bz) using two models of chronic infection. BALB/c mice chronically infected with Sylvio X10/4 or Tulahuen cl2 T. cruzi strains were not treated as control or treated only with the therapeutic vaccine TSNt-ISPA, with a combined treatment TSNt-ISPA+Bz (Bz applied after the vaccine), or only with Bz. The vaccination schedule consisted of TSNt-ISPA administration at days110, 120, and 130 post-infection (pi) and Bz administration was performed daily from day 140 to 170 pi. At day 273 pi, electrocardiographic (ECG) parameters, heart parasite load, myocarditis, and heart fibrosis were assessed. In both models, therapeutic administration of TSNt-ISPA reduced ECG alterations and the cardiac tissue damage observed in the chronic phase. Moreover, vaccine treatment significantly decreased heart parasite load in both Sylvio X10/4 and Tulahuen cl2 infected mice. The combined treatment, but not Bz or vaccine administration alone, allowed to restore ECG parameters in Tulahuen cl2 infected mice. The results indicate the usefulness of the therapeutic TSNt-ISPA formulation in BALB/c mice chronically infected with Sylvio X10/4 or Tulahuen cl2 strain. For the mice infected with T. cruzi Tulahuen cl2 strain, the combined treatment with the vaccine and Bz had a more positive effect on the course of heart disease than the individual treatments with the vaccine or Bz alone.

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 20th 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.

Myeloid-Derived Suppressor Cells in Trypanosoma cruzi Infection

Myeloid-derived suppressor cells (MDSCs) are immature heterogeneous myeloid cells that expand in pathologic conditions as cancer, trauma, and infection. Although characterization of MDSCs is continuously revisited, the best feature is their suppressor activity. There are many markers for MDSC identification, it is distinctive that they express inducible nitric oxide synthase (iNOS) and arginase 1, which can mediate immune suppression. MDSCs can have a medullary origin as a result of emergency myelopoiesis, but also can have an extramedullary origin. Early studies on Trypanosoma cruzi infection showed severe immunosuppression, and several mechanisms involving parasite antigens and host cell mediators were described as inhibition of IL-2 and IL-2R. Another mechanism of immunosuppression involving tumor necrosis factor/interferon γ-dependent nitric oxide production by inducible nitric oxide synthase was also described. Moreover, other studies showed that nitric oxide was produced by CD11b⁺ Gr-1⁺ MDSCs in the spleen, and later iNOS and arginase 1 expressed in CD11b⁺Ly6C⁺Ly6G^lo monocytic MDSC were found in spleen and heart of T. cruzi infected mice that suppressed T cell proliferation. Uncontrolled expansion of monocytic MDSCs leads to L-arginine depletion which hinders nitric oxide production leading to death. Supplement of L-arginine partially reverts L-arginine depletion and survival, suggesting that L-arginine could be administered along with anti-parasitical drugs. On the other hand, pharmacological inhibition of MDSCs leads to death in mice, suggesting that some expansion of MDSCs is needed for an efficient immune response. The role of signaling molecules mediating immune suppression as reactive oxygen species, reactive nitrogen species, as well as prostaglandin E2, characteristics of MDSCs, in T. cruzi infection is not fully understood. We review and discuss the role of these reactive species mediators produced by MDSCs. Finally, we discuss the latest results that link the SLAMF1 immune receptor with reactive oxygen species. Interaction of the parasite with the SLAMF1 modulates parasite virulence through myeloid cell infectivity and reactive oxygen species production. We discuss the possible strategies for targeting MDSCs and SLAMF1 receptor in acute Trypanosoma cruzi infection in mice, to evaluate a possible translational application in human acute infections.