Trypanosoma cruzi-specific CD8+ T cells and other immunological hallmarks in chronic Chagas cardiomyopathy: Two decades of research

Abstract

Trypanosoma cruzi, the causal agent of Chagas disease, has coexisted with humans for thousands of years. Therefore, the parasite has developed several mechanisms of antigenic variability that has allowed it to live inside the cells and evade the host immune response. Since T. cruzi displays an intracellular cycle-stage, our research team focused on providing insights into the CD8⁺ T cells immune response in chronic Chagas cardiomyopathy. We began our work in the 2000s studying parasite antigens that induce natural immune responses such as the KMP11 protein and TcTLE, its N-terminal derived peptide. Different approaches allowed us to reveal TcTLE peptide as a promiscuous CD8⁺ T cell epitope, able of inducing multifunctional cellular immune responses and eliciting a humoral response capable of decreasing parasite movement and infective capacity. Next, we demonstrated that as the disease progresses, total CD8⁺ T cells display a dysfunctional state characterized by a prolonged hyper-activation state along with an increase of inhibitory receptors (2B4, CD160, PD-1, TIM-3, CTLA-4) expression, an increase of specific terminal effector T cells (T_TE), a decrease of proliferative capacity, a decrease of stem cell memory (T_SCM) frequency, and a decrease of CD28 and CD3ζ expression. Thus, parasite-specific CD8⁺ T cells undergo clonal exhaustion, distinguished by an increase in late-differentiated cells, a mono-functional response, and enhanced expression of inhibitory receptors. Finally, it was found that anti-parasitic treatment induces an improved CD8⁺ T cell response in asymptomatic individuals, and a mouse animal model led us to establish a correlation between the quality of the CD8⁺ T cell responses and the outcome of chronic infection. In the future, using OMICs strategies, the identification of the specific cellular signals involved in disease progression will provide an invaluable resource for discovering new biomarkers of progression or new vaccine and immunotherapy strategies. Also, the inclusion of the TcTLE peptide in the rational design of epitope-based vaccines, the development of immunotherapy strategies using T_SCM or the blocking of inhibitory receptors, and the use of the CD8⁺ T cell response quality to follow treatments, immunotherapies or vaccines, all are alternatives than could be explored in the fight against Chagas disease.

Keywords: CD8+ T cell; Chagas cardiomyopathy; Chagas disease; TcTLE peptide; Trypanosoma cruzi; immune response.

Figure 1

Characteristics of the groups of patients included in the different studies. Serology was used to determine IgG anti-Trypanosoma cruzi-specific antibodies.

Figure 2

Binding assays of TcTLE homologous peptides to HLA-A*0201. (A) Binding affinity assay of peptides homologs to the HLA-A*0201 molecule using the original TcTLE peptide as an internal control. The binding affinity of each sequence was defined with different peptide concentrations. (B) Binding stability assay of the peptide/HLA-A*0201 complex normalized to the relative stability of the complex with the TcTLE peptide. The binding stability of each peptide to HLA-A*0201 was analysed at different times using 100 mM of each peptide. The two tests were performed in duplicate. Each point represents the median of 4 experiments performed independently. The p values were calculated using Mann-Whitney U test. *p < 0.05, ****p < 0.0001.

Figure 3

Frequency of CD8⁺ T cells that produce IFN-γ after stimulation with the original TcTLE peptide and homologous peptides after 6 hours of culture with each peptide in CCP (n = 21). The cut-off point for considering a positive response (> 0.05%) was subtracted from each peptide.

Figure 4

Frequency of CD8⁺ T cells in CCP (n = 5) that secrete cytokines or express CD107a/b after 6 hours of culture with the original TcTLE peptide and homologous peptides. Each column shows the median and frequency range of CD8⁺ T cells expressing each molecule tested.

Figure 5

Chronic T. cruzi infection leads to clonal exhaustion characterized by monofunctional CD8⁺ T cell responses and coexpression of inhibitory receptors. In acute infection or after vaccination, antigen-presenting cells (APCs) capture and process antigens to activate naive T cells (T_N). This activation process triggers a specific response to control the infection and eliminate the antigen (sterilizing immunity). After antigen clearance and resolution of inflammation, most T effector (T_Eff) cells die; however, a small cell group persists and differentiates into memory T cells (T_Mem) with remarkable proliferative potential, antigen-independent survival, and the ability to reactivate its effector functions in new encounters with the antigen. In contrast, in an infection where antigenic stimulation persists, although effector and memory T cell subsets are generated, T cells lose effector functions as the infection becomes chronic and increase the expression and co-expression of inhibitory receptors such as PD-1, CTLA-4, CD160, and 2B4. Some functions as cell proliferation or IL-2 production are initially lost. IFN-γ production can be subsequently reduced and TNF-α can decrease or be present as in the case of Chagas disease, and in severe states of exhaustion, cells may die by apoptosis.