Date Published: April 29, 2016
Publisher: Public Library of Science
Author(s): Jonatan Ersching, José R. Vasconcelos, Camila P. Ferreira, Braulia C. Caetano, Alexandre V. Machado, Oscar Bruna–Romero, Monique A. Baron, Ludmila R. P. Ferreira, Edécio Cunha-Neto, Kenneth L. Rock, Ricardo T. Gazzinelli, Maurício M. Rodrigues, Ingrid Müller.
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+CD44highCD62Llow) specific for the previously characterized immunodominant (VNHRFTLV) H-2Kb-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-2Kb-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 cells are important mediators of pathogen control during intracellular infections. Sufficient induction of these cells leads to pathogen elimination [1–8], whereas weak or exacerbated CD8+ T cell stimulation may lead to pathology [9–17]. Therefore, the proper induction of CD8+ T cells must be tightly regulated and may be co-opted in the development of new vaccines against intracellular pathogens [18–22].
The recognition by CD8+ T lymphocytes of antigens displayed on the context of MHC class I molecules is a fundamental requirement for the control of tumors and intracellular infections by viruses, fungi, bacteria and parasites. Proteasomes are of paramount importance in the processing of antigens in the MHC class I pathway. However, the specific contribution of alternative proteasome catalytic subunits β1i, β2i and β5i (as opposed to the canonical active sites) to the generation of MHC class I-restricted epitope repertoire was thought to be, at most, incremental until the recent development of a mouse model simultaneously devoid of β1i, β2i and β5i subunits (TKO mice) . In that study, major differences in the quantity and quality of epitopes recognized by CD8+ T cells was reported, suggesting that models where only one or two subunits are targeted might underestimate the relevance of immunoproteasomes in antigen processing and presentation. Although it is reasonable to assume that immunoproteasome’s role in epitope abundance and specificity may lead to altered resistance to a pathogen, supportive data to this assumption remain relatively scarce. Here we explored the TKO mouse model to further investigate the role of immunoproteasomes in the development of protective immunity to a pathogen. Trypanosoma cruzi, a neglected human parasite, was of didactical use as a model, since protective immunity against it is highly dependent on CD8+ T cell function and IFN-γ (the major inducer of immunoproteasomes). The TKO mice exhibited a drastically reduced response of CD8+ T cells specific for immunodominant and subdominant epitopes after T. cruzi infection, whereas the response of CD4+ T cells was unaltered. Surprisingly, natural resistance to infection in the absence of immunoproteasomes was seriously compromised and even more unexpectedly the protection induced by genetic vaccination was completely abolished, indicating that the immunoproteasome catalytic subunits, rather than the conventional proteolytical sites, are essential for the processing of T. cruzi epitopes related to protective immunity generated during infection or vaccination. Our observations from chimeric mice are consistent with the hypothesis that the defect in TKO mice is the inability of antigen presenting cells to process protective epitopes to CD8+ T cells, rather than the variation of naïve CD8+ T cell repertoire. Also, we were unable to detect any shift in the immunodominance pattern of TKO mice relatively to WT counterparts.