IFNγ-inducible proteasome components in immune responses

Abstract

Protein degradation results in the production of peptides that can be presented to CD8 T cells in MHC class I molecules on the cell surface. The proteasome is the main protease responsible for protein degradation. Especially the IFNγ-inducible proteasome components (proteasome activator PA28 and the immunoproteasome subunits LMP2, LMP7 and MECL-1) play a central role in MHC I antigen processing, as well as in other aspects of immune responses. We studied the function of IFNγ-inducible proteasome components in MHC class I antigen processing and CD8 T cell responses. Deficiency of either PA28 or the immunosubunits leads to decreased MHC class I levels, reflecting lower production of MHC class I ligands. The finding that PA28 deficiency results in decreased MHC class I levels both in the presence or absence of immunosubunits, indicates that the effects of both proteasome components are additive and that they enhance the generation of MHC class I binding peptides through different mechanisms. However, the effect of PA28 on processing of certain immunodominant epitopes did not affect CD8 T cell responses, in contrast to the immunosubunits. The reduced expression of MHC class I molecules found on immunoproteasome-deficient cells could influence the education of NK cells, because MHC class I molecules are ligands for the inhibitory receptors of NK cells. Therefore, we examined whether immunoproteasome-deficient mice have educated NK cells in the periphery. The effector function of NK cells, measured by production of IFNγ and LAMP-1, was similar in RAG1-deficient and RAG1+immunosubunit-deficient mice, indicating that immunoproteasome-deficient mice have functional NK cells in the periphery. Although wildtype NK cells can reject MHC class I-deficient cells, immunoproteasome-deficient cells that were transferred into wildtype recipients were tolerized. However, this tolerance was broken upon infection in a NK cell-dependent way. The increased CD4/CD8 ratio found in immunosubunit-deficient mice is due to absence of MECL-1. To investigate how the effects of immunoproteasomes on CD4/CD8 T cell ratios are caused, T cell compartments of mice that differed in immunosubunit composition and developed in the same environment were analyzed. In this way, environmental factors and thymic selection were similar for all cells, but CD4/CD8 ratios were still enhanced in immunosubunit-deficient cells. Therefore, MECL-1 affects CD4 and CD8 T cell expansion by a T cell intrinsic, yet unknown, mechanism. To study where antigen processing and presentation to prime CD8 T cells occurs, we generated different BM chimeric mice in order to control the expression of immunosubunits in specific cell types, and we used an infection model harboring an epitope that is only efficiently generated by immunoproteasomes. We found that CD8 T cell responses to this immunoproteasome-dependent epitope required expression of immunoproteasomes in BM-derived cells. After depletion of DCs, we still were able to detect CD8 T cell responses to the immunoproteasome-dependent epitope, indicating that DCs are not essential for processing the epitope to prime CD8 T cells. Together, our results provide more insight in the role of proteasome components in antigen processing and immune responses, which could help in the development of CD8 T cell activating vaccins.