The Proteasome Immunosubunit Multicatalytic Endopeptidase Complex-Like 1 Is a T-Cell-Intrinsic Factor Influencing Homeostatic Expansion

Abstract

Homeostatic regulatory mechanisms maintain the constant ratios between different lymphocyte subsets in the secondary lymphoid organs. How this dynamic equilibrium is achieved, in particular following the clonal expansion and subsequent contraction of different cells after infection, remains poorly understood. Expression of the proteasome immunosubunits has been shown to influence not only major histocompatibility complex class I (MHC-I) antigen processing and thereby T-cell responses, but also the CD4/CD8 T-cell ratios in lymphoid organs. We examined the relationships between these different immunosubunit-mediated effects in mice of various proteasome subunit compositions during infection with Listeria monocytogenes. Mice that lacked the immunosubunit multicatalytic endopeptidase complex-like 1 (MECL-1) maintained enhanced CD4/CD8 T-cell ratios during infection, while MHC-I surface levels resembled those in wild-type (wt) mice. LMP7 gene-deficient mice, on the other hand, showed reduced MHC-I expression, while their splenic CD4/CD8 ratios were similar to those in wt mice. Remarkably, analysis of bone marrow-chimeric immunosubunit gene-deficient mice, reconstituted with a mixture of wt and LMP7-plus MECL-1-deficient bone marrow, revealed that the LMP7-plus MECL-1-deficient T-cell population maintained a higher CD4/CD8 T-cell ratio than the wt T-cell population before, during, and after infection and T-cell memory formation. Since in these mice the immunosubunit-positive and immunosubunit-negative T-cell populations were selected in the same thymus and expanded in the same lymphoid environments, our findings indicate that MECL-1 influences the homeostatic equilibrium between T-cell subsets, not through indirect extracellular signals, such as MHC-I expression or the cytokine milieu, but through direct effects on T-cell-intrinsic processes.

Deletion of Immunoproteasome Subunits Imprints on the Transcriptome and Has a Broad Impact on Peptides Presented by Major Histocompatibility Complex I molecules

“…Peptides with the largest fold differences in abundance are listed in Table I, and the full list of peptides is available in supplemental Table S1. In accordance with our data on MIP abundance, flow cytometry analyses revealed that, as previously shown using WT versus dKO spleen lymphocytes (12), expression of cell surface H2D b and H2K b was higher by approximately 2.1-fold on WT than dKO DCs (Fig. 3B).…”

“…Thus, the generation of epitopes like that one would not be blocked by pharmacological inhibitors. Our observation that IPs increase MIP abundance fits well with in vitro proteasome digestion experiments suggesting IPs have greater efflux and cleavage rates than CPs (19,20) and with the decreased cell surface levels of MHC I molecules on Lmp7 Ϫ/Ϫ and Lmp7 Ϫ/Ϫ Mecl1 Ϫ/Ϫ splenocytes (12). For reasons presented in the Introduction, it was not possible to extrapolate from previous in vitro proteasome digestion experiments the overall impact of IPs on the diversity of the MIP repertoire generated in vivo.…”

“…Ϫ/Ϫ mice (12). Furthermore, studies of selected epitopes revealed that some MHC I-associated peptides can be generated only by CPs, some only by IPs, and others by both types of proteasomes (7,(13)(14)(15)(16)(17)(18).…”

Immunoproteasome subunit deficiency has no influence on the canonical pathway of NF-κB activation

“…additional immunological functions. Immunoproteasome deficiency or inhibition affects T cell survival, expansion, and differentiation (Basler et al, 2004;Chen et al, 2001;Kalim et al, 2012;Moebius et al, 2010;Muchamuel et al, 2009;Zaiss et al, 2008), cytokine production (Basler et al, 2011;Basler et al, 2010;Basler et al, 2014;Muchamuel et al, 2009), and progression of autoimmune conditions (Basler et al, 2015). Moreover, mutations in LMP7 and LMP2 in humans cause complex autoimmune and inflammatory phenotypes .…”

Functional Module Search in Protein Networks based on Semantic Similarity Improves the Analysis of Proteomics Data

“…As monocytes replenish the pool of macrophages and dendritic cells in the body, we can assume a presence of immunoproteasome in these cells as well. T cells show a constitutive expression of all three molecules and the immunoproteasome facilitates protein homeostasis and cell proliferation in these cells (63). Because PBMCs are mainly a mixture of macrophages, monocytes, and lymphocytes we can expect that we would identify the immunoproteasome here as well.…”