Summary Proteasomes play a fundamental role in the processing of intracellular antigens into peptides that bind to MHC class I molecules for presentation to CD8 T cells. Three IFNγ-inducible catalytic proteasome (immuno)subunits as well as the IFNγ-inducible proteasome activator PA28 dramatically accelerate the generation of a subset of MHC class I-presented antigenic peptides. To determine whether these IFNγ-inducible proteasome components play a compounded role in antigen processing, we generated mice lacking both PA28 and the immunosubunits β5i/LMP7 and β2i/MECL-1. Analyses of MHC class I cell surface levels ex vivo demonstrated that PA28-deficiency reduced the production of MHC class I-binding peptides both in cells with and without immunosubunits, in the last cells on top of an already diminished production of MHC ligands in the absence of immunoproteasomes. In contrast, the immunosubunits but not PA28 appeared to be of critical importance for the induction of CD8 T cell responses to multiple dominant Influenza and Listeria-derived epitopes. Taken together, our data demonstrate that PA28 and the proteasome immunosubunits use fundamentally different mechanisms to enhance the supply of MHC class I-binding peptides. However, only the immunosubunit-imposed effects on proteolytic epitope processing appear to have substantial effects on the fine-specificity of pathogen-specific CD8 T cell responses.

NK cells play an important role in the early defense against invading pathogens. Although it is well-established that infection leads to a substantial, local increase in NK cell numbers, little is known about the mechanisms that trigger their proliferation and migration. We here investigated the dynamics of NK cell responses following intranasal respiratory virus infection. We show that NK cell numbers increased in the airways following influenza virus infection, but find no evidence of proliferation either at the site of infection or in the draining lymph nodes. Instead, we find that the bone marrow (BM) is the primary site of proliferation of both immature and mature NK cells during infection. Using an adoptive transfer model, we demonstrate that peripheral, long-lived and phenotypically mature NK cells migrate back to the BM and proliferate there, both homeostatically and in response to infection. Thus, the BM is not only a site of NK cell development, but also an important site for proliferation of long-lived mature NK cells.

Aims/hypothesis Incretin-based therapies improve glycaemic control in patients with type 2 diabetes. In animal models of diabetes, glucagon-like peptide-1 receptor agonists (GLP-1RAs) increase beta cell mass. GLP-1RAs are also evaluated in nondiabetic individuals with obesity and cardiovascular disease. However, their effect on beta cell mass in normoglycaemic conditions is not clear. Here, we investigate the effects of the GLP-1RA liraglutide on beta cell mass and function in normoglycaemic mice. Methods C57BL/6J mice were treated with the GLP-1RA liraglutide or PBS and fed a control or high-fat diet (HFD) for 1 or 6 weeks. Glucose and insulin tolerance tests were performed after 6 weeks. BrdU was given to label proliferating cells 1 week before the animals were killed. The pancreas was taken for either histology or islet isolation followed by a glucose-induced insulin-secretion test. Results Treatment with liraglutide for 6 weeks led to increased insulin sensitivity and attenuation of HFD-induced insulin resistance. A reduction in beta cell mass was observed in liraglutide-treated control and HFD-fed mice at 6 weeks, and was associated with a lower beta cell proliferation rate after 1 week of treatment. A similar reduction in alpha cell mass occurred, resulting in an unchanged alpha to beta cell ratio. In contrast, acinar cell proliferation was increased. Finally, islets isolated from liraglutide-treated control mice had enhanced glucose-induced insulin secretion. Conclusions/interpretation Our data show that GLP-1RA treatment in normoglycaemic mice leads to increases in insulin sensitivity and beta cell function that are associated with reduced beta cell mass to maintain normoglycaemia.

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.

FRG1 is considered a candidate gene for facioscapulohumeral muscular dystrophy (FSHD) based on its location at chromosome 4qter and its upregulation in FSHD muscle. The FRG1 protein (FRG1P) localizes to nucleoli, Cajal bodies (and speckles), and has been suggested to be a component of the human spliceosome but its exact function is unknown. Recently, transgenic mice overexpressing high levels of FRG1P in skeletal muscle were described to present with muscular dystrophy. Moreover, upregulation of FRG1P was demonstrated to correlate with missplicing of specific pre-mRNAs. In this study, we have combined colocalization studies with yeast two-hybrid screens to identify proteins that associate with FRG1P. We demonstrate that artificially induced nucleolar aggregates of VSV-FRG1P specifically sequester proteins involved in pre-mRNA processing. In addition, we have identified SMN, PABPN1, and FAM71B, a novel speckle and Cajal body protein, as binding partners of FRG1P. All these proteins are, or seem to be, involved in RNA biogenesis. Our data confirm the presence of FRG1P in protein complexes containing human spliceosomes and support a potential role of FRG1P in either splicing or another step in nuclear RNA biogenesis. Intriguingly, among FRG1P-associated proteins are SMN and PABPN1, both being involved in neuromuscular disorders, possibly through RNA biogenesis-related processes.