Impairment of Immunoproteasome Function by β5i/LMP7 Subunit Deficiency Results in Severe Enterovirus Myocarditis

Abstract

Proteasomes recognize and degrade poly-ubiquitinylated proteins. In infectious disease, cells activated by interferons (IFNs) express three unique catalytic subunits β1i/LMP2, β2i/MECL-1 and β5i/LMP7 forming an alternative proteasome isoform, the immunoproteasome (IP). The in vivo function of IPs in pathogen-induced inflammation is still a matter of controversy. IPs were mainly associated with MHC class I antigen processing. However, recent findings pointed to a more general function of IPs in response to cytokine stress. Here, we report on the role of IPs in acute coxsackievirus B3 (CVB3) myocarditis reflecting one of the most common viral disease entities among young people. Despite identical viral load in both control and IP-deficient mice, IP-deficiency was associated with severe acute heart muscle injury reflected by large foci of inflammatory lesions and severe myocardial tissue damage. Exacerbation of acute heart muscle injury in this host was ascribed to disequilibrium in protein homeostasis in viral heart disease as indicated by the detection of increased proteotoxic stress in cytokine-challenged cardiomyocytes and inflammatory cells from IP-deficient mice. In fact, due to IP-dependent removal of poly-ubiquitinylated protein aggregates in the injured myocardium IPs protected CVB3-challenged mice from oxidant-protein damage. Impaired NFκB activation in IP-deficient cardiomyocytes and inflammatory cells and proteotoxic stress in combination with severe inflammation in CVB3-challenged hearts from IP-deficient mice potentiated apoptotic cell death in this host, thus exacerbating acute tissue damage. Adoptive T cell transfer studies in IP-deficient mice are in agreement with data pointing towards an effective CD8 T cell immune. This study therefore demonstrates that IP formation primarily protects the target organ of CVB3 infection from excessive inflammatory tissue damage in a virus-induced proinflammatory cytokine milieu.

Additive loss-of-function proteasome subunit mutations in CANDLE/PRAAS patients promote type I IFN production

“…This is in agreement with previous work suggesting that immunoproteasome impairment in mice is linked to changed cytokine patterns (17,25) and altered T cell expansion and maintenance (12,26).…”

“…In all CANDLE/PRAAS patients, regardless of their proteasome subunit mutations, the proteasome function is impaired because the mutation-induced subunit defects lead to lower expression and/or impaired proteasome assembly. The impairment in proteolytic function is not limited to chymotrypsin-like activity, as was previously seen their increased proteolytic capacity that is necessary to prevent IFN-induced cell death (9,12,16,17,40). The data in murine models suggest that, while WT cells can adapt to an increased demand for protein degradation through the induction of immunoproteasomes, in mutant cells, there seems to be a threshold at which cells start a survival program followed by apoptotic cell death when the capacity to clear inclusions becomes too low.…”

“…Mouse models link immunoproteasome dysfunction to oxidative stress, accumulation of toxic ubiquitin-rich aggregates, and cytokine dysregulation, suggesting a role for immunoproteasomes in tissue preservation during inflammatory processes (16,17).…”

Lifespan of mice and primates correlates with immunoproteasome expression

“…jci.org Volume 125 Number 5 May 2015 oxidized proteins. All 3 immunoproteasome-specific subunits are upregulated under acute exposure to oxidative stress in cell culture (16,29), and knockout of PSMB8 produces increased protein oxidation and decreased oxidative stress resistance in laboratory mice (29,30). Our results show that immunoproteasome levels, as measured by PSMB8, are significantly higher in fibroblasts from longer-lived primates than in cells from shorter-lived primates.…”

“…The immunoproteasome has been reported to play an important role both in degradation of oxidized proteins (16,29,30) and in generation of peptides for MHC class I antigen presentation (31,32). We wished to determine whether the increase in immunoproteasome in cells of longer-lived primate species was accompanied by an increase in other aspects of MHC class I antigen presentation.…”