PA28 modulates antigen processing and viral replication during coxsackievirus B3 infection

Abstract

The function of the proteasome is modulated at the level of subunit expression and by association with its regulatory complexes. During coxsackievirus B3 (CVB3) myocarditis, IFN-induced formation of immunoproteasomes (ip) is known to be critical for regulating immune modulating molecules. The function of the IFN-γ-inducible proteasome regulator subunits PA28 α and β, however, in this context was unknown. During viral myocarditis, we found an increased abundance of PA28β subunits in heart tissue. PA28α/β exists in PA28-20S-PA28 and PA700-20S-PA28 hybrid proteasome complexes in cells both with either predominant ip and standard proteasome (sp) expression. Being in line with reduced proteasome activity in PA28α/β-deficient cells, we observed increased levels of oxidized and poly-ubiquitinated proteins upon TLR3-activation in these cells. Moreover, PA28α/β is capable to interfere directly with viral replication of CVB3 and facilitates the generation of CVB3-derived MHC class I epitopes by the proteasome. In contrast to a distinct function of PA28α/β in vitro, gene ablation of PA28α/β in mice being on a genetic background with resistance towards the development of severe infection had no significant impact on disease progression. Other than reported for the ip, in this host PA28α/β is dispensable to meet the demand of increased peptide hydrolysis capacity by the proteasome during viral myocarditis.

The immunoproteasome‐specific inhibitor ONX 0914 reverses susceptibility to acute viral myocarditis

“…Another major physiologically relevant function of immunoproteasomes found during viral infection is a more efficient generation of viral peptides resulting in improved antigen presentation of MHC class I epitopes (Schwarz et al , ; Kincaid et al , ). Although facilitated antigen processing by the immunoproteasome is also evident for coxsackievirus peptides in vitro (Jakel et al , ; Voigt et al , ; Respondek et al , ), this immunoproteasome‐dependent improvement of epitope liberation has no effect on the course of CVB3 infection in vivo (Opitz et al , ). Interestingly as shown in this study, ONX 0914 treatment in C57BL/6 mice with hereditary resistance to viral cardiomyopathy slightly deteriorated disease parameters like viral load without affecting overall long‐term course.…”

Proteasomal Protein Degradation: Adaptation of Cellular Proteolysis With Impact on Virus—and Cytokine-Mediated Damage of Heart Tissue During Myocarditis

“…In addition to diverging peptidase activities of the six catalytic subunits, proteasome activity can be regulated upon binding to regulatory particles like the proteasome activator of 28 kDa (PA28). PA28-capped proteasome complexes are equipped with increased peptide hydrolysis capacity ( 91 ), and by as yet unknown mechanisms PA28 suppresses the CVB3 replication machinery ( 73 ). Altogether, a broad spectrum of various viral pathogens exploits the proteasome machinery in cells of the host organism.…”

“…In contrast, even under conditions with cytokine-induced i-proteasome expression, selective i-proteasome inhibitors are advantageous in reducing cardiomyocyte death in comparison to compounds targeting either the standard or both the standard and the i-proteasome with similar efficacy ( 25 ). During viral myocarditis, i-proteasome formation and to a minor extent induction of PA28β also enhance cellular protein turnover reducing the accumulation of oxidant-damaged proteins ( 23 , 73 ). The notion of a minor influence of the i-proteasome regarding the control of pathogens was supported by elimination of virus despite a reduction of T1IFN ( 63 , 72 ) upon i-proteasome inhibitor treatment and induction of immune memory in CVB3 heart disease ( 72 ).…”

Cryo-EM of mammalian PA28αβ-iCP immunoproteasome reveals a distinct mechanism of proteasome activation by PA28αβ

“…PA28αβ has been shown in vitro to affect the generation of peptides by proteasome CPs and is required for efficient presentation of many T cell epitopes from a number of viral, bacterial, and tumor-derived antigens (Rechsteiner et al, 2000a). PA28 deficiency could reduce the production of MHC class I-binding peptides in cells (de Graaf et al, 2011;Respondek et al, 2017;Schwarz et al, 2000).…”