Cardiac cell-specific apoptotic and cytokine responses to reovirus infection: determinants of myocarditic phenotype

Spatiotemporal transcriptomics reveals pathogenesis of viral myocarditis

A significant fraction of sudden death in children and young adults is due to viral myocarditis, an inflammatory disease of the heart. In this study, by using integrated single-cell and spatial transcriptomics, we created a high-resolution, spatially resolved transcriptome map of reovirus-induced myocarditis in neonatal mouse hearts. We assayed hearts collected at three timepoints after infection and studied the temporal, spatial and cellular heterogeneity of host–virus interactions. We further assayed the intestine, the primary site of reovirus infection, to establish a full chronology of molecular events that ultimately lead to myocarditis. We found that inflamed endothelial cells recruit cytotoxic T cells and undergo pyroptosis in the myocarditic tissue. Analyses of spatially restricted gene expression in myocarditic regions and the border zone identified immune-mediated cell-type-specific injury and stress responses. Overall, we observed a complex network of cellular phenotypes and spatially restricted cell–cell interactions associated with reovirus-induced myocarditis in neonatal mice.

NF-κB activation is cell type-specific in the heart

Viral myocarditis is common and can progress to cardiac failure. Cardiac cell pro-inflammatory responses are critical for viral clearance, however sustained inflammatory responses contribute to cardiac damage. The transcription factor NF-κB regulates expression of many pro-inflammatory cytokines, but basal and induced activation of NF-κB in different cardiac cell types have not been compared. Here, we used primary cultures of cardiac myocytes and cardiac fibroblasts to identify cardiac cell type-specific events. We show that while viral infection readily stimulates activation of NF-κB in cardiac fibroblasts, cardiac myocytes are largely recalcitrant to activation of NF-κB. Moreover, we show that cardiac myocyte subpopulations differ in their NF-κB subcellular localization and identify the cis-Golgi as a cardiac myocyte-specific host compartment. Together, results indicate that NF-κB-dependent signaling in the heart is cardiac cell type-specific, likely reflecting mechanisms that have evolved to balance responses that can be either protective or damaging to the heart.

Critical role for death-receptor mediated apoptotic signaling in viral myocarditis

BACKGROUND

Apoptosis of cardiac myocytes plays a key role in the pathogenesis of many cardiac diseases, including viral myocarditis. The apoptotic signaling pathways that are activated during viral myocarditis and the role that these pathways play in disease pathogenesis have not been clearly delineated.

METHODS AND RESULTS

We investigated the role of apoptotic signaling pathways after virus infection of primary cardiac myocytes. The death receptor-associated initiator caspase, caspase 8, and the effector caspase, caspase 3, were significantly activated after infection of primary cardiac myocytes with myocarditic, but not non-myocarditic, reovirus strains. Furthermore, reovirus-induced cardiac myocyte apoptosis was significantly inhibited by soluble death receptors. In contrast, the mitochondrial membrane potential remained unaltered and caspase 9, the initiator caspase associated with mitochondrial apoptotic signaling, was only weakly activated in cardiac myocytes after infection with myocarditic reovirus strains. Inhibition of mitochondrial apoptotic signaling had no effect on reovirus-induced cardiac myocyte apoptosis. In accordance with our in vitro data, caspase 8, but not caspase 9, was significantly activated in the hearts of reovirus-infected mice.

CONCLUSIONS

Death receptor, but not mitochondrial, apoptotic signaling plays a key role in apoptosis after infection of cardiac myocytes with myocarditic reovirus strains.

Proteomic analysis reveals virus-specific Hsp25 modulation in cardiac myocytes

Viruses frequently infect the heart but clinical myocarditis is rare, suggesting that the cardiac antiviral response is uniquely effective. Indeed, the Type I interferon (IFN) response is cardiac cell-type specific and provides one integrated network of protection for the heart. Here, a proteomic approach was used to identify additional proteins that may be involved in the cardiac antiviral response. Reovirus-induced murine myocarditis reflects direct viral damage to cardiac cells and offers an excellent system for study. Primary cultures of murine cardiac myocytes were infected with myocarditic or nonmyocarditic reovirus strains, and whole cell lysates were compared by two-dimensional difference gel electrophoresis (2D-DIGE) and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF/TOF) tandem mass spectrometry. Results were quantitative and reproducible and demonstrated that whole proteome changes clustered according to viral pathogenic phenotype. Moreover, the data suggest that the heat shock protein Hsp25 is modulated differentially by myocarditic and nonmyocarditic reoviruses and may play a role in the cardiac antiviral response. Members of seven virus families modulate Hsp25 or Hsp27 expression in a variety of cell types, suggesting that Hsp25 participation in the antiviral response may be widespread. However, results here provide the first evidence for a virus-induced decrease in Hsp25/27 and suggest that viruses may have evolved a mechanism to subvert this protective response, as they have for IFN.

Interferon regulatory factor 3 attenuates reovirus myocarditis and contributes to viral clearance

Apoptosis is a pathological hallmark of encephalitis and myocarditis caused by reovirus in newborn mice. In cell culture models, the antiviral transcription factor interferon regulatory factor 3 (IRF-3) enhances reovirus-induced apoptosis following activation via retinoic acid inducible gene I and interferon promoter-stimulating factor 1. To determine the role of IRF-3 in reovirus disease, we infected newborn IRF-3(+/+) and IRF-3(-/-) mice perorally with mildly virulent strain type 1 Lang (T1L) and fully virulent strain type 3 SA+ (T3SA+) and monitored infected animals for survival. Both wild-type and IRF-3(-/-) mice succumbed with equivalent frequencies to infection with T3SA+. However, the absence of IRF-3 was associated with significantly decreased survival rates following infection with T1L. The two virus strains achieved similar peak titers in IRF-3(+/+) and IRF-3(-/-) mice in the intestine, brain, heart, liver, and spleen. However, by day 12 postinoculation, titers in all organs examined were 10- to 100-fold higher in IRF-3(-/-) mice than those in wild-type mice. Increased titers were associated with marked pathological changes in all organs examined, especially in the heart, where absence of IRF-3 resulted in severe myocarditis. Cellular and humoral immune responses were equivalent in wild-type and IRF-3(-/-) animals, suggesting that IRF-3 functions independently of the adaptive immune response to enhance reovirus clearance. Thus, IRF-3 serves to facilitate virus clearance and prevent tissue injury in response to reovirus infection.