TL;DR
This study reveals that the age-dependent severity of Coxsackievirus B3 (CV-B3) infection in mice is linked to the expression patterns of the Coxsackievirus and Adenovirus Receptor (CAR). Younger mice, with higher levels of CAR1 and CAR2 (which bind CV-B3), experience more severe encephalitis and myocarditis due to accelerated viral replication and an overactive inflammatory response, highlighting CAR's crucial role in early-life susceptibility.
What Problem Does This Paper Address?
Hand, Foot, and Mouth Disease (HFMD), often caused by enteroviruses like Coxsackievirus B3 (CV-B3), is a significant public health concern, particularly in children. While it's known that young children are disproportionately affected by severe manifestations such as myocarditis and encephalitis, the underlying molecular mechanisms for this age-dependent susceptibility remain poorly understood. Previous explanations focusing solely on immature immune systems are insufficient, especially considering that the same age group shows reduced susceptibility to other viruses like SARS-CoV-2. This paper aims to elucidate the specific factors contributing to heightened CV-B3 vulnerability in neonates and young mice.
Methods
The researchers utilized a mouse model, specifically Balb/c mice at three developmental stages: 7, 14, and 30 days old, to mimic different age groups. They infected these mice with CV-B3 and monitored various parameters including body weight, survival rates, viral loads, and viral titers. Histopathological analysis (HE staining) was performed on cardiac and brain tissues to assess damage. To understand the immune response, they measured cytokine levels (IFN-a, IFN-ẞ, IFN-y, CXCL2, CCL21, CXCL13, IL-6, IL-17). Crucially, they investigated the expression of four CAR splice variants (CAR1-CAR4) at both mRNA and protein levels using quantitative real-time PCR, Western blotting, and immunohistochemistry. To confirm the functional role of CAR, they conducted in vitro experiments involving CAR overexpression and knockdown in SH-SY5Y cells, followed by CV-B3 infection. They also examined the activation of the NLRP3 inflammasome and the infiltration and activation of IL-17-producing γδT cells (γδT17 cells) using immunofluorescence.
Datasets
Not discussed in the paper.
Key Insights and Ideas
- CAR Splice Variants Dictate Age Susceptibility: The core finding is that the differential expression of CAR splice variants is the primary driver of age-dependent susceptibility to CV-B3. CAR1 and CAR2 are the functional receptors for CV-B3, and their high expression in young mice facilitates viral entry and replication.
- Developmentally Regulated CAR Expression: CAR1 and CAR2 expression declines with age, while non-binding variants (CAR3 and CAR4) increase. This suggests a sophisticated developmental regulation of CAR isoforms.
- CAR Orchestrates a Potent Inflammatory Cascade: High CAR1/CAR2 in young mice leads to accelerated viral replication, which in turn triggers a hyperactive NLRP3 inflammasome. This results in excessive production of pro-inflammatory cytokines (IL-1ß, IL-18, IL-17) and the expansion of γδT17 cells, ultimately causing severe tissue damage.
- NLRP3 Dysregulation as a Key Factor: The study establishes NLRP3 inflammasome dysregulation, specifically hyperactivation in young mice, as a critical determinant of CV-B3-induced tissue damage.
- γδT17 Cells as Inflammatory Mediators: The research highlights the significant role of IL-17-producing γδT17 cells, whose recruitment and activation are facilitated by CAR, in driving the inflammatory pathology observed in young, infected mice.
Key Results
- 7-day-old mice are highly susceptible: These mice exhibited 100% mortality, significant weight loss, and the most severe pathological changes in the heart and brain compared to 14- and 30-day-old mice.
- CAR1/CAR2 expression correlates with viral load: A positive association was found between CAR abundance and early viral load in 7-day-old mice, indicating CAR's role in viral proliferation.
- Age-dependent CAR expression: mRNA and protein levels of CAR1 and CAR2 significantly decreased from 7 to 30 days of age, while CAR3 and CAR4 increased.
- CAR manipulation affects viral load: In in vitro experiments, CAR knockdown reduced CV-B3 viral loads, while CAR overexpression increased them.
- NLRP3 inflammasome activation is age-dependent: The 7-day-old group showed the highest NLRP3 inflammasome response, followed by the 14-day-old group, with a significant increase in IL-1ß and IL-18 levels.
- γδT17 cell infiltration and activation are CAR-dependent: Young mice with higher CAR expression showed significantly increased infiltration and activation of γδT17 cells in cardiac and brain tissues.
Limitations and Future Work
The authors acknowledge several limitations:
- Timing of CAR manipulation: Rapid age-dependent changes in CAR splicing occur postnatally, while viral-mediated knockdown or overexpression requires longer timeframes, making direct in vivo manipulation challenging.
- Tissue specificity of viral delivery: Current tools lack tissue specificity for cardiac and brain compartments in neonatal mice, hindering targeted CAR manipulation in these critical organs.
- CAR's broad physiological roles: Constitutive global knockout of CAR via gene editing could lead to confounding developmental effects due to its essential roles in cell adhesion and intercellular signaling.
Future work could involve using cardiac and brain organoids engineered for CAR overexpression or knockdown to further validate these mechanisms in vitro and analyze CV-B3 viral kinetics.