Abstract 17775: Foxo3a Regulates Response to Coxsackievirus B3 Myocarditis via microRNA-155 Triggered NK-cell Activity

Abstract

Background: FoxO3a is a transcription factor crucial in regulation of cell metabolism, stress resistance and immunity. Recently polymorphisms of FOXO3a have been associated with favorable outcome in inflammatory disease. However, mechanistic insight in FoxO3a effects is still limited. Here, we investigated the role of FoxO3a on NK-cell responses in viral myocarditis. Methods: A mouse model of Coxsackie Virus B3 (CVB3) myocarditis in wild-type and FoxO3a-/- mice was used to investigate the effects of FoxO3a on viral load and inflammation. Characterization of NK cells was performed by cytotoxic assay, surface marker- and interferon expression. Results: FoxO3a-/- mice showed significantly reduced myocardial inflammation, lower viral titers and attenuated expression of pro-inflammatory cytokines compared to wild-type mice 7 days post infection. Viral titers were significantly lower in FoxO3a-/- mice at day 3 while IFN γ and NKp46 expression were up-regulated within the heart suggesting early viral control by enhanced NK cell activity. There were no differences in expression of CAR (coxsackie-adenovirus receptor). Additionally NKp46+ splenic NK cells of FoxO3a-/- mice exhibited a superior functional status ex vivo determined by enhanced expression of CD69, higher frequencies of CD11b+CD27+ effector NK cells and increased cytotoxicity compared to NK cells of wild-type littermates. These effects were not due to different levels of apoptosis. MicroRNA-155 expression, recently reported to be essential in NK-cell activation, was significantly elevated in FoxO3a-/- NK cells while its inhibition led to diminished production of IFNγ in Foxo3a deficient animals. Moreover analysis of healthy human donors for the gain of function FoxO3a SNP rs9400239 revealed significantly reduced CD107a dependent degranulation activity in donors homozygous for the allelic variant. Conclusion: Our results implicate FoxO3a directly in regulation of NK-cell function and suggest an important role in innate immune responses to viral infection. While the anti-inflammatory effects of FoxO3a seem beneficial in chronic inflammation the suppression of innate immunity in acute viral myocarditis could have detrimental effects.