Abstract Body (Do not enter title and authors here): Introduction
The Forkhead box O (FOXO) transcription factor FOXO3 plays a pivotal role in longevity, as single nucleotide polymorphisms (SNPs) in FOXO3 are associated with healthy aging in diverse human populations. Moreover, the FOXO3 longevity genotype increases lifespan only in individuals at risk for cardiometabolic disease, suggesting a protective mechanism played by FOXO3 against cardiac stress.
Hypothesis
To test whether FOXO3+ cardiac fibroblasts are inhibiting inflammation in the myocardium and in right ventricle (RV) dysfunction upon pulmonary arterial hypertension (PAH).
Methods
Fibroblast-specific Foxo3 knock-out (Foxo3-FB^KO) mice were subjected to pulmonary artery banding (PAB) for 35 days. Right heart ventricle (RV) function was measured by echocardiography and MRI. Cardiac immune composition was defined by single cell RNA-seq on CD45+ cells, immunohistochemistry and FACS analysis. Levels of cytokines were determined by cytokine array.
Results
Here we show that conditional deletion of Foxo3 in cardiac fibroblasts leads to up-regulation of major histocompatibility complex type 1 (MHC-I) genes, together with a change in the immune composition of the heart. Foxo3-FB^KO hearts display increased numbers of macrophages with high levels of MHC type 2 (MHC-II^hi), increased recruitment of CD8+ T cells and increased expression of cytokines such as IL-1α, IL-1β and IL-17. In spite of this pro-inflammatory signature, Foxo3-FB^KO hearts show no functional phenotype at the baseline. However, upon PAB, Foxo3-FB^KO mice display aberrant RV remodeling with increased cardiomyocyte hypertrophy and reduced RV ejection fraction. This severe phenotype is associated to higher expression of MHC-I and reduced recruitment of MHC-II^hi macrophages. Importantly, FOXO3 is down-regulated in cardiac fibroblasts of patients with decompensated RV hypertrophy due to PAH, and in two different animal models of PAH (monocrotaline rats and PAB mice), indicating a protective role of FOXO3 against RV maladaptive remodeling.
Conclusions
We found that inhibition of FOXO3 in cardiac fibroblasts at late stages of RV remodeling leads to activation of MHC-I antigen presentation genes and recruitment of pro-inflammatory T cells which might further exacerbate maladaptive remodeling via increased cytotoxicity. In conclusion, we describe novel protective functions for FOXO3 in RV remodeling and in cardiac immune composition, with important implications for PAH-induced cardiac disease.