Background and Purpose:
Inflammatory responses of the immune system play a central role in the progression of heart failure (HF). Affected patients are characterized by elevated levels of inflammatory cytokines, which worsen prognosis and correlate with mortality in HF. The aim of this study is to identify long non-coding RNAs (lncRNAs) as potential therapeutic targets for the treatment of heart failure to mechanistically counteract chronic inflammation.

Methods and Results:
Next-generation sequencing revealed significantly elevated lncRNA transcripts in peripheral blood mononuclear cells (PBMCs) of HF patients compared to control cohort. The upregulated (N=4; 2.05-fold increase; p<0.05) Heart failure associated transcript 4 (Heat4) was further functionally characterized. Heat4 is a part of the immunomodulatory gene cluster CD300 and thus maybe involved in regulation of systemic inflammatory disorders. The elevation of Heat4 in HF was validated in a larger patient cohort (HF: N=63; Controls: N=38; p<0.05). Analysis of cellular localization revealed Heat4 as a cytoplasmatic lncRNA being stabilized by a poly(A) tail in its cell compartment (5.30-fold Heat4 enrichment in poly(A+)-fraction). In fact, knockdown of Heat4 resulted in elevated inflammation as measured by TNF-α RNA expression (4.14-fold induction; p<0.05). Conversely, overexpression of Heat4 in human monocytes decreased inflammatory cytokines such as TNF-α on RNA (-38.6% reduction; p<0.05) and protein level, as validated by a cytokine array. Magnetic-activated cell sorting (MACS) revealed Heat4 to be enriched in non-classical monocytes (3.37-fold compared to classical monocytes; p<0.05), which could be phenotypically distinguished from CD16-negative classical monocytes by the additional co-expression of the CD16 cell surface marker. Non-classical monocytes are naturally mobile and patrol the endothelium in search of injury and are considered anti-inflammatory because they maintain vascular homeostasis. Indeed, overexpression of Heat4 promotes vascular regeneration in a humanized mouse model after injury of the carotid artery. Concretely, transplantation of monocytes with increased Heat4-expression led to faster carotid re-endothelialization in NOD-SCID mice compared to injected control-monocytes (N=6; 1.85-fold compared to injection of control monocytes; p<0.05).

Conclusion:
Heart failure-induced lncRNA Heat4 is enriched in the cytoplasm of non-classical monocytes and restricts the inflammatory response here. Through its vascular regeneration-inducing function in vivo, Heat4 represents a promising target for targeted therapy to restore healthy cardiac microcirculation, particularly in HF.