Monocytes and neutrophils promote cardiac fibroblast pro-fibrotic phenotypes through IL-6 and MIF

Cardiac fibrosis, characterized by activation of cardiac fibroblasts and accumulation of extracellular matrix (ECM), is associated with most cardiac pathological conditions, leading to adverse cardiac remodeling and accelerating progression of heart failure. Immune cell recruitment is a hallmark of early fibrosis; however, the underlying mechanisms linking early inflammation, fibroblast activation and perpetuation of cardiac fibrosis remain unclear. In this study, cell-derived matrices (CDM) and collagen gels were used to investigate primary human cardiac fibroblast (HCF) pro-fibrotic response, native ECM synthesis and interactions with immune cells. Direct co-culture of HCF with THP-1 and HL-60 cells, or conditioned media from HCF-immune cell co-cultures, resulted in enhanced HCF-induced contraction of collagen gels - a finding recapitulated using primary human monocytes and neutrophils. Pro-fibrotic cytokines, macrophage migration inhibitory factor (MIF) and interleukin-6 (IL-6), were upregulated in HCF-immune co-cultures and were further confirmed as active regulators of HCF contraction using recombinant purified cytokines and function blocking antibodies. Live imaging revealed that THP-1 cells increased HCF migration, which was suppressed by an IL-6 neutralizing antibody. These findings provide insight into the effects of early exposure of HCF to immune cells and shed light on early pro-fibrotic initiation mechanisms and therapeutic targets.