The ICOS-ICOSL co-stimulatory axis in cardiac fibrosis: distinct pathological outcomes following receptor vs. ligand inhibition

Introduction Cardiac fibroblasts are vital for cardiac damage repair, but their overactivation can lead to maladaptive fibrosis and heart failure. While the Inducible T-cell Co-Stimulator (ICOS) and its Ligand (ICOSL) are known to influence non-immune cells, their specific role in cardiac injury remains largely unexplored. Investigating how this signalling modulates fibroblast behaviour could reveal novel mechanisms to limit fibrosis and improve myocardial recovery. Purpose To investigate the role of the ICOS-ICOSL signalling in modulating fibroblast behaviour and cardiac fibrosis. Methods Six-month-old male Balb/C mice received Angiotensin II (Ang-II; 1.5 mg/kg/day) or saline via osmotic mini-pumps for 15 days. ICOS or ICOSL signalling was inhibited using monoclonal antibodies (150 µg/dose i.p., every 3 days). Mice were divided into six groups: Veh/Isotype (n=6), AngII/Isotype (n=16), Veh/anti-ICOS (n=7), AngII/anti-ICOS (n=16), Veh/anti-ICOSL (n=7), and AngII/anti-ICOSL (n=16). Echocardiography, histochemical characterisation, and qRT-PCR were performed to investigate the morpho-functional changes and molecular features of the heart. Results Combined Ang-II and either anti-ICOS or anti-ICOSL treatment markedly increased mortality rate (40-50%), compared to Ang-II (13%). The combination of Ang-II and ICOS or ICOSL inhibition significantly reduced systolic function, including ejection fraction (EF) and fractional shortening (FS), compared to controls. Notably, anti-ICOSL worsened cardiac remodelling, increasing relative wall thickness (RWT) and left ventricular remodelling index (LVRI), and reduced EF even as a monotherapy. Overall, myocardial fibrosis was increased in AngII/Isotype and AngII/anti-ICOSL groups, which was reflected by elevated CTGF (connective tissue growth factor) mRNA expression level; however, TGF-β (transforming growth factor-β) and Col1a1 (collagen type I alpha 1) expression were significantly increased only with anti-ICOSL. Differential collagen regulation was observed; anti-ICOS increased Col3a1 (collagen type III alpha 1) expression, whereas anti-ICOSL elevated Col1a1 expression and the Col1a1/Col3a1 ratio. Furthermore, the Ang-II-induced reduction in the Myh6/Myh7 ratio (hypertrophy marker) was minimised by anti-ICOS but accelerated by anti-ICOSL treatment. These findings were consistent with NPPB (natriuretic peptide B) expression and serum NT-proBNP levels, which remained significantly elevated in the AngII/anti-ICOSL group but were normalised in the AngII/anti-ICOS group. Conclusions ICOS-ICOSL signalling plays a key regulatory role in cardiac pathology. Whereas ICOS inhibition appears to attenuate fibrotic progression, ICOSL blockade accelerates fibrosis, cardiac dysfunction, and adverse remodelling. These data underscore the complex, cell-specific actions of co-stimulatory immune checkpoints in non-immune cells and suggest that modulating this pathway is a promising therapeutic strategy for heart failure.