A Sino-U.S. study has shown that the increased expression of the small secreted glycoprotein follistatin-like 1 (FSTL1) is critical in the pathogenesis of multiple fibrotic and systemic autoimmune diseases (SADs), the authors reported in the January 6, 2021, print edition of Molecular Therapy after earlier publication online.

The study authors then screened and identified epitope-specific anti-FSTL1 monoclonal neutralizing antibodies (mnAbs), which may represent a multifunctional therapeutic approach for patients with fibrotic diseases or SADs, including rheumatoid arthritis (RA).

"This is the first work to describe the development of anti-FSTL1 mnAbs as potential therapies for multiple fibrotic diseases and SADs," said study co-leader Dianhua Jiang, professor and scientific director of the Pulmonary Fibrosis Research Center at Cedars-Sinai Medical Center in Los Angeles.

Previous studies have suggested that FSTL1 is involved in embryonic development, tissue remodeling and inflammatory processes, while also helping to regulate cell growth, differentiation, apoptosis and migration.

FSTL1 has been shown to function mainly via regulating the transforming growth factor-beta 1/bone morphogenetic protein (TGF-beta1/BMP) signaling pathway at the membrane receptor level.

Activation of multiple signaling pathways and the inflammatory response has also been linked to FSTL1 in disease models, including those of organ fibrosis.


Organ fibrosis involves progressive deposition of extracellular matrix (ECM) components, notably collagen, due to excessive fibroblast activity, which is a common pathological feature of diseases including idiopathic pulmonary fibrosis (IPF), systemic sclerosis (SSc), hypertrophic and keloid scarring, and liver cirrhosis, for which no effective therapies are available.

TGF-beta1 is a key cytokine initiating and perpetuating organ fibrosis through its roles in regulating epithelial injury, myofibroblast differentiation and collagen production, but no drugs directly target the TGF-beta1/BMP pathway.M

Importantly, recent studies have identified FSTL1 as a downstream effector of TGF-beta1-induced fibrosis, which is upregulated in IPF patients' lungs and in bleomycin mouse models.

The SAD story

Studies have also implicated FSTL1 involvement in human SADs, particularly RA pathogenesis, with FTSL1 elevation having been reported in RA patients' synovium and/or serum and in mouse models of collagen-induced arthritis (CIA).

Furthermore, FSTL1 has been shown to enhance proinflammatory cytokine and chemokine synthesis both in vitro and in vivo, suggesting that targeting FSTL1 may treat both human fibrosis and SADs.

Consequently, in the new study researchers led by Jiang, Wen Ning and Honggang Zhou, who are professors at Nankai University in Tianjin, developed epitope-specific mnAbs to functionally block FSTL1, then evaluated their effects in different disease models.

For example, the mnAbs were shown to significantly attenuate bleomycin-induced pulmonary and dermal fibrosis in vivo and TGF-beta1-induced dermal fibrosis ex vivo in human skin.

"The anti-FSTL1 mnAbs ameliorated bleomycin-induced pulmonary and dermal fibrosis in mice, as reflected by biochemically determined 50% reductions in collagen," said Jiang.

Similarly, "they ameliorated TGF-beta1-induced fibrosis ex vivo in human skin, with reductions in collagen content of approximately 40%," he told BioWorld Science.

The mnAbs were then demonstrated to significantly reduce existing lung and skin fibrosis to a similar extent in these experimental models.

Moreover, the mnAbs were shown to exert their potent antifibrotic effects via reduced TGF-beta1 responsiveness and subsequent myofibroblast activation and ECM production, which has important implications for drug development.

Drug development

"TGF-1b plays a central role in fibrogenesis, but due to its pleiotropic nature, blocking TGF-1b signaling may have profound adverse effects," said Jiang. "This has led to a roadblock in the development of a direct targeting strategy with inhibitors or neutralizing antibodies targeting TGF-1b and its receptors in fibrotic diseases."

"Therefore, alternative strategies are needed for modulating TGF-1b signaling, with anti-FSTL1 mnAbs providing an excellent example that effectively targeting one of the key modulators of TGF-1b signaling would benefit patients with multiple fibrotic organ diseases, as well as SADs."

For example, the researchers showed that the mnAbs attenuated the severity of CIA in mice, with "a more than 60% reduction in the arthritis index and paw swelling," said Jiang, noting that this attenuation was accompanied by significantly reduced inflammatory responses in vitro.

"High blood concentrations of FSTL1 have been reported in patients with RA, while elevated expression of FSTL1 has been confirmed in animal models," said Jiang.

"Such aberrant expression of FSTL1 may be related to [the cytokine] interleukin-1beta (IL-1beta), with the current study confirming that FSTL1 is a functional target for RA in animal models," he said.

"IL-1beta blockade stops the inflammatory response commonly seen in autoinflammatory disorders and has been approved by the FDA for the treatment of autoinflammatory disorders including systemic juvenile inflammatory arthritis."

Therefore, "we anticipate that targeting FSTL1 by modulating IL-1beta-induced inflammation would be effective and have fewer adverse effects in patients with SADs than directly targeting IL-1beta."

However, extensive further research work is necessary before anti-FSTL1 mnAbs might enter clinical trials for treating fibrosis or SADs in humans, which could take many years, noted study co-leader Ning.

"Such studies include the generation of humanized anti-FSTL1 mnAbs and subsequent determination of their effects on anti-anti-fibrotic responses," Ning told BioWorld Science.

"Other necessary studies include those of dosage efficacy, systematic and organ-specific inflammatory responses, pharmacokinetic, toxicity, and tissue distribution assessments, large animal studies and antibody characterizations."

Looking forward, concluded Ning, "ultimately we hope that we will be able to develop and introduce humanized anti-FSTL1 mnAbs as an effective and safe therapy for patients with organ fibrosis and SADs."