Noninvasive studies to assist with the diagnosis of pulmonary fibrosis and individual
prognostication represent a “holy grail” for idiopathic pulmonary fibrosis (IPF) and
other interstitial lung diseases (ILDs). Multiple studies have investigated the ability
of physiologic measurements, molecular profiling, circulating cellular markers (i.e.,
monocytes), and imaging biomarkers to predict disease progression. However, there
remains no clinically used disease activity marker for single–time point assessment
to assist with treatment decisions. In this issue of the Journal, Yang and colleagues
(pp. 160–172) evaluate “fibroblast activating protein” (FAP) as a molecular marker
of disease activity with the potential to be leveraged for prognostic purposes (1).
FAP is a transmembrane serine protease that was initially found to be expressed by
cancer-associated stromal cells and subsequently shown to be expressed on other cell
types, including macrophages (2). While associated with fibrosis, FAP has been shown
to have antifibrotic actions linked to its role in the proteolytic degradation and
processing of collagen. Indeed, FAP-deficient mice have increased collagen accumulation
in lungs after radiation- or bleomycin-induced injury (3). However, FAP is also reported
to enhance α2-antiplasmin function, which would be expected to stabilize fibrinogen
and fibronectin and contribute to lung fibrosis (4, 5).
FAP expression is increased by the profibrotic cytokine TGF-β (transforming growth
factor-β), and expression is increased in murine models of lung fibrosis. Within the
lungs of patients with IPF, FAP was upregulated within fibroblastic foci, but not
in the adjacent alveolar epithelium or in normal lung tissues (6). However, increased
expression may not be detrimental; a recent study showed that depletion of FAP+ cells
or genetic deletion of FAP exacerbated fibrosis in a murine model of repetitive bleomycin
injury and had no impact on fibrosis induced by adenoviral overexpression of TGF-β
(7). Regardless of its role in pathogenesis, recent studies have used FAP to direct
delivery of approved and emerging antifibrotic drugs, supporting its potential as
an indicator of active wound repair and fibrosis (8, 9).
Here Yang and colleagues (1) evaluate FAP expression using human lung fibroblasts,
a murine model of pulmonary fibrosis, single-cell sequencing, and a radiolabeled FAP
inhibitor administered to patients with ILD. This study has several major findings.
Fibroblast expression of FAP is induced by TGF-β. Similar findings have been shown
in fibroblasts from several sources and disease states (2, 6). In vivo, increased
FAP expression was detected 1 week after bleomycin-induced lung injury. FAP expression
was increased in fibrotic human lung tissue, with half of explanted IPF samples and
almost all of silicosis lung tissue samples showing elevations, demonstrating that
increased FAP expression is not specific for IPF. Single-cell transcriptomic analysis
indicated that FAP was highly expressed in mesenchymal cells and that there was moderate
correlation with other mesenchymal cell markers associated with fibrosis (α-SMA [α-smooth
muscle actin], fibronectin, and collagen I).
In a small group of healthy volunteers and a larger group of subjects with ILD (IPF
and non-IPF), 68Ga-FAPI-04 positron emission technology (PET) uptake was increased
in the lungs of subjects with ILD compared with healthy volunteers. Differences in
total standardized uptake value (SUVtotal) but not mean standardized uptake value
were seen between the IPF and non-IPF ILD groups, with the IPF group having a higher
SUVtotal. As SUVtotal, a measurement defined by the authors, captures the extent of
probe uptake above a threshold, this finding hints at underlying biologic heterogeneity
among ILD subtypes. SUVtotal correlated moderately with subsequent changes in FVC
and Dl
CO; however, the follow-up times for pulmonary function testing were nonuniform, ranging
from 4 to 24 months.
A major strength of this study lies in the application of molecular imaging to establish
a link between FAP expression as a biologic indicator of fibroblast activation and
a noninvasive indicator of disease activity. PET probes targeting type I collagen,
CCR2 (C-C chemokine receptor 2), αvβ6 integrin, and CXCR4 (C-X-C motif chemokine receptor
4) have been recently applied to patients with IPF to detect disease or to assess
treatment effects (10–13). 68Ga-FAPI-04 PET/computed tomography has been performed
in systemic sclerosis–associated ILD, and the degree of probe uptake was associated
with disease progression (14). This study by Yang and colleagues (1) illustrates several
important applications of molecular imaging to ILD. First, it can be used to obtain
molecular information otherwise not available and thus advance our understanding of
molecular heterogeneity within individuals and ILD subtypes. Second, imaging of molecular
pathways involved in fibrogenesis could provide a window into disease activity at
a single time point. This study also highlights important considerations in using
molecular imaging. PET biomarkers that incorporate both magnitude of uptake and amount
of lung involved may better predict disease progression for diffuse lung diseases
compared with traditional PET measurements such as mean standardized uptake value.
Several important gaps remain in establishing FAP as a reliable marker of disease
activity. This study does not definitively link FAP expression with profibrotic fibroblast
activity. Indeed, although FAP correlated with ACTA2 (actin alpha 2, smooth muscle),
COL1A1 (collagen type I alpha 1 chain), and FN1 (fibronectin 1) in single-cell analysis,
transcriptional expression does not always reflect protein concentration. Given its
role in collagen turnover, FAP may have a functional role in controlling the wound-repair
response to prevent the excessive collagen accumulation that characterizes fibrosis.
Thus, studies will need to determine the ability of molecular imaging for FAP to distinguish
between physiologic and fibrotic repair. More definitive studies are needed to determine
if 68Ga-FAPI-04 PET can reliably predict disease progression in ILD and whether FAP
activity can predict responsiveness to antifibrotic therapies. Although the premise
of differentiating “active” from “stable” disease by noninvasive FAP assessment as
a decision tool to guide pharmacotherapy is appealing, fibrosis progression is not
linear and for some individuals may occur in a stepwise manner. In the liver, FAP
expression in hepatic stellate cells correlates with fibrosis severity (15), and circulating
FAP is increased in patients with increased liver stiffness, supporting a potential
role for FAP in the risk stratification of patients for liver fibrosis (16). In the
lung, one modality may not provide the sensitivity and specificity needed for risk
prediction or guidance of therapy in an individual patient. Nevertheless, the combination
of accessible serum or genetic markers with physiologic measurements and molecular
imaging may improve our ability to risk stratify individuals with ILD.