Discussion
Liquid biopsy provides enhanced sensitivity for biomarker discovery and
ease of repeated sampling throughout treatment in a much more convenient
and noninvasive way13,14. Recent advances in mass
spectrometry (MS)-based proteomics have greatly extended its reach in
biomedical and clinical research, and mass spectrometry (MS) is now
poised to characterize the plasma proteome in great
depth15. Plasma proteomic analysis has proven to be a
promising tool for identifying new and effective biomarkers that can be
used for evaluating the prognosis and treatment response in patients
with various cancers16.
Patients with RO+ LCH have a poorer prognosis than patients with
RO-LCH17. Recent evidence from the myeloid dendritic
cell (DC) model suggested that MS-RO+ LCH results from a driver mutation
in a bone marrow (BM)-resident multipotent hematopoietic
progenitor18. However, the exact mechanism is unknown.
Herein, we focused on the differences in plasma proteome profiles
between RO+ LCH patients and SS LCH patients, and our study is the
first, to our knowledge, to identify a novel soluble form of CSF1R in
the plasma of pediatric LCH patients.
The results of the ELISA validation showed that plasma sCSF1R levels
were associated with disease extent, and high levels of sCSF1R
correlated with more severe disease, as indicated by a younger age;
involvement of the RO, skin, and lung; and the presence of the
BRAF-V600E mutation in lesions or cfDNA. Moreover, a high sCSF1R level
at diagnosis independently predicted inferior PFS and was sustained in
patients who experienced disease progression after 6 weeks of treatment.
Notably, dynamic monitoring of sCSF1R levels provided an early warning
of relapse in patients receiving BRAF inhibitor treatment. Therefore,
the sCSF1R level should be closely monitored, especially during
treatment with targeted inhibitors of BRAF or other proteins in the MAPK
pathway.
IF staining in this study showed that high CSF1R protein expression in
LCH lesions was associated with RO involvement, which was in line with
the plasma sCSF1R levels. We also noted that sCSF1R levels apparently
decreased after dabrafenib administration initiation, suggesting that
extracellular CSF1R in plasma was mainly secreted by cells that carried
the BRAF -V600E driver mutation. In vitro drug sensitivity data
showed that sCSF1R increased resistance to Ara-C in THP-1 cells
expressing ectopic BRAF -V600E. However, the exact biological
function of sCSF1R is unclear. The release of soluble forms may
represent a mechanism for counter regulation of CSF1R. A recent study
revealed that overexpression of sCSF1R significantly decreased the
extent of microgliosis in both the dorsal and ventral horns, indicating
that sCSF1R can reduce the activation of native CSF1R on
microglia19. By acting as a decoy receptor, sCSF1R can
bind to colony-stimulating Factor 1 (CSF1) and prevent it from
interacting with the membrane-bound CSF1R on target cells, possibly
modulating CSF1 signaling and affecting the recruitment, survival, and
differentiation of cells. In contrast, our data showed that sCSF1R was
closely linked to severe disease and relapse in LCH patients, suggesting
that sCSF1R contributes to disease progression. Previous studies have
suggested that several oxidative stress proteins, including thioredoxin
and heat shock proteins, are released from stressed, transformed cells
and act as “endogenous” danger signals by binding TLR4 in the
extracellular microenvironment, which results in the activation of
downstream pathways and the secretion of proinflammatory cytokines20,21. We thus speculated that sCSF1R may act as an
endogenous danger signal by binding to danger signal sensors/receptors,
which is independent of CSF1. However, further investigations are needed
to explore the underlying mechanisms involved.
Several recent studies have shown the good short-term effectiveness and
tolerability of BRAF or MEK inhibitors in recurrent/refractory LCH22-25. However, MAPK pathway inhibition did not appear
to be curative and was associated with a high risk of reactivation
following drug discontinuation 26-28. Pharmacological
inhibition of CSF1R has emerged as a promising antitumor strategy,
and several small-molecule CSF1R
inhibitors have been developed in clinical and preclinical
studies29. Thus, combined inhibition of BRAF/MAPK
signaling and CSF1R may be beneficial for recurrent/refractory LCH
treatment. However, most clinical results on the safety and efficacy of
CSF1R inhibition are limited 30, which may be caused,
in part, by a high level of plasma sCSF1R, as shown in this study.
Therefore, monoclonal antibodies engineered to recognize the
extracellular domains of CSF1R may inspire new ideas for
recurrent/refractory LCH treatment.
One limitation of this study was the lack of a validation cohort for the
prognostic analyses of sCSF1R. Additional studies in independent cohorts
are needed to establish the validity of our findings.
In conclusion, our findings identify plasma sCSF1R as a promising
prognostic indicator for pediatric LCH. Accurate measurements of plasma
sCSF1R at diagnosis and during follow-up have potential academic and
clinical importance, and exploring such effective biomarkers to
facilitate risk stratification and precision medicine is the major
challenge for LCH treatment.