Characterization of cancer-associated fibroblasts in classical Hodgkin lymphoma
Cancer-associated fibroblasts (CAFs) are a heterogeneous cell population with diverse functions. They constitute an important stromal component of the tumor microenvironment (TME), regulate immune responses and promote tumor growth. We have earlier characterized the immunoprofile of cHL and concluded that high proportions of checkpoint protein positive immune cells associated with unfavorable overall survival (OS). Here, we have characterized distinct CAF subsets and their clinical impact in cHL.
CAFs, tumor associated macrophages (TAMs), T cells and checkpoint molecules were phenotyped from the diagnostic cHL samples (n=131) using multiplex immunohistochemistry with digital image analysis. Platelet-derived growth factor receptor (PDGFR) α and β, fibroblast-activation protein (FAP) and α-smooth muscle actin (α-SMA) were utilized as CAF markers. Nanostring-based data from a 770-gene immune panel was used to correlate CAF proportions with gene expression.
Median age of the patients was 30 years, 78% had nodular sclerosis subtype and 56% advanced stage. Primary treatment (85%) was mostly doxorubicin, bleomycin, vinblastine, dacarbazine, and in limited stage, in combination with radiotherapy. At the median follow-up time of 55 months, 22% of the patients relapsed and 8% died. Five-year freedom from treatment failure (FFTF) was 79% and OS 91%.
From all cells in the TME, 20% were PDGFRβ (range 0.2-92%), 19% FAP (range 0-88%), 16% α-SMA (range 2-56%), and 1% PDGFRα (range 0.1-58%) positive. High proportion of FAP+ cells associated with inferior FFTF (HR 1.25, 95%CI 1.0-1.5, P=0.020).
In a small subgroup of patients, PDGFRα and FAP were expressed on the same cells defining an PDGFRα+FAP+ double positive cHL phenotype, which translated to better outcome (5-y FFTF 100 % vs. 76%, P=0.035; 5-y OS 100% vs. 90%, P=0.270). Clinical characteristics were equally distributed between these subgroups. PDGFRα+FAP+ phenotype associated positively with high proportion of TAMs (P=0.026), but negatively with T cells (P=0.006) and checkpoint molecule indoleamine 2,3 dioxygenase 1 (IDO-1)+ cells (P=0.002). In addition, focal adhesion and extracellular matrix pathway genes were enriched in PDGFRα+FAP+ cHLs.
Our data identifies distinct CAF subsets in cHL and suggests that the newly characterized subset of PDGFRα+FAP+ CAFs has prognostic impact in cHL patients. Validation of the results is ongoing.