Distinctive Histogenesis and Immunological Microenvironment Based on Transcriptional Profiles of Follicular Dendritic Cell Sarcomas

Follicular dendritic cell (FDC) sarcomas are rare mesenchymal tumors (MTs) with variable clinical, morphologic and phenotypic characteristics. Transcriptome analysis was performed on multiple FDC sarcomas and compared to other MTs, microdissected Castleman FDCs, and normal fibroblasts. Using unsupervised analysis, FDC sarcomas clustered with microdissected FDCs, distinct from other MTs and fibroblasts. The specific endowment of FDC-related gene expression programs in FDC sarcomas emerged by applying a gene signature of differentially expressed genes (n=1,289) between microdissected FDCs and fibroblasts. Supervised analysis comparing FDC sarcomas with microdissected FDCs and other MTs identified 370 and 2,927 differentially expressed transcripts, respectively, and based on pathway enrichment analysis ascribed to signal transduction, chromatin organization, and extracellular matrix organization programs. Since the transcriptome of FDC sarcomas retained similarity with FDCs, the immune landscape of FDC sarcoma was investigated by applying the CIBERSORT algorithm to FDC sarcomas and non-FDC MTs, and demonstrated that FDC sarcomas were enriched in T follicular helper (Tfh) and T regulatory (Treg) cell populations, as confirmed in situ by immunohistochemistry. The enrichment in specific T-cell subsets prompted investigating the mRNA expression of the inhibitory immune receptor PD-1 and its ligands PD-L1 and PD-L2, which were found to be significantly upregulated in FDC sarcomas as compared with other MTs, a finding also confirmed in situ. Here it is demonstrated for the first time the transcriptional relationship of FDC sarcomas with non-malignant FDCs and their distinction from other MTs.