Folate Receptor (FR) Expression in Tissue-Resident and Tumor-Associated Macrophages Associates with and Depends on the Expression of PU.1

As macrophages exhibit a huge functional plasticity under homeostasis and pathological conditions, they have become a therapeutic target for chronic inflammatory diseases. Hence, the identification of macrophage subset-specific markers is a requisite for the development of macrophage-directed therapeutic interventions. In this regard, the macrophage-specific Folate Receptor beta (FR beta, encoded by theFOLR2gene) has been already validated as a target for molecular delivery in cancer as well as in macrophage-targeting therapeutic strategies for chronic inflammatory pathologies. We now show that the transcriptome of human macrophages from healthy and inflamed tissues (tumor; rheumatoid arthritis, RA) share a significant over-representation of the "anti-inflammatory gene set", which defines the gene profile of M-CSF-dependent IL-10-producing human macrophages (M-Mo). More specifically,FOLR2expression has been found to strongly correlate with the expression of M-Mo-specific genes in tissue-resident macrophages, tumor-associated macrophages (TAM) and macrophages from inflamed synovium, and also correlates with the presence of the PU.1 transcription factor. In fact, PU.1-binding elements are found upstream of the first exon ofFOLR2and most M-Mo-specific- and TAM-specific genes. The functional relevance of PU.1 binding was demonstrated through analysis of the proximal regulatory region of theFOLR2gene, whose activity was dependent on a cluster of PU.1-binding sequences. Further, siRNA-mediated knockdown established the importance of PU.1 forFOLR2gene expression in myeloid cells. Therefore, we provide evidence that FR beta marks tissue-resident macrophages as well as macrophages within inflamed tissues, and its expression is dependent on PU.1.