The Plexiform Neurofibroma Microenvironment

Dynamic interactions between tumorigenic cells and surrounding cells, including immunomodulatory hematopoietic cells, can dictate tumor initiation, progression, and transformation. Hematopoietic-stromal interactions underpin the plexiform neurofibroma, a debilitating tumor arising in individuals afflicted with Neurofibromatosis type 1 (NF1), a common genetic disorder resulting from mutations in the NF1 tumor suppressor gene. At the tissue level, plexiform neurofibromas demonstrate a complex microenvironment composed of Schwann cells, fibroblasts, perineural cells, mast cells, secreted collagen, and blood vessels. At the cellular level, specific interactions between these cells engender tumor initiation and progression. In this microenvironment hypothesis, tumorigenic Schwann cells secrete pathological concentrations of stem cell factor, which recruit c-kit expressing mast cells. In turn, activated mast cells release inflammatory effectors stimulating the tumorigenic Schwann cells and their supporting fibroblasts and blood vessels, thus promoting tumor expansion in a feed-forward loop. Bone marrow transplantation experiments in plexiform neurofibroma mouse models have shown that tumorigenesis requires Nf1 haploinsufficiency in the hematopoietic compartment, suggesting that tumor microenvironments can depend on intricate interactions at both cellular and genetic levels. Overall, our continued understanding of critical tumor-stromal interactions will illuminate novel therapeutic targets, as shown by the first-ever successful medical treatment of a plexiform neurofibroma by targeted inhibition of the stem cell factor/c-kit axis.