ON THE RELATIONSHIP OF MAST CELLS TO VARIOUS SOFT TISSUE TUMOURS

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.

Pathogenesis of plexiform neurofibroma: tumor-stromal/hematopoietic interactions in tumor progression

Neurofibromatosis type 1 (NF1) is a genetic disease that results from either heritable or spontaneous autosomal dominant mutations in the NF1 gene. A second-hit mutation precedes the predominant NF1 neoplasms, which include myeloid leukemia, optic glioma, and plexiform neurofibroma. Despite this requisite NF1 loss of heterozygosity in the tumor cell of origin, nontumorigenic cells contribute to both generalized and specific disease manifestations. In mouse models of plexiform neurofibroma formation, Nf1 haploinsufficient mast cells promote inflammation, accelerating tumor formation and growth. These recruited mast cells, hematopoietic effector cells long known to permeate neurofibroma tissue, mediate key mitogenic signals that contribute to vascular ingrowth, collagen deposition, and tumor growth. Thus, the plexiform neurofibroma microenvironment involves a tumor/stromal interaction with the hematopoietic system that depends, at the molecular level, on a stem cell factor/c-kit-mediated signaling axis. These observations parallel findings in other NF1 disease manifestations and are clearly relevant to medical management of these neurofibromas.

Pathological evaluation of uterine leiomyomas treated with gonadotropin-releasing hormone agonist (GnRH-a) therapy: role of mast cells and a possible mechanism of GnRH-a resistance in leiomyomas

Gonadotropin-releasing hormone agonist (GnRH-a) therapy is frequently applied to reduce the volume of uterine leiomyomas (UL). In addition, the possible relationship between mast cells (MC) within UL and the development of UL has been suggested, but the role of MC in UL remains to be determined. UL with or without GnRH-a therapy in 121 premenopausal patients were reviewed. The number of MC was evaluated between the two groups, immunohistochemistry was done for insulin-like growth factor-I (IGF-I), and the association between the IGF-I immunoreactivity in UL and the GnRH-a therapy was analyzed. The number of MC significantly increased in UL in GnRH-a therapy, while IGF-I immunoreactivity was significantly reduced in smooth muscle cells of these UL. Furthermore, IGF-I immunoreactivity in MC was inversely correlated with the size reduction rate of UL in GnRH-a therapy. Although GnRH-a therapy is considered to reduce the size of UL transiently, the regression of UL was in part hampered by the increased IGF-I secretion from the increased MC after GnRH-a therapy. Therefore, the more the IGF-I secretion from MC in UL increases, the less effective the GnRH-a therapy is on the size reduction of UL. Thus, the present study may provide an explanation of the possible mechanism of GnRH-a resistance in UL.

It takes two to tango: mast cell and Schwann cell interactions in neurofibromas

Neurofibromas are benign tumors comprised primarily of Schwann cells and fibroblasts. Mast cell infiltration is a well-known phenomenon; however, their role in tumor pathogenesis has been enigmatic. In an elegant set of experiments using cells derived from a murine model of neurofibromatosis 1 (NF1), Yang et al. dissect the molecular pathways involved in mast cell migration to neurofibromin-deficient Schwann cells. These results set the stage for rational development of therapeutics that could influence the multicellular microenvironment of neurofibromas to inhibit the development and/or progression of these tumors in human NF1.

Arteriovenous hemangioma: a clinicopathological and immunohistochemical study

The clinicopathological and immunohistochemical properties of 6 examples of arteriovenous hemangioma, including 2 intraoral lesions, were reviewed. This distinct benign, acquired vascular lesion, infrequently encountered in the literature, is characterized by multiple thick- and thin-walled vascular spaces resembling arteries and veins, respectively. In our study, we performed elastic stains that revealed a prominent venular component, whereas the arterial aspect was inconspicuous to absent. Our aim was also to elucidate the possible histogenesis of this lesion. Previous reports suggest as pathogenetic mechanisms hamartomatous proliferation either of the subpapillary vascular plexus or of the Suquet-Hoyer canal of the true glomus. Our immunohistochemical studies failed to identify typical glomus cells. In addition, we investigated the mast cell count in all lesions and it was found increased. These findings, as well as recent evidence directly implicating mast cells in angiogenesis, can support the theory of hamartomatous proliferation of the subpapillary plexus. One should also not exclude the possibility of a reactive process resulting in the characteristic features of arteriovenous hemangioma.

Mast cell frequency in soft tissue tumors. Relation to type and grade of malignancy

A high content of mast cells (MC) is considered characteristic of neurofibromas but not of malignant schwannomas and neurilemmomas. We examined the extent and reliability of this finding by counting MC in 61 peripheral nerve sheath tumors and in 103 non-neurogenic soft tissue sarcomas. We furthermore investigated correlations between the amount of MC and various features of the tumors (e.g. grades of malignancy). Neurofibromas had very high mast cell counts. However, this result only applied to about 70% of these tumors. Malignant schwannomas, malignant fibrous histiocytomas and leiomyosarcomas had remarkably high median values of MC counts with a wide dispersion within the histological groups. Synovial sarcomas were the only group that contained MC in every case, though often in small numbers. In univariate analyses the number of MC was negatively correlated to grades of malignancy, cellularity and mitotic activity of the sarcomas and tended to correlate positively to the amount of myxoid and collagenous connective tissue and lymphocytic infiltrates. Multiple linear regression analysis revealed a significant correlation to the grade of malignancy and the amount of connective tissue.

Ultrastructure of cerebellar capillary hemangioblastoma. II. Mast cells and angiogenesis

The topographic distribution, population density, and ultrastructural features of mast cells were studied in six cases of cerebellar capillary hemangioblastoma. The vascular area of tumor tissue contained large numbers of mast cells (6.3 cells/high power field, X 400) in comparison with hyalinized area (0.3 cell) and adjacent cerebellar tissue (less than 0.1 cell). Close association of mast cells with endothelial cells and stromal cells was found. The morphology of mast cell granules and their degranulation through dissolution of granule contents and exocytosis were illustrated. The findings suggest that an increased number of mast cells may represent one of the characteristic histological features of capillary hemangioblastoma, and continuous degranulation of mast cell granules with release of heparin may play an important pathophysiologic role in the vascular proliferation and expansion of the tumor.

Ultrastructure of mast cells in growing and involuting stages of hemangiomas

Mast cells from 11 patients with growing strawberry (capillary) hemangiomas and cellular hemangiomas and from ten patients with involuting stages of strawberry hemangiomas were investigated electron microscopically. Two types of mast cells were evident: numerous elongated, fibroblastic cells and a few round, classic cells. The mast cells in growing hemangiomas contained many small, simple-type granules in various stages of development. It is possible that, owing to the immaturity of these granules, mast cells have different, probably weaker, functional activities. Mast cells in involuting stages of hemangiomas were filled primarily with compound-type granules, showing typical parallel, concentric lamellae or crystalloid structures, which were similar to the mature mast cells in normal skin.

[Studies of the mucopolysaccharid metabolism in the juvenile angiofibroma (author's transl)]

The mucopolysaccharide content of juvenile angiofibromas is investigated using histochemical and ultrahistochemical methods (histochemical in ten cases, ultrahistochemical in four cases). The investigations result in typical histochemical features and show the localization of acid mucopolysaccharides at electronmicroscopical level. The endothelium of the vessels exhibits a strong PAS-reaction and also shows acid groups at the cell surfaces. The interstitial tissue demonstrates neutral perjodatreactive substances as well as acid mucopolysaccharides. The latter substances mainly reacted in perifibrillar position. Electronmicroscopically the perjodreactive material was localized in the cytoplasma of typical fibroblasts and their modulations resembling histiocytes. The acid groups of carboxylated and sulfated mucopolysaccharides of the intercellular substance could be determined as hyaluronic acides and chondroitinsulfates (Chondroitin-4 and Chondroitin-6 sulfate) using the critical-electrolyte-concentration method after Scott and Dorling. Ultrastructurally these acid groups are localized as cell coat covering the fibroblasts and between the collagen fibrils. This histochemical investigation also supports the thesis of an organoid growth pattern of juvenile angiofibroma.

Mast cells in benign nerve sheath tumours

The concentration of mast cells was estimated, and their distribution noted, in 132 benign nerve sheath tumours. With few exceptions, neurofibromas were characterised by a high concentration of diffusely distributed mast cells. In neurilemmomas fewer mast cells were present and these were restricted to Antoni type-B tissue. Based on these observations, and on the behaviour of mast cells in injured nerves, it is postulated that neurilemmomas are true neoplasms of Schwann cells sometimes incorporating reactive endoneurial tissue (Antoni type-B tissue). Neurofibromas probably consist of reactive endoneurial tissue and may not be true neoplasms. The distinction between neurofibromas and neurilemmomas thus seems to be a real one based on a difference in histogenesis.