Immunohistochemical study of the presence of mast cells in idiopathic orbital inflammatory pseudotumor: possible role of mast cells in the course of its pathogenesis

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Abstract
It is well known that mast cells (MC) are involved in fibrosis and many forms of chronic inflammation. Chronic inflammatory cells infiltration and fibrosis in various orbital tissues are the main histopathologic features in patients with idiopathic orbital inflammatory pseudotumor (IOIP). Whether MC is involved in the course of chronic inflammatory conditions of IOIP is not yet clear. We sought to investigate the distribution of MCs in samples of IOIP and to explore the possible role of MC in the course of its pathegenesis. Immunohistochemistry with tryptase monoclonal antibody (a specific mast-cell surface marker) was used in 53 different subtypes of IOIP specimens including 19 of the lymphocyte infiltrative type, 22 of the fibrotic type, 12 of the mixed type and 4 specimens of normal orbital tissue. The number of positive stained MC was counted by light microscopy. The differences of the number of MC between various subtypes of IOIP were analyzed. The average number of positive stained MC in the normal control group was 33.33 ± 4.72 /mm3, whereas the average numbers of positive stained MC in the lymphocyte infiltrative subtype group, the mixed subtype group and the fibrotic subtype group were 306.35 ± 55.81 /mm3, 662.93 ± 115.28 /mm3 and 813.44 ± 146.56 /mm3, respectively. Compared with the normal control, the number of MC increased significantly in all three subtypes of IOIP samples (P < 0.01). The number of MC in fibrotic subtype IOIP was the largest, followed by mixed subtype IOIP, and the lymphocyte infiltrative subtype IOIP (P < 0.05). The MC were distributed mainly around small vessels and in collagen fibers. Mast cells may play an underappreciated role in the fibrosis of IOIP.

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