[2055] Expression of Thymoproteasome Subunit β5t in Type AB Thymoma

Yosuke Yamada, Utano Tomaru, Kanako Kubota, Akihiro Ishizu, Takayuki Kiuchi, Tomoko Mitsuhashi, Yoshihiro Matsuno, Masanori Kasahara. Hokkaido University Hospital, Sapporo, Hokkaido, Japan; Hokkaido University Graduate School of Medicine, Sapporo, Hokkaido, Japan; Hokkaido University, Sapporo, Hokkaido, Japan

Background: Among the subtypes of thymoma, type AB thymoma, which is composed of a lymphocyte-poor type A component mixed with a lymphocyte-rich type B component, shows a broad range of morphology despite being a single entity. However, the functional characteristics of the neoplastic cells responsible for this morphological variety remain to be clarified. β5t, which is expressed specifically by thymic cortical cells, and involved in T-cell maturation, was discovered recently. We have already reported that most cases of type B thymoma and some cases of type AB thymoma express β5t, whereas all cases of type A thymoma are β5t-negative. More detailed study of β5t expression in type AB thymomas would provide a better understanding of the natural history of thymomas.
Design: A total of 20 cases of type AB thymoma were retrieved. In specimens stained with hematoxylin and eosin, we divided the tumor area into the type A component (a) and type B component (b), the latter being further subdivided into 3 components: those with spindle neoplastic cells (b-1), those with polygonal neoplastic cells (b-2), and those with polygonal neoplastic cells and medullary differentiation (b-3). We then performed immunohistochemistry using anti-β5t and anti-TdT antibodies, and evaluated the following parameters for each of these 4 components: the ratio of β5t-positive neoplastic cells among total neoplastic cells (0-100%), the predominant staining intensity (0 = negative, 1 = weak, 2 = intermediate, and 3 = strong), and the ratio of TdT-positive lymphocytes in infiltrating lymphocytes. For β5t, immunostaining was interpreted as positive when >20% of the neoplastic cells exhibited an intensity of 2 or 3.
Results: Among the 20 cases, 19 cases contained the a component, 15 cases the b-1 component, 12 cases the b-2 component, and 4 cases the b-3 component. In the a, b-1, b-2, and b-3 components, the β5t-positivity ratio was 5%, 20%, 42%, and 75%, respectively. These positivity ratios were significantly different (P = 0.001, χ2 test). The corresponding mean ratios of TdT-positive lymphocytes were 9%, 59%, 75%, and 80%, respectively. These mean ratios were significantly different (P<0.001, Wilcoxon-test).
Conclusions: Considering the distribution of a functional molecule of thymic cortical epithelial cells, β5t, as well as morphology, it is suggested that type AB thymoma is not an entity distinct from type A thymoma and type B thymoma, but forms a spectrum and shares its natural history with these subtypes.
Category: Pulmonary

Wednesday, March 21, 2012 9:30 AM

Poster Session V # 291, Wednesday Morning

 

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