Phenotypical characteristics and proliferative capabilities of thymocyte subsets in human thymoma

The phenotypical and functional characteristics of the lymphoid component present in ten human thymomas have been analyzed. Thymomas were classified according to the predominant epithelial cell type present in the neoplastic gland. In thymomas of the cortical type a large proportion, clearly higher than that present in mixed thymomas, of T6-positive lymphocytes was present. This T6 subset did not proliferate to mitogen but contained almost all thymocytes able to spontaneously proliferate. Higher proportions of T3-positive cells were found in mixed types of thymomas than in cortical-type tumors. This T3+ subset responded to mitogen stimulation and constituted the more mature intrathymic pool. Surprisingly we identified in thymomas a further subset, lacking the cortical and medullary markers T6 and T3, capable of responding to mitogen. The occurrence of higher proliferative responses to phytohemagglutinin (PHA) in the unfractionated population of cells from mixed thymomas than in the cells of cortical thymomas was judged to be attributable to the relatively higher content of both T3-positive and T3, T6-negative thymocytes in the former. Unlike T6+ thymocytes, T3-T6- as well as T3+ cells were practically devoid of spontaneous proliferative capacity. The expansion of the intrathymic lymphoid component in human thymomas should then be considered to be attributable to the "spontaneous" proliferative capacity of the T6+ cell pool. In this respect, cortical thymomas not only contained more T6+ cells than the mixed type but also exhibited a higher lymphocyte/epithelial cell ratio and more frequent mitotic figures.