Human thymomas: evidence of immunohistologically defined normal and abnormal microenvironmental differentiation.

Fifteen human thymomas were analyzed by immunoperoxidase studies on frozen and paraffin-embedded tissue sections in an attempt to identify the existence of immunologically defined microenvironments. All nine lymphocyte predominant thymomas contained a predominance of lymphocytes bearing the phenotype of cortical thymocytes and dendritic Class II major histocompatibility complex antigen-positive epithelial cells, thus defining cortical-like microenvironments. Medullary-like foci were also seen in all of these cases. Minor phenotypic abnormalities in Leu-2 and -3 antigen expression were seen in three cases. In contrast, the two epithelial predominant thymomas and four mixed thymomas all exhibited features of aberrant microenvironmental differentiation, with only two cases showing demarcation into cortical and medullary foci. A lack of Class II major histocompatibility complex antigens was associated with a decrease in the lymphoid populations and an increase in Leu-1 antigen expression by T cells of otherwise normal cortical phenotype when lymphocytes were present. In contrast, lack of Class I antigen on epithelial cells was not associated with any abnormality in lymphocyte phenotype or microenvironmental organization. We document for the first time abnormal microenvironments in thymomas that may offer insights into understanding normal thymic differentiation.