Factor XIIIa and the classic histiocytic markers in malignant fibrous histiocytoma: a comparative immunohistochemical study.

Fifteen cases of malignant fibrous histiocytoma (MFH) and 79 cases of differential-diagnostically related soft tissue tumors were evaluated for immunoreactive cells for the subunit A of factor XIII (F-XIIIa) in comparison with the staining obtained by the classic histiocytic markers: lysozyme, alpha 1-antitrypsin (AAT) and alpha 1-antichymotrypsin (AACT). Ubiquitous and focal staining patterns were distinguished. Only three cases of MFH were characterized by an ubiquitous positive reaction for AAT and AACT, in contrast to the obligatory positive staining of MFH for F-XIIIa. This low ratio is probably related to the high proportion of predominantly fibroblastic and myxoid types of MFH (11/15). In the three cases of MFH characterized by ubiquitous positive reactions for both antiproteases and for F-XIIIa, the frequency of positive cells for AAT and AACT exceeded that for F-XIIIa. Thus, the positive cells for antiproteases and those for F-XIIIa represent different levels of fibro-histiocytic differentiation; the F-XIIIa-positive cells are fibro-histiocyte precursors. F-XIIIa-positive stromal cells are present in the normal mesenchyme, but their significance is unknown. The fact that these cells are a constant feature of MFH argues for a histiocytic pathway of their differentiation. The ubiquitous presence of F-XIIIa-positive cells in MFH distinguishes them from the histologically similar soft tissue tumors. However, the focal presence of F-XIIIa-positive cells indicate only a host response to an unspecified tissue injury that may occur in all kinds of soft tissue tumors.