Malignant fibrous histiocytoma tumor cells resemble fibroblasts.

The malignant fibrous histiocytomas (MFHs) are a histologically heterogeneous group of sarcomas that have been postulated to be derived from, or have the capacity to differentiate into, histiocytes. To determine whether MFH tumor cells actually express the features of histiocytes, i.e., bone marrow-derived cells of monocyte-macrophage lineage, we studied the antigenic and enzymatic phenotype of 13 MFHs in situ using frozen and plastic sections, respectively. Five pleomorphic three fibrous, two myxoid, two giant cell, and one histiocytic MFH were studied. While tumor cells in 12 of 13 cases were positive for HLA-A,B,C, tumor cells in all cases failed to express antigens present on bone marrow-derived macrophages, i.e., leukocyte common antigen (L3B12), HLA-DR, Leu-M3, and Leu-3a. Interestingly 8 of 13 cases were positive for CALLA. Although nonspecific, this may prove useful in differential diagnosis. Enzyme histochemistry demonstrated that tumor cells in 9 of 13 cases were positive for membrane 5' nucleotidase (5'N+). Four of these were also alkaline phosphatase positive (ALKP+). All cases were either negative or weakly positive for acid phosphatase (ACIDP) and alpha-naphthyl acetate esterase (ANAE). Tumor cells were unreactive for alpha-naphthyl butyrate esterase (ANBE) and adenosine triphosphatase (ATP). These findings indicate that MFH tumor cells do not express the enzymatic profile of cells of monocyte/macrophage lineage which are membrane 5'N-/ALKP- and ACIDP+/ANAE+/ANBE+/ membrane ATP+. In fact, these data suggest a similarity to fibroblasts which are membrane 5'N+, variably ALKP+, weakly ACIDP+/ANAE+, and ANBE-/membrane ATP-. Osteoclast-like giant cells present in two cases did express a histiocytic phenotype, suggesting that they are reactive elements not derived from admixed tumor cells. These results suggest that MFHs are primitive mesenchymal neoplasms, most likely sarcomas composed of poorly differentiated fibroblasts, and are unrelated to true histiocytic neoplasms.