LN-2 (CD74). A marker to distinguish atypical fibroxanthoma from malignant fibrous histiocytoma.

In this study, the authors examined the expression of LN-2, an antigen expressed by B cells, macrophages, and Reed-Sternberg cells, in a variety of spindle cell lesions of the skin to determine whether LN-2 immunoreactivity can be used to differentiate among these tumors. For comparison, they examined CD34 antigen expression in these lesions, which has been shown to be a useful marker in differentiating dermatofibrosarcoma protuberans from dermatofibroma. Immunocytochemistry with anti-LN-2 and anti-CD34 monoclonal antibodies on formalin fixed, paraffin embedded material was performed on 102 spindle cell lesions, including dermatofibroma, dermatofibrosarcoma protuberans, atypical fibroxanthoma, malignant fibrous histiocytoma, leiomyoma, and neurofibroma. LN-2 immunoreactivity did not distinguish between dermatofibroma and dermatofibrosarcoma protuberans, both of which showed weak immunoreactivity. In marked contrast, 90% of cases of malignant fibrous histiocytoma showed strong staining for LN-2, whereas the vast majority (90%) of cases of atypical fibroxanthoma were negative or stained only weakly with anti-LN-2 antibodies. Of the two cases of atypical fibroxanthoma that stained strongly for LN-2, both lesions were > 2 cm in size and extended deep into the subcutaneous fat. Differential expression of the LN-2 antigen by atypical fibroxanthoma and malignant fibrous histiocytoma distinguishes these two lesions and suggests that acquisition of LN-2 positivity may be a marker of tumor progression.