Monoclonal antibody MB2: a potential marker for Ewing's sarcoma and primitive neuroectodermal tumor.

The small round-cell tumors of childhood present difficulties in diagnosis when differentiation is not apparent. Immunohistochemistry is helpful; however, the only antigen consistently detected in Ewing's sarcoma is vimentin, which may also be detected in the other types of small-cell neoplasms. The monoclonal antibody (MAb) MB2 is marketed as a B-lymphocyte marker that can be used on paraffin-embedded tissue. To determine its specificity, we performed immunohistochemical staining on pediatric tumors with MB2. These included 55 cases of small round-cell tumors (lymphomas, Ewing's sarcoma, peripheral primitive neuroectodermal tumors [PNET], neuroblastomas, rhabdomyosarcomas, and nephroblastomas). MB2 positivity was detected in all B-cell lymphomas and in seven of nine cases of Ewing's sarcoma and three PNET. In neuroblastomas only differentiating ganglion cells were positive. In rhabdomyosarcomas only large rhabdomyoblasts were positive. Blastema of nephroblastomas was negative. Thus, in cases of poorly differentiated small round-cell tumors, MB2 was positive in all B-cell lymphomas, most Ewing's sarcomas and all cases of PNET. Lymphomas were distinguished by staining for leukocyte-common antigen and PNET by neuron-specific enolase. Therefore, the addition of MB2 to a discrete panel of antibodies may prove useful in the diagnosis of Ewing's sarcoma and PNET.