Diagnosis of rhabdomyosarcomas with HHF35, a monoclonal antibody directed against muscle actins.

The authors have recently developed a monoclonal antibody, HHF35, that recognizes the muscle-specific isoforms of actin. To determine its potential usefulness in the differential diagnosis of "small, round, blue cell" tumors of childhood, they immunolabeled formalinor B-5-fixed tissue sections from known cases of rhabdomyosarcoma or rhabdomyoma (30), neuroblastoma (9), retinoblastoma (2), and Ewing's sarcoma (9) with HHF35 and with antibodies to creatine kinase M, myoglobin, vimentin, and neuron-specific enolase. HHF35 reacted with 29 of 30 cases of rhabdomyosarcoma, whereas antibodies to creatine kinase M and myoglobin were positive on only 12 and 7 tumors, respectively. HHF35 did not react with any case of neuroblastoma, retinoblastoma, or Ewing's sarcoma when the antibody diluent contained 50 mM EDTA. These results indicate that HHF35 is a highly sensitive and specific marker for myogenic differentiation and that it will be useful in the differential diagnosis of rhabdomyosarcomas.