Smooth Muscle‐associated Contractile Protein in Human and Experimental Acute Leukaemias*

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Abstract
Cells from 10 human myeloblastic leukemias and spontaneous rat myeloblastic and lymphoblastic leukemias were examined by indirect immunofluorescence with human serum containing smooth muscle antibody. Strong positive cell outline staining of blast cells was seen in cryostat sections of rat leukemic lymph nodes, spleen and liver. In cell smears of human and rat leukemias, cell outline fluorescence was restricted to cells in close contact with each other. Membrane immunofluorescence tests of suspensions of viable leukemic cells were negative. Specificity of the immunofluorescent staining reaction was established by failure to obtain staining with normal serum, or with smooth muscle antibody serum neutralized by homogenates of smooth muscle or extracts containing actin derived from smooth muscle. These observations suggest that smooth muscle-associated microfilaments are present in leukemic blast cells and expression of the antigen is dependent on cell-cell contact. The presence of this antigen may facilitate leukemic infiltration of tissues.