A rat monoclonal antibody (MAb) termed RS‐11 (Ig M) was obtained by syngeneic immunization with rat bladder tumor cells induced by N‐butyl, N‐hydroxybutylnitrosamine (BBN). immunocytochemical analysis showed that RS‐11 is also reactive with mouse, dog and human bladder tumor‐cell lines and some other human tumor‐cell lines but not myeloma or leukemia cells. Immunohistochemical examination of paraffin‐embedded tissues has shown that RS‐11 is reactive with mouse, rat, dog and human bladder tumors (5/5, 5/5, 1/1, 31/ 49) and some other tumors, but not with normal human uro‐thelium or normal rat tissues. The antigen is expressed on the majority of low‐grade or well‐differentiated tumors, but less on advanced, invasive tumors. The immunofluorescence staining pattern of cultured cells was cytoplasmic and filamentous. Immunoblotting revealed that the antigen is a Mr54,000 to Mr 56,000 protein which was extracted with difficulty from the cultured cells with Triton X‐100. The antigen was also detected in the culture supernatant by means of ELISA. Our results suggest that the epitope is expressed on cytoskeletons common to a wide variety of mammalian cells at a certain stage of differentiation.
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