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On the nature of serological tissue polypeptide antigen (TPA); monoclonal keratin 8, 18, and 19 antibodies react differently with TPA prepared from human cultured carcinoma cells and TPA in human serum.

A panel of monoclonal antibodies which recognize the rod domains of human keratin 8, 18, or 19 was tested for its capacity to replace TPA antiserum as anchoring antibody in a two-site TPA immuno-radiometric assay kit. All six antibodies could anchor a TPA preparation, with varying efficiencies, when iodinated TPA antiserum monitored antigen binding. Conversely, the soluble keratin 8, 18, and 19 rod complex could efficiently replace the TPA preparation as antigen when TPA antiserum anchored and detected antigen binding. Moreover, in immunoblots the TPA antiserum recognized the rod domains of human keratins 8, 18, and 19. In contrast to the general reaction of the keratin antibodies with a TPA preparation, the reactions with TPA in human serum were antibody and sample dependent. A keratin 19- and a keratin 8-specific antibody anchored TPA in all sera consistently and efficiently, while the other 4 antibodies anchored serological TPA inefficiently. TPA in human serum was found in immunoblots to be somewhat more heterogeneous in size than the standard TPA preparation. The data presented here indicate that TPA in patients' sera is a degradation product of keratins 8, 18, and 19. The significance of these findings is discussed.