A Comparison of the Content and Immunohistochemical Patterns of CEA-like Activity in Human Colorectal Tumours and Nude Mouse Xenografts

The immunohistochemical pattern of CEA-like material was examined in sections of five human colonic tumour xenograft lines in nude mice and in the primary tumours from which these lines were derived. Conventional tissue sections were stained by a PAP immunoperoxidase technique, using affinity purified goat anti-CEA as the primary reagent. Normal human and nude mouse tissues were also examined. Saline extracts were made from three of the human tumours and from examples of the respective xenograft lines. The CEA content of extracts was determined by radioimmunoassay. The pattern of CEA-like material was similar in primary tumours and xenografts, but in 3/5 xenograft lines the intensity of specific staining was reduced when compared to that of the respective primary tumour. The three xenograft lines from which extracts were made contained mean values of 22%, 63% and 68% of the CEA activity found in the respective primary tumours, and the data suggested that reduced immunoperoxidase stain intensity seen in some xenografts resulted from the presence of far less antigen than in the corresponding primary tumour. We suggest that some xenograft lines represent subpopulations of cells in the primary tumour which produce less CEA than those of the primary tumour sample examined, although histologically no differences can be identified. No CEA-like activity was detected in normal nude mouse tissues.