Novel immunohistochemical localization of 28 000 molecular-weight (Mr) calcium binding protein (calbindin-D(28k)) in enterochromaffin cells of the human appendix and neuroendocrine tumors (carcinoids and small-cell carcinomas) of the midgut and foregut

Calbindin-D(28k) is a highly conserved 28 000 (dalton) molecular-weight (Mr) calcium binding protein with broad tissue distribution, yet cell-type- specific expression predominantly in subpopulations of central and peripheral nervous system neurons, distal tubular cells of the kidney, and enteric neuroendocrine cells. A polyclonal antiserum against rat renal calbindin- D(28k) and a monoclonal antibody to calbindin-D(28k) purified from chicken intestine (clone CL-300) were used for immunohistochemical evaluation of formalin-fixed, paraffin-embedded tissues from multiple areas of the human small and large intestines and 93 primary neoplasms of the gastrointestinal tract (foregut, midgut, and hindgut derivatives) and the lung (foregut derivative). Calbindin-D(28k) immunostaining was obtained in a minority of enterochromaffin (neuroendocrine) cells, predominantly of the appendix and small intestine, as well as in autonomic neurons of the neural plexuses. Focal cytoplasmic Golgi-type staining was obtained with monoclonal antibody CL-300 in the appendiceal surface epithelium and dendritic macrophages confined to the appendiceal lymphoid follicles. Epithelial progenitor cells in enteric crypts and absorptive, goblet, and Paneth cells were calbindin- D(28k) negative, while no immunoreactivity was demonstrated in the mucosae of the colon and rectum. Calbindin-D(28k) staining was consistently detected in subpopulations of neuroendocrine phenotypes in midgut (appendiceal/ileal) and foregut (bronchial) carcinoids and small-cell carcinomas, but was absent in adenocarcinomas, squamous cell carcinomas, leiomyomas/leiomyosarcomas, schwannomas, and lymphomas. Our observations suggest that calbindin-D(28k) is a novel adjuvant neuroendocrine marker that is potentially useful in diagnostic tumor immunohistochemistry.