Expression of X hapten immunoreactivity by human and rat adenohypophyseal cells

The monoclonal antibody Hy2D4 was found to label a previously undescribed subset of rat and human anterior pituitary cells. The antibody binding site appears to be a carbohydrate moiety previously named "X hapten." Double-label immunofluorescence studies in both normal rat and postmortem human pituitaries showed that this antigen is distributed on a subset of adrenocorticotropic hormone (ACTH)-positive cells, but is not detectable in cells immunoreactive for growth hormone, prolactin (PRL), thyroid-stimulating hormone, or luteinizing hormone. Since X hapten labeling revealed a biological subdivision of corticotroph cells, it was expected that some ACTH-positive tumors would be immunoreactive, but that tumors of other hormonal types would be negative. Instead, in 21 surgical specimens examined, tumors of all hormonal types were found to show immunoreactivity. To determine whether experimental proliferative changes in the pituitary could explain the shift in the cell type expressing the antigen, PRL-cell hyperplasia was induced in rats through chronic (8-week) exposure to diethylstilbestrol. The fraction of X-positive cells increased in these hyperplastic glands almost ninefold and, as in human adenomas, many non-corticotroph cells expressed the X marker in this model. However, the non-corticotroph cells expressing X were predominantly growth hormone cells, not the proliferative PRL cells. Thus, expression of the antigen does not necessarily imply that a cell is in a proliferative mode. While it is not known what role an altered expression of this antigen might play, the antibody offers a probe into cellular biology of human and experimental pituitary tumors.