Reduced expression of cyclin-dependent kinase inhibitor p27Kip1 in feline lymphoma.

Expression of p27Kip1 was identified in feline lymphoid tissues by immunohistochemistry. In normal lymphoid tissues, p27Kip1 was detected as a distinct nuclear stain in lymphocytes of the follicular mantle zone and interfollicular small lymphocytes, whereas activated lymphoblasts in the germinal center were negative. Lymphoid hyperplasia was similarly immunolabeled but with an expanded mantle zone and marginal zone of p27Kip1-reactive lymphocytes. Both T- and B-cell lymphomas lacked p27Kip1 immunolabel and were determined to be proliferative based on immunohistochemical detection of the Ki-67 antigen. Scattered p27Kip1-immunolabeled lymphocytes were detected throughout the lamina propria of most specimens characterized as lymphoplasmacytic enteritis. The results of this study suggest that the antiproliferative effect of the cell cycle regulator p27Kip1 is abrogated in feline lymphoma, presumably allowing cells to bypass the G1-S checkpoint of the cell cycle.