[Malignant lymphoma with medium-sized macronucleolated cells in the dog: involvement of an original cell from the marginal zone of the reactive lymph node].

Among the non-Hodgkin's malignant lymphomas of the dog, which are largely dominated by the centroblastic heterogeneous type, there is an original form of malignant lymphoma which is homogeneous and diffuse, with macronucleolated medium-sized cells. These cells seem to be morphologically very similar to those which constitute the majority population in the marginal zone of the secondary follicle of the lymph node in the dog, and which appear in the course of certain conditions: systemic lupus erythematosus, leishmaniasis, satellite lymph nodes in benign or malignant tumors. The aim of this study was twofold: on the one hand to establish, in the canine species, the identity of the lymphomatous cells and the reactive cells that make up the marginal zone, i.e. the filiation between the hyperplastic marginal zones and the macronucleolated malignant lymphoma with medium-sized cells, and, on the other hand, to compare this type of malignant lymphoma with those which are reputed to originate in the marginal zone in humans, for example the malignant lymphoma of the lymphoid tissue associated with the mucous membranes, and the monocytoid malignant B-cell lymphomas. Ninety four malignant lymphomas were observed between 1989 and 1994 at the Veterinary School in Lyon; these consisted of 71 cases showing medium or high-grade malignancy, 17 cases with small cells, of low-grade malignancy, and 6 cases of mycosis fungoides. Among the 71 cases of medium and high-grade malignancy, 8 were immunoblastic, 5 centroblastic homogeneous, 50 centroblastic heterogeneous, and 8 homogeneous with macronucleolated medium-sized cells. The methods used in these 94 cases were of a morphological type: cytology, histology, transmission microscopy and immunohistochemistry. The cytohistological, ultrastructural and immuno-phenotypical characteristics (CD3-, CIg-, Ki-67- phenotype) of the lymphomatous cells and the cells of the marginal zone were found to be identical, in the dog; this strongly suggests B-lineage cells which do not secrete cytoplasmic immunoglobulins and are not involved in the cell cycle. Finally, these cells seem to us to be morphologically very similar to the minority population described by Van den Oord in the marginal zone of the secondary follicles in the lymph node in humans, in certain reactive situations.