Histological progression of follicular centre cell lymphomas to immunoglobulin-producing tumours in two pigs.

Follicular centre cell lymphomas in two crossbred sows were examined histologically, immunologically and ultrastructurally. In one case, most of the involved tissues showed a diffuse growth pattern and follicular structures were present in some lymph nodes. In some areas, immunoblastoid cells were scattered or focal (immunoblastic transformation) and contained only cytoplasmic IgM (CIgM) or both CIgM and cytoplasmic IgG. In the other case, the neoplastic cells grew diffusely and an incompletely affected lymph node showed separate neoplastic follicles as well as direct transition into a diffuse growth pattern, without the stage of accumulated follicles. Scattered immunoblastoid or plasmacytoid cells with CIgM were present in some sections (plasmacytic differentiation). The origin and histological progression of these neoplasms are discussed and it is suggested that the presence of two immunoglobulins in single neoplastic cells could be the result of heavy chain class switch. As these swine lymphomas resembled human lymphomas they may offer useful models for the study of the latter.