OMENTAL MILKY SPOTS IN THE LOCAL IMMUNE-RESPONSE IN THE PERITONEAL-CAVITY OF RATS

Background: Milky spots have been described as reactive structures, th eir classification varying from inflamed or haematopoietic tissue to l ymphoid organs. In this study we investigated the reactivity of the mi lky spots in the omentum of rats upon induction of a chronic immune re sponse in the peritoneal cavity.Methods: At different time points afte r intraperitoneal administration of Bacillus Calmette-Guerin (BCG), a peritoneal lavage was made, and the omentum and the draining parathymi c lymph nodes were taken out. The cellular composition of these tissue s was examined on the light microscopic level, using a panel of monocl onal antibodies, and also by electron microscopy. Results: During the first 4 months after administering BCG, the number and size of the mil ky spots increased enormously. Separate macrophage, T, and B cell area s were formed, but interdigitating cells and follicular dendritic cell s were not observed. The number of cells in the peritoneal cavity also increased, and the cellular composition showed a strong similarity wi th that of the milky spots. Especially during the onset of the experim ent, most bacteria were observed in the macrophages in the milky spots rather than in the draining lymph nodes. A cellular immune response w as observed in the parathymic lymph nodes but not in the milky spots. Conclusions: Milky spots, either unstimulated or stimulated, should be classified as perivascular infiltrates. They play a role in the initi al clearance of bacteria from the peritoneal cavity. Although the larg e increase in cell number is predominantly caused by immigration of ce lls, the results do support the role of milky spots as a site for loca l proliferation and maturation of especially macrophages and also B ce lls. The obtained data, however, do not support the earlier made assum ption that milky spots function as a secondary lymphoid organ in the p eritoneal cavity. (C) 1996 Wiley-Liss, Inc.