Omental milky spots in the local immune response in the peritoneal cavity of rats.

BACKGROUND: Milky spots have been described as reactive structures, their classification varying from inflamed or haematopoietic tissue to lymphoid organs. In this study we investigated the reactivity of the milky spots in the omentum of rats upon induction of a chronic immune response in the peritoneal cavity.
METHODS: At different time points after intraperitoneal administration of Bacillus Calmette-Guérin (BCG), a peritoneal lavage was made, and the omentum and the draining parathymic lymph nodes were taken out. The cellular composition of these tissues was examined on the light microscopic level, using a panel of monoclonal antibodies, and also by electron microscopy.
RESULTS: During the first 4 months after administering BCG, the number and size of the milky spots increased enormously. Separate macrophage, T, and B cell areas were formed, but interdigitating cells and follicular dendritic cells were not observed. The number of cells in the peritoneal cavity also increased, and the cellular composition showed a strong similarity with that of the milky spots. Especially during the onset of the experiment, most bacteria were observed in the macrophages in the milky spots rather than in the draining lymph nodes. A cellular immune response was 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 initial clearance of bacteria from the peritoneal cavity. Although the large increase in cell number is predominantly caused by immigration of cells, the results do support the role of milky spots as a site for local proliferation and maturation of especially macrophages and also B cells. The obtained data, however, do not support the earlier made assumption that milky spots function as a secondary lymphoid organ in the peritoneal cavity.