Formation of lymph follicles and germinal centers in draining lymph nodes after local injection of phytohemagglutinin and lipopolysaccharide in mice.

Changes in the number of lymph follicles and germinal centers in draining popliteal lymph nodes were investigated in 8-week-old mice injected with either phytohemagglutinin (PHA) or lipopolysaccharide (LPS) into the footpad of the left hind leg. The dose of PHA injected ranged from 10 micrograms to 1 mg, and that of LPS, from 2 to 200 micrograms. In unstimulated animals, the popliteal lymph nodes contained only a small number of germinal centers, and many of the lymph follicles in the nodes were in the form of primary follicles. In the draining lymph nodes, regardless of the dose injected, PHA induced germinal center development in existing primary follicles, but eventually failed to stimulate the formation of new lymph follicles and germinal centers. On the other hand, LPS not only induced germinal centers in the existing follicles, but also stimulated the formation of new primary follicles, many of which then developed germinal centers. The occurrence of new follicles in the LPS-treated lymph nodes was dose-dependent, and LPS appeared to cause de novo formation of follicles.