Development of immune complex trapping: experimental study of lymphoid follicles and germinal centers newly induced by exogenous stimulants in mouse popliteal lymph nodes.

The development of immune complex trapping in newly-induced lymphoid follicles of draining popliteal lymph nodes was investigated in young adult mice, which had been given bilateral injection of hemocyanin (KLH) or phytohemagglutinin (PHA), each absorbed onto alumina. HRP-anti-HRP immune complex was injected into the footpad 1 day before sacrifice. Using three series of semi-serial cryostat sections prepared from each popliteal node, the number of lymphoid follicles in each node was counted, and follicular localization of the in vivo injected and in vitro applied immune complexes in each follicle was determined. By day 5, a large germinal center had developed within each preexisting follicle. A large number of 'new' secondary follicles, each containing a small PNA-positive germinal center, appeared outside pre-existing follicles, from day 5 through day 11 in KLH-treated nodes, and from day 7 through day 14 in PHA-treated nodes. Shortly after their appearance, new secondary follicles showed no in vitro or in vivo trapping, but subsequently, many of the new follicles began to display in vitro trapping, at first weakly but later intensely. Occurrence of in vivo trapping in new follicles took some time and was first recognized when new follicles showed intense in vitro trapping. At day 21 or 25, many of the new follicles showed both in vitro and in vivo trapping. It was concluded that in lymph nodes treated with a stimulant, secondary follicles containing germinal centers can be formed de novo in the extrafollicular zone where the follicular trapping microenvironment is absent, but subsequently the microenvironment capable of trapping immune complexes develops at the site of formation of new follicles.