Follicular dendritic cell and ICCOSOMES in germinal center reactions.

The development and destination of immune complex coated bodies (ICCOSOMES) was investigated at the ultrastructural level in the germinal centers (GC) of murine lymph nodes after secondary-immunization with horseradish peroxidase. Immediately after a booster injection, large amounts of immune complex (IC) were trapped on the surface of the cytoplasmic extensions of follicular dendritic cells (FDC). Cytoplasmic extensions, consisting mainly of FDC processes with associated IC, formed complex lamellar labyrinth-like structures (LBS). We describe here morphological changes of LBS. Lamellar or stationary LBS before the injection sequentially changed into distorted, filiform, glomerated and transitional LBS. Around day 1, IC-coated cytoplasmic extensions developed into glomerated LBS (G-LBS) which were invaginated into their own cytoplasmic ends, which swelled up to several microns in diameter. Around day 2, tufts of the glomerated LBS were unlaced and dispersed as (ICCOSOMES) in the interstitium. Some ICCOSOMES were endocytosed by germinal center cells (GCCs) and transported to the Golgi apparatus and perinuclear space. These ICCOSOME-bearing GCCs were B220-positive and some produced specific antibody (spAb) in the perinuclear space indicating they are in the B cell lineage. ICCOSOME-ingesting GCCs are thought to be able to process the antigen and present it to T cells. This may be important in the development of plasma cells and memory cells which developed from the GC B cells.