Follicular dendritic cell-derived antigen and accessory activity in initiation of memory IgG responses in vitro

Follicular dendritic cells (FDC) release Ag in developing germinal centers in the form of immune complex-coated bodies (iccosomes). Iccosomes are endocytosed by B cells, and the B cells process and present the FDC-derived Ag to T cells. By 3 days after Ag challenge, Ab-forming cells producing IgG specific for the iccosomal Ag emerge from the developing germinal centers and home to the bone marrow, where most Ab in a secondary response are produced. In addition to providing Ag, FDC exhibit potent accessory activities that promote B cell proliferation. These observations prompted the hypothesis that both FDC-derived Ag and FDC-derived Ag-independent secondary signal(s) are essential for optimal secondary Ab responses. To test this hypothesis, methods were developed to separate Ag-bearing iccosomes from intact FDC, and we then examined the ability of isolated iccosomes to elicit secondary Ab responses in vitro in the presence and absence of intact FDC. In the absence of FDC, iccosomes bearing the appropriate Ag elicited only minimal levels of specific IgG. Proliferation studies revealed that iccosomes lacked the FDC accessory activity necessary to augment B cell proliferation. When a source of FDC lacking the relevant Ag but exhibiting accessory activity was added to the iccosomal/Ag/lymphocyte mixture, dramatic increases in IgG specific for the iccosomal Ag were obtained (increases were from low ng/ml to microg/ml levels of specific IgG). The results suggest the concept that Ag on FDC or on iccosomes signals B cells through B cell Ag receptor, the iccosome provides these B cells with Ag necessary to process and elicit T cell help, and a secondary signal(s) necessary to optimize the memory response is delivered to B cells by FDC in an Ag-nonspecific fashion.