Follicular dendritic cell (FDC)-FcγRIIB engagement via immune complexes induces the activated FDC phenotype associated with secondary follicle development

Follicular dendritic cell (FDC)-Fc gamma RIIB levels are up-regulated 1-3 days after challenge of actively immunized mice with Ag. This kinetics suggested that memory cells are not driving this response, prompting the hypothesis that immune complex (IC)-FDC interactions lead to FDC activation. To test this, mice passively immunized with anti-OVA Ab were OVA challenged to produce IC. After 3 days, levels of IC, Fc gamma RIIB, ICAM-1, and VCAM-1 on FDC were analyzed. FDC were also stimulated with IC in vitro, and mRNA for Fc gamma RIIB, ICAM-1, and VCAM-1 was quantified by quantitative RT-PCR. IC labeling in passively immunized WT and Fc gamma RIIB-/- mice revealed five to six FDC-reticula per LN midsagittal section. In WT mice, these IC-bearing FDC-reticula corresponded with FDC-reticula labeling for Fc gamma R11B, ICAM-1, and VCAM-1. Increases in these molecules on IC-stimulated FDC were confirmed by flow cytometry. In marked contrast, in Fc gamma RIIB-/- mice, no increased VCAM-1 or ICAM-1 was seen on IC-bearing FDC-reticula or on purified FDC. Addition of IC in vitro resulted in dramatic increases in mRNA for Fc gamma RIIB, ICAM-1 and VCAM-1 in WT FDC, but not in FDC from Fc gamma RIIB-/- mice, 2.4G2-pretreated WT FDC, B cells, or macrophages. Thus, although FDC-Fc gamma RIIB was not essential for IC trapping, engagement of FDC-Fc gamma RIIB with IC initiated an FDC activation pathway.