Isolation of CD35⁺ follicular dendritic cells and its role in the differentiation from B cells to IgA⁺GL7⁺ cells

Follicular dendritic cells (FDCs) are non-hematopoietic cells that are localized in the germinal centers (GCs) of lymph nodes (LNs) and are involved in humoral immunity. FDCs are a rare population that are sensitive to mechanical and chemical stimuli, making their isolation for analysis difficult. In Peyer's Patches, which are the main IgA-inductive sites, FDCs have been reported to be activated by retinoic acid receptor (RAR) and toll-like receptor (TLR) signals to induce IgA production. However, little is known about FDCs in mesenteric LNs (MLNs), although MLNs are also an IgA-inductive site. In this study, we efficiently isolated FDCs as CD35⁺ cells using anti-CD35 antibodies (Abs) and magnetic bead sorting. We found that CD35⁺ FDCs facilitated differentiation from B220⁺ B cells into IgA⁺GL7⁺ GC B-like cells but not IgA⁺CD138⁺ plasma cells. Furthermore, using CD35⁺ FDCs from LPS-resistant C3H/HeJ mice, the generation of IgA⁺GL7⁺ GC B-like cells was not altered significantly between wild-type and LPS-resistant mice. Moreover, the addition of RAR antagonists and agonists revealed that differentiation into IgA⁺GL7⁺ GC B-like cells required the activation of RAR, especially RAR-β, in FDCs. The differentiation of IgA⁺GL7⁺ cells was promoted by FDCs in peripheral LNs as well as MLNs in our in vitro assay. Taken together, these results indicate that magnetic bead sorting with anti-CD35 Abs enable the efficient isolation of FDCs. Our data suggested that CD35⁺ FDCs can support differentiation of B cells into IgA⁺GL7⁺ GC B-like cells in environments that are not limited to MLNs, which can be stimulated by retinoic acid.