16,17 Mice deficient in tumour necrosis factor-α (TNF-α) or lymphotoxins (LTs) reveal profound defects in FDC development.15,18,19 In addition, other cytokines including IL-4 and IL-6 appear to be associated with FDC development.20,21 In this report, we present evidence that IL-15 enhances the proliferation of human FDCs and regulates chemokine secretion of human FDCs. Interleukin-15 is an IL-2-like T-cell proliferation factor that is required for the generation

of cytotoxic T lymphocytes and natural killer cells.22–24 It is also important in humoral immunity.25–27 Interleukin-15 enhances the proliferation and immunoglobulin secretion of human peripheral B cells and is involved in B-cell lymphomagenesis.28–34 The heterotrimeric IL-15 receptor (IL-15R) specifically binds IL-15. The IL-15 receptor α-chain (IL-15Rα) is the distinctive component for this Smoothened antagonist specific binding, whereas the IL-15 receptor β-chain (IL-2Rβ)

and IL-15 receptor γ-chain (IL-2γ) chains in the receptor complex, which are shared with find more the IL-2 receptor, are involved in signal transduction.35 Unlike IL-2, however, IL-15 is expressed in various cell types including dendritic cells, keratinocytes,36 monocytes,37,38 thymic epithelial stromal cells,39 bone marrow stromal cells40 and fibroblasts.41 The membrane-bound form of IL-15 plays an essential role in proliferation, or apoptosis of various kinds of cells in an autocrine fashion.37,42–44 Previously, we showed that IL-15 is produced by human FDCs and presented on the surface in a membrane-bound form.13 The IL-15 enhances U0126 nmr GC-B-cell proliferation rather than protecting GC-B cells from apoptosis. Furthermore, the level of IL-15 on the surface of FDCs increased following the cellular interaction with GC-B cells. However, the functional role of IL-15 in FDCs has not been investigated. In this study, we show that IL-15 augments the proliferation of human primary FDCs in vitro. The FDCs express the IL-15R complex that is functional

because anti-IL-15 or anti-IL-15R antibodies that block IL-15 signalling reduced FDC proliferation. In addition, blocking of FDC IL-15 signalling reduced FDC secretion of CCL-2, CCL-5, CXCL-5 and CXCL-8, suggesting potentially important roles for recruitment of other cellular components required for GC reaction. Because IL-15 is expressed by FDCs within the GC microenvironment and enhances the proliferation of both GC-B cells and FDCs, IL-15 may contribute to the rapid expansion and formation of the GC structure, suggesting an important role of IL-15 in the humoral immune response. Anti-IL-15 monoclonal antibodies (mAbs) [M110, M111 and M112: immunoglobulin G1 (IgG1)] were kindly provided by Dr R. Armitage (Amgen Inc., Seattle, WA). Anti-IL-2Rβ (Mik-β2) was purchased from BD Biosciences, (San Jose, CA). Mouse IgG1 (MOPC 21; used as an isotype control) was purchased from Sigma (St Louis, MO).