Background IL-7 is a potent T cell activating cytokine that has been shown to cause proliferation, survival and differentiation of T cells as well as T cell-dependent activation of myeloid cells in numerous conditions. In inflamed tissues of patients with several autoimmune diseases (RA, pSS, psoriasis) increased IL-7 production by tissue cells and immune cells has been documented. Although reduced serum immunoglobulin levels in IL-7R-deficient individuals suggested that IL-7 might play a role in activation of mature human B cells, direct evidence for this is lacking. Previously, it has been demonstrated that EBV, one of the suggested triggers of pSS, induces TLR7-dependent B cell activation and that EBV-transformed B cells express significant levels of the IL-7R as well as IL-7. Furthermore, we have shown that both intracellular IL-7R and IL-7 expression is up regulated in vitro upon TLR7 triggering of B cells.
Objectives To investigate the potential synergy of IL-7-driven T cell-dependent and TLR7-mediated B cell activation and to assess the additive effects of monocyte/macrophages in this respect.
Methods Isolated CD19 B cells and CD4 T cells from HC (n=7) were co-cultured (1:1) with and without IL-7, TLR7 agonist (TLR7A, Gardiquimod) or the combination of IL-7/TLR7A with and without CD14 monocytes/macrophages (T/B/mono; 1:1:0,1). Proliferation of T and B cells was measured using 3H-thymidine incorporation and by Ki67 expression (FACS analysis). Activation markers (CD19, HLA-DR, CD25) expression were measured by FACS analysis.
Results Exogenously added IL-7 did not activate B cells directly, in line with the absence of surface IL-7R. However, in the presence of T cells, IL-7 activated both T and B cells (Ki67+ CD4 cells from 1.1±0.2% to 14.4±3.7%, p<0.01 and Ki67+ B cells from 1.9±0.3% to 4.1±0.9%, p<0.05). TLR7A induced B cell activation, as measured by increased proliferation (%Ki67 from 1.2±0.2% to 9.3±1.4%) and up regulation of activation markers on B cells, which was facilitated in the presence of monocytes. TLR7-induced B cell activation in T/B or T/B/monocyte co-cultures was not associated with T cell activation. IL-7 added to TLR7A synergistically increased both B cell (TLR7A vs. IL-7/TLR7A; 9.3±1.4% vs. 33.4±7.3%) and T cell proliferation (IL-7 vs. IL-7/TLR7A; 0.8±0.1% vs. 29.2±5.2%), which for B cells again was further increased by monocytes (TLR7A vs. IL-7/TLR7A; 30.2±8.9% vs. 63.0±8.0%). Similar results were observed for activation marker expression on B cells (CD19, HLA-DR CD25) and on T cells (HLA-DR, CD25).
Conclusions IL-7-induced T cell activation and TLR7A-induced B cell activation synergistically activate B cells, which is markedly enhanced by the presence of monocytes. Our results indicate that previously described increased local expression of IL-7 and TLR7 and increased numbers of macrophages could contribute to enhanced lymphocyte activation in patients with pSS.
Disclosure of Interest None Declared
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