Human follicular dendritic cells and vascular cells produce interleukin-7: a potential role for interleukin-7 in the germinal center reaction.

Interleukin-7 (IL-7) has long been known as a potent growth factor in lymphocyte development. However, recent data obtained in vitro revealed additional functions for this cytokine, e.g. IL-7 influences the generation of cytotoxic T cells and NK cells, and in higher concentrations, activates monocytes in a manner similar to bacterial lipopolysaccharide. Furthermore, human tonsillar B cells are able to proliferate upon stimulation by IL-7 and cross-linking of the B cell receptor. Considering the latter role of IL-7 for B cell proliferation, we investigated which cells of the human immune system express IL-7 in vivo. mRNA expression of tonsillar cells was analyzed using the reverse transcription polymerase chain reaction, and protein expression was assessed by immunohistochemical staining of frozen sections. Among a variety of different immune cell types isolated from human tonsils, only follicular dendritic cells (FDC) expressed specific IL-7 products, whereas B and T cells were consistently negative. Immunohistochemical staining of tonsillar sections revealed the expression of immunoreactive IL-7 protein on FDC, and a pronounced expression could also be detected in oral mucosa and vascular endothelial cells. For the latter, we could also demonstrate mRNA expression in primary cultured cells. In light of the previous finding that IL-7 can act as a co-stimulus for the induction of proliferation in tonsillar B cells, these data suggest a role of IL-7 in the germinal center reaction. It is tempting to speculate that FDC may function as regulatory cells in B cell development in the tonsil, as epithelial nurse cells are thought to govern T cell development in the thymus.