Article by O. Schulz et al. JEM 2009

The Rockefeller University Press $30.00 J. Exp. Med. Vol. 206 No. 13 3101-3114 www.jem.org/cgi/doi/10.1084/jem.20091925 3101 The intestinal mucosa contains large numbers of mononuclear cells, including macrophages and DCs, which together are believed to play a central role in regulating mucosal innate and adaptive immune responses in both the steadystate and inflammatory setting (Kelsall, 2008). Expression of CD11c and CD11b has been used to distinguish DC subsets from macrophages in the intestine. However, separating subsets of mononuclear cells based on CD11c is complicated because this marker is also expressed on tissue macrophages (Hume, 2008). Furthermore, there is currently little evidence to suggest that subdividing mononuclear lamina propria (LP) cells (LPCs) based on CD11c and CD11b defines populations with distinct turnover rates, origins, and functional properties. Based on the observation that DC can penetrate epithelial monolayers in vitro and a close association of CD11c-expressing cells with the gut epithelium in vivo, Rescigno et al. (2001) put forward the idea of epithelial-associated DC that is capable of capturing luminal bacteria and transporting them into the intestinal LP. Further studies using transgenic mice expressing GFP under the control of the CX3CR1promoter (CX3CR1+/GFP mice) confirmed that a major subset of mononuclear cells in the intestinal LP are capable of extending processes across the epithelial layer into the intestinal lumen (Niess et al., 2005). In vivo invasion-defective Salmonella enterica were associated with CX3CR1-expressing cells in the intestinal LP of transepithelial-proficient, but not transepithelial dendrite-deficient, CX3CR1GFP/GFP mice, suggesting that transepithelial dendrites play an CORRESPONDENCE Oliver Pabst: [email protected] OR William W. Agace: [email protected]