RESULTS AND DISCUSSION Crtam/ and Cadm1/ mice have markedly fewer CD4+CD8+ T cells in the intestinal mucosa under steady-state conditions

Previous studies showed that mouse Crtam is expressed on the surface of recently activated CD8+ T cells and NK cells (Arase et al., 2005; Boles et al., 2005; Takeuchi et al., 2009) as well as on a subset of activated CD4+ T cells (Yeh et al., 2008). We asked whether Crtam is expressed on T cells in the intestinal mucosa and mLN. All gut T cells expressed very low to undetectable amounts of Crtam ex vivo. Upon stimulation in vitro, robust expression of Crtam was detected on CD8+ T cells from all tissues examined, including intestinal epithelium and lamina propria, Peyer’s Patches, and mLN (Fig. 1 A). In contrast, only activated CD4+ T cells from the intestinal epithelium strongly expressed Crtam. Intraepithelial CD4+CD8+ T cells also expressed high levels of Crtam, consistent with the recent detection of Crtam mRNA in these cells (Mucida et al., 2013). Cadm1, the ligand of Crtam, is expressed on epithelial cells (Sakisaka and Takai, 2004; Murakami, 2005; Mizutani et al., 2011) and CD8 DCs (Galibert et al., 2005; Poulin et al., 2010), yet the distribution of Cadm1 in the intestinal mucosa has not been determined. To address this, we stained the LPLs from the small intestine and the mLNs with a fusion protein consisting of the extracellular domain of murine Crtam and the Fc portion of human IgG1 (mCrtam-hFc). We found that mCrtam-hFc bound CD11c+ MHCII+ DCs, particularly the CD103+ DC subset (Fig. 1 B). The binding of mCrtamhFc to gut CD103+ DC was abrogated in Cadm1/ mice