One unresolved issue in gut immunity is how the mucosal T lymphocytes are activated and which antigen presenting cell (APC) is critical for regulating this process. We have identified a unique population of APCs that is exclusively localized in the lamina propria (LP). These APCs constitutively expressed the costimulatory molecule CD70 and exhibited antigen presenting functions. After oral Listeria monocytogenes infection, the expansion and differentiation of antigenspecific T cells occurred in the gut mucosa in situ and blockade of CD70 costimulation abrogated the mucosal T cell expansion and effector functions. Thus, a potent CD70-dependent stimulation via specialized tissue-specific APC is required for the expansion and differentiation of gut mucosal T cells after an oral infection.