To investigate the environmental signals directing CD4+ T cell heterogeneity, we adoptively transferred naïve CD90.1+ C7 T cells into B6 hosts and either parked them in the new host without challenge for 7 days or subjected them to antigenic challenge the next day upon infection with an attenuated strain of L.  monocytogenes engineered to secrete the mycobacterial protein ESAT-6 (L.m.-ESAT) containing the cognate antigen for the C7 TCR (Fig. 3 a) (Gallegos et al., 2008). This evidence concerns the gene CD4 and infection.