To rule out whether lymph node colonization is specific and is not due to the high dose of infection, we first monitored albino IFN-b +/Db-luc reporter mice for IFN-b production after low dose Listeria monocytogenes infection (Fig. 3A) . Luciferase expression at 24 hours p.i. peaked in spleen. At 48 and 72 hours p.i. we observed the signal both in spleen and in cervical lymph nodes. At 96 hours p.i. the IFN-b production pattern was almost identical to the 48 hour time point after high dose infection (Fig. 1A) , although the overall luminescent signal was lower (note the different scales). Quantification of luminescence intensity in regions of interest ( Fig. 3B ) in spleen and cervical lymph nodes (Fig. 3A , selected circled areas 1 and 2, respectively) confirmed qualitative in vivo data presented in Fig. 3A .