These three antigenically related porcine coronaviruses with distinct differences in virulence and tissue tropisms (enteric TGEV-A or TGEV-V or respiratory PRCV) provided an ideal model to study interactions between bronchus-associated lymphoid tissues (BALT) and gut-associated lymphoid tissues (GALT) in the induction of mucosal immunity and protection against virulent TGEV challenge (Brim et al., 1995; Saif, 1996; VanCott et al., 1993 VanCott et al., , 1994 .