CD4 and infection: A chronic M. tuberculosis bacterial model was contrasted to infection with an acute respiratory virus, influenza A. A recombinant influenza A virus (rIAV) was chosen that expresses P25 (PR8-P25), the dominant I-Ab-restricted CD4+ T-lymphocyte epitope derived from the M. tuberculosis Ag85B protein, that confers protection against M. tuberculosis as a mucosal vaccine (26).