Immune responsiveness in Mycobacterium avium ‐infected mice: changes in the proportion of T cell subsets and antibody production during the course of infection

SUMMARY The C57B1/6 susceptible (Beg S ) and its resistant (Beg r ) congenic mouse, previously developed by retrogressive backcrossing, were infected with 1 × 10 6 colony‐forming units (CFU) of Mycobacterium avium and bacterial growth and their immune responses during the early and prolonged periods of infection were examined. There was a high proliferation in the liver and spleen of Beg S mice, whereas no proliferation was observed in the Beg r mice. Similarly, the sizes and weights of these organs were much higher than those of their Beg r counterparts. The size and number of granulomas in Beg r were also found to be higher than those of Beg r . The CD3 + and CD4 + subsets increased dramatically in both mice during the early stage of infection. However, in the later phase of the infection, these populations decreased dramatically in Beg r mice, but not in Beg r mice, resulting in a depression in cell‐mediated immune responses. No significant decrease in cellmediated immune responses was observed in Beg r mice even after prolonged infection. ELISA was performed to determine the antibody levels in both mice, and it was found that serum IgG and IgM levels in Beg s were comparatively higher than those in Beg r mice throughout the period of infection. The Beg gene therefore may have an important role in the maintenance of resistance not only in the early phase but also in the later phase of Myco. avium infection.