Colonial Morphotype as a Determinant of Cytokine Expression by Human Monocytes Infected with Mycobacterium avium

Mycobacterium avium is an intracellular pathogen that causes disseminated infection in patients with AIDS. Colonial morphotype (smooth-transparent (SmT) vs smooth-domed (SmD)) is a key determinant of virulence in mice and the capacity for replication in human monocytes. Some cytokines (IL-1 and IL-6) promote, whereas others (IFN-γ and TNF) inhibit intracellular M. avium growth. The specific factors that determine virulence of M. avium, however, are not clear. In this study, we examined cytokine expression by human monocytes stimulated with isogeneic cloned isolates of M. avium. Monocytes were prepared from healthy donors and cultured with or without isogeneic M. avium for up to 7 days. Cytokine levels (IL-1, IL-6, and TNF-α) in monocyte supernatants and cell lysates were measured by immunoassay using an ELISA. The expression of cytokine mRNA by monocytes infected with M. avium also was determined by extracting total RNA and subjecting it to Northern blot analysis. Optimal cytokine release occurred at 24 h. SmD induced higher levels of the following cytokines in supernatants than SmT: IL-1α (140 plusmn; 32 (mean plusmn; SE) vs 47 plusmn; 16 pg/ml, p <0.02), IL-1β (4.0 plusmn; 0.9 vs 1.7 plusmn; 0.5 ng/ml, p <0.01), and TNF-α (2725 plusmn; 546 vs 1464 plusmn; 409 pg/ml, p <0.01). IL-6 production was comparable for both strains. SmD and SmT isolates induced comparable levels of steady state mRNA for IL-1β, TNF, and IL-6. Pulse-chase analysis indicated that differences in cytokine expression between SmT and SmD occurred in monocyte lysates at the earliest time point (immediately after pulse-labeling). The dissociation of the expression of specific mRNA from production of IL-1 and TNF suggests that translational capacity for the expression of certain cytokines was reduced by the more virulent SmT. Differential induction of cytokine may be a factor in the pathogenicity of M. avium strains isolated from patients with AIDS.