Potentiation of interferon-mediated inhibition of Chlamydia infection by interleukin-1 in human macrophage cultures.

One mechanism by which interferons (IFNs) can inhibit chlamydial infection is by the induction of the enzyme indoleamine 2,3-dioxygenase (IDO), which restricts the availability of tryptophan, which is required for chlamydial growth. Other immunomodulating agents, including interleukin-1 (IL-1), can interact synergistically with IFNs, resulting in increased IDO activity in macrophages. The objectives of this study were to establish that IL-1 can enhance IFN-mediated inhibition of chlamydial growth by increasing the amount of IDO activity induced by IFNs and to identify immunomodulatory agents in culture supernatants from chlamydia-infected macrophages that interact synergistically with IFNs in restricting chlamydial growth. Monocyte-derived macrophages were treated with IL-1 combined with gamma IFN (IFN-gamma) or IFN-beta. The ability of treated cells to support the growth of Chlamydia psittaci was directly related to the amount of IDO activity induced; as IDO activity increased, so did inhibition of chlamydial growth. Furthermore, concentrations of IFNs were identified at which little IDO activity was induced and chlamydial growth was permitted yet which in the presence of IL-1 resulted in increased IDO activity and restriction of chlamydial growth. The addition of exogenous tryptophan reversed the effect of combined IFN and IL-1 treatment, indicating that IDO activity induced by combined cytokine treatment was responsible for chlamydial inhibition. Supernatants from chlamydia-infected macrophages were capable of potentiating IDO induction by IFN-gamma and of restricting the growth of C. psittaci. Antibody to IL-1 beta neutralized the potentiating effects of supernatants from chlamydia-infected cells on both IDO induction and chlamydial inhibition. Thus, IL-1 produced in response to chlamydial infection may contribute to the elimination of the infection.