Interferons in immunity to chlamydia pneumoniae

Abstract

The cytokine IFN-gamma is the architect behind an amazing immunological program of host resistance to intracellular bacterial and protozoan infections. IFN-gamma activates macrophages, making them into inhospitable habitats for parasites attempting to grow inside them. The family of obligate intracellular Gram-negative bacteria Chlamydia is an example of such pathogens. The overall aim of this thesis was to unravel resistance to infection with the human respiratory pathogen C. pneumoniae. Specific focus was placed on innate immune responses to C. pneumoniae and the regulation and role of IFN-gamma in the outcome of infection. An experimental mouse model of lung infection and a macrophage model of in vitro infection were used for this purpose.

A protective role for infection-induced IFN-gamma in restricting C. pneumoniae growth in vivo was observed, though IFN-gamma was not required for resolution of infection. IL-12 and/or IL-23 was a necessary but not an absolute requirement for expression of IFN-gamma. IFN-gamma-dependent protection was in part mediated by iNOS expression. TNF-alpha, known to be synergistic with IFN-gamma, was not required for restricting Chlamydial growth. Innate immune cells in the lung constituted an important source of IFN-gamma and were essential for restricting C. pneumoniae growth and for containment of bacteria in the lungs. However, NK cells were not implicated in such protective IFN-gamma release. On the other hand, lung macrophages isolated from C.pneumoniae-infected mice expressed IFN-gamma. Moreover, bone marrow-derived macrophages (BMMphi) conferred upon transfer to RAG-1-/-/IFN-gamma-/-mice, enhanced resistance to C. pneumoniae infection via their ability to release IFN-gamma. Innate IFN-gamma was however not required for protection conferred by CD4+ or CD8+ T cells. Innate and T cell-derived IFN-gamma are also non-redundant (complementary) in protesting mice against C. pneumoniae.

C. pneumoniae-infected BMMphi also expressed IFN-gamma in vitro. Such IFN-gamma release was IL-12independent but required instead IFN-alpha/beta and restricted Chlamydial growth. IFN-alpha/beta, and not IFNgamma, was required for iNOS-mediated protection in BMMphi. The molecular details of BMMphi-derived IFNgamma expression revealed a TLR4-MyD88-dependent pathway of IFN-alpha and IFN-gamma induction. Also surprising was the presente of a TLR4- and MyD88-independent, infection-induced NF-kappaB activation and proinflammatory cytokine expression. Phosphorylation of STAT1 during infection was IFN-alpha/beta-dependent, and necessary for increased IFN-gamma expression and for restricting Chlamydial growth. Expression of IFN-gamma and restriction of C. pneumoniae growth also required NF-kappaB activation, but such activation was independent of IFN-alpha/beta, revealing a dual pathway of C.pneumoniae-induced IFN-gamma expression in BMMphi: a TLR4-MyD88-IFNalpha/beta-STAT1 -dependent pathway, and a TLR4-independent pathway leading to NF-kappaB activation.

IFN-alpha/beta was also protective in vivo by cooperating with IFN-gamma for activation of STAT1, which was required for restricting Chlamydial growth. Different from the in vitro situation, IFN-gamma was sufficient on its own for this effect and did not require IFN-alpha/beta for its expression.

In summary, IFN-gamma is important for restricting C. pneumoniae growth. Innate IFN-gamma is protective both in lungs and in BMMphi. IFN-alpha/beta are pivotal in regulating protective responses in BMMphi, including IFNgamma release, but are dispensable for IFN-gamma expression and protection in vivo. This discrepancy may be a qualitative feature in C. pneumoniae pattern recognition by different cell types; lung cells convey the generation of protective, IL-12-driven responses, while IFN-alpha/beta-driven protection in BMMphi is essential.