Comparison of gamma interferon-mediated antichlamydial defense mechanisms in human and mouse cells.

Gamma interferon (IFN-gamma)-induced effector mechanisms have potent antichlamydial activities that are critical to host defense. The most prominent and well-studied effectors are indoleamine dioxygenase (IDO) and nitric oxide (NO) synthase. The relative contributions of these mechanisms as inhibitors of chlamydial in vitro growth have been extensively studied using different host cells, induction mechanisms, and chlamydial strains with conflicting results. Here, we have undertaken a comparative analysis of cytokine- and lipopolysaccharide (LPS)-induced IDO and NO using an extensive assortment of human and murine host cells infected with human and murine chlamydial strains. Following cytokine (IFN-gamma or tumor necrosis factor alpha) and/or LPS treatment, the majority of human cell lines induced IDO but failed to produce NO. Conversely, the majority of mouse cell lines studied produced NO, not IDO. Induction of IDO in human cell lines inhibited growth of L2 and mouse pneumonitis agent, now referred to as Chlamydia muridarum MoPn equally in all but two lines, and inhibition was completely reversible by the addition of tryptophan. IFN-gamma treatment of mouse cell lines resulted in substantially greater reduction of L2 than MoPn growth. However, despite elevated NO production by murine cells, blockage of NO synthesis with the l-arginine analogue N-monomethyl-l-arginine only partially rescued chlamydial growth, suggesting the presence of another IFN-gamma-inducible antichlamydial mechanism unique to murine cells. Moreover, NO generated from the chemical nitric oxide donor sodium nitroprusside showed little direct effect on chlamydial infectivity or growth, indicating a natural resistance to NO. Finally, IFN-gamma-inducible IDO expression in human HeLa cells was inhibited following exogenous NO treatment, resulting in a permissive environment for chlamydial growth. In summary, cytokine- and LPS-inducible effectors produced by human and mouse cells differ and, importantly, these host-specific effector responses result in chlamydial strain-specific antimicrobial activities.

Chlamydial IFN-gamma immune evasion is linked to host infection tropism.

Chlamydiae are obligate intracellular pathogens that can exhibit a broad host range in infection tropism despite maintaining near genomic identity. Here, we have investigated the molecular basis for this unique host-pathogen relationship. We show that human and murine chlamydial infection tropism is linked to unique host and pathogen genes that have coevolved in response to host immunity. This intimate host-pathogen niche revolves around a restricted repertoire of host species-specific IFN-gamma-mediated effector responses and chlamydial virulence factors capable of inhibiting these effector mechanisms. In human epithelial cells, IFN-gamma induces indoleamine 2,3-dioxygenase expression that inhibits chlamydial growth by depleting host tryptophan pools. Human chlamydial strains, but not the mouse strain, avoid this response by the production of tryptophan synthase that rescues them from tryptophan starvation. Conversely, in murine epithelial cells IFN-gamma induces expression of p47 GTPases, but not indoleamine 2,3-dioxygenase. One of these p47 GTPases (Iigp1) was shown by small interfering RNA silencing experiments to specifically inhibit human strains, but not the mouse strain. Like human strains and their host cells, the murine strain has coevolved with its murine host by producing a large toxin possessing YopT homology, possibly to circumvent host GTPases. Collectively, our findings show chlamydial host infection tropism is determined by IFN-gamma-mediated immunity.