Chlamydiae are obligate intracellular pathogens that are sensitive to pro-inflammatory cytokine interferon-γ. IFN-γ-inducible murine p47 GTPases have been demonstrated to function in resistance to chlamydia infection in vivo and in vitro. Because the human genome does not encode IFN-γ-inducible homologues of these proteins, the significance of the p47 GTPase findings to chlamydia pathogenesis in humans is unclear. Here we report a pair of IFN-γ-inducible proteins, the human guanylate binding proteins (hGBPs) 1 and 2 that potentiate the anti-chlamydial properties of IFN-γ. hGBP1 and 2 localize to the inclusion membrane, and their anti-chlamydial functions required the GTPase domain. Alone, hGBP1 or 2 have mild, but statistically significant and reproducible negative effects on the growth of Chlamydia trachomatis, whilst having potent anti-chlamydial activity in conjunction with treatment with a sub-inhibitory concentration of IFN-γ. Thus, hGBPs appear to potentiate the anti-chlamydial effects of IFN-γ. Indeed, depletion of hGBP1 and 2 in cells treated with IFN-γ led to an increase in inclusion size, indicative of better growth. Interestingly, chlamydia species/strains harboring the full-length version of the putative cytotoxin gene, which has been suggested to confer resistance to IFN-γ was not affected by hGBP overexpression. These findings identify the guanylate binding proteins as potentiators of IFN-γ inhibition of C. trachomatis growth, and may be the targets of the chlamydial cytotoxin.
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