Abstract
Neisseria gonorrhoeae is an exclusive human pathogen that evades the host immune system through multiple mechanisms. We have shown that N. gonorrhoeae suppresses the capacity of antigen-presenting cells to induce CD4+ T cell proliferation. In this study, we sought to determine the gonococcal factors involved in this adaptive immune suppression. We show that suppression of the capacity of antigen-pulsed dendritic cells to induce T cell proliferation is recapitulated by administration of a high-molecular-weight fraction of conditioned medium from N. gonorrhoeae cultures, which includes outer membrane vesicles that are shed during growth of the bacteria. N. gonorrhoeae PorB is the most abundant protein in N. gonorrhoeae-derived vesicles, and treatment of dendritic cells with purified recombinant PorB inhibited the capacity of the cells to stimulate T cell proliferation. This immunosuppressive feature of purified PorB depended on proper folding of the protein. PorB from N. gonorrhoeae, as well as other Neisseria species and other Gram-negative bacterial species, are known to activate host Toll-like receptor 2 (TLR2) signaling. Published studies have demonstrated that purified Neisseria PorB forms proteinacious nanoparticles, termed proteosomes, when detergent micelles are removed. Unlike folded, detergent-solubilized PorB, PorB proteosomes stimulate immune responses. We now demonstrate that the formation of PorB proteosomes from structurally intact PorB eliminates the immunosuppressive property of the protein while enhancing TLR2 stimulation. These findings suggest that gonococcal PorB present in shed outer membrane vesicles plays a role in suppression of adaptive immune responses to this immune-evasive pathogen.
Highlights
Neisseria gonorrhoeae is an exclusive human pathogen that evades the host immune system through multiple mechanisms
We show that suppression of the capacity of antigen-pulsed dendritic cells to induce T cell proliferation is recapitulated by administration of a high-molecular-weight fraction of conditioned medium from N. gonorrhoeae cultures, which includes outer membrane vesicles that are shed during growth of the bacteria
Our results suggest that, the native PorB trimer from N. gonorrhoeae can stimulate signaling in some immune cells through activation of Toll-like receptor 2, PorB overcomes this stimulation by profoundly inhibiting dendritic cell–promoted T cell proliferation when presented to cells in its native, properly folded state
Summary
N. gonorrhoeae– conditioned medium inhibits dendritic cell–induced, antigen-specific T cell proliferation. E. coli LPS at these concentrations demonstrated no detectable effect on OVA-pulsed dendritic cell–induced T cell proliferation (Fig. S3) These experiments demonstrated an inhibitory effect on dendritic cell–mediated T cell proliferation when recombinant PorB1B was added to the culture, it did so at concentrations much higher than the levels of PorB1B present in either the Ng-CM or live gonococci that produced an equivalent inhibitory effect (data not shown). We observed more cell-associated PorB1B in cells treated with 5 g/ml of recombinant porin than with live N. gonorrhoeae at an m.o.i. of 1, even though the former caused little inhibition, whereas the latter decreases T cell proliferation by over 50% These data indicate that N. gonorrhoeae likely produces additional inhibitory factors that complicate the comparison between live bacteria and a single purified protein. In contrast to recombinant PorB1B, neither protein impacted the capacity of OVA-pulsed dendritic cells to induce OT-II T cell proliferation (Fig. 4E) [25]
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