PPE18 protein of Mycobacterium tuberculosis downregulates LPS-induced IgG production by inhibiting nitric oxide synthesis.

Mycobacterial PPE18 is a virulence factor. Previous studies have demonstrated that recombinant PPE18 protein (rPPE18) downregulates inflammation-induced nitric oxide synthase (iNOS) and nitric oxide (NO) production. Given its ability to inhibit iNOS, we investigated whether rPPE18 can also affect IgG secretion, which is modulated by NO levels. Mouse splenocytes and purified B cells were stimulated with lipopolysaccharide (LPS) to induce iNOS-mediated NO as well as IgG and IgM production. The addition of rPPE18 to LPS-stimulated cells reduced NO and iNOS at both the transcript and protein levels. This was accompanied by a drop in total secreted IgG, which could be reversed by the addition of SNAP, an NO donor. Importantly, rPPE18 did not affect secreted IgM levels. These data suggest that rPPE18 interferes with NO-dependent antibody class-switching from IgM to IgG. This was further confirmed by significantly reduced transcript levels of activation-induced cytidine deaminase (AID) in rPPE18-treated LPS-stimulated splenocytes. AID is crucial for class-switch recombination and is regulated by NO levels. These data highlight a novel mycobacterial virulence mechanism, in which rPPE18 inhibits IgG secretion and dampens protective humoral immunity, which is a critical component of the host anti-mycobacterial response.