The role of lytic transglycosylases LtgA and LtgD in innate immune recognition and pathogenesis of Neisseria gonorrhoeae

During growth, Neisseria gonorrhoeae releases inflammatory 1,6-anhydro peptidoglycan monomers through the action of lytic transglycosylases LtgA and LtgD. N. gonorrhoeae lacking LtgA and LtgD release peptidoglycan multimers rather than monomers. Because peptidoglycan is a known activator of the innate immune system, we sought to understand how the activity of LtgA and LtgD influenced host responses to N. gonorrhoeae. We tested inflammatory responses to N. gonorrhoeae using isolate FA1090 and FA1090 with ltgA and ltgD deleted, FA1090 ΔltgA/ΔltgD. Both live N. gonorrhoeae and culture supernatants from FA1090 ΔltgA/ΔltgD caused increased production of IL-1β and TNF-α in THP1 cells and primary human blood dendritic cells. Culture supernatants from FA1090 ΔltgA/ΔltgD, which contain multimeric peptidoglycan fragments, were more potent activators of host NOD2 and TLR2, but not TLR4, TLR9, or NOD1. NOD1 was activated equally by peptidoglycan monomers and multimers from N. gonorrhoeae, but NOD2 was activated only by multimers. To explain this, we showed that multimeric peptidoglycan digested by LtgA, which produce anhydro monomers, were poor stimulators of NOD2, while reducing peptidoglycan monomers produced by host lysozyme were potent stimulators of NOD2. Increased TLR2 activation in response to FA1090 ΔltgA/ΔltgD culture supernatants was not the result of activation by peptidoglycan, but the presence of higher levels of TLR2-activating proteins. These data indicate that LtgA and LtgD allow N. gonorrhoeae to evade detection by host TLR2 and NOD2. We next tested the roles of NOD2 as well as LtgA and LtgD in N. gonorrhoeae infection in vivo. Nod2-/- mice infected with FA1090 had similar bacterial burdens and persistence of infection as wild type mice, confirming in vitro data that wild type N. gonorrhoeae interacts minimally with NOD2. When wild type mice were infected with FA1090 ΔltgA/ΔltgD we observed a significantly lower bacterial burden compared to mice infected with FA1090. Competitive co-infection with FA1090 revealed a fitness defect for FA1090 ΔltgA/ΔltgD. There was not difference in infection persistence between FA1090 and FA1090 ΔltgA/ΔltgD, though less mice were successfully infected with FA1090 ΔltgA/ΔltgD than wild type FA1090. Overall our data is supportive of an important role for LtgA and LtgD in pathogenesis.Doctor of Philosoph