Distinct Lipid A Moieties Contribute to Pathogen-Induced Site-Specific Vascular Inflammation

Abstract

Several successful pathogens have evolved mechanisms to evade host defense, resulting in the establishment of persistent and chronic infections. One such pathogen, Porphyromonas gingivalis, induces chronic low-grade inflammation associated with local inflammatory bone loss and systemic inflammation manifested as atherosclerosis. P. gingivalis expresses an atypical lipopolysaccharide (LPS) structure containing heterogeneous lipid A species, that exhibit Toll-like receptor-4 (TLR4) agonist or antagonist activity, or are non-activating at TLR4. In this study, we utilized a series of P. gingivalis lipid A mutants to demonstrate that antagonistic lipid A structures enable the pathogen to evade TLR4-mediated bactericidal activity in macrophages resulting in systemic inflammation. Production of antagonistic lipid A was associated with the induction of low levels of TLR4-dependent proinflammatory mediators, failed activation of the inflammasome and increased bacterial survival in macrophages. Oral infection of ApoE ^−/− mice with the P. gingivalis strain expressing antagonistic lipid A resulted in vascular inflammation, macrophage accumulation and atherosclerosis progression. In contrast, a P. gingivalis strain producing exclusively agonistic lipid A augmented levels of proinflammatory mediators and activated the inflammasome in a caspase-11-dependent manner, resulting in host cell lysis and decreased bacterial survival. ApoE ^−/− mice infected with this strain exhibited diminished vascular inflammation, macrophage accumulation, and atherosclerosis progression. Notably, the ability of P. gingivalis to induce local inflammatory bone loss was independent of lipid A expression, indicative of distinct mechanisms for induction of local versus systemic inflammation by this pathogen. Collectively, our results point to a pivotal role for activation of the non-canonical inflammasome in P. gingivalis infection and demonstrate that P. gingivalis evades immune detection at TLR4 facilitating chronic inflammation in the vasculature. These studies support the emerging concept that pathogen-mediated chronic inflammatory disorders result from specific pathogen-mediated evasion strategies resulting in low-grade chronic inflammation.

Author Summary

Several human pathogens express structurally divergent forms of lipid A, the endotoxic portion of lipopolysaccharide (LPS), as a strategy to evade host innate immune detection and establish persistent infection. Expression of modified lipid A species promotes pathogen evasion of host recognition by Toll-like receptor-4 (TLR4) and the non-canonical inflammasome. The Gram-negative oral anaerobe, Porphyromonas gingivalis, expresses lipid A structures that function as TLR4 agonists or antagonists, or are immunologically inert. It is currently unclear how modulation of P. gingivalis lipid A expression contributes to innate immune recognition, survival, and the ability of the pathogen to induce local and systemic inflammation. In this study, we demonstrate that P. gingivalis expression of antagonist lipid A species results in attenuated production of proinflammatory mediators and evasion of non-canonical inflammasome activation, facilitating bacterial survival in the macrophage. Infection of atherosclerosis-prone ApoE ^−/− mice with this strain resulted in progression of chronic inflammation in the vasculature. Notably, the ability of P. gingivalis to induce local inflammatory bone loss was independent of lipid A modifications, supporting distinct mechanisms for induction of local versus systemic inflammation. Our work demonstrates that evasion of immune detection at TLR4 contributes to pathogen persistence and facilitates low-grade chronic inflammation.