Interleukin-17A Contributed to the Damage of Blood-CNS Barriers During Streptococcus suis Meningitis

Vascular Endothelial Growth Factor A Contributes to Increased Mammalian Respiratory Epithelial Permeability Induced by Pasteurella multocida Infection

Pasteurella multocida infection can cause significant zoonotic respiratory problems in both humans and animals, but little is known about the mechanisms used by P. multocida to invade and cross the mammalian respiratory barrier. In this study, we investigated the influence of P. multocida infection on the dysfunction of the respiratory epithelial barrier. In vivo tests in mouse infection models demonstrated that P. multocida infection significantly increased epithelial permeability and increased the expression of vascular endothelial growth factor A (VEGFA) and endothelial nitric oxide synthase (eNOS) in murine tracheae and lungs. In murine lung epithelial cell (MLE-12) models, P. multocida infection decreased the expression of tight junctions (ZO-1) and adherens junctions (β-catenin and E-cadherin) proteins but induced the activation of hypoxia-inducible factor 1α (HIF-1α) and VEGFA signaling. When the expression of HIF-1α is suppressed, the induction of VEGFA and ZO-1 expression by P. multocida infection is decreased. We also found that intervention of HIF-1α and VEGFA signaling affected infection outcomes caused by respiratory bacteria in mouse models. Most importantly, we demonstrate that P. multocida infection increases the permeability of human respiratory epithelial cells and that this process is associated with the activation of HIF-1α and VEGFA signaling and likely contributes to the pathogenesis of P. multocida infection in humans. IMPORTANCE The mammalian respiratory epithelium forms the first line of defense against infections with P. multocida, an important zoonotic respiratory pathogen. In this study, we found that P. multocida infection increased respiratory epithelial permeability and promoted the induction of the HIF-1α-VEGFA axis in both mouse and murine cell models. Similar findings were also demonstrated in human respiratory epithelial cells. The results from this study provide important knowledge about the pathogenesis of P. multocida causing infections in both animals and humans.

IL-17 and IL-17-producing cells in protection versus pathology

IL-17 cytokine family members have diverse biological functions, promoting protective immunity against many pathogens but also driving inflammatory pathology during infection and autoimmunity. IL-17A and IL-17F are produced by CD4⁺ and CD8⁺ T cells, γδ T cells, and various innate immune cell populations in response to IL-1β and IL-23, and they mediate protective immunity against fungi and bacteria by promoting neutrophil recruitment, antimicrobial peptide production and enhanced barrier function. IL-17-driven inflammation is normally controlled by regulatory T cells and the anti-inflammatory cytokines IL-10, TGFβ and IL-35. However, if dysregulated, IL-17 responses can promote immunopathology in the context of infection or autoimmunity. Moreover, IL-17 has been implicated in the pathogenesis of many other disorders with an inflammatory basis, including cardiovascular and neurological diseases. Consequently, the IL-17 pathway is now a key drug target in many autoimmune and chronic inflammatory disorders; therapeutic monoclonal antibodies targeting IL-17A, both IL-17A and IL-17F, the IL-17 receptor, or IL-23 are highly effective in some of these diseases. However, new approaches are needed to specifically regulate IL-17-mediated immunopathology in chronic inflammation and autoimmunity without compromising protective immunity to infection.

IL-18 Signaling Is Essential for Causing Streptococcal Toxic Shock-like Syndrome (STSLS)

Streptococcus suis (S. suis) is an emerging zoonotic pathogen that can cause multiple diseases, including streptococcal toxic shock-like syndrome (STSLS). The S. suis SC-19 strain could cause NOD-like receptor thermal protein domain-associated protein 3 (NLRP3) inflammasome hyperactivation, then induce a cytokine storm and STSLS. Although IL-18 is the downstream effector of NLRP3 signaling, the role of IL-18 signaling on STSLS remains to be elucidated. Thus, il18r1 gene knockout mice were constructed and challenged with the SC-19 strain. Alleviated clinical signs and tissue damages, as well as improved survival were observed in il18r−/− mice compared with the WT mice post-SC-19 challenge. Meanwhile, an obvious decrease in the inflammatory cytokine levels in blood was observed in the il18r-/- mice infected with SC-19. Therefore, IL-18, the downstream effector of NLRP3 inflammasome activation, was responsible for the cytokine storm and STSLS development caused by S. suis, suggesting that IL-18/IL-18Rα signaling could serve as a new target for STSLS.