Interleukin-35 is upregulated in response to influenza virus infection and secondary bacterial pneumonia

Postinfluenza pneumococcal pneumonia is an important cause of global morbidity and mortality. What causes this increased susceptibility is not well elucidated. IL-35 is a newly described cytokine in infectious tolerance. A murine model was established to study postinfluenza pneumococcal pneumonia and evaluate the role of IL-35 in host defense against postinfluenza pneumococcal pneumonia. Pulmonary IL-35 was rapidly up-regulated during murine influenza infection, which was partially mediated by type I IFN-α/β receptor signaling pathway. Secondary pneumococcal infection led to a synergistic IL-35 response in influenza-infected mice. Clinical analysis showed that IL-35 levels were significantly elevated in the patients with influenza infection compared with healthy individuals and influenza infection could induce IL-35 production from human peripheral blood mononuclear cells. These data suggest that IL-35 contributes to the increased susceptibility to secondary pneumococcal pneumonia at least in part by inhibiting the early immune response.