Neutrophil-mediated Suppression of Influenza-induced Pathology Requires CD11b/CD18 (MAC-1)

Severe influenza virus infection can lead to life-threatening pathology through immune-mediated tissue damage. In various experimental models, this damage is dependent on T cells. There is conflicting evidence regarding the role of neutrophils in influenza-mediated pathology. Neutrophils are often regarded as cells causing tissue damage, but, in recent years, it has become clear that a subset of human neutrophils is capable of suppressing T cells, which is dependent on macrophage-1 antigen (CD11b/CD18). Therefore, we tested the hypothesis that immune suppression by neutrophils can reduce T cell-mediated pathology after influenza infection. Wild-type (WT) and CD11b^-/- mice were infected with A/HK/2/68 (H3N2) influenza virus. Disease severity was monitored by weight loss, leukocyte infiltration, and immunohistochemistry. We demonstrated that CD11b^-/- mice suffered increased weight loss compared with WT animals upon infection with influenza virus. This was accompanied by increased pulmonary leukocyte infiltration and lung damage. The exaggerated pathology in CD11b^-/- mice was dependent on T cells, as it was reduced by T cell depletion. In addition, pathology in CD11b^-/- mice was accompanied by higher numbers of T cells in the lungs early during infection compared with WT mice. Importantly, these differences in pathology were not associated with an increased viral load, suggesting that pathology was immune-mediated rather than caused by virus-induced damage. In contrast to adoptive transfer of CD11b^-/- neutrophils, a single adoptive transfer of WT neutrophils partly restored protection against influenza-induced pathology, demonstrating the importance of neutrophil CD11b/CD18. Our data show that neutrophil CD11b/CD18 limits pathology in influenza-induced, T cell-mediated disease.