Lowering the Threshold of Lung Innate Immune Cell Activation Alters Susceptibility to Secondary Bacterial Superinfection

Background. Previous studies have shown that the interaction of CD200R, a myeloid inhibitory receptor, with its ligand, CD200, is critical in the control of innate immune activation in the lung. Methods and Results. Using a mouse model of bacterial superinfection following influenza, we show that an absence of CD200R (a negative regulator highly expressed by macrophages and dendritic cells), restricts commensal and exogenous bacterial invasiveness and completely prevents the mortality observed in wild-type mice. This benefit is due to a heightened innate immune response to influenza virus in cd200r knockout mice that limits immune pathogenesis and viral load. In wild-type mice, apoptotic cells expressing CD200 that we believe contribute to the suppressed innate immune response to bacteria dominate during the resolution phase of influenza-induced inflammation. We also show for the first time the presence of a variety of previously unidentified bacterial species in the lower airways that are significantly adjusted by influenza virus infection and may contribute to the pathophysiology of disease. Conclusions. The interaction of CD200 with CD200R therefore contributes to the hyporesponsive innate immune state following influenza virus infection that predisposes to secondary bacterial infection, a phenomenon that has the potential for immune modulation.