To Investigate Molecular Mechanisms of CCR2 and CXCR3 Ligand Induction and Its Impacts on Influenza Infection-Mediated Pathogenesis

Influenza virus is the most common respiratory virus which casues seasonal epidemic and pandemic infections. Mechanistic understanding of triangular relationships among high viral load, heavy leukocyte infiltration and strong cytokine storm in the severe viral infection remains unclear. Using mouse models with mild (141 strain), moderate (SOIV strain) and severe (PR8 strain) infections, we find that predominately infiltrating Gr1+CD11b+ myeloid cells in lung are highly associated with disease severity and these cells strongly produce CCR2 and CXCR3 ligands. The results from knockout mice reveal that type I and type II IFNs are responsible for CCR2 and CXCR3 induction. Infiltrating Gr1+CD11b+ myeloid cells are composed of 20% granulocytes and 80% CCR2+ inflammatory monocytes. Next, we attend to understand the relationships of CCR2 ligand induction and excessive accumulation of CCR2+ inflammatory monocyte in severe infection. We observe that 141- and SOIV-infected mice can efficiently eliminate viruses at day 7 post-infection, compared to PR8-infected mice. Thus, mRNA expression of influenza NS1 and IFN is only detected in PR8-infected Gr1+CD11b+ myeloid cells. Mechanistically, impaired viral clearance prolongs production of type I IFN and results in the massive production of CCR2 ligands. Blockage of IFNAR signaling by neutralizing antibody or knockout mice interrupts the recruitment of CCR2+ inflammatory monocytes. Using CCR2-/- mice, we demonstrate that monocyte mobilization is dependent on CCR2 ligand-mediated chemotaxis. CXCR3 chemokine receptors are mainly expressed on CD4 T cells. To date, it is not clear how CXCR3+ CD4 T cells leave lymph nodes and then enter inflamed site to orchestrate anti-viral responses. Our results show that infiltrating Gr1+CD11b+ myeloid cells of lung are the main producers of CXCR3 ligands and generate a concentration gradient to attract CXCR3+ CD4 T cells from lymph compartments into lung. CXCR3+ CD4 T cells produce IFN, which is a strong inducer for CXCR3 ligand expression. IFN plays a crosstalk mediator between CXCR3+ CD4 T cells and CCR2+ inflammatory monocytes. Taken together, recruitment of CCR2+ inflammatory monocytes and CXCR3+ CD4 T cells in inflamed lung are highly associated with infections of viral virulence. Based on our study, CCR2+ inflammatory monocytes play an important role to amplify inflammatory signals especially in high pathogenic influenza infection. Thus, CCR2+ inflammatory monocytes may be applied as a predictive biomarker of influenza virus virulence and pathogenesis in future influenza pandemics.

Project ID:PC10308-1715
External Project ID:MOST103-2320-B182-028-MY3