Because AMΦ production of chemokines, such as MIP-2, is an important determinant of PMN migration, we also addressed the role of LPS/TLR4-mediated up-regulation of TLR3 in regulating PMN migration in the lungs. PMNs were counted in the BALF of WT, TLR4−/−, and TLR3−/− mice. In the LPS (Group 3) and Poly I:C (Group 2) only injection groups, MIP-2 caused a slight increase in PMN migration in WT mice when compared with the saline control (Group 1). Sequential injection of Poly I:C at 4 h after the initial LPS injection (Group 4) markedly increased PMN migration in WT mice compared with the other groups. However, sequential challenge with LPS and Poly I:C did not significantly increase PMN migration in TLR4−/− mice compared with WT mice (Fig. 5). Moreover, there was a similar increase in AMΦ after sequential challenge with LPS and Poly I:C in TLR3−/− mice (Group 6) when compared to LPS alone (Group 3). Additionally, the observed MIP-2 expression level in the lung was consistent with PMN migration. Together, these data show the important role of TLR4 signaling and AMΦ MIP-2 expression in mediating PMN migration in response to the up-regulation of TLR3 expression.