MyD88-mediated instructive signals in dendritic cells regulate pulmonary immune responses during respiratory virus infection.

Respiratory syncytial virus (RSV) is the leading cause of respiratory disease in infants worldwide. The induction of innate immunity and the establishment of adaptive immune responses are influenced by the recognition of pathogen-associated molecular patterns by TLRs. One of the primary pathways for TLR activation is by MyD88 adapter protein signaling. The present studies indicate that MyD88 deficiency profoundly impacts the pulmonary environment in RSV-infected mice characterized by the accumulation of eosinophils and augmented mucus production. Although there was little difference in CD4 T cell accumulation, there was also a significant decrease in conventional dendritic cells recruitment to the lungs of MyD88(-/-) mice. The exacerbation of RSV pathophysiology in MyD88(-/-) mice was associated with an enhanced Th2 cytokine profile that contributed to an inappropriate immune response. Furthermore, bone marrow-derived dendritic cells (BMDC) isolated from MyD88(-/-) mice were incapable of producing two important Th1 instructive signals, IL-12 and delta-like4, upon RSV infection. Although MyD88(-/-) BMDCs infected with RSV did up-regulate costimulatory molecules, they did not up-regulate class II as efficiently and stimulated less IFN-gamma from CD4(+) T cells in vitro compared with wild-type BMDCs. Finally, adoptive transfer of C57BL/6 BMDCs into MyD88(-/-) mice reconstituted Th1 immune responses in vivo, whereas transfer of MyD88(-/-) BMDCs into wild-type mice skewed the RSV responses toward a Th2 phenotype. Taken together, our data indicate that MyD88-mediated pathways are essential for the least pathogenic responses to this viral pathogen through the regulation of important Th1-associated instructive signals.