Interaction of the HA protein with SLAM suppresses TLR4-mediated IL-12 production, but not IL-6 and TNF-α production. However, IL-12 production mediated by other TLR signaling pathways (i.e., not via TLR4) is unaffected by the HA protein [37] . These observations suggest that SLAM is a coupling factor for TLR4, and that the HA protein inhibits this function. The HA protein also interacts with a C-type lectin, dendritic cell-specific ICAM-3grabbing nonintegrin (DC-SIGN) on DCs, and activates the serine/threonine protein kinase Raf-1 via the Ras signaling pathway. DC-SIGN-mediated Raf-1 activation induces phosphorylation of NF-κBp65 on Ser-276, and its subsequent acetylation. This leads to the enhanced transcription of IL-10 [38] . During infection stage that virus proteins are synthesized de novo, the host cells, such as monocytes and DCs, infected with measles virus show suppressed IL-12 production and TLR signaling, via TLR2, TLR4, TLR7, and TLR9. pDCs infected with the measles virus showed suppressed IFN production and dysfunction of maturation [39, 40] . Both monocytic cell lines U937 and THP-1 and human peripheral blood mononuclear cells infected with measles virus show markedly suppressed TLR2-and TLR4-mediated proinflammatory cytokine induction via NF-κB and AP-1 [41] . However, epithelial cells infected with measles virus show constitutive activation of NF-κB and proinflammatory cytokine production, and these are further enhanced by treatment with TLR agonists such as lipopolysaccharide (LPS). Monocytic cell lines infected with the mumps virus, which also belongs to the Orthomyxoviridae family, show constitutive activation of NF-κB and constitutively high levels of IL-8 production. In monocytic cells infected with the measles virus, LPS-induced ubiquitination (probably K63linked type) of TNF receptor-associated factor 6 (TRAF6) is suppressed and does not form active complexes of TAK1, TAB2, and TRAF6. An ubiquitin-modifying enzyme A20, 8 Mediators of Inflammation which is a host NF-κB negative regulator, is upregulated in measles virus-infected monocytic cells, but not in infected epithelial cells. The promoter region of the A20 gene shares two NF-κB binding sites and a negative regulatory motif, ELIE, which is located upstream of, and adjacent to the two NF-κB binding motifs. Measles virus P protein (phosphoprotein) interacts with the ELIE motif, and activates transcription of A20. P protein is thought to release the suppressed A20 transcription machinery independently of activated NF-κB [42] . The reason for cell-type specific A20 expression is unclear. However, the measles virus V protein, which is formed by RNA editing of the P genome and has an N terminal amino acid sequence identical to that of P protein, does activate NF-κB. The balance of the expression levels and time courses of the P and V proteins may also contribute to the cell-type specific suppression of TLR signaling pathways.