To assess the role of Toll-like receptor (TLR) signaling in host resistance to Mycobacterium avium infection, mice deficient in the TLR adaptor molecule myeloid differentiation factor 88 (MyD88), as well as TLR2(-/-) and TLR4(-/-) animals, were infected with a virulent strain of M. avium, and bacterial burdens and immune responses were compared with those in wild-type (WT) animals. MyD88(-/-) mice failed to control acute and chronic M. avium growth and succumbed 9-14 wk postinfection. Infected TLR2(-/-) mice also showed increased susceptibility, but displayed longer survival and lower bacterial burdens than MyD88(-/-) animals, while TLR4(-/-) mice were indistinguishable from their WT counterparts. Histopathological examination of MyD88(-/-) mice revealed massive destruction of lung tissue not present in WT, TLR2(-/-), or TLR4(-/-) mice. In addition, MyD88(-/-) and TLR2(-/-), but not TLR4(-/-), mice displayed marked reductions in hepatic neutrophil infiltration during the first 2 h of infection. Although both MyD88(-/-) and TLR2(-/-) macrophages showed profound defects in IL-6, TNF, and IL-12p40 responses to M. avium stimulation in vitro, in vivo TNF and IL-12p40 mRNA induction was impaired only in infected MyD88(-/-) mice. Similarly, MyD88(-/-) mice displayed a profound defect in IFN-gamma response that was not evident in TLR2(-/-) or TLR4(-/-) mice or in animals deficient in IL-18. These findings indicate that resistance to mycobacterial infection is regulated by multiple MyD88-dependent signals in addition to those previously attributed to TLR2 or TLR4, and that these undefined elements play a major role in determining bacterial induced proinflammatory as well as IFN-gamma responses.