Mammalian Target of Rapamycin Inhibition in Macrophages of Asymptomatic HIV+ Persons Reverses the Decrease in TLR-4-Mediated TNF-α Release through Prolongation of MAPK Pathway Activation

TLR-4-mediated signaling is significantly impaired in macrophages from HIV+ persons, predominantly owing to altered MyD88-dependent pathway signaling caused in part by constitutive activation of PI3K. In this study we assessed in these macrophages if the blunted increase in TLR-4 mediated TNF-alpha release induced by lipid A (LA) is associated with PI3K-induced upregulation of mammalian target of rapamycin (mTOR) activity. mTOR inhibition with rapamycin enhanced TLR-4-mediated TNF-alpha release, but suppressed anti-inflammatory IL-10 release. Targeted gene silencing of mTOR in macrophages resulted in LA-induced TNF-alpha and IL-10 release patterns similar to those induced by rapamycin. Rapamycin restored MyD88/IL-1R-associated kinase interaction in a dose-dependent manner. Targeted gene silencing of MyD88 (short hairpin RNA) and mTOR (RNA interference) inhibition resulted in TLR-4 mediated 70-kDa ribosomal protein S6 kinase activation and enhanced TNF-alpha release, whereas IL-10 release was inhibited in both silenced and nonsilenced HIV+ macrophages. Furthermore, mTOR inhibition augmented LA-induced TNF-alpha release through enhanced and prolonged phosphorylation of ERK1/2 and JNK1/2 MAPK, which was associated with time-dependent MKP-1 destabilization. Taken together, impaired TLR-4-mediated TNF-alpha release in HIV+ macrophages is attributable in part to mTOR activation by constitutive PI3K expression in a MyD88-dependent signaling pathway. These changes result in MAPK phosphatase 1 stabilization, which shortens and blunts MAPK activation. mTOR inhibition may serve as a potential therapeutic target to upregulate macrophage innate immune host defense responsiveness in HIV+ persons. The Journal of Immunology, 2011, 187: 6052-6058.