Lipopolysaccharide- and lipoteichoic acid-induced tolerance and cross-tolerance: Distinct alterations in IL-1 receptor-associated kinase

Human Toll-like receptor (TLR) 4 and TLR2 receptors recognize LPS or lipoteichoic acid (LTA), respectively. Prolonged exposure of human macrophages/monocytes to bacterial LPS induces a state of adaptation/tolerance to subsequent LPS challenge. Inflammatory gene expressions such as IL-1β and TNF-α are selectively repressed, while certain anti-inflammatory genes such as secretory IL-1R antagonist are still induced in LPS-adapted/tolerant cells. In this report, we demonstrate that LPS-tolerized human promonocytic THP-1 cells develop cross-tolerance and no longer respond to LTA-induced IL-1β/TNF-α production, indicating that disruption of common intracellular signaling is responsible for the decreased IL-1β/TNF-α production. We observe that down-regulation of IL-1R-associated kinase (IRAK) protein level and kinase activity closely correlates with the development of cross-tolerance. IRAK protein levels and kinase activities in LPS-tolerized cells remain low and hyporesponsive to subsequent LPS or LTA challenges. We also demonstrate that THP-1 cells with prolonged LTA treatment develop LTA tolerance and do not express IL-1β/TNF-α upon further LTA challenge. Strikingly, cells tolerized with LTA are only refractory to subsequent LTA challenge and can still respond to LPS stimulation. Correspondingly, stimulation of TLR2 by LTA, although activating IRAK, does not cause IRAK degradation. IRAK from LTA-tolerized cells can be subsequently activated and degraded by further LPS challenge, but not LTA treatment. Our studies reveal that LTA-induced tolerance is distinct compared with that of LPS tolerance, and is likely due to disruption of unique TLR2 signaling components upstream of MyD88/IRAK.