TLR4 as a TRIF-biased receptor (P1212)

Abstract We reported previously that the vaccine adjuvant monophosphoryl lipid A (MPLA) could behave as a TRIF-biased agonist of TLR4, with mostly intact TRIF-dependent outcomes but less MyD88/TRIF co-dependent activation of proinflammatory cytokines, and proposed that TRIF-bias was an important component of MPL’s success as a clinically safe immunostimulant. Taking a pharmacological approach, we have now found that synthetic MPLA (sMLA) has approximately 10% the potency of synthetic Lipid A (sLipid A) for all TRIF and MyD88/TRIF co-dependent signaling outcomes tested, revealing that sMLA is not a TRIF-biased agonist of TLR4. We observed instead that both agonists’ potencies for TRIF-dependent outcomes were markedly higher than MyD88/TRIF co-dependent outcomes. This suggests that the receptor, not the agonist, is TRIF-biased. TLR4’s TRIF-bias was not restricted to "highly-inducible" chemokines because potency measurements using TRIF-dependent CD86 showed the same result. Tests of tetra-acylated lipid IVa also showed TRIF-biased signaling by TLR4, indicating that the phenomenon was not unique to hexa-acylated structures such as sMLA and sLipid A. Lower potency TLR4 agonists like MPLA may be useful as vaccine adjuvants because they take advantage of a TRIF-biased receptor system, providing enough signal strength to activate TRIF-dependent adaptive priming with little MyD88/TRIF co-dependent inflammation.