Activation of human macrophages by the endogenous toll-like receptor 4 ligand high molecular weight hyaluronan (101.20)

Rheumatoid arthritis [RA] is caused in part by cytokines secreted by classically activated synovial macrophages. We reported that IFNγ, typically produced by CD4 T cells within synovitis, primes macrophages for activation by high molecular weight hyaluronan [HMW-HA], a major component of joint synovial fluid. HMW-HA is a ligand for both toll-like receptor 4 [TLR4] and CD44. The purpose of this study was to compare the molecular mechanism of HMW-HA (endogenous TLR ligand) versus LPS (exogenous TLR ligand) induced macrophage activation. Transcriptome bioprofiles were determined by cDNA arrays, and the roles of CD44 and TLR4 were determined using blocking antibodies and cytokine measurement by Milliplex®. Significant differences in gene expression profiles were revealed when comparing IFNγ priming versus mock priming under both LPS and HMW-HA activating conditions, although nearly identical gene expression profiles were observed when comparing HMW-HA to LPS stimulation under both mock and IFNγ priming conditions. Blockade of CD44 enhanced inflammatory cytokine secretion while blockade of TLR4 inhibited inflammatory cytokine secretion in response to either HMW-HA or LPS. HMW-HA provides the stimulus required for classical macrophage activation when cells are primed with IFNγ in a response that is essentially identical to the LPS response. Type 1 T cells in proximity to endogenous TLR ligands may provide sufficient signals to initiate classical macrophage activation in RA.