TLR4 (not TLR2) dominate cognate TLR activity associated with CoCrMo implant particles.

Innate immune reactions to orthopedic implant debris are the primary cause of total joint replacement (TJR) failure over the long term (15-20 years). The role of pathogen associated pattern recognition receptors (i.e., TLRs) in regulating immune reactivity to metal implant particles remains controversial. Do different TLRs (i.e., TLR2 vs. TLR4) activated by their respective ligands in concert with metal implant debris elicit equivalent innate immune responses? In this investigation, our in vitro and in vivo data indicate that Gram-negative PAMPs are more pro-inflammatory than Gram-positive PAMPs. In vitro results indicated TLR4 activation in concert with CoCrMo orthopedic implant debris (CoCrMo/LPS+) challenged primary macrophages resulted in significantly greater inflammatory responses than CoCrMo/PAM3CSK+ (TLR2). Similarly, in vivo results indicated CoCrMo/LPS+ TLR4 challenge induced a twofold increase in inflammation-induced bone resorption (osteolysis) than CoCrMo/PAM3CSK+ (p < 0.01) or CoCrMo (p < 0.03) alone in an established murine calvaria model. This points to a more potent TLR4-based effect of CoCrMo/LPS+ on innate immune responses, that is, IL-1ß, TNF-α, and resulting osteolysis. Differential CoCrMo/LPS+ induced osteolysis compared to CoCrMo/PAM3CSK+, reveals inherent differences in TLR4 versus TLR2 activation which are relevant to (i) how different types of implant debris elicit differential reactivity, (ii) how TLR2 Gram-positive bacteria benefits from less immune activation possibly due to the down-regulation of TLR2 surface expression, that subsequently impacts Gram-positive infections in TJRs, and (iii) how using TLR4 LPS (a Gram-negative agonist) may not accurately model Gram-positive bacteria responses, alone and/or with specific types of implant particles, particularly CoCrMo alloy.