Nat. Mater. https://doi.org/10.1038/s41563-018-0271-6 (2018)

Orthopedic implants composed of cobalt-chrome — generally considered biocompatible — can occasionally elicit inflammation and contribute to implant failure. In Nature Materials, Gause and colleagues investigate the ability of cobalt-chrome microparticles to initiate inflammation in mouse models. Micrometer-sized microparticles, much like those frequently seen in patients with worn cobalt-chrome implants, induce caspase-1-independent type 2 immunity, with macrophages being central to this response. Mechanistically, this type 2 immunity is driven by the uptake of microparticles by macrophages, which then activates SYK–BTK signaling, macrophage death and release of the cytokine IL-33. Interestingly, this microparticle-triggered type 2 response seems to be initiated by pathways qualitatively distinct from those associated with helminth infection. Failed human implants show a similar macrophage response and type 2 immunity around implants.