**2.4 Macrophages**

Macrophages are tightly regulated by microenvironment signals including presence of injured cells, microbial debris, pro/anti-inflammatory cytokine or mechanical forces [19]. Depending on the cues, macrophages tend to polarize into characterized phenotypes like pro-inflammatory or immunomodulatory [20]. Taking advantage of the fundamental homing capability of macrophages to migrate to the injured/inflamed arthritic synovium, macrophages can be exploited as delivery vehicles to target specific macrophage populations to carry payloads.
