C5AR1 and bone cancer: In response to C5a in fact, C5aR1 interacts with TLR2 in osteoblasts, promoting the upregulation of CXCL10 (Kwak et al., 2008; Mödinger et al., 2018), a chemokine that is critical for bone cancer cells recruitment, to support osteoclast differentiation and to promote the formation of osteolytic bone metastases (Lee et al., 2012).