KDR and rheumatoid arthritis: Interestingly, NRP1 is known to act as a coreceptor for vascular endothelial growth factor (VEGF)-R40 in MOs/macrophage, and VEGFR expression and VEGF-mediated signalling has been associated with MO differentiation into OCs.41 42 Evaluation of RA patient serum revealed that high levels of VEGF correlate with response to TNF (figure 6F).