RA ST SCID mouse chimera. After a four-week engraftment period, mice were injected i.v. with 2.5 × 105 fluorescently dye-tagged human endothelial progenitor cells (EPCs) while receiving simultaneous intragraft injections of rheumatoid arthritis (RA) synovial fluid (SF) (100 μl) that was either “sham” immunoneutralized with non-specific IgG or immunoneutralized with specific antibody to human Id1. In some experiments, RA synovial tissue (ST) SCID chimeric mice received dye-tagged EPCs while receiving simultaneous intragraft injections of human Id1 (10 nM) or PBS. A) Section from a SCID mouse injected with Id1 or sham immunodepleted RA SF showing significantly elevated EPC migration to the engrafted RA ST injected with “sham” immunodepleted RA SF. B) Approximately 50% fewer EPCs migrated to engrafted RA ST injected with Id1 neutralized from RA SF compared to sham injected RA SF (n = 10 sections from five mice (that is, two sections/mouse)). C) Less than 50% fewer EPCs migrated to engrafted RA ST in chimeric mice when injected intragraft with phosphate-buffered saline (PBS) compared to Id1 (n = 10 sections from five mice (that is, two sections/mouse)). D) Engrafted RA ST in the SCID mouse chimera.