Anti-Epo antibodies detection. As described in “Materials and methods,” plasma samples from C57Bl/6 mice (A) and from Balb/c mice (B) that had received implants of matrix-embedded Epo⁺ MSCs derived from C57Bl/6 mice were diluted and incubated onto mouse recombinant Epo-coated 96-well plates, and following blocking, anti-Epo antibodies were revealed using anti-mouse Ig-HRP antibody and TMB substrate (n = 4-5, mean ± SEM). OD indicates optical density. The filled circle on the left side of the arrow indicates C57Bl/6-derived MSCs as donor; the filled circle on the right side of the arrow indicates C57Bl/6 mouse as recipient, whereas the open circle on the right side of the arrow indicates Balb/c mouse as recipient.