Mixed chimerism and transplant tolerance are not effectively induced in C3a-deficient mice

• We studied mixed chimerism induction in C3a-deficient mice.
• Mixed chimerism was not effectively established in C3a−/− mice.
• Numbers of HSC were similar in C3a−/− and WT mice.
• C3a is essential for achieving tolerance through mixed chimerism phenomenon.

Mixed chimerism, a phenomenon involved in the development of specific alloantigen tolerance, could be achieved through the transplantation of hematopoietic stem cells into properly prepared recipients. Because the C3a complement component modulates hematopoietic cell trafficking after transplantation, in the present study, we investigated the influence of the C3a deficiency on mixed chimerism and alloantigen tolerance induction. To induce mixed chimerism, C57BL/6J (wild-type strain; H-2Kb; I-E−) and B6.129S4-C3tm1Crr/J (C3a-deficient) mice were exposed to 3 G total body irradiation (day -1). Subsequently, these mice were treated with CD8-blocking (day -2) and CD40L-blocking (days 0 and 4) antibodies, followed by transplantation with 20 × 106 Balb/c (H-2Kd; I-E+) bone marrow cells (day 0). The degree of mixed chimerism in peripheral blood leukocytes was measured several times during the 20-week experiment. The tolerance to Balb/c mouse antigens was assessed based on the number of lymphocytes expressing Vβ5 and Vβ11 T-cell receptor and on skin-graft (day 0) acceptance. Applying our experimental model, mixed chimerism and alloantigen tolerance were effectively induced in C57BL/6J (wild-type) mice, but not in C3a−/− animals. The present study is, to our knowledge, the first to demonstrate that C3a is vital for achieving stable mixed chimerism and related to this induction of transplant tolerance.