CD6-Depletion in the Dog Model: New Insights into Chimerism and Tolerance.

Adoptive immunotherapy with donor lymphocyte infusion (DLI) following bone marrow transplantation may produce a graft-versus-leukemia (GVL) effect after transplantation of T-cell-depleted bone marrow. However, this method is limited by a high risk of GvHD. We studied the occurrence of graft-versus-host disease (GVHD) and chimerism after DLI at various times after CD6-depleted bone marrow transplantation in a canine model of DLA-homozygous donor and DLA-heterozygous littermate recipient. Using a cross reactive mouse-anti-human antibody which recognizes the canine CD6 antigen, a simple and economical depletion method could be established, which makes use of the ability of the CD6 antibody MT-606 to activate the complement cascade. In vitro experiments using MT-606 and rabbit complement showed the effective depletion of CD6-positive cells whereas hematopoietic progenitor cells remained unaffected at the same time. For in vivo studies, DLA heterozygous recipients with one DLA haplotype being identical to the donor were treated with 10 Gy total body irradiation (TBI) and received CD6-depleted bone marrow of their DLA-homozygous littermate donors. In the control group 7 dogs given unmanipulated marrow died of GVHD within 28 days of transplantation. Two dogs showed sustained engraftment without the occurrence of GvHD after the infusion of 1,8 · 10⁸/kg – 2,0· 10⁸/kg CD6-depleted mononuclear marrow cells. The establishment of tolerance to the bone marrow donor was shown by the transplantation of skin from the donor to the host. Transfusion of donor lymphocytes (1.0 · 10⁸ MNCs/kg body weight) on day 3, 7 or 14 induced GvHD in the host. However, with DLI on day 20, only 2 out of 4 dogs developed GvHD, but DLI always converted mixed into complete chimerism. We conclude from this study that the cross reactive CD6 antibody MT606 effectively depletes T cells in the presence of rabbit complement and induces GVH tolerance without jeopardizing engraftment, and the risk of GVHD after DLI is reduced with time after transplantation.