A novel depleting anti-CD2 monoclonal antibody for induction of mixed chimerism and allograft tolerance in nonhuman primates

Purpose: Selection of T cell–depleting agents for conditioning regimens is critical for inducing allograft tolerance via mixed chimerism. Polyclonal anti–T cell antibodies can produce broad immune depletion and global immune modulation, whereas monoclonal antibodies typically provide more targeted but potent T cell depletion. In prior studies, horse anti-thymocyte globulin (ATGAM) enabled long-term renal allograft tolerance. However, substitution with rabbit anti-thymocyte globulin (Thymoglobulin) resulted in inferior outcomes, including donor-specific antibody (DSA) development and chronic rejection. Here, we evaluated immune modulation and chimerism induction using a novel depleting anti-CD2 monoclonal antibody (mAb).
Methods: MHC-mismatched cynomolgus macaques (n=5) underwent conditioning with 1.5 Gy total body irradiation, peri-transplant Bcl-2 inhibition, 7 Gy thymic irradiation, and infusion of anti-CD2 mAb. Animals then underwent combined kidney and bone marrow transplantation (CKBMT) and received anti-CD154 (2–4 doses of 20 mg/kg during the first two post-transplant weeks) and a 28-day course of cyclosporine, after which immunosuppression was withdrawn. Lymphocyte subsets and chimerism were assessed by flow cytometry. Outcomes were compared with animals receiving ATGAM (n=8) or Thymoglobulin (n=6) under identical conditioning protocols.
Results: Overall depletion and recovery of T cell subsets were similar across groups, except for CD8 effector memory T cells (TEMs), which recovered more rapidly in Thymoglobulin-treated animals than in ATGAM or anti-CD2 recipients. All groups demonstrated regulatory T cell enrichment, with no differences in B cell or NK cell depletion or recovery. Compared with Thymoglobulin, both anti-CD2 and ATGAM recipients achieved higher and more durable myeloid chimerism, with greater peak levels and higher time-averaged chimerism. Lymphoid chimerism approached 10% in all anti-CD2–treated animals and persisted up to one month post-transplant, whereas Thymoglobulin-treated animals exhibited lower peak lymphoid chimerism (<10% in 4/6 animals) with rapid decline after day 15. This decline temporally correlated with CD8 TEM recovery. There was no difference in myeloid or lymphoid chimerism induction between ATGAM and anti-CD2.
Conclusions: This novel anti-CD2 mAb induces higher and more durable myeloid and lymphoid chimerism than Thymoglobulin and achieves chimerism comparable to ATGAM. These effects appear to be associated with prolonged depletion of CD8 TEMs. While long-term graft outcomes are still under evaluation, the superior chimerism induction observed with anti-CD2 suggests the potential for improved transplant outcomes compared with Thymoglobulin.