Background: Alemtuzumab (Campath-1H) is a humanized mAb directed against CD52 that is expressed on all lymphocytes, including CD4⁺FoxP3⁺ regulatory T cells (Treg), and that depletes lymphocytes and other leukocytes, mainly by complement-dependent mechanisms.

Methods: We investigated the influence of alemtuzumab on (i) ex vivo-expanded naturally-occurring Treg (nTreg) that could potentially be used in adoptive cell therapy of allograft rejection, and (ii) nTreg following infusion of alemtuzumab in Indonesian cynomolgus monkeys. (i) Treg were flow-sorted from PBMC and lymph nodes and expanded for two rounds using anti-CD3/CD28 beads, IL-2 and TGF-Β. CD52 expression, binding of alemtuzumab, and killing of cells were compared between freshly-isolated and expanded Treg and effector T cells. (ii) Monkeys undergoing allogeneic heart transplantation received 3 doses of alemtuzumab and were monitored for Treg and serum alemtuzumab levels.

Results: (i) ex vivo-expanded nTreg were characterized by CD25^hi/CD127^low and FoxP3⁺ expression and suppressive capacity by CFSE-MLR. Expanded Treg exhibited progressive downregulation of CD52 expression, no change in CD46 (complement inhibitory factor) expression, absence of alemtuzumab binding and no complement-dependent killing. (ii) In vivo infusion of alemtuzumab resulted in potent depletion of all lymphocytes, but a transient increase in the proportion of circulating Treg. After infusion of alemtuzumab, monkey serum killed fresh PBMC, but not expanded Treg.

Conclusions: Expanded cynomolgus monkey nTreg are resistant to alemtuzumab-mediated, complement-dependent cytotoxicity. Our in vitro data suggest that expanded nTreg can be infused into graft recipients given alemtuzumab without complement-mediated killing. Furthermore, our in vivo data show that alemtuzumab infusion results in transient relative increase in circulating Treg.

« Back to 2013 American Transplant Congress