Ex Vivo Generated Cytotoxic T Cells (CTL) for Adoptive Immunotherapy across MHC Barriers Retain Anti-Leukemic Capacity (GVL) without Eliciting Graft Versus Host Disease (GVHD): Crucial Role of Recipient-Derived Antigen Presenting Cells (APC).

BACKGROUND: While recipient-derived APC have proven to be crucial in mediating GVL after allogeneic hematopoietic stem cell transplantation (HCT), this desired effect seemed to be inevitably linked to the risk of developing severe GVHD. It has been elegantly shown in murine models that delayed donor lymphocyte infusions (DLI) into mixed rather than full hematopoietic chimeras produce dramatically improved GVL effects and virtually no GVHD. However, avoidance of GVHD has been more difficult when this intelligent concept was transferred into the clinical setting. Here we demonstrate that donor-derived T cells of MHC-mismatched origin retain strong anti-leukemic activity and lose nonspecific alloreactivity upon specific in vitro priming and consecutive short term expansion.

METHODS: We chose a fully MHC-mismatched allogeneic bone marrow transplantation model using B10.A mice (H-2^a) as donors and C57BL/6 (H-2^b) mice as recipients to produce either full (controls) or stable mixed hematopoietic chimeras. After full hematopoietic recovery mice were challenged with a lethal dose of a C57BL/6-derived leukemic cell line (C1498) and consecutively injected with either naive DLI, ex vivo generated CTL or PBS only. CTL were generated by specifically priming lymphocytes of MHC-mismatched donor origin on bone marrow-derived dendritic cells (DC) (either donor- or recipient-derived) that had been pulsed with recipient-derived leukemia cell lysates. This was followed by expansion using αCD3/αCD28 coated microspheres.

RESULTS: We show that

1. ex vivo generated CTL of allogeneic origin can mediate anti-leukemic effects only when primed on recipient-derived DC that had been pulsed with leukemia cell lysates (CTL versus controls, p < 0.05 ). In contrast, loading recipient-derived leukemia lysates on donor-derived DC (T cells and DC syngeneic to each other) abrogates the anti-leukemia effect of adoptively transferred CTL in vitro and in vivo.

2. CTL exhibit a strong proliferative response in vitro when stimulated with irradiated leukemic cells of allogeneic origin (C1498), however, this response is significantly weaker (3-fold) when compared to naive DLI. Remarkably, the nonspecific proliferative response (alloreactivity) of CTL is completely abolished when irradiated C57BL/6 splenocytes are used as stimulators.

3. CTL maintain their responsiveness to leukemia in vivo as demonstrated by cure rates from leukemia in up to 40% of mice. Importantly, upon adoptive transfer into leukemia bearing mice after MHC-mismatched HCT, CTL lose their unspecific alloreactivity in vivo as well as proven on microscopic sections of GVH target organs.

4. Anti-leukemia effects of CTL across MHC-barriers are comparable to naive DLI in leukemia bearing mice (40% versus 50% cure rate).

5. CTL are able to convert mixed hematopoietic chimerism into full hematopoietic chimerism (30% donor →100% donor). In contrast, mixed chimerism remains unchanged when CTL are transferred that had been primed ex vivo on donor-derived DC. This data is encouraging in that it provides further evidence that strong GVL effects can be obtained across MHC barriers without eliciting severe GVHD.