Memory T Cells from Minor Histocompatibility Antigen-Vaccinated Donors Improve Engraftment with Low Intensity Conditioning

While allogeneic hematopoietic stem cell transplantation (alloSCT) is largely employed to treat malignant disorders, it can also cure inherited and acquired nonmalignant disorders. For example, inherited diseases such as sickle cell anemia, thalassemias, and varied immunodeficiencies can be cured with alloSCT. Autoimmune diseases can also be cured via the abilities of both the conditioning regimen and alloimmune T cells to eradicate autoimmune T and B cells. And importantly, tolerance to solid organ grafts can be created via transplantation of hematopoietic cells from the organ donor. However, the utility of alloSCT in these settings is limited by the morbidity and mortality of the procedure, including graft-vs-host disease (GVHD). Some progress has been made in lowering the toxicities in these settings. However, these approaches frequently result in mixed chimerism, which may not be sufficient to treat autoimmune diseases, and may lead to unreliable engraftment overall. Previously we reported that in mouse models of alloSCT memory CD8⁺ T cells from mice vaccinated against the single minor histocompatibility antigen (miHA) H60, which is mostly expressed on hematopoietic cells, dramatically improves graft-vs-leukemia (GVL) against H60-expressing leukemias, without causing GVHD. In the present work we have tested the utility of memory CD8 cells reactive to H60 (T_MH60) in promoting engraftment. Following 6 Gy or 8Gy of total body irradiation (TBI), B6.H60 (H-2b, Ly9.1^-, H60⁺) mice were transplanted with C3H.SW (H-2b, Ly9.1⁺, H60^-) bone marrow (BM), with memory T cells from H60 vaccinated mice (T_MH60) containing 3500 tetramer⁺ cells (Tet^H60) or with T_M from unvaccinated mice as negative control. At TBI doses of 8 Gy and 6 Gy, the addition of T_M from H60 vaccinated mice (T_MH60), as compared to T_M from unvaccinated mice, modestly improved engraftment. We next lowered the dose of TBI further. At 5 Gy, recipients of T_MH60 had multilineage engraftment of donor-derived hematopoietic cells in bone marrow (BM), spleen and lymph nodes (LN) nine weeks after transplantation. In contrast, mice with T_M failed to engraft. Importantly, mice did not develop clinical GVHD. At a TBI dose of 4 Gy, recipients with T_MH60 had poor donor engraftment, while T_MH60 recipients treated with an anti-CD40 agonist antibody (FGK45) on day 0 achieved full multilineage engraftment in spleen, LN and BM. In addition we achieved mixed chimerism in some T_MH60 and anti-CD40 treated mice conditioned with 3 Gy. We also tested the utility of T_M targeting the K^b-restricted SIINFEKL peptide derived from OVA (T_M-OVA) in a mouse model wherein OVA expression is restricted to hematopoietic cells. We used B6.OVA mice, which express OVA ubiquitously, as BM donors and created B6.OVA-->B6 BM chimeras (H-2b, Ly9.1^-, OVA⁺). We retransplanted these chimeras with C3H.SW (H-2b, Ly9.1⁺, OVA^-) BM with C3H.SW T_M-OVA, containing 3500 tetramer⁺ cells, or T_M from unvaccinated mice following 5 Gy or 4 Gy of TBI. T_M-OVA, but not T_M promoted full multilineage engraftment in mice conditioned with 5 Gy in blood of mice analyzed two weeks after transplantation. At dose of 4 Gy, T_M-OVA induced full donor engraftment, but only with the addition of anti-CD40. This experiment is ongoing. In sum, our data demonstrate that T_M reactive against a miHA expressed on hematopoietic cells can dramatically promote donor multilineage engraftment without GVHD, and that the effectiveness of these T_M can be augmented by stimulating CD40. Future experiments will test miHA-reactive T_M with chemotherapy-based conditioning, in haploidentical models and the utility of blocking PD-1, which we have found greatly augments GVL mediated by miHA-reactive T_M. This approach holds the promise of achieving consistent donor engraftment without GVHD, a prerequisite for the widespread use of alloSCT in treatment of autoimmune diseases and for tolerance induction in tandem with organ transplantation.