Minor Histocompatibility Antigen HA-1-Specific Immune Reactions after Hematopoietic Cell Transplantation (HCT).

Minor histocompatibility antigens can be responsible for both GvHD and GvT effects after HCT, depending on their tissue distribution. HA-1 is expressed on hematopoietic cells and several types of solid tumors, but not on GvHD target cells. Two alleles have been defined, of which HA-1^H is immunogenic in the presence of HLA-A*0201. Here, we investigate HA-1-specific immune responses in HLA-A*0201 HA-1^H individuals after related, HLA-matched, HA-1 mismatched HCT.

Methods: A total of 9 patients with hematological malignancies received HCT from HLA-matched, HA-1-mismatched donors, in seven cases following reduced intensity conditioning (RIC). The appearance of HA-1-specific T-cells was monitored with HLA-A*0201/HA-1^H Tetramers and with the IFN-γ Elispot Assay at different time points before and after HCT (days 0, 28, 56, 84 and monthly thereafter). T-cell lines were generated from 2 patients at different time points after engraftment.

Results: HA-1-specific immune reactions were detected in 3 patients in the months following HCT, 2 after RIC and 1 after conventional HCT. Frequencies of HA-1-specific T-cells increased further with time. Both RIC-transplanted patients had secondary AML. The first patient had a complete remission (CR) of his malignancy coincident with a grade I skin GvHD. HA-1-specific T-cells persisted for at least 3 years after engraftment at an average of 0.05% total mononuclear cells. After discontinuation of immunosuppression at day 1000 post HCT, frequencies of up to 0.24% were detected and strong HA-1-specific reactions were observed by Elispot. The second RIC patient required immunosuppression to control acute GvHD. On day 298 after HCT, CD34⁺ donor chimerism (DC) declined to 15% as an early indication of relapse. After reducing cyclosporine, a reversion to 100% DC occurred simultaneously with an increase of HA-1-specific T-cells from 0.05 to 0.14%. The patient went into remission, but again experienced GvHD. Increased immunosuppression was associated with a decrease in HA-1-specific T-cells. A similar decrease in CD34⁺ DC occurred at day 591 and 726 after HCT and was treated with low-dose chemotherapy and reduction of immunosuppression. On these occasions, the level of HA-1-specific T-cells increased from 0.08% to 0.21% and from 0.05% to 0.18% respectively, and was accompanied by the restoration of full DC. Interestingly, the fluctuations of HA-1-specific T-cells observed with Tetramer staining were not detected in the Elispot assay. T-cell lines were generated from PBMC of the 2 patients during periods of different HA-1 specific T-cell frequency. They reacted similarly with HA-1 peptide pulsed target cells and HA-1^H EBV-LCL in the Elispot and Cr-release assay. Tetramer staining revealed up to 77,2% HA-1-specific CD8⁺ T-cells after 6 weeks of culture. Of the 2 patients with conventional HCT, 1 patient with CML was in CR with up to 0.17% HA-1 specific T-cells 557 days after tranplant.

Conclusions: HA-1-specific immune reactions are observed in vivo after HA-1 mismatched HCT. Reactivity to HA-1 increases during the first 3 months after HCT and after reduction/discontinuation of immunosuppression. In one patient, increases of HA-1 specific T-cell frequency were associated with reductions in tumor cell burden (leukemic host CD34⁺ cells) and decreases with reappearance of tumor cells. Tetramer staining was clearly more sensitive than the Elispot assay in monitoring the endogenous HA-1-specific immune response. T cell lines can be established from PBMC independent of the frequency of HA-1-specific T cells.