In HLA-matched allogeneic hematopoietic stem cell transplantation (SCT), donor T cells can mediate graft-versus-leukemia/lymphoma (GvL) reactivity and graft-versus-host disease (GvHD) by recognition of minor histocompatibility antigens (MiHA). 1-4 Only a minority of MiHA shows hematopoiesis-restricted expression, and donor T cells for these MiHA may induce beneficial GvL reactivity without GvHD. The number of well-characterized MiHA with therapeutic relevance based on hematopoiesis-restricted expression remains limited and only 25% and 40% of recipients transplanted with grafts from sibling and unrelated donors, respectively, are eligible for therapies targeting known hematopoietic MiHA. 3, 4 Thus, in order to increase the efficacy and applicability of cellular therapy for selective GvL induction, more hematopoiesis-restricted MiHA with balanced population frequencies in common HLA molecules must be identified. Here, we investigated the therapeutic significance of a MiHA encoded by ARHGDIB. 5 We demonstrated hematopoiesis-restricted gene expression with the exception of intermediate mRNA expression in endothelial cells and showed that T cells recognized LB-ARHGDIB-1R presented by HLA-B* 07: 02 on primary leukemic cells, but not on [interferon-gamma (IFN-γ)]-treated fibroblasts and keratinocytes. To evaluate potential toxicity against endothelial cells, we tested T cell recognition of LB-ARHGDIB-1R on human umbilical vein endothelial cells (HUVEC) and found only limited reactivity under inflammatory conditions. Furthermore, we demonstrated in vivo targeting of LB-ARHGDIB-1R in eight out of ten patients who were screened for posttransplant specific T-cell responses. In one patient with relapsed lymphoma, high T-cell frequencies were induced after donor lymphocyte infusion (DLI), coinciding with long-lasting anti-lymphoma immunity without GvHD. Our data thus support the relevance of LB-ARHGDIB-1R as a therapeutic target with the potential to induce selective GvL reactivity.

We previously demonstrated that CD8 T cells specific for a MiHA (LB-ARHGDIB-1R) encoded by the ARHGDIB gene were induced in a patient with myelodysplastic syndrome who responded to DLI after HLA-matched allogeneic SCT. 5 LB-ARHGDIB-1R is translated from the normal ARHGDIB transcript (NM_001175) in an alternative reading frame. Since ARHGDIB has been described to be expressed in hematopoietic cells, 6, 7 we investigated the therapeutic value of LB-ARHGDIB-1R to stimulate GvL reactivity after allogeneic SCT without GvHD. We first examined ARHGDIB expression by microarray gene expression analysis using Illumina HT-12 v3/4 BeadChips8 and compared gene expression between (malignant) hematopoietic and non-hematopoietic cells, which were cultured in the absence or presence of IFN-γ to mimic a state of inflammation. ARHGDIB showed strong overexpression in the majority of (malignant) hematopoietic versus (IFN-γ pre-treated) nonhematopoietic cells. The ARHGDIB expression profile was comparable to the strictly hematopoietic HMHA1