Identification of New Hematopoietic Minor Histocompatibility Antigen UTA2-1; Ready for Application in Antitumor Immunotherapy

Abstract 2979

Allogeneic stem cell transplantation (allo-SCT), alone or followed by donor lymphocyte infusion (DLI), is a potentially curative treatment for various hematological malignancies. In an HLA-matched transplantation setting, the therapeutic graft-versus-tumor (GvT) effect is mediated by donor T-cells directed at minor histocompatibility antigens (mHags), which are HLA-bound polymorphic peptides. Since mHags are also involved in the induction of graft-versus-host disease (GvHD), the immunotherapeutic potential of a mHag depends on its hematopoietic tissue restriction. The identification of more relevant mHags, which are presented by frequent HLA molecules and display an equally balanced population frequency, is imperative to enable broad implementation of mHag-based immunotherapy. We now report the discovery of a novel mHag that fulfills all these criteria and thus has evident clinical and immunotherapeutic relevance.

The mHag was identified by analysis of a cloned CD8⁺ cytotoxic T lymphocyte (CTL), isolated from a multiple myeloma (MM) patient at the peak of his clinical response. This response started just after a second DLI following an HLA matched sibling transplantation and resulted in a complete remission that at present persists for over 8 years, without other treatment. The CTL 503A1 was specifically selected for analysis since it displayed strong cytotoxic activity toward a mHag which was presented by the frequent HLA-molecule HLA-A2*0101. Phenotype analyses revealed a phenotype frequency of around 40% in the Caucasoid population, providing a possibility to apply this mHag in immunotherapy in nearly 12% of allo-SCT patients. Using genome wide zygosity-genotype correlation analysis, followed by fine epitope mapping with synthetic peptides, we identified the antigen recognized by this clone as a nonameric peptide encoded by the SNP rs2166807 on the gene C12orf35. This new mHag was designated as UTA2-1. Quantitative PCR experiments confirmed the hematopoietic tissue restriction of gene C12orf35, with high expression on malignant B and T cells. In a detailed analysis exploring the clinical relevancy of UTA2-1, CTL 503A1 lysed not only benign hematopoietic cells but also primary and immortalized multiple myeloma cells without affecting fibroblasts, keratinocytes or stromal cells derived from the original patient. Tetramer analyses executed on original recipient peripheral blood samples taken after allo-SCT and the DLIs, demonstrated significant expansion of UTA2-1 specific T-cells, coinciding with strong clinical responses after allo-SCT and the second DLI.

In summary, UTA2-1 has an ideally balanced population frequency, HLA-A2 restriction, hematopoietic-specific tissue distribution, optimal expression on malignant cells and the clinical capacity to evoke effective T-cell responses during an anti-myeloma effect. With these properties, UTA2-1 is the most clinically relevant mHag identified so far, next to the well-studied mHag HA-1, and has potential of expanding mHag-based adoptive immunotherapy for a great number of patients.