Alternative Splicing Due to an Intronic SNP Defines a Novel HLA-B44-Restricted-Hematopoiesis-Restricted Minor Histocompatibility Antigen.

Minor histocompatibility antigens (mHAgs) with expression limited to hematopoietic cells represent attractive targets for immunotherapy to induce selective graft-versus-leukemia (GVL) reactions. Here we report the identification of a novel mHAg which is recognized by an HLA-B*4403-restricted CTL clone. Microsatellite allele image analysis of two DNA pools generated from CTL-defined mHAg positive and mHAg negative groups was performed using microsatellite markers set at 100 kb intervals within the segment initially mapped by two-point genetic linkage analysis and detailed mapping of the chromosomal recombinant points. This approach defined a 0.53 Mbp region of chromosome 18q21–22 containing 12 candidate genes potentially encoding the mHAg, although the target gene could not be identified. Subsequently, cDNA expression cloning studies demonstrated that the CTL epitope of interest was encoded by a novel allelic splice variant of XM_209104, hereafter designated as XM_209104-av. Indeed, this gene was found to lie within the region predicted by microsatellite allele image analysis. The immunogenicity of the epitope was generated by differential protein expression due to alternative splicing, which was completely controlled by one intronic single nucleotide polymorphism (SNP) located in the consensus 5′ splice site adjacent to an exon. To our knowledge, this is the first example of a mHAg controlled by a SNP located in a region other than coding sequences. Because the CTL lysed also HLA-B*4402 positive, mHAg positive B-LCLs, this novel epitope peptide can bind to not only HLA-B*4403 but also HLA-B*4402 which is a relatively common HLA-B allele in Caucasian populations. Finally, the finding that the novel XM_209104-av showed low or no expression in normal tissues including resting hematopoietic cells, but significantly higher expression in primary acute leukemia cells, especially those of myeloid lineage, suggest that this novel epitope may be an attractive therapeutic target for immunotherapy not only as a minor H antigen but also as a leukemia-associated antigen.