The Y-Specific Gene PRY Is Expressed in Normal Blood Cells as Well as Leukemia Cells and Can Elicit a Specific Antibody Response in Male Recipients of Hematopoietic Stem Cells from Female Donors.

H-Y minor histocompatibility antigens (mHA) are common targets of immune responses following allogeneic hematopoietic stem cell transplantation (HSCT) in male patients who receive stem cells from female donors. These H-Y antigens are encoded by a group of genes located on the non-recombining portion of the Y chromosome. H-Y genes are ubiquitously expressed with 1 to 13% disparity at the protein level with their X homologues. The same portion of the Y chromosome also contains a distinct group of 11 Y-specific genes, for which there are no X homologues. Expression of Y-specific genes is reportedly restricted to the testis thereby limiting the potential relevance of these proteins as immune targets following allogeneic HSCT. However, atypical expression of Y-specific genes has recently been reported in prostate cancer, suggesting that these genes might follow a pattern of expression characteristic of cancer-testis antigens. In this study, we investigated the expression of a representative Y-specific gene, PRY, in male leukemia cell lines, and examined the immunogenicity of this protein in male patients who received allogeneic HSCT from female donors. Using DNAse treated RNA in RT-PCR experiments we showed that PRY gene is expressed in 3 of 6 male leukemia cell lines tested but not in 4 female cell lines. Results were confirmed by southern blotting of PCR products using an internal specific probe. We next assessed PRY expression in normal blood cells collected from 3 male and 3 female donors. In contrast to what has previously been reported, PRY gene was found expressed at low levels in blood cells from all male donors but not from female donors. Although the precise phenotype of cells expressing PRY remains unknown, expression in normal hematopoietic as well as tumor cells suggested that Y-specific gene products could also elicit immune responses after sex mismatched allogeneic HSCT. We used a series of overlapping peptides encompassing the entire sequence of PRY in ELISA assays to examine the antibody response to PRY antigen in male recipients of female transplant. Thus far, 1 of 13 serum samples has been positive for antibody to PRY. This sample, collected approximately one year post-transplant, strongly reacted with a single peptide, indicating that the patient had developed a B cell response to the PRY antigen. This patient with AML had received a HSCT from his HLA identical female sibling. After transplant, he developed acute and chronic GVHD. Remarkably, this patient also developed a strong CD4+ T cell and B cell response to another H-Y antigen, DBY, following HSCT. Studies are ongoing to further characterize the immune response elicited towards this newly defined antigen. Overall, our data indicate that the Y-specific gene PRY is expressed in normal blood cells as well as leukemia cells. Following allogeneic HSCT, PRY can also trigger B cell immunity in male patients with female donors. Further studies are required to determine whether other Y-specific gene products are also immunogenic and constitute a new category of mHA with significant implications in the development of GVL and GVH reactions.