Characterization of GYP*Mur and novel GYP*Bun-like hybrids in Thai blood donors reveals a qualitatively altered s antigen

Background and objectives: The Mi(a+) GP(B-A-B) hybrid phenotypes occur with a prevalence of 2%–23% across Southeast Asia. While the s antigen is alleged to be altered, no evidence for specific variants is known. Screening using a monoclonal IgM anti-s mistyped six S‒s+ RBC units as S‒s‒. Further, alloanti-s was identified in an S+s+ patient. Our objective was to investigate the s antigen further. Materials and methods: DNA from 63 Thai blood donor samples PCR-positive for a GYP(B-A-B) hybrid was sequenced with primers spanning GYPB exons 3–4. Flow cytometry was used for semiquantitative analysis of s expression and correlated with the glycophorin genotype. Results: DNA sequencing showed that GYP*Mur was carried by 56/63 samples (88·9%) of which 5/56 lacked normal GYPB: three of these were GYP*Mur homozygotes, one was a compound heterozygote carrying GYP*Mur and a GYP*Bun-like allele (designated GYP*Thai), and the fifth sample carried GYP*Mur and another GYP*Bun-like allele. Seven samples (7/63) were GYP*Thai heterozygotes. IgM monoclonal anti-s (P3BER) did not react with the s antigen carried by GP.Mur or GP.Bun, whereas two IgG anti-s showed enhanced reactivity. Conclusions: We confirmed that GYP*Mur is the most frequent variant in Thai blood donors and also identified GYP*Thai with a frequency of 1·1%. We showed that s antigen on Mi(a+) GP(B-A-B) hybrids is qualitatively altered and should be considered when selecting reagents for phenotyping where such hybrids are prevalent, endemically and in blood centres worldwide.