Studies of the ABO and FORS Histo-Blood Group Systems: Focus on Flow Cytometric and Genetic Analysis
Sammanfattning: ABO is the clinically most important blood group system and its antigens are carbohydrate moieties present on the surface of the red blood cell (RBC) but also on other tissues throughout the body. The ABO gene encodes an enzyme, a glycosyltransferase (GT),that adds a terminal monosaccharide to the precursor structure, H antigen, to define the A or B antigens. Blood group O is due to a non-functional GT that leaves the precursor unchanged. Weak expression of ABO antigens can be acquired or be due to polymorphisms in the ABO gene. The aim of this study was to characterise normal/weak/aberrant expression of ABO and related structures on RBCs in light of genetic variations of the ABO gene or acquired changes in the setting of transfusion, pregnancy or stem cell transplantation. The O2 allele has been proposed sometimes to give rise to weak A expression. In 40 group O blood donors heterozygous for the O2 allele no A antigen expression could be detected nor could any GTA activity be noted in enzyme activity testing. (Paper I) Flow cytometry was used to examine genetically-defined ABO subgroups and flow cytometric patterns were shown to correlate with many genetic changes seen at the ABO locus. In addition, this method was found invaluable for semi-quantification of ABO antigens in clinical samples from individuals with weak antigen expression, both acquired or inherited but genetically unexplained. (Paper II) Thirteen new ABO alleles were defined based on an A2-allelic backbone. A combination of genetics, flow cytometry and 3D-modelling gave an insight to possible mechanisms underlying the diminished GT activity in these samples. For instance, the first reported change in the important DVD motif was noted in this cohort. (Paper III) Synthetic A- and B-glycolipids called Functional-Spacer-Linker (FSL) derivatives were used to modify group O RBCs. Different amounts of FSL were used for upload, and flow cytometry was applied to semi-quantify the acquired A or B expression. The purpose of the modification was to create RBCs that mimic the ABO antigen expression of naturally-occurring subgroups and to use these for control purposes. Certain concentrations of FSL gave similar flow cytometric patterns to genetically defined Ax and Bw RBCs included as controls in the study. (Paper IV) The phenomenon of donor-derived group O RBCs acquiring weak A/B expression following transfusion or ABO-incompatible stem cell transplants were examined. By flow cytometry, antigen levels ranging from very low in A1 non-secretor individuals to levels almost equivalent to the Ax control sample in secretor individuals were noted. The major role of adsorption of A/B antigen from plasma as the probable mechanism was supported but our findings also indicate a secretor-independent mechanism.(Paper VI) The enigmatic ABO subgroup Apae was examined and shown to be ABO-independent. The A-like antigen was proven by structural analysis to be the Forssman (Fs) antigen and shown to be expressed in normal haematopoietic tissue. The Fs gene (GBGT1) is thought to be inactive in humans but in Apae individuals a missense mutation in the GBGT1 gene was identified. Transfection studies showed a significant difference in antigen expression between the wildtype and the mutant GBGT1. Naturally-occurring anti-Fs exists in plasma and was shown to cause haemolysis of Fs-positive cells in vitro, hinting at potential risk for intravascular lysis of transfused RBCs. Furthermore, some E. coli strains are known to bind specifically to the Fs antigen, which suggests biological implications. (Paper V) In summary, the combination of a sensitive flow cytometry method for semi-quantification of ABO antigens, genetic analysis and 3D-modelling provide good tools to examine ABO subgroups. The elucidation of the Apae subgroup provided insight into the complex world of glycobiology and established a novel blood group system designated FORS.
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