Background - Cromer antigens are carried on decay accelerating factor (DAF, CD55), for which the crystal structure is available. We investigated two samples with an unidentified antibody to a high prevalence antigen and evaluate the location and characteristics of amino acids associated with antigens on the CD55 by 3D modelling.
Materials and methods - Antigen typing and antibody identification were by standard methods. CD55 was sequenced, and Cromer variants were generated using the protein’s crystal structure (1OK3, chain A). Antigen-associated residues and intraprotein interactions were investigated in 3D (Naccess, Protein Interactions Calculator).
Results - The antibody in the sample from a woman of Kashmiri descent was identified as anti-IFC (anti-CROM7). Her RBCs were negative for high-prevalence Cromer antigens including IFC. CD55 sequencing revealed a silent c.147G>A (p.Leu49=) and c.148G>T (p.Glu50Ter) changes, designated CROM*01N.05. The antibody in the sample from a woman of Greek ancestry was only compatible with IFC– RBCs but her RBCs were positive for known high-prevalence Cromer antigens. CD55 sequencing found she was homozygous for c.173A>G (p.Asp58Gly). The high prevalence antigen was named CRAG (ISBT CROM18 or 021018) and the allele designated CROM*01.-18. By 3D analysis, all known antigen-associated residues, including the new CRAG antigen, were exposed at the protein surface. Interactions between antigen-associated residues within the same CD55 domains were
Discussion - Identification of antibodies to high prevalence Cromer antigens can be challenging. The surface exposure of antigen-associated residues likely accounts for their immunogenicity. 3D analysis of CD55 provides insight into previous serologic observations regarding the influence of some Cromer antigens on the expression of others.
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