Background. Single antigen bead testing is currently accepted as standard practice for evaluating HLA alloantibodies. However, it is well established that the expression of HLA antigens, on beads, is not representative of actual cellular expression. It is well recognized that the cellular surface expression of certain HLA loci are expressed at relatively low levels. As such, single antigen bead data may overrepresent the impact of HLA antibodies for certain loci. Importantly, for clinical assessments, the cell surface expression of target donor HLA antigens is not routinely considered unless it is a known null allele. Methods. Using a panel of variably reactive DQ4 allosera, we have evaluated DQ4 antigen assembly and surface expression on cell lines and in transfectants carrying DQ4 variants. Results. A single amino acid change at residue 137 in the α2 domain of the DQα chain can significantly affect the stability of the αβ heterodimer and result in the reduced surface expression of the affected DQ antigens, resulting in changes in anti-HLA antibody reactivity. Conclusions. A single amino acid residue in the DQα chain can influence the expression of a DQ4 molecule. Importantly, such surface expression variability does significantly impact allo-antibody reactivity. As a result, amino acid sequence variations in the α2 and β2 domains of the HLA class II serological groups and alleles within serological groups should be further investigated to understand the scope of allelic expression variability. Identifying low surface expressing HLA antigens/alleles might facilitate the understanding of certain discordant clinical observations.
