Quantitative and thermodynamic study of weak A erythrocyte phenotypes]

The analysis of more than 140 "weak A" samples: A3, Ax, Aend, Am, Ay and Ael, support the classical distinction between each subgroup which has been established on serological and genetical data. Accordingly, a valuable classification of these rare phenotypes must take into account, (i) the mode of inheritance, (ii) the agglutination pattern of the RBC by anti-A reagents, (iii) the presence or absence of soluble A substances in the saliva of secretors. The question is then open to know if such related erythrocytic antigens, whose specificity appears to be very similar, could be described on a quantitative basis or on qualitative structural variations. Evidence for quantitative differences was first demonstrated by a gradual decrease in the standard agglutinability of "weak A" RBC with human anti-A (B) sera, from A3 red cells (63 +/- 10%) to Ax (33 +/- 10%), Aend (10 +/- 5%) then Am, Ay and Ael (0%), and secondly by direct measurement of A antigen site densities, the mean values being respectively 35.10(3) A sites/RBC (A3); 4.8 10(3) (Ax); 3.5 10(3) (Aend) and 0.7 10(3) (Am, Ael). Further investigation on A3, Ax and Aend RBC agglutinability lead also to the demonstration of a large heterogeneity in the A antigenic content of red cells inside one individual sample. The most striking result was obtained with Aend phenotypes which appeared like A + O transmitted mosaicisms. However, heterogeneity was also observed, but to a lesser extent, among A3 and Ax RBC. The significance of this heterogeneity is discussed and used to explained the typical picture of agglutinability commonly observed with such red cells and anti-A antibodies. Qualitative difference were also studied by estimation of equilibrium constants (Ko) and thermodynamic parameters (delta Fo, delta Ho and delta So) associated with the binding of rabbit 125I-IgG anti-A molecules onto A RBC determinants. Only small variations of thermodynamic parameters were observed between each subgroup, but the high Ko values (greater than 10(8)M-1) measured, strongly suggest that "weak A" RBC determinants would process a common antigenic structure of the type: alpha-GalNAc (1 leads to 3) [alphaLFuc (1 leads to 2) beta Gal. However, the small differences of reactivity observed from one sample to an other could be related to slight variations in tridimensional configurations of oligosaccharides chains bearing the A specificity, associated with their variable antigenic content.