Avidity as a parameter for diphtheria vaccine efficacy

The avidity of antibodies (Abs) to diphtheria Toxin (DT), toxoids (DTo) and the DT binding site (DTBS) was investigated in sera from guinea bigs vaccinated with various DTo. Measurement of avidity by the thiocyanate procedure revealed that, the maturation patterns of Abs to the corresponding DTo were not much different whereas the maturation patterns of Abs to DT were different. Differences up to 20 folds were observed in the affinity to the DTBS as measured by the tissue culture procedure and expressed as equilibrium constant (K). The avidity of Abs to either the corresponding DTo or the DT could not be correlated to the vaccine efficacy, whereas the vaccine efficacy could be ranked according to its affinity to the DTBS. This may be explained in terms of the specificity of measurement in each procedure. The thiocyanate procedure measures the average avidity to complex antigens with several epitopes whereas the tissue culture procedure allows determination of the affinity of Abs to the DTBS. It is concluded that in assessment of new vaccines, attention should be given to including measurement of the avidity of Abs to known protective epitopes rather than to complex antigens or a mixture of antigens.