Antibody endpoint titer determination by ELISA

Endpoint ELISA titers were determined by following the same ELISA protocol for antibody detection, with serum dilutions extending to 25,600 fold. The absorbance was measured at 450 nm, and the optical density (OD) values were graphed using GraphPad Prism V6.05. The endpoint OD cutoff was defined as 3 times of the average background OD value. For example, if the background OD was 0.05, then the endpoint OD cutoff is 0.15. For the samples whose first dilutions were below OD 0.15, their endpoint titers were defined as one dilution up. For instance, the endpoint titer of a sample with OD 0.10 at the first dilution (1:100) was defined at 50. Statistics analysis results are presented as mean with SEM. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001 for αGal positive versus αGal negative vaccines (by unpaired Student t test).