The ELISA assay used in this study was previously described in detail [22, 29, 30] . Briefly, ELISA plates were coated with antigens (100 ml/well) of the following: End-point titers were determined by linear regression analysis of the absorbance values (OD 450) as previously described [32] [33] [34] , with R 2 .0.9 obtained from a series of three-fold dilutions, as the cut-off value was set as 0.3.