(Lab 5). The microplate cytopathic effect method (CPE method) was used to analyze EV71 (or CA16) NTAb titer  and . Serum was inactivated for 30 min at 56 °C. Samples were
serially diluted from 1:8 to 1:2048, mixing with equal volumes of 100 TCID50 EV71 or CA16 viruses. After incubation at 37 °C for 2 h in selleck 96-well plates, rhabdomyosarcoma (RD) cell suspension (final concentration: 1–2 × 105 cells/ml) was added. Cell control, serum control, and virus control were set on each plate, and virus backdrops were set for each test. Tests were considered successful if backdrop results were 32–320 TCID50/well. They were then placed in a CO2 incubator at 35 °C for 7 days. CPEs were observed by microscopy. Neutralizing titers were defined as the highest dilution capable of inhibiting 50% of the CPEs. Neutralization titers ≥1:8 were considered positive
for NTAb. Fifty plasma samples from healthy individuals (provided by Lab 5) with glutamic-pyruvic transaminase (ALT) <25 U and confirmed to be negative for see more HBsAg, HIV antibody, HCV antibody, and syphilis antibody (tests made by kits from Abbot Inc., US) were assayed for EV71–NTAb titers. Eight candidate standards with different levels of neutralizing activities (Nos. J4, J10, J15, J16, N3, N12, N25, and N30) were chosen from these fifty samples (Supplementary Fig. 2). Among them, four plasmas (Nos. J4, J10, J15, and J16) were determined to be negative EV71–NTAb standards and two EV71–NTAb plasmas with titer >1:100
(Nos. N3 and N30) were selected as weak positive EV71–NTAb standards. Two EV71–NTAb plasmas Olopatadine with titers >1:500 (Nos. N12 and N25) were selected as strong positive EV71–NTAb standards. Eight EV71–NTAb standards were lyophilized by a vacuum lyophilizer according to instructions specified in Chinese Pharmacopoeia, 3rd edition  and . Lyophilized standards were kept at −20 °C before use. Samples were blinded and distributed by Lab 1 for collaborative calibration. Samples were reconstituted in 0.2 ml sterile water. Analysis was carried out according to SOPs specific for this collaborative calibration. Three independent assays were performed by Lab 1 to determine CA16–NTAb titer in candidate standards. These were performed using the G-10 virus strain of CA16 provided by Lab 2. Eight EV71 virus strains (1 strain from genotype A, 1 strain from genotype B3 and 6 strains from subtype C4a) were included in this study. BrCr (type A) was isolated from California (US) in 1969 . Genotype B3 strain was adapted to infect mouse brains . Six C4 strains were isolated from an HFMD epidemic region in China between 2007 and 2009. These strains were first verified by EV71 and CA16 NTAb standard serum and specific PCR. Virus titers were between 106.8 and 107.9 TCID50/ml. Except for genotypes A and B, which are genetically distant, VP1 from six strains of C4a subtype showed over 92% homology. The passage background of eight virus strains was clearly documented (Supplementary Fig. 3).