Statistical analysis
Statistical analysis was performed using SPSS 20.0 (IBM, Chicago, IL, USA). Kolmogorov-Smimov test was used to evaluate the normal distribution of the data. Results were expressed as mean ± SD when the data were normally distributed, or as median (quartile) when the data were non-normally distributed. Inter-groups differences were examined by independent t -test when the data were normally distributed, or by Mann-Whitney U test when the data were non-normally distributed. The difference in NK cytotoxicity between groups was analyzed by a general linear model. The difference was analyzed using Pillai’s Trace test when Mauchly’s test of sphericity resulted in a value of P ≤ 0.05, or Sphericity Assumed test when the sphericity test resulted in a value of P > 0.05. A two-tailed P < 0.05 was considered statistically significant.