Total RNA was extracted from spleen cells using RNeasy mini kit (Qiagen) according to the manufacturer's instructions. DNA contamination in the total RNA preparation was eliminated using DNase I (Qiagen). RevertAid First Strand cDNA Synthesis Kit (Fermentas) with oligo(dT) primers was used for reverse transcription of purified RNA. PCR was performed using the Promega kit according to the instructions of the supplier. 