Mixed lymphocyte reaction (MLR) assay
The two-way mouse mixed lymphocyte reaction was conducted as described in our previous study(Diao et al., 2012). In brief, single-cell suspensions of spleens from C57BL/6 and BALB/C were prepared as foregoing mentioned. Each set of 2×105 cells were seed into 96-wells plates in RPMI-1640 with 10% FBS and 50 μM β-mercaptoethanol. Cells were incubated for 4 days with indicated concentrations of PIP in the presence or absence of uridine (100 μM). Cell proliferation was evaluated by MTT as described above and the proliferation of BALB/C spleen cells alone was served as background.