The MEK/ERK pathway plays an important role in APRIL-induced PD-L1 expression in MM cells. (A-B) PD-L1 expression was examined by flow cytometry (A) and immunoblotting (B) in indicated RPMI 8226 transfectants. (C) PD-L1 mRNA expression was examined in BCMA knockdown (MM1R-TRIPZ-BCMA and H929-TRIPZ-BCMA) vs control MM cells by real-time qRT-PCR. Fold changes of PD-L1/18S to control were shown. (D) MM cells were treated with APRIL and/or U0126 molecules (100 nM) for 4 hours. PD-L1 expression was examined using real-time qRT-PCR; (F) PD-L1, pERK, and pMEK was examined by immunoblotting in indicated cells. (E) Serum-starved MM1S cells were pretreated with U0126 (100 nM) followed by APRIL stimulation. Cell lysate was collected and subjected to immunoblotting with indicated Abs. Shown is mean ± SD from 3 independent experiments. *P < .05; **P < .001; by unpaired 2-sided Student t test.