PDCD1 and plasma cell myeloma: Since multiple immune checkpoint proteins are upregulated on T cells in myeloma-bearing mice (Figure 1), and blocking the PD-L1/PD-1 axis in lymphodepleted animals induces increased expression of LAG-3, TIM-3 and 2B4 checkpoint proteins (Figure 2), we hypothesized that anti-myeloma immunity would be enhanced by blocking combinations of immune checkpoints.