Fig. 3: The mechanism underlying the therapeutic effects of checkpoint inhibitors in multiple myeloma.

In patients with MM, the expression of PD-1 on various subsets of immune cells, including T cells, NK cells, and macrophages, is upregulated. The interaction between PD-1 on immune cells and PD-L1 on MM cells leads to the inhibition of immune cell proliferation and activation, resulting in the formation of an immunosuppressive microenvironment. Moreover, plasmacytoid dendritic cells (pDCs) exhibit high levels of PD-L1 expression, which further hampers the immune killing function when combined with PD-1 on T or NK cells. However, the administration of either PD-1 or PD-L1 inhibitors can disrupt the binding between PD-1 and PD-L1 molecules and restore the cytotoxicity of immune cells against MM.