Coinhibitory Molecule PD-1 As a Potential Target for the Immunotherapy of Multiple Myeloma

The adaptive immune system is clearly capable of recognizing and attacking malignant plasma cells in patients with multiple myeloma (MM). However, MM patients evidence severe defects of humoral and cellular immunity, and it is likely that the profound immune dysregulation typical for this malignancy contributes to its eventual escape from natural immune control. One of the factors responsible for the immune dysfunction in MM might be the programmed death 1 (PD-1) protein. The physiological role of PD-1 is to guarantee T-cell homeostasis by limiting T-cell activation and proliferation. Accordingly, binding of the ligand PD-L1 to PD-1 expressed on the surface of activated T cells delivers an inhibitory signal, reducing cytokine production and proliferation. Using the same mechanism, PD-L1/PD-1 interactions have been shown in a number of animal models to confer tumor escape from immune control. Recently, clinical trials have suggested a significant therapeutic impact of PD-1/PD-L inhibition on a variety of solid tumors-for example, by the application of monoclonal antibodies. We show here that based on (1) the broad expression of PD-1 and its ligands in the microenvironment of myeloma, (2) data indicating an important role of the PD-1 pathway in the immune evasion by MM cells and (3) preclinical results providing a strong rationale for therapeutic PD-1/PD-L inhibition in this malignancy, MM may be very well suited for immunotherapy, for example, a monoclonal antibody, targeting PD-1 and/or its ligands.