PDCD1 and monoclonal gammopathy: Understanding the distribution of PD-1/PD-L1 molecules within BM niche of patients with monoclonal gammopathies and the contribution of the immune resistance mechanisms to PD-1/PD-L1 blockade represents a critical step in order to identify the best subset of patients which could benefit from this checkpoint blockade and to give a rationale for new combined therapeutic strategies.