HGF and plasma cell myeloma: As an example, in multiple myeloma, HGF expression is related to a worse immune response deficiency through the upregulation of indoleamine 2,3-dioxygenase 1 (IDO1) [59], and this could be an important issue to consider, giving the role of PD-1 and PD-L1 in multiple myeloma immune evasion and progression [60] (Figure 2).