Frequent expression of PD-L1 in BLS-type diffuse large B-cell lymphoma: implications for aggressiveness and immunotherapy

BLS-type diffuse large B-cell lymphoma (DLBCL) denotes an uncommon, aggressive variant of DLBCL presenting initially in bone marrow, liver and spleen without lymphadenopathy or mass lesion. Patients with BLS-type DLBCL present frequently with haemophagocytic syndrome which often leads to early patient demise. Programmed death ligand 1 (PD-L1) plays a negative regulatory role on effector T cells and is an important target of immunotherapy. Assessment of PD-L1 expression in BLS-type DLBCL may carry therapeutic implications and provide mechanistic insights. Standard immunohistochemical analysis for PD-L1 was performed in seven cohorts for this study: (1) DLBCL-not otherwise specified (NOS) (n=201); (2) Epstein–Barr virus (EBV)-positive DLBCL (n=26); (3) thymic (primary mediastinal) DLBCL (n=12); (4) intravascular LBCL (n=3); (5) high-grade B-cell lymphoma, NOS (n=12); (6) BLS-type DLBCL (n=37); and (7) systemic DLBCL involving bone marrow (n=28). We found that PD-L1 was positive in 12.9% of DLBCL-NOS cases, 46.2% of EBV-positive DLBCL, 91.7% of thymic LBCL, none of intravascular LBCL, 8.3% of high-grade B-cell lymphoma-NOS, and 56.8% of BLS-type DLBCL. By comparison, only 14.3% of bone marrow cases involved by systemic DLBCL were positive for PD-L1 (p<0.001). Interestingly, BLS-type DLBCL more frequently showed activated B-cell phenotype (86.5% vs 65.2%, p=0.010), a high Ki-67 proliferative index (97.1% vs 63.3%, p<0.001), MYC overexpression (90.9% vs 56.2%, p=0.023), presence of haemophagocytic syndrome (86.5% vs 4.0%, p<0.001), and poorer overall survival (p<0.001) than DLBCL-NOS. These data suggest that the poor prognosis of BLS-type DLBCL may be explained by both extrinsic tumour microenvironment factors and intrinsic genetic factors of tumour cells, such as PD-L1-associated inactivation of anti-tumour immunity for the former, and MYC pathway activation-related aggressiveness for the latter.