PD-1 Expression in Chronic Lymphocytic Leukemia and Large B-Cell Richter Transformation (DLBCL-RT): A Distinguishing Feature for DLBCL-RT and Promising Surrogate Marker for Clonal Relatedness

Purpose: The programmed death-1 (PD-1) pathway is crucial in tumor immunity evasion and its blockade has emerged as an effective anti-cancer immunotherapy. PD-1 and PD-L1 expression has not been thoroughly examined in chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) and their associated large B-cell Richter transformation (DLBCL-RT). This study investigates PD-1 and PD-L1 expression and explores its clinicopathological significance in CLL/SLL and DLBCL-RT.

Methods: PD-1 and PD-L1 expression was assessed by immunohistochemistry in 39 CLL/SLL, 15 DLBCL-RT with prior history of CLL/SLL, and 26 DLBCL without prior history of CLL/SLL (22 de novo DLBCL and 4 with prior history of other low-grade B-cell lymphoma). The clonal relationship of DLBCL and the corresponding preceding CLL/SLL was molecularly determined in 10/15 DLBCL-RT. Clinical outcomes were evaluated by Kaplan-Meier analysis.

Results: PD-1 expression in CLL/SLL neoplastic B-cells was weak and restricted to the larger prolymphocytes/paraimmunoblasts within the proliferation centers (PCs) and accentuated PCs of all sizes. Expression was not observed on the background monotonous-appearing small B-cells. Neoplastic B-cells PD-1 expression was highly prevalent in DLBCL-RT and demonstrated increased intensity in comparison to the CLL/SLL prolymphocytes/paraimmunoblasts. In contrast, neoplastic B-cell PD-1 expression was only rarely seen in DLBCL without prior history of CLL/SLL (12/15 vs. 1/26, P<0.0001). In DLBCL-RT, neoplastic B-cell PD-1 expression demonstrated excellent concordance (90%) with the molecularly-defined CLL/SLL clonal relatedness. Strong PD-1 expression was observed in the scattered background small tumor infiltrating T-cells (TIL) in the entire study cohort, and served as an internal control for the PD-1 immunostain. PD-L1 expression was observed on the background immune cells histiocytes and dendritic cells, absent on the CLL/SLL neoplastic B-cells, and rarely seen on the large neoplastic B-cells in DLBCL-RT or other DLBCL (1/15 vs. 1/26, P>0.05). DLBCL-RT prognosis was significantly inferior to the other DLBCL (median overall survival 18.3 vs. 167.3 months, P=0.001) and PD-1 expression was associated with adverse outcome in the combined DLBCL cohort (median overall survival 18.3 vs. 167.3 months, P=0.02).

Conclusions: PD-1 expression is weak and restricted to the PC prolymphocytes/paraimmunoblasts in CLL/SLL and increased on the large neoplastic B-cells in DLBCL-RT, suggesting a biological continuum. It is highly prevalent in DLBCL-RT but rare in other DLBCL. Neoplastic B-cell PD-1 expression is a distinguishing feature for DLBCL-RT and a promising surrogate marker for CLL/SLL clonal relatedness in DLBCL-RT, which have important prognostic and therapeutic implications. The restricted PD-1 expression in PCs may also aid in precise PC evaluation in CLL/SLL.