PD-1 and PD-L1 Immunohistochemistry as a Diagnostic Tool for Classic Hodgkin Lymphoma in Small-volume Biopsies.

It is becoming increasingly important to obtain detailed diagnostic information on small-volume tissue biopsies, such as core needle biopsies. This is particularly crucial in the workup and diagnosis of classic Hodgkin lymphoma (CHL) and other morphologically similar lymphomas such as T-cell/histiocyte-rich large B-cell lymphoma (THRLBL), where small-volume lymph node biopsies often represent the frontline tissue source, and the differential diagnosis includes a reactive process. Immunohistochemical markers could be helpful to differentiate CHL from reactive lymph node changes (RLN) in this setting. The use of programmed cell death-1 (PD-1) and its ligand (PD-L1) immunohistochemistry has historically focused on prognostic and therapeutic value when evaluating CHL. However, the present study seeks to determine the diagnostic utility of these markers in core needle biopsies of CHL (25), THRLBL (3), and RLN (31). The cases of CHL and THRLBL were previously diagnosed and confirmed with standard immunohistochemistry, allowing the utility of PD-1 and PD-L1 to be tested in this setting. Different PD-1 and PD-L1 expression patterns were observed between the reactive process of RLN and the malignant lymphomas (CHL and THRLBL). CHL cases overall showed the greatest expression of PD-L1 within the malignant Reed-Sternberg cell population, with 40% of CHL cases exhibiting >50% PD-L1 expression. This degree of PD-L1 expression was not seen in the lymphocytic cell population of any RLN (P<0.001). Conversely, CHL cases showed an overall lower expression of PD-1, as 96% of CHLs had <5% PD-1 expression in Reed-Sternberg cells compared with only 10% expression within the lymphocytic population of RLN (P<0.001). THRLBL cases followed a similar trend to CHL. These results demonstrate that upfront PD-1 and PD-L1 immunohistochemistry can aid in the diagnosis of CHL in small-volume tissue biopsies.