PD-1 Blockade in Mediastinal Gray-Zone Lymphoma

TO THE EDITOR

Mediastinal gray-zone lymphoma lies intermediate between nodular-sclerosis classic Hodgkin’s lymphoma and primary mediastinal B-cell lymphoma (a subtype of diffuse large B-cell lymphoma), with overlapping clinical, histologic, and molecular features.^1,2 Genetic characteristics shared among these tumors include frequent 9p24.1 copy-number alterations, which occur in approximately 61% of patients with mediastinal gray-zone lymphoma.³ The programmed death 1 (PD-1) pathway is proposed to play a key role in lymphomagenesis, providing escape from immune surveillance. We report three cases of refractory mediastinal gray-zone lymphoma that were successfully treated with PD-1 blockade.

An 18-year-old woman (Patient 1) received a diagnosis of mediastinal gray-zone lymphoma after biopsy of a mediastinal mass showed an immunophenotype intermediate between diffuse large B-cell lymphoma and classic Hodgkin’s lymphoma (positive for CD20, CD30, and CD15). After a partial response to dose-adjusted etoposide, doxorubicin, and cyclophosphamide with vincristine and prednisone plus rituximab (DA-EPOCH-R), she had disease progression 6 weeks after salvage radiotherapy and had a complete metabolic response (i.e., normalization of an abnormal positron-emission tomographic scan) after treatment with pembrolizumab (Fig. 1A). After 235 days of treatment, she underwent allogeneic transplantation. Rearrangement of the genes encoding the PD-1 and PD-2 ligands (PD-L1/ PD-L2) was detected by fluorescence in situ hybridization (FISH) analysis (Fig. 1B).

Figure 1. ¹⁸F-Fluorodeoxyglucose (FDG) Positron-Emission Tomographic (PET) Response, 9p24.1 (PD-L1/PD-L2) Fluorescence In Situ Hybridization (FISH) Analysis, and PD-L1 Immunohistochemical Analysis.

Panels A, C, and E show maximum-intensity projection images from FDG-PET scans of three patients with refractory mediastinal gray-zone lymphoma who were treated with programmed death 1 (PD-1) blockade. As shown in Panel A, Patient 1 had a complete metabolic response after treatment with pembrolizumab, with no residual hypermetabolic uptake on post-treatment imaging. As shown in Panel C, Patient 2 had a complete metabolic response after treatment with pembrolizumab, with residual uptake in the neck and upper chest that was consistent with normal-variant brown fat. As shown in Panel E, Patient 3 had a complete metabolic response after treatment with nivolumab, with diffuse uptake in the bowel that was compatible with inflammation. Amplification and rearrangement of the PD-L1/PD-L2 locus was investigated by FISH with the use of a PD-L1/PD-L2 Break Apart probe (labeled with Spectrum Red and Spectrum Green) and a CEP 9 probe (labeled with Spectrum Aqua). Panel B shows rearrangement of the PD-L1/PD-L2 locus in Patient 1, with a split green signal suggesting the presence of an inversion within this region. Panel D shows amplification of the PD-L1/PD-L2 locus in Patient 2, with evidence of multiple fusion signals (69% of cells with ≥6 copies per cell, 20% with 3 to 5 copies, and 11% with ≤2 copies). In Panel F, immunohistochemical analysis in Patient 3 shows focal membranous expression of PD-L1 in the lymphoma cells, with additional expression seen on the small background cells surrounding the malignant lymphoma cells.

A 76-year-old man (Patient 2) received a diagnosis of mediastinal gray-zone lymphoma after biopsy of a subcarinal mass showed diffuse large B-cell lymphoma with an immunophenotype of classic Hodgkin’s lymphoma (positive for CD30 and PAX5 and negative for CD10, CD20, and CD15). After a partial response to DA-EPOCH-R, he had disease progression. He had a complete metabolic response after treatment with pembrolizumab and continues to be in remission on day 381 of treatment (Fig. 1C). Amplification of PD-L1/PD-L2 was detected by FISH, with 69% of cells having at least six copies per cell (Fig. 1D).

An 80-year-old woman (Patient 3) received a diagnosis of mediastinal gray-zone lymphoma after biopsy of an axillary lymph node showed an immunophenotype intermediate between diffuse large B-cell lymphoma and classic Hodgkin’s lymphoma (positive for CD20, CD30, and CD15). Despite a complete metabolic response to DA-EPOCH-R, she had a relapse and disease progression after treatment with brentuximab; a combination of rituximab, gemcitabine, and oxaliplatin; and mediastinal radiation. She had a complete metabolic response after treatment with nivolumab and continues to be in remission on day 161 of treatment (Fig. 1E). Immunohistochemical analysis showed focal membranous PD-L1 expression (Fig. 1F).

Genetic susceptibility to immune-checkpoint inhibition that is conferred by 9p24.1 copy-number alterations is best characterized in classic Hodgkin’s lymphoma. Near-uniform (97%) concordant copy-number alterations in PD-L1/PD-L2 have been shown in cases of classic Hodgkin’s lymphoma by means of FISH, with 2% having a translocation at this locus.⁴ Nivolumab⁵ and pembrolizumab have a high frequency of response in relapsed or refractory classic Hodgkin’s lymphoma but not in primary mediastinal B-cell lymphoma. Recent findings indicate that mediastinal gray-zone lymphoma may be more closely related to classic Hodgkin’s lymphoma than to primary mediastinal B-cell lymphoma.² These findings suggest that PD-1 inhibitors may be therapeutically important for mediastinal gray-zone lymphoma, which is more resistant to treatment than classic Hodgkin’s lymphoma or primary mediastinal B-cell lymphoma.²

The high frequency of 9p24.1 copy-number alterations across mediastinal lymphomas suggests a disease-specific, genetically determined dependence on PD-1 for survival. These cases provide early evidence for using PD-1 inhibition in relapsed or refractory mediastinal gray-zone lymphoma, which warrants further testing.