Challenges and perspectives in the immunotherapy of Hodgkin lymphoma

Hodgkin lymphoma (HL) was one of the first few cancers to be cured first with radiotherapy alone and then with a combination of chemotherapy and radiotherapy. Around 80% of the patients with HL will be cured by first-line therapy. However, the ionising radiation not only produces cytotoxicity but also induces alterations in the microenvironment, and patients often struggle with the long-term consequences of these treatments, such as cardiovascular disorders, lung diseases and secondary malignancies. Hence, it is essential to improve treatments while avoiding delayed side-effects. Immunotherapy is a promising new treatment option for Hodgkin lymphoma, and anti- programmed death-1 (PD1) agents have produced striking results in patients with relapsed or refractory disease. The microenvironment of Hodgkin lymphoma appears to be unique in the field of human disease: the malignant Reed-Sternberg cells only constitute 1% of the cells in the lymphoma, but they are surrounded by an extensive immune infiltrate. Reed-Sternberg cells exhibit 9p24.1/PD-L1/PD-L2 copy number alterations and genetic rearrangements associated with programmed cell death ligand 1/ ligand 2 (PD-L1/2) overexpression, together with major histocompatibility complex-I (MHC-I) and major histocompatibility complex-II (MHC-II) downregulation (which may facilitate the tumour's immune evasion). Although HL may be a situation in which defective immune surveillance is restored by anti-PD1 therapy, it challenges our current explanation of how anti-PD1 agents work because MHC-I expression is required for CD8-T-cell-mediated tumour antigen recognition. Here, we review recent attempts to understand the defects in immune recognition in HL and to design an optimal evidence-based treatment for combination with anti-PD1.