Disturbed antigen presentation in classical Hodgkin Lymphoma; implications for immune checkpoint inhibitor therapy

Immune checkpoint inhibitors are being tested in clinical trials and show great promise in the treatment of classical Hodgkin lymphoma (cHL). The proposed mechanism of action of these inhibitors consists of reactivating T lymphocytes that have become unresponsive as a consequence of inhibitory mechanisms exerted by the tumor cells. These reactivated T cells are expected to kill tumor cells after recognizing tumor cell derived antigens that are presented by Human Leukocyte Antigens (HLA) at the tumor cell surface. Consequently, lack of tumor cell surface expression of HLA may be one of the explanations why immune checkpoint inhibitor therapy does not always result in a complete remission in cHL. We evaluated expression of HLA class I, HLA class II and HLA-DM in primary diagnostic tissue in a large population based cohort of cHL, by using immunohistochemistry. HLA class I cell surface expression was lacking in tumor cells in 229 out of 361 cases (63.3%), more often in EBV- cHL (83.2%) than in EBV+ cHL (27.4%). HLA class II cell surface expression was missing in 147 out of 361 cHL cases (40.8%; EBV- 46.8% and EBV+ 29.7%). In addition, we scored lack of cytoplasmic expression of the non-polymorphic HLA-DM. This molecule is essential in the intracellular assembly of HLA class II-antigenic peptide complexes. HLA-DM displaces the invariant chain peptide CLIP from the antigen binding groove of HLA class II molecules, to make this groove accessible for loading of antigens. We show that in the absence of HLA-DM, CLIP is not displaced and is aberrantly expressed on the cell surface of Hodgkin tumor cells in frozen sections (n=8). We found that in HLA class II cell surface expressing tumor cells, HLA-DM expression was lacking in 44 out of 89 cases of cHL (49.4%), indicating that no immunogenic peptides are being presented. Combined results for HLA class I, class II and DM show that only 12.4% of cHL cases show HLA expression that is compatible with normal antigen presentation. In conclusion, antigen presentation is often disturbed in cHL. It is expected that this will influence the success of immune checkpoint inhibitors. We therefore propose to take tumor cell HLA expression into account in the evaluation of (lack of) clinical response to these inhibitors.