Preclinical in vitro and in vivo evidence for CD74-targeting as effective treatment strategy for cutaneous T cell lymphomas.

Background: Prognosis and quality of life of advanced cutaneous T cell lymphoma (CTCL) patients, in particular those with Sézary syndrome (SS) and advanced-stage mycosis fungoides (MF), are poor. Monoclonal antibodies or antibody-drug conjugates (ADCs) have been implemented into CTCL therapy algorithms, but the spectrum of antibody-targetable cell-surface antigens on T cell non-Hodgkin lymphomas (T-NHL) is limited.

Objectives: To evaluate expression of the MHC-II chaperone CD74 across common subtypes of CTCL by various methods, and to explore the efficacy of CD74-targeting of CTCL cells by anti-CD74 antibody-drug conjugate (ADC) in vitro and in vivo.

Methods: We comprehensively investigate expression of CD74 in well-defined CTCL cell lines by PCR analyses, immunoblotting and flow cytometry. More than 140 primary CTCL samples of all common entities are analyzed by immunohistochemistry, flow cytometry, immunofluorescence and 'co-detection by indexing' (CODEX) multiplexed tissue imaging as well as single-cell RNAseq analyses. DNA methylation of CTCL cell lines is interrogated by generation of genome-wide methylation profiling. The effect of a maytansinoid-conjugated humanized ADC against CD74 is investigated on CTCL cell lines in vitro, alone or in combination with gemcitabine, and in vivo after xenotransplantation of CTCL cell lines in NOD-scid Il2rgnull (NSG) mice.

Results: We demonstrate by different experimental approaches in CTCL cell lines and a broad collection of primary CTCL samples that CD74 is widely and robustly expressed in CTCL cells. Additionally, CD74 expression in SS and MF is confirmed by analyses of single cell (sc)RNA-seq data, and correlates in CTCL cell lines with CD74 gene DNA hypomethylation. CD74 is rapidly internalized in CTCL cells, and CD74 targeting by the ADC STRO-001 efficiently kills CTCL-derived cell lines. Finally, CD74 targeting synergizes with conventional chemotherapy in vitro, and eradicates murine xenotransplants of CTCL cell lines in vivo.

Conclusions: CD74 is expressed across common CTCL subtypes, and CD74-targeting efficiently kills CTCL cells in vitro and in vivo. Our data thus identify CD74-targeting as highly promising treatment strategy for CTCL.