Immune cell topography predicts response to PD-1 blockade in cutaneous T cell lymphoma

Anti-PD-1 immunotherapies have transformed cancer treatment, but the determinants of clinical response are largely unknown. We performed CODEX multiplexed tissue imaging and RNA sequencing on 70 tumor regions of advanced cutaneous T cell lymphoma (CTCL) from 14 patients enrolled in a clinical trial of pembrolizumab therapy. Clinical response was not associated with the frequency of tumor-infiltrating T cell subsets, but rather with striking differences in the spatial organization and functional immune state of the tumor microenvironment (TME). After treatment, pembrolizumab responders had a localized enrichment of tumor and CD4+ T cells, which coincided with immune activation and cytotoxic PD-1+ CD4+ T cells. In contrast, non-responders had a localized enrichment of Tregs pre- and post-treatment, consistent with a persistently immunosuppressed TME and exhausted PD-1+ CD4+ T cells. Integrating these findings by computing the physical distances between PD-1+ CD4+ T cells, tumor cells, and Tregs revealed a spatial biomarker predictive of pembrolizumab response. Finally, the chemokine CXCL13 was upregulated in tumor cells in responders post-treatment, suggesting that chemoattraction of PD-1+ CD4+ T cells towards tumor cells facilitates a positive outcome. Together, these data show that T cell topography reflects the balance of effector and suppressive activity within the TME and predicts clinical response to PD-1 blockade in CTCL. We Laser Capture Microdisected regions containing approximately 1000 cells from FFPE-tissue of CTCL lesions from responders and non responders before and after treatment with pembrolizumab.