Abstract Background: Nodal T-follicular helper-phenotype lymphomas (TFHL) are defined by epigenetic dysregulation and microenvironmental dependence. Across multiple solid tumor types, the presence of tertiary lymphoid structures (TLS) have been shown to predict for improved response to immune checkpoint blockade, with B-cell signatures identified as the cellular fingerprint of functional TLS. We present the correlative analysis from a study of peripheral T-cell lymphoma (PTCL) patients receiving pembrolizumab plus romidepsin suggesting that the mechanism of improved responses lies in the reactivation of the TLS. Methods: We conducted a phase I/II trial (NCT03278782) of pembrolizumab plus romidepsin in 38 relapsed/refractory PTCL patients. RNAseq was performed on pretreatment tumor specimens from 10 patients (6 responders, 4 non-responders). Immune cell proportions were estimated from RNAseq data using CIBERSORT (v0. 1. 0) R package implementing the CIBERSORT algorithm. Gene set enrichment analysis was performed using MSigDB Hallmark gene sets, comparing non-responders with responders. CODEX multiplexed immunofluorescence (n=10) was performed using a custom 33-marker lymphoma panel. Results: Thirteen (34. 1%) patients had TFHL and achieved a median progression free survival of 39. 3 months, longer than PTCL-NOS (2. 25 months; log-rank p=. 013) and other subtypes (. 92 months; p=. 035). No hyperprogression events occurred among patients with TFHL. CIBERSORT immune cell deconvolution of 10 pretreatment patient samples showed M1 macrophages were significantly enriched in responders (Wilcoxon p=0. 036). Gene set enrichment analysis identified significantly enriched hallmark pathways in non-responders including MYC (NES=1. 45; p. adj=0. 021). Hierarchical clustering of CODEX-derived immune cell densities revealed CD3+CD4+CD8+ T cells were enriched in responders (4. 7-fold; p=0. 042). Macrophage populations including CD68+ (p=0. 010), CD68+CD163+ (5. 3-fold, p=. 019), CD68+MHCII+ (6. 8-fold, p=. 038), and CD68+CD163+PD-L1+ (15. 2-fold, p=. 038) macrophages were all significantly higher in responders. Total CD19+ B cells were 6. 2-fold higher in responders (p=0. 019) ; proliferating CD19+Ki-67+ B cells were 21. 5-fold higher (p=0. 010) ; CD19+CD30+ activated B cells were 21. 2-fold higher (p=0. 010) ; and CD20+CD30+ B cells were 5. 7-fold higher (p=0. 038) in responders compared to non-responders. Conclusion: The CODEX finding of greater than 20-fold enrichment of proliferating and activated B cells in responders is of translational significance as this spatial signature corresponds with cellular markers that define TLS, which may be the mechanism by which patients in this study, the majority with TFHL, have improved responses to romidepsin (causing epigenetic priming leading to TLS reactivation) and pembrolizumab. These analyses reveal that response to pembrolizumab + romidepsin are governed by the organization of the tumor microenvironment, specifically by a B-cell-enriched architecture consistent with TLS and M1 macrophage polarization rather than by individual somatic mutations. Citation Format: Neha Akkad, Daniela E. Duenas, Ranjit Nair, Luis Malpica, Francisco Vega, Chitra Hosing, Cara Haymaker, Christopher R. Flowers, Sattva S. Neelapu, Jie Xu, Swaminathan P. Iyer. Immune checkpoint sensitivity through spatial reprogramming of the tumor microenvironment in T cell lymphoma abstract. In: Proceedings of the Fifth AACR International Meeting on Advances in Malignant Lymphoma: From Discovery to Clinical Impact; 2026 Jun 24-27; Philadelphia, PA. Philadelphia (PA): AACR; Blood Cancer Discov 2026;7 (3Suppl): Abstract nr A050.
Akkad et al. (Wed,) studied this question.