Abstract 3975: Regulating dendritic cells to promote mature tertiary lymphoid structures and enhance anti-tumor immunity
Authors: Elizabeth E. Evans, Crystal Mallow, Terrence L. Fisher, Elaine Gersz, Holm Bussler, Maria Scrivens, Vuyani Matsenjwa, Ellen Giampoli, Saurabh K. Garg, Colin W. Snyder, Brian Czerniecki, Conor Steuer, Nabil Saba, Michael Lowe, Gregory B. Lesinski, Chrystal Paulos, Maurice Zauderer
Journal: Cancer Research (2025)

Abstract Mature tertiary lymphoid structures (TLS) characterized by the presence of germinal centers are strong predictors of response to immunotherapy. DCs play critical and decisive roles in determining the outcome of the immune responses in the TME and within TLS. Therefore, strategies to recruit and activate DC have the potential to induce TLS and enhance response to immunotherapy. Pepinemab, semaphorin 4D (SEMA4D) blocking antibody, reversed myeloid suppression and promoted infiltration of activated DC and CD8+ T cells into the TME in preclinical and clinical studies. New data demonstrates the presence of mature TLS following pepinemab treatment in combination with immune checkpoint inhibitors (ICI) in three separate clinical studies. KEYNOTE-B84 study (NCT04815720) evaluated the safety and efficacy of pepinemab in combination with pembrolizumab as a first-line treatment for immunotherapy-naïve patients with R/M HNSCC. Neoadjuvant studies in patients with surgically resectable metastatic melanoma (NCT03769155) and HNSCC (NCT03690986) received pepinemab treatment combined with nivolumab and/or ipilimumab. Screening and on-treatment tumor biopsies were evaluated using multiplex immunohistochemistry, RNAseq and spatial transcriptomics. TLS identified in biopsies of patients treated with pepinemab in combination with ICI were larger and more mature, containing high density of activated HLA-DR+ DCs, FDC, follicular B cells, and Tfh cells expressing CXCR5, characteristic of germinal centers. Importantly, comparison of matched biopsy pairs before and after treatment indicated that TLS were induced by the pepinemab combination treatment. Increased density and maturity of lymphoid aggregates, as well as significant changes in DC activation markers and the Flt3/Wnt pathway correlated with clinical benefit. We hypothesized that effects of pepinemab could be further enhanced by increasing the population of DC available in the TME. In preclinical studies, intra-tumoral vaccination with ex vivo expanded DC-1 enhanced effects of systemic treatment with SEMA4D-blocking antibody, resulting in improved DC trafficking to draining lymph nodes, and inducing tumor regression in both target and distant tumors. For in vivo expansion of DC, treatment with Flt3L improved anti-tumor activity when combined with SEMA4D-blocking antibody. Pepinemab represents a novel strategy to boost dendritic cell infiltration, activation, and organization of functional TLS to overcome limitations of immune checkpoint therapies. When combined with ICI, pepinemab induced de novo formation of TLS that correlated with clinical benefit, even in poorly immunogenic tumors that do not respond well to ICI, including HPV-negative and PD-L1 low HNSCC. When combined with modalities to further expand DC, SEMA4D blocking antibodies inhibited tumor growth in preclinical models, including in ICI-resistant models. Citation Format: Elizabeth E. Evans, Crystal Mallow, Terrence L. Fisher, Elaine Gersz, Holm Bussler, Maria Scrivens, Vuyani Matsenjwa, Ellen Giampoli, Saurabh K. Garg, Colin W. Snyder, Brian Czerniecki, Conor Steuer, Nabil Saba, Michael Lowe, Gregory B. Lesinski, Chrystal Paulos, Maurice Zauderer. Regulating dendritic cells to promote mature tertiary lymphoid structures and enhance anti-tumor immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3975.