Abstract 1033: Leronlimab induces PD-L1 expression and is associated with long-term survival with an ICI in PD-L1 low metastatic TNBC

Abstract Introduction. Patients with metastatic triple-negative breast cancer (mTNBC) have poor survival but may be eligible for immune check point inhibitor (ICIs). CCR5 is overexpressed in ∼95% of TNBC. Data suggest that combining the CCR5 inhibitor leronlimab with an ICI may improve survival in mTNBC. Methods. Analysis of patient gene expression, tumor histology, cancer-associated macrophage-like cells/circulating tumor cells (CAML/CTC) from clinical studies and tissue cultures of TNBC were conducted. Findings. In breast cancer cohorts (N=1,096) CCR5 expression correlated with gene signatures of T cell immune exhaustion. Across public TNBC cohorts (N=73; after deduplication), CCR5 expression correlated with both GSEA and gene signatures of T cell infiltration and T cell immune exhaustion. TNBC subtype analysis showed CCR5 enrichment in epithelial cells of MLIA (Mesenchymal-like Immune-Altered, Jézéquel subtype) and IM (immune modulatory, Lehmann subtype). TNBC Subtype analyses showed higher CCR5-related signals in tumors classified as MLIA and IM subtypes. In cultured MDA-MB-231 TNBC cells, CCR5 expression suppressed glycosylated PDL1; CCR5 inhibition increased the abundance of PDL1 (18 kDa, 35 kDa, and glycosylated 55 kDa forms). To understand the mechanisms by which CCR5 may promote immune exhaustion, we investigated a heterotypic signal between TNBC cultured cells and the tumor microenvironment (TME) using a proteomic approach. CCR5 activity induced sB7-H3 (CD276), sTyro3 and the Tyro3 ligand Pros1. Both CD276 and Tyro3 are associated with ICI resistance; and abundance of both were attenuated by CCR5 blockade with leronlimab. In Vivo. Leronlimab induced PD-1 expression in CD8+ T cells in lymph node of rhesus macaques and had variable modulation on expression of several T cell exhaustion markers. In a retrospective analysis of data pooled from 28 patients with mTNBC leronlimab induced PD-L1 in CTC/CAMLs. Leronlimab was generally well tolerated. Higher leronlimab dose (550-700 mg once weekly), induction of PD-L1, and the formation of CCR5 dots in CTC/CAMLs, and treatment with leronlimab in combination, or subsequently, with an ICI were associated with improved survival. The median age of the 28 patients was 48.5 years (range 32-83), patients had a median of 2 prior lines of therapy in the metastatic setting (range 0-5), 18 patients had visceral metastases (64%), of which 8 had brain metastases, and 10 had non-visceral metastases, 17.9% of heavily pretreated mTNBC patients are currently alive after median 60 months of follow-up. Conclusions. Leronlimab is well tolerated, inducing PDL1 expression on CTC/CAMLs, which may prime tumors for PDL1 blockade. CCR5 may promote ICI resistance in TNBC by upregulating immune checkpoints (sB7-H3) and sTyro3. Overall, 17.9% (5/28) of patients with mTNBC treated with leronlimab are currently alive after a median of 60 months follow-up. Citation Format: Richard G. Pestell, Ritika Harish, Zhiping Li, Danni Li, Xuanmao Jiao, Hallgeir Rui, Massimo Cristofanilli, Daniel L. Adams, Neil E. Buss, Jonah B. Sacha, Jacob P. Lalezari. Leronlimab induces PD-L1 expression and is associated with long-term survival with an ICI in PD-L1 low metastatic TNBC abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1033.