Development of an MSC-based immune checkpoint inhibiting cell therapy

Cancer research has focused on targeted therapy, particularly immune checkpoint therapies, since programmed cell death protein 1 (PD1) monoclonal therapies demonstrated notable efficacy with reduced toxicity. Despite its progress to date, several challenges persist, including a non-inflammatory tumour microenvironment (TME), constraints on drug delivery, and suboptimal binding of recombinant antibodies. To overcome these challenges, we propose a pragmatic cell therapy approach in which mesenchymal stem cells (MSCs) are utilised as carriers of a soluble, high-affinity PD1 variant (sPD1HAC). This approach aims to harness the tumour tropism and plasticity of MSCs while providing a sustained source of a highly competitive PD1 in the TME. Our study demonstrates successful insertion of sPD1HAC into MSCs, leading to sufficient secretion without compromising cell viability and maintaining unperturbed cytokine profile, as shown by ELISA, apoptosis, and cytokine array assays. Across solid-phase and cell-based binding assays, including cross-species competitions, sPD1HAC outcompeted recombinant PD1 and anti-PDL1 monoclonal antibodies. It reduced PD1-PDL1 binding by 82-94 per cent at one-fifth the comparator concentration. Furthermore, a single intravenous dose of MSC.sPD1HAC significantly reduced pulmonary metastatic burden in C57BL/6 mice with B16.F10 melanoma. Additionally, at the cohort scale, TCGA PanCancer Atlas mRNA Z-scores were analysed for T-cell abundance, activation, progenitor-exhausted, and exhausted states to stratify samples as hot or cold. Melanoma showed the highest T-cell-defined inflammatory state within the dataset. We also examined a predefined set of 323 immune-related genes and their correlations with PDL1 to identify a general signature for high-PDL1 tumours. Although no universal signature emerged, PDL1 aligned with IFNγ-linked activation, as confirmed by flow cytometry in IFNγ-stimulated cancer cell lines. This reinforces the idea that delivery and continuous ligand competition may be decisive, especially in colder settings. Together, these findings highlight an approach that is both principled and practical. Cohort data clarify where inflammation is limited and why a universal biomarker may be elusive. The MSC platform then addresses that challenge by providing tumour-tropic, continuous delivery of a high-affinity PD1 variant, achieving robust competition in vitro and efficacy in vivo. In doing so, it augments the antibody therapy and offers a credible route to strengthen responses in cancers that are currently difficult to inflame.