Marker-Agnostic Tumor Anchoring Chimeras Enable pH-Gated Immune Engagement

Targeted cancer immunotherapies typically depend on cell surface antigens to direct immune responses. However, many tumors either lack tumor-specific markers or downregulate their expression, limiting the applicability of antigen-directed strategies. In contrast, tumor acidosis represents a nearly universal hallmark of solid cancers, arising from dysregulated metabolism that acidifies the extracellular tumor microenvironment (TME). Here, we leverage this feature to develop Marker-Agnostic Tumor Anchoring Chimeras (MATRACs), pH-sensitive fusion proteins that mediate ternary complex formation between cancer cells and immune effector cells in an antigen-independent manner. MATRACs harness the diphtheria toxin translocation domain (tdDT), whose pH-dependent conformational switch drives membrane insertion, allowing tdDT and its Fc-fused form (tdDT-Fc) to anchor selectively to cells in TME-like acidity while remaining inactive at physiological pH. To demonstrate pH-dependent immune activation, we designed a fluorescein-tagged tdDT-Fc variant that anchors to tumor cells in acidic conditions and presents a synthetic epitope enabling recruitment of antifluorescein CAR T cells. These findings establish tdDT-based MATRACs as a modular, TME-responsive platform for antigen-agnostic immune engagement and support their potential for next-generation pH-targeted immunotherapies.