MARPLE: A Proximity‐Triggered CRISPR‐Cas13 Platform for Ultrasensitive Antibody Detection

ABSTRACT Monitoring clinically relevant antibodies—as biomarkers of disease or therapeutic response—is essential for informed clinical decision‐making. Traditional immunoassays like ELISA offer reliable quantification but often involve multistep workflows and limited point‐of‐care utility. New approaches coupling antibody recognition with signal amplification are therefore highly desirable. The CRISPR‐Cas13 system, known for its potent collateral cleavage activity, has emerged as a powerful diagnostic tool for nucleic acid detection. However, its application to protein biomarkers such as antibodies remains underdeveloped. Here, we introduce MARPLE (Modular Antibody Recognition via Proximity‐triggered Linker Exchange), a modular CRISPR‐Cas13–based platform for ultrasensitive antibody detection. MARPLE harnesses antibody‐induced proximity to trigger a strand displacement reaction that releases a sequestered RNA target, activating Cas13‐mediated collateral cleavage of fluorescent RNA reporters. This cascade enables detection of antibodies at femtomolar concentrations. We demonstrate MARPLE's versatility across diverse targets—including anti‐digoxigenin, anti‐cholesterol, anti‐HA, trastuzumab, and anti‐MUC1—highlighting applications in infectious disease monitoring, cancer diagnostics, and therapeutic drug tracking. The assay is isothermal, one‐pot, and retains robust performance in complex matrices such as human serum. These features establish MARPLE as a promising tool for immunodiagnostics, extending CRISPR‐based sensing beyond nucleic acids to protein biomarker detection.