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

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.