A Versatile Cytochemical Labeling Technique as a Forensic Decision-Support Tool for In Situ Detection of Cell-Derived Markers in Invisible Biological Traces.

Forensic genetic analysis of biological traces collected at a crime scene can aid in the rapid identification of perpetrators. Touch DNA samples consist of invisible biological traces left on an object or person after skin contact. Although these samples represent a significant portion of DNA evidence, they often fail to yield usable DNA profiles due to challenges in detection and collection. A decision-support tool capable of highlighting such traces would optimize sampling strategies and improve forensic outcomes. We previously identified cell-derived markers that persist on shed corneocytes. Here, we propose an in situ cytochemistry-based tool using a dual labeling technique that combines chromogenic and fluorescent visualization to enhance the detection of otherwise invisible traces. Chromogenic labeling of cell-derived mannose residues and keratin 10 allows the visualization of up to 100 cells on surfaces and swabs and enables the acquisition of usable DNA profiles. A combination of FITC-conjugated primary probes and an HRP-conjugated anti-FITC antibody enables versatile visualization, allowing both chromogenic and fluorescent detection of otherwise invisible biological traces and offering adaptability across various substrates. Corneocytes from fingermarks were successfully detected on both light and opaque surfaces (ceramic and metal) using a single cytochemistry process. In addition, the presence of biological material after swabbing a fingermark was highlighted, notably by chromogenic labeling. This approach could improve on-field touch DNA sample detection, proving valuable both before and after sampling. It can highlight areas for collection or prioritize swabs likely to contain biological material, thereby increasing the chances of obtaining usable DNA profiles for criminal investigation.