A cell surface code mediates tissue-intrinsic defense against aberrant cells in epithelia

Tissue-intrinsic error-correction mechanisms allow epithelial cells to detect aberrant neighboring cells and cause their removal from the tissue. The molecular mechanisms which grant cells the ability to compare their internal states is unknown. Here we demonstrate that comparison of cell identity, created by cell-fate-specifying transcription factors and patterning pathways, is conveyed through a specific set of cell surface molecules. We demonstrate that Drosophila imaginal discs express a range of cell surface molecules previously implicated in neuronal axon guidance processes, such as members of the Robo, Teneurin, Ephrin, Toll-like or atypical Cadherin families. Expression of these molecules is regulated by intrinsic fate-patterning pathways of the disc but also by aberrant expression of oncogenic RasV12. Importantly, mosaic clones deregulating individual cell surface molecules are sufficient to induce all hallmarks of interface surveillance, a tissue-intrinsic error-correction mechanism previously shown to be induced by cells with aberrant activation of fate-patterning pathways. Specifically, cells with deregulated expression of Robo2 and Robo3 induce actomyosin enrichment, bilateral JNK signaling and apoptosis at mosaic clone interfaces in imaginal discs. Moreover, deregulation of Robo2 levels, which is normally expressed in a complex endogenous pattern, induces these interface surveillance hallmarks in a Robo2-pattern-specific manner. Taken together, our work indicates that these cell surface molecules mediate cell fate recognition in epithelial tissues and thereby contribute to the maintenance of epithelial health by initiating detection and removal of aberrant cells during development and adult tissue homeostasis.