An intermediate activation state primes Langerhans cell migration from the epidermis

Langerhans cells (LCs) are a specialized subset of dendritic cells in the epidermis, forming a dense network that acts as a frontline defense through immune surveillance. Upon antigen uptake, LCs become activated and orchestrate subsequent immune responses by migrating to lymphatics. However, how transcriptional programs are regulated during activation and how LCs behave in vivo during this transition remain poorly understood. Here, we combine single-cell transcriptomic analysis and intravital imaging to reconstruct the activation trajectory of epidermal LCs. We present a high-resolution single-cell transcriptomic dataset of over 22,000 high-quality epidermal LCs in both homeostatic and injured conditions. We define specific LC subpopulations representing sequential activation stages, characterized at the level of pathways and transcription factors. Notably, we identify a distinct intermediate state that precedes their migration. Integrating our data with an external dataset from homeostatic and injured skin reveals that wound-specific, WNT-modulated fibroblasts are the primary source of C3, the central component of the complement cascade. Intravital imaging of C3-deficient mice demonstrated that C3 is essential for effective recruitment of activated LCs to wound sites. Together, our findings uncover a novel population of activated epidermal LCs and highlight complement signaling as a critical mediator of LC recruitment during skin injury.