Activated stellate cells are the main driver of fibrosis. The signals that convert quiescent stellate cells into pathogenic myofibroblasts remain largely unknown. Spatial analysis has revealed that each quiescent stellate cell is paired with a Kupffer cell (KC) in the steady-state liver. Upon KC depletion, stellate cells promote the recruitment of monocytes and their differentiation into KCs through the production of instructive signals. In fibrotic livers of mice and humans the progressive activation of stellate cells is coupled with the disappearance of KCs and pro-fibrotic stellate cells are no longer paired with KCs. I hypothesize that the KC-Stellate cell relationship is mutually beneficial and essential for liver homeostasis and that KCs play a key role in preventing stellate cell activation. I have recently identified a mouse model in which the knock-down of a receptor for one of the stellate cell-derived instructive factors leads to KC depletion, yielding a liver where most stellate cells are not paired with a KC. These mice spontaneously develop fibrosis, strengthening my hypothesis that KCs are crucial to maintain stellate cells quiescent. In this project, I will use scRNAseq and spatial transcriptomics to study stellate cells paired or unpaired with a KC to define and study gene targets involved in the KC-stellate cell crosstalk, in the hope to unravel the KC-derived signals that maintain stellate cells quiescent, as this may lead to novel anti-fibrotic strategies.