Recruitment and local granulopoiesis of human hematopoietic stem/progenitor cells in a skin-on-a-chip.

Extramedullary hematopoiesis is increasingly recognized as an important mechanism by which peripheral tissues augment immune responses during inflammation and infection. However, the mechanisms governing recruitment, retention, and local differentiation of circulating hematopoietic stem and progenitor cells (HSPCs) within human tissues remain poorly understood due to the lack of physiologically relevant experimental models. Here, we developed a human skin-on-a-chip microphysiological platform to investigate the role of circulating hematopoietic stem and progenitor cells (HSPCs) in cutaneous immune responses and their potential contribution to extramedullary hematopoiesis. The device recapitulates key features of human skin, including a perfusable vascular endothelium, fibroblast-populated dermis, and keratinocyte epidermis. Upon stimulation with pro-inflammatory cytokines or a TLR2 agonist, endothelial activation significantly increased ICAM-1 and VCAM-1 expression and enhanced recruitment of both neutrophils (PMNs) and HSPCs. While PMNs readily underwent transendothelial migration into the dermal compartment, HSPCs remained localized to the vascular niche. Notably, HSPCs adhered robustly and persisted on inflamed endothelium independent of SDF-1/CXCR4 signaling. Under granulopoietic conditions, these retained HSPCs differentiated locally within the vascular compartment into PMN-like cells capable of phagocytosis and exhibiting pathogen-responsive gene expression profiles. These findings provide evidence in a human model that circulating HSPCs can contribute to host defense via localized granulopoiesis without tissue infiltration, and suggests that the vascular niche itself may serve as a site of immune cell production during infection. The platform further offers a foundation for developing HSPC-based therapeutic strategies to combat antibiotic-resistant skin infections.