Adaptation of an in vitro generated human skin model for tick feeding to study Borrelia burgdorferi transmission and colonization

Abstract The Lyme Disease pathogen Borrelia burgdorferi is estimated to cause nearly half a million human infections in the U.S. every year. Transmitted by the blacklegged deer tick (Ixodes scpaularis), B. burgdorferi is passed between rodents, ticks and large mammals to complete its complex enzootic cycle. Humans get infected as an incidental host, usually in the springtime by nymph-stage ticks. In this proposal we adapt an established lab-generated in vitro organotypic stratified keratinocyte skin model to induce attachment and feeding of ticks. In Aim 1, we continue to modify the skin to increase tick feeding efficiency and demonstrate transmission and acquisition of B. burgdorferi by nymph-stage ticks. In Aim 2, we characterize the kinetics of bacterial growth and spread within organotypic skin following direct injection and validate that B. burgdorferi responds to environmental cues of the bloodmeal (pH, temperature) to increase expression of mammalian stage virulence factors. This novel organotypic human skin model is highly adaptable and can accommodate introduction of other cell types. For instance, in Aim 1 we introduce endothelial cells, macrophages and extracellular matrix factors into the skin to induce vascularization and provide a biologically-relevant blood delivery system. This is the first demonstration of arthropod feeding on in vitro organotypic human skin, and its development will enable new avenues of research not only for Lyme Disease research, but also for other tick-borne diseases. We expect this model will reduce the need for animals in Lyme Disease research and will provide a system to answer long-standing questions in tick-pathogen biology, such as the role of resident immune cells in containing infection and the contribution of specific bacterial virulence factors to skin colonization and dissemination into the bloodstream. Given the scalable and reproducible nature of this model, we can use it as a platform to screen therapies that could prevent pathogen colonization following a tick bite or prophylactics that would prevent tick feeding.