Label-free imaging of macrophage phenotypes and phagocytic activity in the human dermis in vivo using two-photon excited FLIM

Abstract

AbstractMacrophages (MΦs) are important immune effector cells that promote (M1 MΦs) or inhibit (M2 MΦs) inflammation and are involved in numerous physiological and pathogenic immune responses. Their precise role and relevance, however, is not fully understood because of the lack of non-invasive quantification methods. Here, we show that two-photon excited fluorescence lifetime imaging (TPE-FLIM), a label-free non-invasive method, can visualize MΦs in human dermis in vivo. We demonstrate in vitro that human dermal MΦs exhibit specific TPE-FLIM properties that distinguish them from the main components of the extracellular matrix and other dermal cells. We visualized MΦs, their phenotypes and phagocytosis in the skin of healthy individuals in vivo using TPE-FLIM. Additionally, machine learning identified M1 and M2 MΦs with a sensitivity of 0.88±0.04 and 0.82±0.03 and a specificity of 0.89±0.03 and 0.90±0.03, respectively. In clinical research, TPE-FLIM can advance the understanding of the role of MΦs in health and disease.