Striking a balance: new perspectives on homeostatic dendritic cell maturation

Abstract

Dendritic cells (DCs) are crucial gatekeepers of the balance between immunity and tolerance. They exist in two functional states, immature or mature, that refer to an information-sensing versus an information-transmitting state, respectively. Historically, the term DC maturation was used to describe the acquisition of immunostimulatory capacity by DCs following their triggering by pathogens or tissue damage signals. As such, immature DCs were proposed to mediate tolerance, whereas mature DCs were associated with the induction of protective T cell immunity. Later studies have challenged this view and unequivocally demonstrated that two distinct modes of DC maturation exist, homeostatic and immunogenic DC maturation, each with a distinct functional outcome. Therefore, the mere expression of maturation markers cannot be used to predict immunogenicity. How DCs become activated in homeostatic conditions and maintain tolerance remains an area of intense debate. Several recent studies have shed light on the signals driving the homeostatic maturation programme, especially in the conventional type 1 DC (cDC1) compartment. Here, we highlight our growing understanding of homeostatic DC maturation and the relevance of this process for immune tolerance. Dendritic cells (DCs) act as gatekeepers between immunity and tolerance. Initially, it was postulated that mature DCs promote effector T cell responses and immature DCs promote tolerance. Recent studies have shown instead that two distinct modes of DC maturation exist — homeostatic and immunogenic. Here, Bosteels and Janssens discuss our current understanding of homeostatic DC maturation and how this contributes to immune tolerance, with a focus on the cDC1 compartment.