“Our paper was the first to describe in a living mammal the cellular dynamics of T cell interactions with antigen-presenting dendritic cells (DCs) in lymph nodes.”

Papers get typically cited because they report a novel observation, concept, or technical advance that is relevant to the rationale, execution, or interpretation of someone else’s research. Our paper was the first to describe the cellular dynamics of T cell interactions with antigen-presenting dendritic cells (DCs) in lymph nodes in a living mammal. This process is a critical event in the initiation of both protective and pathological (e.g., autoimmune) responses and, therefore, of considerable interest to immunologists. We discovered that T cell-DC interactions during the first 48 hours after the initial encounter are divisible into three distinct phases. This observation has helped to reconcile seemingly contradictory results from a number of earlier in vitro studies, which can now be understood as reflecting different phase-dependent interactive behaviors.

Immunologists have long been wondering how DCs interact with naive T cells to induce antigen-specific T cell activation and the generation of effector cells. Some previous in vitro studies had reported that short-lasting, serial, and dynamic interactions are sufficient to activate T cells. Others found that long-lasting, stable contacts were necessary and that these were required for the formation of a highly ordered molecular interface between DCs and T cells, the so-called immunological synapse. We have developed an intravital imaging approach using multiphoton microscopy to generate three-dimensional time-lapse movies of interacting T cells and DCs in lymph nodes of anesthetized mice. By synchronizing the observed T cell populations with regard to their residence time in the lymph node we could show that short-lasting, serial encounters predominate during the first ~8 hours after T cell entry into a lymph node containing antigen-presenting DCs. These transient encounters induce some signs of T cell activation. Subsequent to this first phase until about 24 hours, tight, long-lasting contacts become prevalent with T cells showing high levels of activation markers and beginning to produce effector cytokines. After the first day, the now-activated T cells revert to mostly transient interactions and begin to proliferate vigorously. Thus, our study provides a chronological roadmap of T cell activation that may serve as a guideline for further investigations of T cell activation at the molecular level. Furthermore, the new intravital multiphoton microscopy model enables researchers to study other immunological events by direct real-time observation.

The activation of immune cells, such as T lymphocytes, by so-called antigen-presenting cells is an essential event for protective immune responses to vaccines, infectious microbes, and tumors, but also for misguided autoimmune attacks on the body’s own tissues. Lymph nodes are important sites for all forms of T cell education, which is provided by dendritic cells (DCs), a type of leukocyte that gathers antigenic material in peripheral tissues and transports it to lymph nodes. We have developed a new microscopy technique that has allowed us to analyze the first 48 hours of this activation process in intact lymph nodes of a live, anesthetized mouse. Our study reveals that T cell activation is a highly orchestrated process that we call sometimes "dating, mating, and procreating." During the initial "dating phase," T cells migrate vigorously and form multiple contacts with DCs, each lasting only a few minutes. After about eight hours, the T cells enter the "mating phase" during which they form a tight conjugate with just one DC that lasts several hours. During the second day, the T cells detach from the DCs and begin to divide rapidly (the "procreating phase").

To explore how immune cells migrate in the body and, in particular, in lymph nodes, has been the focus of our group for over 10 years. Intravital fluorescence microscopy has always been a key tool for our investigations, but the generation of three-dimensional recordings of cell-cell interactions deep within solid tissues was not possible until recently. Multi-photon microscopy allows such recordings to be generated at subcellular resolution. We spent several years setting up and developing our approach before this could be conducted.