Cells are the smallest units of life and their behavior is modulated by a complex interplay of many intracellular mechanisms. However, “no cell is an island”, because tissues and organs consist of multiple, sometimes even millions, of cells. In this regards, organs and organisms constitute “republics of living elementary units” (Mazzarello, 1999). Such multicellular entities require additional layers of regulation beyond those operating solely within cells. This is because the function of most tissues and organs builds on behavioral coordination of multiple cells. An example in this regard is the brain, which consists of individual neurons, but whose full functionality only emerges through the communication among millions of cells. Conversely, loss of behavioral coordination among cells is a hallmark of cancer.
The focus of our research is the delineation of how interactions among cells of the immune system protect us from infections and malignancies, and at the same time avoid immune-mediated tissue damage. Our work combines bioinformatic approaches with reductionist in vitro as well as comprehensive in vivo systems to elucidate how immune cells mutually interact with each other and how such communication enables behavioural coordination. Additional fields of interest within our group concern genetic manipulation of T lymphocytes as well as the development of genetic reporter systems.
Quorum regulation of lymphocytes
A prototype for behavioral control across a population of cells is quorum regulation, which has been extensively studied in different bacterial species (e.g., Water & Bassler, 2005). It builds on the ability of population members to mutually sense each other and adjust their behavior accordingly. This enables collective decision-making based on cellular abundance, density and activation state.
Based on the hypothesis that quorum-regulation may also occur within the immune system, we have investigated whether activated T lymphocytes act entirely independent of each other or whether they also mutually regulate their behavior. Initial time-lapse video-microscopy experiments revealed that activated T cells seek contact with each other. Furthermore, we developed a bioinformatic pipeline to predict candidate receptor-ligands pairs that could mediate mutual regulation of activated T cells. Subsequent in vitro as well as in vivo experiments confirmed the physiological relevance of several predicted candidate molecules in the regulation T cell population dynamics. Together our work revealed that self-organization within a population does not only occur in bacteria, but can also be found in T lymphocytes. Specifically, we have delineated how the local density of T lymphocytes modulates the balance between expansion and apoptosis in order to promote population homeostasis. This enables activated CD8+ T lymphocytes to mutually regulate their population dynamics and act in a concerted manner. We are currently extending this work to study the role of T cell quorum-regulation in different disease scenarios.
Intercellular molecule transfer between immune cells
Cells do not only communicate via secreted signalling molecules, but are sometimes also able to exchange cell membrane-associated molecules. Specifically, cell surface receptors or ligands synthesized in one (donor) cell can be integrated into another (recipient) cell. Within the recipient cell the acquired molecules can be degraded, but even more interesting, they may also continue to exert their function. This has for example been shown for oncogenic Ras-molecules, which can be transferred from tumor cells to non-malignant neighboring lymphocytes and induce ERK-phosphorylation in the recipient lymphocytes (Vernitsky et al, 2012). Within the immune system it has been proposed that regulatory T cells acquire and degrade co-stimulatory molecules from antigen-presenting cells (Qureshi et al, 2011). It is, however, unknown whether this is also the main mechanism operating in conventional T cells or whether the acquired molecules follow different fates in them.
In one of our research projects, we study the transfer of cell membrane-anchored receptors from antigen-presenting cells to T lymphocytes. Specifically, we investigate the transfer and fate of peptide-major-histocompatibility complexes (pMHC) as well as co-stimulatory molecules. For this we have generated libraries of different fluorescence-tagged pMHC and co-stimulatory molecules as well as different genetic reporter systems. Using confocal and total-internal reflection microscopy as well as flow cytometry as readout systems, we can track the dynamics of intercellular molecule transfer. Observations made in these in vitro systems are complemented by in vivo experiments to confirm their physiological relevance.