Salk Institute for Biological Studies


Björn F. Lillemeier

Björn F. Lillemeier

Assistant Professor
Nomis Foundation Laboratories for Immunobiology and Microbial Pathogenesis
Waitt Advanced Biophotonics Center



One of the central challenges in biology is to elucidate how microenvironments modulate molecular mechanisms and thus global cellular function. The Lillemeier lab studies signal transduction in the plasma membrane of T lymphocytes (T cells) upon their activation by Antigen Presenting Cells (APCs). Major rearrangements of signaling molecules take place during this event, which is most dramatically seen in the formation of signaling microclusters and the immunological synapse (see movie). The lab uses cutting edge super-resolution and dynamic fluorescence microscopy techniques (e.g. PALM and FCCS) in combination with traditional biochemical and molecular biological approaches to study the molecular patterns that regulate and are required for T cell activation and function.

We have found a new type of plasma membrane domains, termed protein islands, and the specific segregation of all membrane-associated proteins within them. These findings inspire a new and unsuspected role for the plasma membrane in the spatio-temporal regulation of T cell activation and membrane biology in general. Specifically, we found that signaling cascades are prearranged into 'building blocks', through the localization of signaling molecule subsets within specific protein islands. Redistribution of these protein islands in response to stimuli can lead to either concatenation and assembly of signal transduction pathways or their dissociation and disassembly. These rearrangements are not diffusion limited but active and directed through cytoskeletal forces and pathway-specific protein-protein interactions.

We welcome students and postdocs to join our lab. For more information please contact Björn Lillemeier.

Movie: A primary T cell becomes activated on a glass supported lipid bilayer containing its natural ligands. T cell receptor (TCR) molecules, labeled with enhanced green fluorescent protein (eGFP), form microclusters at the entire the contact site between T cell and bilayer. TCR microclusters are transported towards the center of the contact site in an actin dependent fashion and form the center of an immunological synapse. The movie was acquired in total internal reflection (TIRF) mode to eliminate intracellular fluorescence.

"While signal transduction is traditionally seen as a sequence of protein interactions and modifications, it has become clear that these events are also spatially controlled through plasma membrane compartmentalization. To further understand this, my lab studies the architecture of the plasma membrane in general, as well as its contribution to signal transduction in T cells."

In eukaryotes, the plasma membrane–a double layer of lipid molecules that encloses all cells–not only segregates the cell from its environment, but also serves as the principal interface for communication between cells. Not surprisingly, the plasma membrane's structure and properties impact many biological processes. T cells, whose main job is to fight infection, for example, utilize and reorganize their plasma membrane constantly during activation and effector functions. This is most dramatically seen in the establishment of signaling microclusters and the formation of the immunological synapse between T cells and antigen-presenting cells upon activation of the former by the latter.

Despite a lot of interest in the past in the precise architecture of the plasma membrane, studies of plasma membrane-associated signaling had been hampered by technical barriers such as cell lysis and limited resolution in microscopy. Lillemeier overcame these limitations through the use of novel high-resolution imaging techniques such as photo-activated localization microscopy (PALM) and dual color fluorescence crosscorrelation spectroscopy (dcFCCS), which allowed him to observe directly the spatial and temporal distribution of membrane-associated molecules on a nanometer scale.

He discovered that all membrane-associated proteins in the cells that he examined are clustered into what he refers to as "protein islands," which led him to postulate a novel concept for the general architecture of plasma membranes. Lillemeier also found that the T cell receptor signaling cascade is spatially and temporally controlled through the segregation and association of distinct membrane microdomains (protein islands) that contain specific subsets of T cell signaling molecules. He believes that this type of signal control may be a general feature of membrane-associated signaling and is probably used in a variety of signaling processes.

Lillemeier will expand his research to understand how this higher order in the plasma membrane is achieved and what molecular mechanisms are in place to utilize it during signal transduction. His studies will help to expand knowledge of spatio-temporal signaling control, which will suggest new approaches in manipulating the response of the immune system to pathogens and diseases.

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