October 22, 1999-Researchers have illuminated a crucial step in the immune system's response to infection by using live cell imaging to follow the movement of immune system cells that have been genetically manipulated to produce a fluorescent jellyfish protein.
The north Pacific jellyfish Aequoria victoria is thought to startle predators by emitting a bioluminescent glow using the green fluorescent protein (GFP). Molecular biologists have used GFP to their own advantage by splicing the gene for GFP into the genes of a number of proteins, thereby creating luminous proteins that are easily observed in living cells.
In recent years, researchers have used GFP to study a variety of cellular processes, including cell division in yeast and development of the nervous system in worms. Now HHMI associate Brian Schaefer and colleague Colin Monks, who are both at the National Jewish Medical and Research Center in Colorado, have used GFP and live cell imaging techniques to probe how T cells respond to infection.
The research team, which also included HHMI investigators Philippa Marrack and John Kappler and National Jewish investigator Gary Johnson, published their findings in the October 22, 1999, issue of the journal Immunity.
Monks and Johnson had been studying signal transduction in the facilitator cells of the immune system-so-called helper T cells. When helper T cells encounter foreign molecules, they kick the immune system into gear, alerting it to the presence of invaders and prompting the production of the appropriate antibodies.
But T cells don't find antigens on their own-the foreign molecules must be proffered by antigen-presenting cells (APCs), which recognize invaders, chew them up, and display parts of the destroyed enemy on their outer surface.
When an APC displaying antigen meets a T cell bearing the appropriate antigen receptor, the two stick together. This meeting alters gene expression in both cells and triggers an immune response.
Monks wanted to know how the signal travels from a receptor on the T cell surface to the T cell's nucleus where molecules that control gene expression are located. He had preliminary evidence that suggested that the protein MEKK2 concentrated around the T cell receptor after the receptor bound to its antigen. However, MEKK2's function in immune cells remained a mystery because MEKK2 could only be studied within its complex milieu by killing the activated T cells. "We were only able to look at a frozen snapshot of time in a very dynamic system," Monks said.
Enter Schaefer, who was working in the Marrack and Kappler lab to develop methods of using GFP-tagged proteins to observe molecular movements in immune system cells. When approached by Monks with the MEKK2 problem, Schaefer turned his attention to the problem of visualizing MEKK2 in active, living T cells. The two researchers, with help from others in the Johnson laboratory, developed a system to record digital movies of MEKK2 fused to GFP under conditions that did not disturb the activity of the helper T cells.
The resulting images bore out Monks' initial hypotheses about MEKK2. Within seconds of a helper T cell binding to an antigen, the green-tagged MEKK2 protein could be seen moving toward the T cell receptors.
Further experiments showed that interfering with MEKK2 blocked subsequent pathways that normally carry molecular signals to the nucleus. With these pathways blocked, the helper T cells failed to respond as they normally do after binding to an antigen, indicating that MEKK2 is crucial for the delivery of information.
In addition to sending the signal for altered gene expression to the nucleus, MEKK2 also contributes to the active attachment between the two cells. "MEKK2 is directly involved in the event that causes a T cell to become sticky and to hold onto an APC," said Monks.
This active attachment of the T cell to the APC is critical for immune response activation. "Stabilization is biologically important in order for the T cell to remain attached to the APC long enough to get all the information it needs," Schaefer said.
Schaefer and other scientists in the Marrack and Kappler laboratories plan to use GFP and live cell imaging to find other signaling proteins involved in triggering immune cell activity. They also plan to use this technique to examine the dynamics that take place within cells during immune cell activation.