Led by immunologist Ronald Germain, M.D., Ph.D., the scientists took videos through a microscope to document what happens inside the lymph nodes of a living mouse shortly after a vaccination. The videos reveal that the movement of a specific type of immune cell known as a CD8+ T cell, also called a cytotoxic T cell, is not random as was previously thought, but instead is guided by chemical signals released from other cells.
Scientists have long recognized the importance of understanding how CD8+ T cells move through the lymph nodes and become activated. Once active, CD8+ T cells roam throughout the body destroying cells infected with bacteria or viruses--a process known as cell-mediated immunity. When these CD8+ T cells encounter an infected cell, they unleash a torrent of substances that poke holes in the cell's membrane, chew up its proteins and ultimately cause it to die. They also produce molecules such as interferon-gamma that help activate other immune cells.
After they fight the initial infection, some of these CD8+ T cells remain in the circulation as memory cells, primed to fight if the host is re-infected with the same pathogen. Memory cells are key to vaccine strategies being studied for infectious agents such as HIV. But the CD8+ T cells can only become effective, long-lived memory cells after they encounter certain other cells in the lymph node that can activate them.
The new research, conducted largely by senior postdoctoral fellows Flora Castellino, M.D., and Alex Huang, M.D., Ph.D., with Dr. Germain's guidance, shows that when CD8+ T cells enter the lymph node, a combinatio n of specific physical and chemical cues guides them to sites where they receive activation signals. Specifically, two molecules known as chemokines help guide them toward the cells that release these activation signals.
"Understanding the processes whereby CD8+ T cells find their way in the lymph nodes is important because their activation is essential for eliminating infected cells and for providing, together with antibodies, long-lasting protection following vaccinations," says NIAID Director Anthony S. Fauci, M.D.
The body contains hundreds of millions of CD8+ T cells, but only a tiny fraction of them become activated during an infection. These are selected because each CD8+ T cell carries a unique surface protein called a T-cell receptor, which recognizes only specific antigens (pieces of virus or bacteria that trigger the immune response). During an infection, CD8+ T cells that recognize antigens from the infecting pathogen are activated. These antigen-specific CD8+ T cells expand into a large population of active clones, which then sweep through the body, hunting down and killing infected cells.
For CD8+ T-cell activation to occur in the lymph node, the cell must encounter its target antigen--but that antigen must be displayed on the surface of another immune system cell, called a dendritic cell. Usually a third type of cell, known as a "helper" T cell, must be involved as well. But how do the CD8+ T cells find the right dendritic cells presenting the specific antigen they need to see? Moreover, how do they find the particular dendritic cells that have been properly stimulated by helper T cells?
Dr. Germain and his colleagues determined that naïve CD8+ T cells do not wander aimlessly through the lymph node but instead are steered towards areas in which dendritic cells concentrate. Think of the lymph node as a large airport terminal and the CD8+ T cells as the arriving passengers, says Dr. Germain. If passengers know that limo drivers will meet them in the terminal, they will look for their drivers upon arrival. Rather than hoping to run into each other by chance, the drivers crowd around the arrival gates and hold up signs that the passengers can read from a distance.
Moreover, CD8+ T cells are chemically attracted to the cells that might activate them by the chemokines these other cells produce. Dr. Germain and his colleagues demonstrated that when CD8+ T cells enter the lymph nodes and detect a potential infection, they express receptors that allow them to detect and follow these chemokines.
The NIAID team also showed that when dendritic cells interact in specific fashion with helper T cells, the activated cell pair releases the chemokines CCL3 and CCL4. It is the combination of these two chemokines that the CD8+ T cells receive best as a signal, says Dr. Germain. By interfering with the action of these chemokines, he and his colleagues demonstrated that CD8+ T cells lost their ability to home in on the dendritic cells interacting with the helper T cells. The result was a marked impairment of memory cell generation.
These new findings not only provide insight into the fundamental behavior of the immune system, but also suggest that attention needs to be paid to chemokines and chemokine receptor function when designing new vaccine strategies and evaluating whether drugs targeting chemokines might have unanticipated effects on immune function.