New mouse model shows how news of pathogen reaches immune system

Answer revealed in glowing live cells

Using a new mouse model that literally glows with health- protecting molecules, researchers have rewritten part of the textbook tale about how the immune system knows when to fight germs.

Time-lapse video from a pair of Harvard Medical School labs shows how pieces of captured germs may work their way to the surface of live dendritic cells. Dendritic cells are immune cells that alert other immune cells about invading germs. Inside a dendritic cell, the video shows, the molecules with the job of carrying germ fragments shoot toward the T cell along surprisingly long tubules. The fragments, called antigens, alert T cells to kill the invading germs.

These are the first studies using cells from a new genetically engineered mouse, whose antigen-carrying molecules have been genetically tagged with green fluorescent protein. The study is published in the August 29 issue of the weekly journal Nature.

“We assume this dialogue between the dendritic cell and the T cell improves the efficiency of the immune response,” said senior author Hidde Ploegh, Edward Mallinckrodt Jr. professor of immunopathology at Harvard Medical School.

The action was recorded by a special microscope for viewing living cells recently developed in the laboratory of Tomas Kirchhausen, HMS professor of cell biology and senior investigator at the Center for Blood Research.

In the body, dendritic cells and other cells of its type initially handle all infections in the body. Dendritic cells lurk in the skin, lungs, gut and other tissues. On sentry duty, they continually snack on things around them, which might include a pathogenic bacteria that has sneaked past the skin barrier, stomach acid or other innate defenses. When a dendritic cell has gobbled a harmful germ, it stops snacking and races off to the nearest lymph node to alert the T cells that command the more complex immune responses.

The details have been fuzzy, but scientists believed dendritic cells kept busy on the journey by digesting bacteria in its bowels, loading the resulting pieces of antigen onto antigen-carrying molecules known as major histocompatibility complex molecules (MHC) class II, and shipping the complex out to the surface of the cell. The dendritic cell supposedly arrived in the lymph node with the antigen complex in full view, like a peacock with all its feathers on display, an act referred to as antigen presentation. Each T cell senses a different kind of germ antigen. In the lymph node, many varieties of T cells swarmed around until the right one came along. Or so the story went.

Now, it appears that dendritic cells may save most of their antigen cargo for the right T cells. “Within minutes of contacting T cells, class II molecules are directed from the compartment inside the cell via extraordinarily long (up to 50 microns) tubules along the microtubule railroad, right to the dendritic cell-T cell interface,” said Jonathan Yewdell, an immunologist at the National Institute of Asthma and Infectious Diseases in a commentary accompanying the two new studies. “In the case of a promiscuous dendritic cell interacting with multiple T cells, multiple tubules form simultaneously to deliver class II molecules to each T cell.” In cell cultures where an unsuitable T cell was added, the dendritic cell kept it class II cargo to itself.

“It’s the first time we have seen that dendritic cells are active players in stimulating antigen presentation to antigen-specific T cells,” said postdoctoral fellow Marianne Boes, the first author of the paper at Harvard Medical School.

If the tubule talk between the dendritic cell and T cell is as important to the immune system as the researchers think it is, then it’s likely that some bacteria can disable this mechanism. “We should be on the lookout for bugs that throw a spin in the works and prevent this dialogue from happening,” Ploegh said. “It’s a recurring theme of host-pathogen interactions: Every move by the immune system is counteracted by a clever adaptation of the bug. It’s a never-ending game.”

Designed by Boes, a postdoctoral fellow in the Ploegh lab, the mouse is believed to be the first whole-animal model of antigen presentation in real time. Tests show the mouse’s immune system is normal and apparently unaffected by the green glowing protein. For these experiments, Boes began with stem cells from the mouse’s bone marrow and cultured them in a test tube to become dendritic cells.

About the time the mouse cells were ready to study, Ploegh and Boes heard about a fancy new microscope for viewing living cells. Boes teamed up with Jen Cerny and Ramiro Massol, postdoctoral fellows in Kirchhausen’s lab, who collaborated on the imaging studies. In early experiments, the tubules showed up as glowing streams of class II MHC molecules. Other experiments found the tubules formed like train cars linking up on the tracks of the cell’s cytoskeleton.

In a related paper, time-lapse video from Yale researchers uses another new microscope technique to show that antigen-hauling MHC class II molecules carried their cargo from the acidic bowels of the dendritic cells to the surface. Once the cargo arrives on the surface, it can presumably alert a T cell to fight the infection. For these experiments, graduate student Amy Chow, first author of the paper, used a virus to genetically tag mouse stem cells in culture and turn them into dendritic cells in a test tube.

“This is important to the average person,” said senior author Ira Mellman, cell biology department chair and professor of immunobiology at Yale. “The entirety of your ability to respond to any foreign antigen, your ability to become vaccinated and the ability of your immune system to maintain a balance between identifying foreign substances and its own self proteins all are critical activities controlled to a surprising extent by dendritic cells. The entire world of immunology has been T-cell-centric for 20 years, but it’s becoming clear that antigen presenting cells play an equally important role in initiating or suppressing an immune response.”

The studies are part of a new era in immunology research where scientists can see live elements of the immune system, Yewdell said.