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The saying goes that a picture is worth a thousand words. But these pictures could one day be worth much more -- the lives saved by the development of new vaccines.

Harvard University researcher Ulrich H. von Andrian showed the immune system of a live mouse being challenged by a foreign microbe during a presentation at Experimental Biology 2006. The footage gave many of the physiologists in attendance their first glimpse of these cells in action in a live animal.

Scientists have long studied the immune system, imagining how many of the component cells proceed on their microscopic duties. Until recently, there has been no way to directly view much of this complex cellular teamwork in vitro and in real time.

The von Andrian videos provided a real-time look at what happens inside a mouse lymph node when a foreign microbe enters the body. The videos were made possible by recent technological advances together with techniques the von Andrian lab developed.

*Henry Pickering Bowditch Award Lecture: "Migrants on a single-minded mission: How T cells find their antigen," 5:45 p.m., Sunday, April 2, Room 134, Convention Center Room 134. Speaker: Ulrich H. von Andrian, Harvard Medical School and CBR Institute for Biomedical Research, Boston, MA.

The American Physiological Society presents the Henry Pickering Bowditch Award for early-career achievement to a scientist younger than 42, whose accomplishments are both original and outstanding. It is the Society's second highest award.

A medical doctor who also earned a Ph.D. in neurology and neurosurgery, von Andrian was originally interested in microvascular integrity in the brain. But during a fellowship early in his career, he discovered his interest in the immune system, and made it his life's work.

Rare footage highlights presentation

A live mouse's T cells and dendritic cells are the stars of the von Andrian videos, as the immune system marshals its forces to fight off a new infection. His laboratory hones in on how dendritic cells -- professional antigen-presenting cells -- teach T cells to respond to an infection.

"This had never been observed in a live system before we obtained it," von Andrian said of the footage "Before this, what had gone on inside the lymph node was largely a black box." His team published their first work using the new approach in January 2004.

von Andrian's research has focused on how

  • tissues of the immune system recruits T cells
  • dendritic cells get to the right spot, the lymph node, to meet the T-cells
  • T cells and antigen-presenting cells find each other once in the lymph node
  • dendritic cells "educate" the T cells while in the lymph node
  • T cell and dendritic cell interactions change over time

In addition, the research focuses on how T cells, after their "education" is complete, migrate to effector sites elsewhere in the body to eliminate pathogens or tumors, but also may cause inflammatory diseases, von Andrian said.

Dendritic cells teach, T cells learn

"We specialize in various types of intravital microscopy to find out how leukocytes -- the white blood cells of the immune system that fight infection -- find their way around the body and how they communicate with other cells," von Andrian explained. "My lab is interested in leukocyte recruitment and trafficking, particularly in the lymph nodes."

The lymph nodes are the school, the dendritic cells the teachers and the T cells the students, von Andrian explained. Dendritic cells find foreign invaders, rip them apart and bring the pieces (antigens) via the lymphatic system to the nearest lymph node. When the dendritic cell arrives at the lymph node, it presents the dismembered invader to the T and B white blood cells in a way that rings the alarm about the intruder.

"The dendritic cells alert the T cells that there is something foreign entering the body," von Andrian explained. "They tell the T cells what to do, how to respond. We have known about some aspects of this process from tissue culture studies, but this has never been observed before in vivo. That has largely been a black box," he said.

His group has explored the first days after a T cell encounters the one antigen it will learn to recognize, von Andrian said. During this time interval, T cells first migrate rapidly and touch every dendritic cell for only a few minutes. This initial phase lasts about eight hours. Then the T cells pick one antigen-presenting dendritic cell and sticks to it for many hours.

After one day, the T cells disengage from the dendritic cell and begin to rapidly reproduce. About three days later, they depart from the lymph node on their search-and-destroy mission.

Homing in on the target

T cells patrol the body by hitching a ride in the blood, methodically stopping at each lymph node like a night security guard making the rounds and checking the doors. There are dendritic cells in the lymph nodes, waiting to present antigens in a way the T cells can recognize.

Each T cell recognizes only one antigen, but preserves that knowledge in its progeny. On average there are 6,000 T cells circulating in the human body that recognize a given antigen. Another 6,000 recognize a different antigen, etc. The human body is estimated to contain 25-100 million distinct T cell clones, he said.

When dendritic cells capture a foreign invader and bring it to the lymph node, many T cells stop by before one is able to recognize it. "The system, while cumbersome, works because of the amazing trafficking where they circle around and look for the presence of anything that might tickle their antigen receptor," von Andrian said.

If the T cells don't recognize the antigen, they hop back in the blood stream and head for the next lymph node. T cells and B cells can live for months, repeating this process and then passing the information on to their successors. The interaction between T cells and dendritic cells in the lymph node changes over time, von Andrian said.

"It's nice that the T cell learns to recognize the antigen, but that doesn't get rid of the microbe that has entered the body," von Andrian said. He traced the path of the educated T cells leaving the lymph node, when they become known as effector cells. In addition, von Andrian addressed how these T cells kill those cells they identified as harboring a pathogen without going on a rampage and annihilating everything in sight.

"There must be a control mechanism that can keep these cytotoxic cells in check," von Andrian noted. The immune system does this by creating regulatory T cells to control the effector cells, he said. Previous research had shown that the regulatory T cells can also receive their education in the lymph node before proliferating and entering the blood stream.

May lead to vaccines

"The events I have described are at the heart of any vaccination," von Andrian said. Many pathogens take advantage of the lymph system to gain access to the body, he noted, including pathogens used in biological warfare, such as anthrax, or yersinia pestis, the agent that causes the plague.

Bubonic plague, for instance, starts with a flea that deposits a few bacteria, which travel via the lymphatic system to the lymph node and proliferate to huge numbers. One of the puzzles von Andrian has been working on, is how foreign invaders manage to proliferate in the very area, the lymph node, where the immune system is marshaling its troops.

"So understanding how these (foreign) cells survive in the lion's den of the lymph node might allow us to develop strategies to combat infections," von Andrian said. "Somehow, they do this without ringing the alarm bells. Possibly we could manipulate the process to ring the alarm bell for the immune system when it fails to recognize the danger."

Technological leap makes it possible

Intravital microscopy, invented in the early 19th century, allows scientists to look at the tissues and cells of live animals. The technology has been in use for generations. But particularly advanced and powerful multi-photon microscopes using infrared beams to image fluorescent cells in living tissues became commercially available in the late 1990s.

That prompted von Andrian to embark on a project to use the technology to study the immune system. He set up his lab at the CBR Institute and developed a strain of mouse that has fluorescent T-cells.

"We are among a handful of labs which started to do this at the same time," von Andrian said of using the technology to study the immune system. The microscopes produce optical sections through solid organs, similar to a CT-scan. Hundreds, even thousands of these sections are assembled into digital time-lapse videos to generate a 3-dimensional look at an immune response in real time.

The organism remains intact, providing a live look at what is happening in the immune system. The challenge in adapting the technology was that it takes 15-30 seconds to generate the necessary series of photographs to create the 3-dimensional footage and the animal must remain perfectly still for it to work.

It took a year to figure out how to keep the anesthetized animal from moving, he said. Others have also worked to develop and advance the technology, von Andrian said. His lab's distinction was in adapting the technology for use in lymphoid organs in live animals.

Source : American Physiological Society

April 3, 2006 02:47 AMMolecular & Cell Biology

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