"Our findings suggest a potentially important design principle for vaccines and challenge the prevailing theory used for vaccine design," said Christopher C. Norbury, Ph.D., assistant professor of microbiology and immunology, Penn State College of Medicine, Penn State Milton S. Hershey Medical Center. "Ultimately, our information suggests that vaccines should target both pathways that generate T cells, which are our killer cells, to allow the most efficient protection against viruses."
The study, conducted by Penn State College of Medicine and National Institutes of Health investigators and titled, "CD8+ T Cell Cross-Priming via Transfer of Proteasome Substrates," was published May 28, 2004, in the journal Science.
Some viruses, like chickenpox, travel outside of cells in the body's bloodstream. When introduced to the body, these viruses trigger antibodies that destroy the virus. For example, the chickenpox vaccine contains dead virus that, when administered, trains the body to recognize and create antibodies to kill the virus. The body is then prepared to react if exposed to live virus.
However, some viruses, like human immunodeficiency virus (HIV) and smallpox, travel inside cells where antibodies cannot penetrate. These viruses are transferred from cell to cell and only specialized white blood cells called T cells, which can penetrate cell walls, can kill them.
"There's a great deal of research going into the design of vaccines to target these types of difficult-to-destroy viruses," Norbury said. "Most focus on how to boost or improve the body's own T cell response to the virus."
CD8+ T cells play an important role in the elimination of tumor cells and disease-causing agents such as viruses. For the T cells to respond to a viral infection, "professional" antigen-presenting cells (pAPCs) must present small pieces of viral proteins, called peptides, on their surfaces. This triggers a seek-and-destroy response in T cells.
There are two pathways through which to activate the T cells. In one pathway, called direct priming, the small pieces of viral protein presented on the pAPCs surfaces can be generated when the infected protein is made by pAPC. Alternately, the other pathway, known as cross-priming, involves transfer of some form of protein, previously thought to be peptides, to the pAPC where it can be presented to T cells.
"Although direct priming is relatively well-understood, less is known about cross-priming," Norbury said. "The prevailing hypothesis for cross-priming suggests that peptides are transported by carrier proteins from virus infected cells to pAPC."
Norbury and colleagues used vaccinia virus - the virus in the smallpox vaccine - and influenza A virus, in a mouse model to further investigate how the cross-priming pathway works. The team found that it is not peptides that are shared with recipient cells, but rather full-length, intact proteins. And, whereas, the previous theory suggested that proteasomes in the donor cell cut up the proteins into peptides prior to transfer to the recipient cell, Norbury's team found that the proteins are taken up by pAPCs and cut up after they are transferred to the recipient cells.
In a previous study, the team found that the protein pieces presented via direct priming are usually very short-lived but produce peptides very efficiently. However in cross-priming, because transfer to another cell takes time, rapidly degraded proteins are inefficient at generating peptides.
"Some viruses can only be seen via cross-presentation. Thus, it's important to make vaccines targeting both pathways," Norbury said. "If the proteins used in the vaccine are rapidly degrading, they will only target one pathway. Therefore vaccines should include both types of proteins."
In addition to Norbury, study investigators were: Jonathan W. Yewdell, Sameh Basta, David C. Tscharke, Michael F. Princiotta, Peter Berglund, James Gibbs, and Jack R. Bennink, Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, and Keri B. Donohue, graduate student, Department of Microbiology and Immunology, Penn State College of Medicine, and Integrative Biosciences Graduate Program, Penn State University Huck Institute for Life Sciences.