New research shows that a protein made by a cancer virus causes infected immune cells to cling to other immune cells, enabling the virus to spread.
The virus, the human T lymphotropic virus type 1 (HTLV-1), is transmitted mainly when infected cells known as T lymphocytes, or T cells, touch uninfected T cells.
The finding helps explain how this cell-to-cell transmission happens. It suggests that an HTLV-1 protein known as p12 activates infected T cells and causes them to become sticky and adhere to other T cells.
The greater stickiness happens because the p12 viral protein causes special adherence proteins found on the surface of T cells to cluster in large groups - something that normally happens when T cells touch to communicate with one another during an immune response.
The findings also suggest that a drug that inhibits the p12 protein might also help prevent HTLV-1 transmission.
The research, published in the May issue of the Journal of Immunology, was led by scientists with The Ohio State University Comprehensive Cancer Center and the OSU College of Veterinary Medicine.
"This study indicates that the p12 protein plays an important role in programming infected cells for cell-to-cell transmission," says principal investigator Michael D. Lairmore, professor and chair of veterinary biosciences and a member of the OSU Comprehensive Cancer Center.
"It shows that this virus takes advantage of something that T cells do normally, but, in this case, the virus is stimulating the interaction with other T cells rather than a normal immune response."
HTLV-1 infects an estimated 15 to 25 million people worldwide. About 5 percent of those infected develop adult T cell leukemia or lymphoma (ATLL), an aggressive disease characterized by a long latent period and the proliferation of T cells. The infected cells are spread from person to person during sexual activity and by blood and breast milk.
In the body, the HTLV-1 mainly targets immune-system cells known as CD4 T lymphocytes. These immune cells coordinate immune responses in part through physical contact with other immune cells. The cells adhere to one another using a protein known as LFA-1, which is found on the cells' surface.
In this study, Lairmore and his collaborators examined the influence of the p12 protein on LFA-1 adhesion. The researchers compared cells infected with HTLV-1 that lacked the p12 protein to cells that were infected by normal HTLV-1.
They found that the p12 protein not only activated the T cells, but caused the cells infected with normal HTLV-1 to have far greater adherence than cells infected with viruses that lacked p12 in a standardized adherence test.
In addition, they showed that the greater adherence did not occur because the infected cells made more of the LFA-1 protein, but rather because already existing LFA-1 protein molecules gathered into large clusters on the cell surface.
(LFA-1 proteins float in the cell membrane like buoys in semisolid gelatin. They can move across the surface of the cell and form clusters when directed to do so by signals from within the cell.)
"Our study is the first to show that HTLV-1 p12 not only enhances the activity of infected T cells, but that it promotes the spread of the virus from cell to cell by causing LFA-1 receptors to cluster on the cell surface," Lairmore says.