MEMPHIS, Tenn. -- Investigators at St. Jude Children's Research Hospital have shown in mice how the potentially deadly cooperation between influenza virus and bacterial pneumonia infections can be foiled, even if treatment is delayed and flu virus levels in the lung have peaked.
The St. Jude team showed that the flu virus enzyme neuraminidase (NA) strips lung cells of their protective forest of molecules called sialic acid. The unprotected cells are then vulnerable to subsequent infection with Streptococcus pneumoniae bacteria. But treatment of the mice with the NA- inhibitor drug oseltamivir blocked the activity of flu virus NA and offered significant protection against pneumonia. The study's findings appear in the March 15 issue of the Journal of Infectious Diseases.
"We previously showed that when Streptococcus pneumoniae was given to mice seven days after they had been infected with flu virus, the mortality was extremely high," said Jonathan McCullers, MD, the lead author of the report. "Our current study using tissue in lab dishes, as well as live mice, showed that viral NA is the key to this deadly synergy between flu virus and pneumonia bacteria. The streptococcus bacteria also have NA," McCullers said, "but it is much less efficient than the viral enzyme in cutting away at the lung cell's sialic acid barrier."
The St. Jude finding is important because a combined infection with influenza and pneumonia is the sixth leading cause of all deaths worldwide, and is the top cause of death due to an infection, according to McCullers assistant member in the Department of Infectious Diseases at St. Jude. Moreover, the overall death rate associated with pneumonia and influenza increased by 59 percent between 1979 and 1994, according to a 1996 report in the Journal of the American Medical Association.
"The flu virus targets the sialic acid molecules on the lung cell membrane surface so it can get inside the lung cell and reproduce," McCullers said. "After the virus multiplies inside the cell, the new viruses cut through the sialic acid on the way out."
"By blocking the virus NA we protect the sialic acid molecules covering the lung cells," says McCullers. "That leaves the pneumonia bacteria stranded outside the cells and gives the immune system time to destroy them before they can cause respiratory disease."
In the study, one group of mice received prophylactic (protective) doses of oseltamivir for five days, beginning four hours before being infected with flu virus. Another group got oseltamivir for five days starting 48 hours after being infected with flu.
Mice that received delayed treatment lost weight and had greatly increased levels of virus in their lungs by the time they received oseltamivir. However, these mice still had significant protection against subsequent infection by streptococcus bacteria compared to control mice that received no treatment.
"The key to significantly reducing the rate of death in these mice was not reducing virus levels or maintaining weight," McCullers explains. "What was important was blocking the virus NA enzyme with oseltamivir and preserving the protective sialic acid molecules."
The St. Jude team also studied the effect of oseltamivir on bacterial pneumonia using live mice infected with genetically modified streptococcus. These modified bacteria were engineered to carry on a light-producing chemical reaction-the same chemical reaction used by fireflies. The light, too faint for humans to see, passed through the skin of the mice and was picked up by a special camera.
In mice given flu virus and then the "glowing" streptococcus but no oseltamivir, the camera captured images of extensive bacterial infections throughout the thorax-the part of the body containing the lungs. In mice that also received oseltamivir as a delayed treatment after infection with flu virus, the light emitted from the lungs was less intense and not localized in specific areas. This suggested that the immune system of the infected mice was able to kill many of the bacteria that were "stranded" outside the cell membrane in oseltamivir-treated animals.
Untreated mice developed pneumonia earlier than treated mice and died. In treated mice, even when infection spread, the progression of the disease was slower than in untreated mice, and up to half of the treated animals survived. When the researchers studied the effect of oseltamivir using human lung cells in a dish, they found that the drug blocked the binding of streptococcus to these cells by interfering with NA, similar to the findings in mice.
The article's co-author is Rhodes College pre-med student Kimberly A. Bartmess, who works in McCullers' lab as a part of the St. Jude - Rhodes Summer Plus Program. This project was supported by the National Institute of Allergy and Infections Diseases at the National Institutes of Health, a Cancer Center Support Grant and ALSAC.
Source: St. Jude Children's Research Hospital