Cell discovery might lead to therapies to shrink death toll in flu pandemic

    Robert G. Webster, PhD

    St. Jude researchers have discovered a key factor of fatal influenza virus infection that suggests a possible new way to treat the disease and to greatly reduce the death toll of a potential worldwide pandemic like the 1918 Spanish flu.

    “We have pinpointed one of the cell types that damage the lung during influenza infection, and we have shown a method to at least partially reduce these cells,” said Jerry Aldridge Jr., PhD, Infectious Diseases, the study’s first author.

    The discovery, recently published in the advance online issue of Proceedings of the National Academy of Sciences, takes on particular significance because of concerns that the H5N1 avian influenza A virus, commonly known as bird flu, might cause the most deadly pandemic since 1918. The Spanish flu killed more than 40 million people worldwide, about 2.5 percent of Earth’s population at the time. The toll included many healthy young adults, an age group that rarely dies from influenza.

    “If the current H5N1 virus does acquire the ability for consistent human-to-human transmission, either by mutation or by reassortment with a current human virus, then the death toll would likely be much more catastrophic than the Spanish influenza of 1918 and underlies the importance of these studies,” said Robert Webster, PhD, Infectious Diseases, a co-author of the study.

    Birds can transmit H5N1 to humans, but no case of human-to-human spread has been confirmed. Avian influenza has killed 62 percent of the 413 people with confirmed cases, many of them otherwise healthy young adults. Scientists fear that if the virus mutates to allow human-to-human transmission, a pandemic with the potential to be more deadly than the Spanish flu might rapidly engulf the world.

    In a series of lab experiments, the scientists used lethal strains of H1N1 or H5N1 and two sublethal influenza viruses to see what immune system components changed significantly in fatal influenza infections. The investigators identified a subtype of dendritic cells that proved lethal when markedly increased in numbers. Known as tipDCs, these cells were first identified in 2003.

    In additional experiments, the researchers discovered that pioglitazone, a drug prescribed for type II diabetes, significantly reduced the level of tipDCs and protected flu-infected mice from death.

    “I don’t think pioglitazone is the absolute answer in the event of a pandemic, but we have laid the groundwork for developing better therapies by defining one mechanism by which influenza viruses kill,” Aldridge said.

    Any successful treatment will need to address a surprising paradox discovered by the St. Jude team. In the researchers’ experiments, infected mice died when tipDCs numbered 50 percent to nearly 100 percent higher than the number of cells found in mice with nonfatal influenza infections.

    “We now need to understand what specific features of these cells are contributing to their highly pathogenic effect. It is likely a result of multiple mechanisms that we want to tease apart,” said Paul Thomas, PhD, Immunology, the paper’s senior author.

    TipDCs are regulated by a cell-surface receptor known as CCR2, without which the cells cannot leave the bone marrow. Working with a strain of bioengineered knock-out mice that had no CCR2 receptors, the researchers expected to find that when they infected the animals with lethal H1N1 or H5N1 viruses, the CCR2 deficient mice would fare better than those with the receptors.

    “We were shocked,” Aldridge said. “When we looked at infection in these knock-out mice, we found they didn’t fare better. This finding was likely due to the fact that the knock-out mice were impaired in their ability to clear virus infection.”

    The team then assessed all components of the immune system in an effort to determine how the mice without the CCR2 receptor differed from normal mice. The scientists found that CCR2-deficient-mice had significantly fewer antigen-specific killer T cells than those with the receptor. Once an influenza virus infects the body, killer T cells constitute the body’s major defense against the disease.

    “The benefit of a T cell is that it will only kill host cells infected with the virus,” Aldridge said. “The tipDCs, however, kill not only infected cells, but bystander cells, thus inducing unnecessary host damage.”

    Dendritic cells present invading antigens—in this case, protein particles from the influenza virus—to T cells, which stimulate the killer immune cells’ response. The researchers showed that one role of tipDCs in the lungs is to present influenza virus antigen to the T cells. Without tipDCs to activate killer T cells, the body cannot mount an effective attack against influenza virus.

    The work demonstrated that eliminating tipDCs in the lungs of mice did not protect against death. The team then undertook a medical literature search for something that might limit but not eliminate tipDCs. This led to the suspicion that pioglitazone might reduce the number of tipDCs in the airways and provide protection against deadly influenza strains.

    When the researchers tested the drug, they found mice treated with pioglitazone and exposed to lethal H1N1 influenza virus were less severely affected and recovered faster.

    The researchers have no answer to how tipDCs increase mortality in mice. They hypothesize, however, that the killer cells trigger a biochemical reaction called apoptosis, or programmed cell death, in uninfected bystander cells, which ends in those cells’ demise.

    Clues uncovered by the St. Jude team expose a weakness in lethal influenza infections.

    “We need to pinpoint how pioglitazone reduces inflammation without compromising the ability of the immune response to clear the virus,” Aldridge said. “Once we determine how it works, new therapeutic targets that finely modulate the immune response to the flu may be the result.”

    Other authors of this paper include David Boltz, PhD, Nicholas Negovetich, PhD, and John Franks, all of Infectious Diseases; Scott Brown and Peter Doherty, PhD, of Immunology; Carson Moseley, Pediatric Oncology Education student; and Cory Reynolds, formerly of St. Jude.

    This research was sponsored in part by the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Department of Health and Human Services and ALSAC.

    April 2009