Dec 9, 2010 (CIDRAP News) – A team of US and Argentine researchers has proposed an unusual biological mechanism to explain severe pandemic H1N1 influenza cases in nonelderly adults, involving antibodies that react with but fail to stop the virus.
The scientists, from Vanderbilt University in Nashville and several institutions in Buenos Aires, say that the antibody response in some middle-aged adults, shaped by previous exposures to influenza, not only failed to block the new virus but also made things worse by activating the complement system, leading to a damaging accumulation of proteins in the lungs.
The researchers arrived at their conclusions by analyzing and comparing samples from adults and children infected with the pandemic H1N1 virus and with recent seasonal flu viruses and by studying archived samples from patients in the 1957 flu pandemic. They reported their findings this week in Nature Medicine.
The scientists, led by senior author Fernando P. Polack, say their observations "provide a previously unknown biological mechanism for the unusual age distribution of severe cases during influenza pandemics."
Younger adults hit hard
The burden of severe and fatal 2009 H1N1 cases fell most heavily on nonelderly adults, while the elderly were relatively spared, and young children had milder disease, the authors note. The elderly were believed to be protected because of their exposure before 1957 to H1N1 viruses related to the 2009 H1N1 strain.
A possible part of the explanation for this age pattern, the authors write, is that in the nonelderly adults, "an antibody repertoire shaped by seasonal infections may recognize but fail to neutralize the new pandemic strain, leading to immune complex–mediated disease." (Immune complexes are antibodies bound to antigens).
To test the hypothesis, the researchers studied respiratory and blood samples from 75 adults with pandemic H1N1 infections, including 21 outpatients and 54 who were hospitalized, with a median age of 39. Twenty-three of the patients died, and 15 of those who survived needed intensive care. Also studied were samples from infants and young children who had pandemic H1N1 and from adults hospitalized with seasonal flu infections in 2007 and 2008.
The investigators also examined the lungs of H1N1 patients who died, and found evidence of hemorrhaging, swelling of the alveoli, and other kinds of damage.
The scientists found antibodies to the pandemic virus in both elderly and nonelderly adults, but antibody "avidity" for the virus was lower in the younger adults, and this group lacked sufficient levels of antibody to neutralize the virus, the report says. The younger adults' antibodies had a greater ability to bind to an earlier (1999) seasonal H1N strain than to the 2009 virus. Further, severely ill patients had antibodies with lower avidity for the 2009 virus than did mildly ill patients.
In light of the role of nonprotective antibody responses in certain other diseases, such as respiratory syncytial virus, the authors examined lung sections for a complement system protein called C4d. The complement system, part of the innate immune system, consists of a number of proteins that circulate in the blood and, when triggered by any of various factors, initiate a cascade of events that help antibodies clear pathogens from the system.
"We detected extensive C4d deposition in bronchioles of individuals infected with 2009 H1N1, matching the distribution in immune complex mediated diseases due to other viruses," the report says. In contrast, the team found only traces of C4d in the lungs of persons who had seasonal flu.
The scientists also examined archived lung sections from adults who died in Tennessee during the H2N2 flu pandemic of 1957. They found C4d deposition in and around the bronchioles. As a control, they also looked at an archived sample from a person who had no lung infection, and found no C4d.
No cytokine storm
In other steps, the researchers examined their samples for evidence of "cytokine storm"—an immune-mediated outpouring of chemical messengers that triggers intense lung inflammation. Cytokine storm has been suggested as a cause of or contributor to death from influenza.
The team found little evidence of cytokine storm, with "low cytokine concentrations" in secretions from pandemic H1N1 patients. For example, samples from pandemic and seasonal flu patients had similar amounts of such cytokines as tumor necrosis factor-alpha, interleukin-6 (IL-6), IL-10, and IL-12.
"Taken together, these observations indicate that 2009 H1N1 influenza virus leads to immune complex–mediated disease in adults through high titers of low-avidity nonprotective antibody and immune complex–mediated complement activation in the respiratory tract," the report states.
"Immune complex–mediated disease also contributed to fatal cases cause by 1957 H2N2 pandemic influenza. We speculate that this phenomenon contributes to severe symptoms in the middle-aged adult population during all pandemics," it adds.
Findings called intriguing
Vincent Racaniello, PhD, a virologist at Columbia University and author of Virology Blog, said he found the report "intriguing." Racaniello, who is Higgins professor in Columbia's Department of Microbiology and Immunology, was not involved in the study.
"I have always wondered if immune-complex disease is a component in influenza, as it is in respiratory syncytial virus and dengue virus infections," he commented by e-mail. "In particular, non-neutralizing antibodies are believed to lead to severe dengue disease during second infections with a different serotype.
"In this case, non-neutralized virus-antibody complexes appear to lead to deposition of complement, which causes tissue damage. This is not a cytokine storm; in fact the authors found no evidence for such in the lungs of victims of severe H1N1 disease."
To substantiate the authors' hypothesis, it will be important to identify, in patients previously exposed to flu, antibodies that bind to but fail to neutralize a new flu strain, Racaniello said.
"These observations make it even more compelling to immunize against pandemic strains. Presumably this would lower virus load and prevent deposition of immune complexes," he added. "In this vein the authors note that they have not determined whether there is also a contribution of direct viral injury to severe disease, an important point."
Racaniello said the findings imply that severe flu cases could be treated by disrupting immune complexes or interfering with the complement system. "Perhaps a better approach is to produce universal influenza vaccines, which would neutralize any strain, thus eliminating the pathogenic problem of non-neutralizing antibodies," he added.
Monsalvo AC, Batalle JP, Lopez MR, et al. Severe pandemic 2009 H1N1 influenza disease due to pathogenic immune complexes. Nature Med 2010; early online publication Dec 5 [Abstract]
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