Immune System of Healthy Adults May be Better Prepared Than Expected to Fight 2009 H1N1 Influenza

A new study shows that molecular similarities exist between the 2009 H1N1 influenza virus and other strains of seasonal H1N1 virus that have been circulating in the population since 1988. These results suggest that healthy adults may have a level of protective immune memory that can blunt the severity of infection caused by the 2009 H1N1 influenza virus.

The study team was led by Bjoern Peters, PhD, and Alessandro Sette, PhD, of La Jolla Institute for Allergy and Immunology, grantees of the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health.

The investigators looked at molecular structures known to be recognized by the immune system—called epitopes—on 2009 H1N1 influenza and seasonal H1N1 viruses. Viral epitopes are recognized by immune cells called B and T cells: B cells make antibodies that can bind to viruses, blocking infection, and T cells help to eliminate virus-infected cells.

Using data gathered and reviewed from the scientific literature and deposited into the NIAID-supported Immune Epitope Database and Analysis Resource, the investigators found that some viral epitopes are identical in both the 2009 and seasonal H1N1 viral strains. Those epitopes that could be recognized by two subsets of T cells, called CD4 and CD8 T cells, are 41 percent and 69 percent identical, respectively.

Subsequent experiments using blood samples taken from healthy adults demonstrated that this level of T-cell epitope conservation may provide some protection and lessen flu severity in healthy adults infected with the 2009 H1N1 influenza virus.

Analysis of the database also found that among six viral surface epitopes that can bind antibody, thereby preventing infection, only one is conserved between 2009 and seasonal H1N1 viral strains.

These results suggest that healthy individuals may have immune memory that recognizes the 2009 H1N1 strain and therefore can mount some measure of an immune attack. The findings also may help explain why the 2009 H1N1 influenza pandemic affects young children more severely than it does healthy older adults and also why two H1N1 vaccinations are needed to protect children ages nine years and under.

Reference: Greenbaum J, et al. Pre-existing immunity against swine-origin H1N1 influenza viruses in the general human populace. Proceedings of National Academy of Sciences. DOI: 10.1073/PNAS.0911580106 (2009).