How Severe Will RSV Be? Immune Factors Make a Difference

Newswise — Respiratory syncytial virus (RSV) is a very common virus that causes severe respiratory illness in a small percentage of infants. Which babies will develop severe RSV illness? Low levels of certain types of immune system cells may have an impact, according to a study in the October issue of The Pediatric Infectious Disease Journal. The journal is published by Lippincott Williams & Wilkins, a part of Wolters Kluwer Health, a leading provider of information and business intelligence for students, professionals, and institutions in medicine, nursing, allied health, and pharmacy.

"In infants younger than six months, severe RSV infection is associated with a peripheral impaired immune response, especially during the first days of illness," according to the study by Dr. Carmen L. Larrañaga and colleagues of University of Chile, Santiago.

Differences in Cell Counts Linked to Severity of RSV IllnessThe researchers performed in-depth studies of immune activity in 69 infants hospitalized for RSV pneumonia. All were between one and six months old—a time at which the infant's developing immune system is still immature. The goal was to determine the immune system characteristics of infants with severe RSV illness, compared to those with less severe RSV disease or healthy babies.

For infants with severe RSV illness, certain types of immune system activity were significantly reduced. In particular, severely ill infants had reduced levels of CD8+ cells and certain types of "natural killer" (NK) cells—both of which play a critical role in the immune system's ability to kill foreign invaders. The lower their numbers of CD8+ and NK cells, the more supplemental oxygen the infants required.

Babies with severe RSV disease also had reduced levels of certain cytokines—substances that carry signals for different types of immune system responses. In particular, infants with severe RSV had low levels of inflammation-promoting cytokines. Inflammatory responses play a central role in fighting infections.

Severe RSV illness during infancy may be a risk factor for asthma later in childhood. However, the reduced immune responses in babies with severe RSV were unrelated to their rates of wheezing in the year after hospitalization.

Reduced Immune Function Could Be Key Risk FactorEighty percent of infants become infected with RSV during the first two years of life—in most, the virus causes no symptoms or only a mild cold. However, about five percent of babies with RSV infection develop more serious illness, requiring hospitalization. Risk factors for more severe RSV disease have been identified, including premature birth, exposure to cigarette smoke, and certain types of lung and heart disease.

However, many infants hospitalized for RSV disease do not have any of these risk factors. The researchers sought to determine which characteristics of the immature immune system affected the risk of severe RSV illness.

The results suggest that reductions in CD8+ and NK cells and in inflammation-promoting cytokines may be the critical factors that determine why some babies are more likely to become severely ill from RSV infection. Dr. Larrañaga and coauthors hope their study will lead to new insights in the management of this common but potentially serious cause of respiratory illness in infants.

About The Pediatric Infectious Disease JournalThe Pediatric Infectious Disease Journal® (http://www.pidj.com) is a peer-reviewed, multidisciplinary journal directed to physicians and other health care professionals who manage infectious diseases of childhood. The journal delivers the latest insights on all aspects of infectious disease in children, from state-of-art diagnostic techniques to the most effective drug therapies and other essential treatment protocols. The Pediatric Infectious Disease Journal is official journal of the Pediatric Infectious Diseases Society (www.PIDS.org) and the European Society for Paediatric Infectious Diseases (www.ESPID.org).

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