The role of bacterial toxins in sudden infant death syndrome (SIDS).

There is increasing evidence for the involvement of bacterial toxins in some cases of sudden infant death syndrome (SIDS), particularly the pyrogenic toxins of Staphylococcus aureus. This had led to the hypothesis that some SIDS deaths are due to induction of inflammatory mediators by infectious agents or their products during a period in which the infant is unable to control these normally protective responses. The genetic, developmental and environmental risk factors identified for SIDS are assessed in relation to frequency or density of mucosal colonisation by toxigenic bacteria and their effects on induction and control of inflammatory responses to the toxins.

Why is smoking a risk factor for sudden infant death syndrome?

Smoking is a major risk factor for both Sudden Infant Death Syndrome (SIDS) and respiratory tract infections. Such infections, both viral and bacterial, also increase the SIDS risk. This study investigated the effect of cigarette smoke at two stages of infection: 1) mucosal surface colonization; 2) induction and control of inflammatory responses. For colonization, RSV or influenza A infected cells bound several bacterial species in significantly higher numbers due to increased expression of host cell antigens. Buccal epithelial cells from smokers bound significantly more bacteria. For Staphylococcus aureus, this was associated with increased tar levels. Some SIDS deaths have been proposed to result from high levels of pro-inflammatory mediators elicited by infection and/or cigarette smoke during a developmental period when infants are less able to control inflammatory responses. Inflammatory reponses were compared between blood samples from smokers (n = 42) and non-smokers (n = 60) stimulated with TSST-1 or LPS. Non-smokers had significantly higher IL-6 (P = 0.011), IFN (P = 0.003) and IL-10 (P = 0.000) baseline levels. Non-smokers had higher IFN (P = 0.008) and IL-1 (P = 0.001, 0.007) responses to LPS and higher IL-10 responses to TSST-1 (P < 0.05) and LPS (P < 0.000). This study highlights that smoking increases the SIDS risk by greater susceptibility to viral and bacterial infections and enhanced bacterial binding after passive coating of mucosal surfaces with smoke components. In animal models, IL-10 reduced the lethal effect of staphylococcal toxins. In this study, smokers had lower IL-10 responses toTSST-1 and LPS. Dose response effects of cigarette smoke exposure needs to be established in relation to inflammatory response control and infantile infections.

Evidence for a genetic component in sudden infant death syndrome.

There is increasing evidence that inflammatory responses have been elicited in some Sudden Infant Death Syndrome (SIDS) infants and that these responses are under genetic control. The objective of this study was to investigate the hypothesis that the cytokine responses of SIDS parents (n = 41) differed significantly from control donors (n = 61). Blood samples were stimulated with the staphylococcal toxin TSST-1 and LPS from Eschericia coli and assessed for production of TNF, IL-1, IL-6, IFN and IL-10. In response toTSST-1 (P < 0.02) and LPS (P < 0.002), SIDS parents produced higher levels of IL-1 than the controls. SIDS parents produced higher levels of IFN in response to TSST-1 compared to LPS (P < 0.001) although in response to LPS, the IFN (P = 0.0008) and IL-6 (P < 0.0002) responses of the SIDS parents were lower than those of the controls. For TNF and IL-10, there was little difference between the two groups unless the effect of smoking was considered. As part of this work, a small pilot genotyping study was carried out using DNA from SIDS parents (n = 10), control donors (n = 10) and Bangladeshi subjects (n = 10). An IFN polymorphism (3/3) was found in 40%,15.4% and 0% of donors respectively. Staphylococcal toxins have been identified in SIDS infants therefore this study highlights the importance of assessing IL-1 levels. Determination of cytokine polymorphisms and consideration of interactions between these and environmental factors such as smoking in high, average and low risk ethnic groups will assist in establishing the contribution of these factors to an infant's susceptibility to SIDS.

Binding of bacteria to HEp-2 cells infected with influenza A virus.

Epidemiological studies indicate influenza virus infection increases susceptibility to bacterial respiratory pathogens and to meningococcal disease. Because density of colonisation is an important factor in the development of bacterial disease, the objectives of the study were to use flow cytometry methods for assessment of bacterial binding and detection of cell surface antigens to determine: (1) if HEp-2 cells infected with human influenza A virus bind greater numbers of bacteria than uninfected cells; (2) if influenza infection alters expression of cell surface antigens which act as receptors for bacterial binding; (3) if neuraminidase affects binding of bacteria to HEp-2 cells. There was significantly increased binding of all isolates tested regardless of surface antigen characteristics. There were no significant differences between virus-infected and -uninfected Hep-2 cells in binding of monoclonal antibodies to Lewisb, Lewisx or H type 2. There were significant increases in binding of monoclonal antibodies to CD14 (P < 0.05) and CD18 (P < 0.01). Treatment of cells with monoclonal antibodies significantly reduced binding of Neisseria meningitidis strain C:2b:P1.2, CD14 (P < 0.001) and CD18 (P < 0.001). No reduction in binding of a strain of Streptococcus pneumoniae (12F) was observed in these experiments. Neuraminidase treatment of HEp-2 cells increased binding of monoclonal antibodies to CD14 (P < 0.01) and CD18 (P < 0.01). In three experiments, the increase in binding of meningococcal strain C:2b:P1.2 to neuraminidase-treated cells was not significant, but binding of Staphylococcus aureus strain NCTC 10655 was significant (P < 0.05).

Infection with respiratory syncytial virus enhances expression of native receptors for non-pilate Neisseria meningitidis on HEp-2 cells.

Respiratory virus infections have been suggested to be predisposing factors for meningococcal disease. Respiratory syncytial virus (RSV) affects young children in the age range at greatest risk of disease caused by Neisseria meningitidis. It has been previously shown that glycoprotein G expressed on the surface of RSV-infected HEp-2 cells (a human epithelial cell line) contributed to higher levels of binding of meningococci compared with uninfected cells. The aim of the present study was to examine the effect of RSV infection on expression of surface molecules native to HEp-2 cells and their role in bacterial binding. Flow cytometry and fluorescence microscopy were used to assess bacterial binding and expression of host cell antigens. Some molecules analysed in this study have not been reported previously on epithelial cells. RSV infection significantly enhanced the expression of CD15 (P < 0.05), CD14 (P < 0.001) and CD18 (P < 0.01), and the latter two contributed to increased binding of meningococci to cells but not the Gram-positive Streptococcus pneumoniae.

The effect of cigarette smoke on adherence of respiratory pathogens to buccal epithelial cells.

Smoking is associated with an increased risk of respiratory tract infection in adults. In children, exposure to cigarette smoke is a risk factor for respiratory tract infection and bacterial meningitis: Active smoking and passive exposure to cigarette smoke is also associated with carriage of some potentially pathogenic species of bacteria in both adults and children. The aims of the study were to determine the effect of active smoking on: (1) bacterial binding to epithelial cells; (2) expression of host cell antigens that act as receptors for some species; and (3) the effects of passive exposure to water-soluble components of cigarette smoke on bacterial binding. Flow cytometry was used to assess binding to buccal epithelial cells of the following species labelled with fluorescein isothiocyanate: Neisseria meningitidis, Neisseria lactamica, Streptococcus pneumoniae, Bordetella pertussis, Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus. Flow cytometry was also used to assess expression of host cell antigens which have been identified as bacterial receptors. For each species, binding to cells of smokers was significantly higher than to cells of non-smokers; however, expression of host cell antigens was similar on epithelial cells of both groups. Non-dilute cigarette smoke extract reduced binding of bacteria to epithelial cells, but dilutions between 1 in 10 and 1 in 320 enhanced binding. We conclude that smokers might be more densely colonised by a variety of potentially pathogenic bacteria. The enhanced bacterial binding to epithelial cells of smokers is not related to enhanced expression of host cell antigens that can act as receptors for some species, but possibly to components in the smoke that alter charge or other properties of the epithelial cell surface. Passive coating of mucosal surfaces with components of cigarette smoke might enhance binding of potentially pathogenic bacteria.

Developmental and environmental factors that enhance binding of Bordetella pertussis to human epithelial cells in relation to sudden infant death syndrome (SIDS).

Asymptomatic infection due to Bordetella pertussis has been suggested to be one cause of sudden infant death syndrome (SIDS). We examined developmental and environmental factors previously found to affect binding of another toxigenic species, Staphylococcus aureus, to human epithelial cells: expression of the Lewis(a) antigen; infection with respiratory syncytial virus (RSV); exposure to cigarette smoke; and the inhibitory effect of breast milk on bacterial binding. Binding of two strains of B. pertussis (8002 and 250825) to buccal epithelial cells was significantly reduced by treating the cells with monoclonal antibodies to Lewis(a) (P < 0.05) and Lewis(x) (P < 0.01) antigens. Both strains bound in significantly greater numbers to cells from smokers compared with cells from non-smokers (P < 0.05). HEp-2 cells infected with RSV subtypes A or B had higher binding indices for both 8002 (P < 0.001) and 250825 (P < 0.01). On RSV-infected cells, there was significantly enhanced binding of monoclonal antibodies to Lewis(x) (P < 0.05), CD14 (P < 0.001) and CD18 (P < 0.01); and pre-treatment of cells with anti-CD14 or CD18 also significantly reduced binding of both strains of B. pertussis. Pre-treatment of the bacteria with human milk significantly reduced their binding to epithelial cells. The results are discussed in relation to our three-year survey of bacterial carriage among 253 healthy infants, their mothers and local SIDS cases between 1993-1995 and in relation to the change to an earlier immunisation schedule for infants and the recent decline in SIDS in Britain.