Immune and inflammatory responses in sudden infant death syndrome

Infancy is a time of unparalleled infection exposure. Coming from the privilege of the uterus, the newborn infant must make appropriate immune responses following infection that eliminates the infection but protects the host. There is evidence that in sudden infant death syndrome (SIDS) subjects there is a background of recent 'trivial' infection and immunological/inflammatory reactivity. This immunological/inflammatory reactivity is seen in enhanced pulmonary immunoglobulins and T-cell activation. It may be that in certain SIDS cases a trivial infection triggers an exaggerated inflammatory response, inducing cytokine cascades and eventual demise of the infant.     

Oral DNA vaccination in utero induces mucosal immunity and immune memory in the neonate

Infectious diseases are responsible for a significant number of deaths during the first weeks of life. Some of the salient pathogens include HSV, HIV, hepatitis B virus, group B streptococcus, Haemophilus sp., and Chlamydia sp. The vertical transmission of many of these pathogens significantly increases the risk of neonatal infection. We recently reported that oral DNA immunization in utero induced high serum Ab titers and cell-mediated immunity in fetal lambs. In this study, we demonstrate immune memory and mucosal immunity in newborn lambs following oral DNA immunization of the fetus. A single oral exposure in utero to plasmid DNA encoding a truncated form of glycoprotein D of bovine herpesvirus-1 induced detectable immune responses in 80% (12 of 15) of newborn lambs. There was no evidence for the induction of immune tolerance in nonresponding lambs. Responding lambs displayed both systemic and mucosal immune responses and reduced virus shedding following intranasal challenge. Furthermore, strong anamnestic responses were evident for at least 3 mo after birth. The efficacy of in utero oral DNA immunization was further demonstrated with the hepatitis B surface Ag, and protective serum Ab titers occurred in 75% of immunized lambs. Thus, the present investigation confirms that oral DNA immunization in utero can induce both mucosal and systemic immune responses in the neonate and that this immunity has the potential to prevent vertical disease transmission.     

Sudden infant death syndrome, infection and inflammatory responses

Sudden infant death syndrome (SIDS) is sudden unexpected death in infancy for which there is no explanation after review of the history, a death scene investigation and a thorough autopsy. The use of common diagnostic criteria is a prerequisite for discussing the importance of infection, inflammatory responses and trigger mechanism in SIDS. Several observations of immune stimulation in the periphery and of interleukin-6 elevation in the cerebrospinal fluid of SIDS victims explain how infections can play a role in precipitating these deaths. Finally, these findings and important risk factors for SIDS are integrated in the concept of a vicious circle for understanding the death mechanism. The vicious circle is a concept to elucidate the interactions between unfavourable factors, including deficient auto-resuscitation, and how this could result in death.     

Alteration in regulation of inflammatory response to influenza a virus and endotoxin in suckling rat pups: a potential relationship to sudden infant death syndrome

Data increasingly implicate a possible role of immune and inflammatory responses to infection in sudden infant death syndrome (SIDS). We have previously described a dual challenge model that results in pathology, organ damage, vascular collapse and unexplained death similar to that seen in SIDS. In this study, we examined changes in inflammatory cytokine mRNA in the lung and liver and regulation of pathways associated with nitric oxide production. Our data suggest that priming of the immune system by mild viral infection disturbs normal inflammatory response to endotoxin. This results in an increased nitric oxide synthase production, most likely the cause of liver pathology and clotting abnormalities.     

Detection of specific antibodies in cord blood, infant and maternal saliva and breast milk to staphylococcal toxins implicated in sudden infant death syndrome (SIDS)

The common bacterial toxins hypothesis of sudden infant death syndrome (SIDS) is that nasopharyngeal bacterial toxins can trigger events leading to death in infants with absent/low levels of antibody that can neutralise the toxins. The aim of this study was to investigate nasopharyngeal carriage of Staphylococcus aureus and determine levels of immunity in the first year of life to toxic shock syndrome toxin (TSST-1) and staphylococcal enterotoxin C (SEC). Both toxins have been implicated in SIDS cases. Seventy-three mothers and their infants (39 males and 34 females) were enrolled onto the study. The infants had birth dates spread evenly throughout the year. In infants, S. aureus carriage decreased significantly with age (P<0.001). Between 40% and 50% of infants were colonised with S. aureus in the first three months of life and 49% of the isolates produced one or both of the staphylococcal toxins. There was a significant correlation between nasopharyngeal carriage of S. aureus in mothers and infants in the three months following the birth (P<0.001). Carriage of S. aureus in infants and their mothers was not significantly associated with levels of antibody to TSST-1 or SEC in cord blood, adult saliva or breast milk. Infants colonised by S. aureus had higher levels of salivary IgA to TSST-1 than infants who were culture negative. Analysis of cord blood samples by a quantitative ELISA detected IgG bound to TSST-1 and SEC in 95.5% and 91.8% of cases respectively. There was a marked variation in levels of maternal IgG to both TSST-1 and SEC among cord blood samples. Maternal age, birth weight, and seasonality significantly affected the levels of IgG binding to TSST-1 or SEC. Analysis of infant saliva samples detected IgA to TSST-1 and SEC in the first month after birth; 11% of samples tested positive for salivary IgA to TSST-1 and 5% for salivary IgA to SEC. By the age of two months these proportions had increased to 36% and 33% respectively. More infants who used a dummy tested positive for salivary IgA to TSST-1 compared to infants who did not use a dummy. Levels of IgA to TSST-1 and SEC detected in the breast-milk samples varied greatly among mothers. There was a trend for infants receiving breast milk with low levels of antibody to TSST-1 or SEC to have higher levels of salivary antibody to the toxins. In conclusion, passive immunity to toxins implicated in SIDS cases varies greatly among infants. Infants are able to mount an active mucosal immune response to TSST-1 and SEC in the first month of life.     

Mucosal immune responses to infections in infants with acute life threatening events classified as 'near-miss' sudden infant death syndrome

This study examined the hypothesis that dysregulation of mucosal immune responses to respiratory infections is a critical event, which could be causal in respiratory arrest of some previously healthy infants. To examine this hypothesis, a prospective study was undertaken of infants presenting to the emergency department of a major teaching hospital with acute life threatening events (ALTE) of unknown cause and classified as "near-miss" SIDS. Salivary immunoglobulin concentrations were measured on admission and again after 14 days. The salivary immunoglobulins were compared with three control groups: infants with a mild upper respiratory tract infection (URTI); bronchiolitis; and healthy age-matched infants. The salivary IgA and IgM concentrations in the ALTE infants at presentation to hospital indicated a significant mucosal immune response had already occurred, with nearly 60% of the IgA concentrations significantly above the population-based reference ranges. The hyper-immune response was most evident in the ALTE infants with pathology evidence of an infection; 87% of these infants had salivary IgA concentrations on average 10 times higher that the age-related median concentration. The most prevalent pathogen identified in the ALTE infants was respiratory syncytial virus (RSV) (64%). RSV was also identified in all subjects with bronchiolitis. Risk factors for SIDS were assessed in each group. The data indicated that the ALTE infants diagnosed as 'near-miss' SIDS were a relatively homogeneous group, and most likely these ALTE infants and SIDS represent associated clinical outcomes. The study identified exposure to cigarette smoke and elevated salivary IgA concentrations as predictors of an ALTE. The study findings support the hypothesis of mucosal immune dysregulation in response to a respiratory infection in some infants with an ALTE. They provide a plausible explanation for certain SIDS risk factors. The underlying patho-physiological mechanism of proinflammatory responses to infections during a critical developmental period might be a critical factor in infants who have life-threatening apnoea or succumb to SIDS. The study raises the possibility of using salivary IgA to test infants who present with mild respiratory infections to identify a substantial number of infants at risk of developing an ALTE or SIDS, thus enabling intervention management to prevent such outcomes.     

Cortisol levels and control of inflammatory responses to toxic shock syndrome toxin-1 (TSST-1): the prevalence of night-time deaths in sudden infant death syndrome (SIDS)

There is evidence that inflammatory responses have been induced in the tissues and body fluids of many SIDS infants. We suggested that some of these deaths are due to uncontrolled inflammatory responses to infectious agents and possibly cigarette smoke. The majority of SIDS deaths occur during the 2-4 month age range when infants have decreasing levels of maternal antibodies to infectious agents. Most deaths occur during the early hours of the morning. Adults are more susceptible to inflammatory responses at night due to lower levels of cortisol associated with circadian rhythm patterns. Infants develop these patterns between the ages of 7 weeks and 4 months, at which time their night-time cortisol levels drop dramatically. The objective of this study was to use an in vitro model system to assess the effects of different cortisol levels on proinflammatory cytokine production in response to the staphylococcal toxic shock syndrome toxin-1 (TSST-1) which has been identified in a significant number of SIDS infants. Levels of cortisol present in infants at night and during the day before and after the development of the circadian rhythm pattern were examined. Human buffy coats (n = 9) were stimulated with TSST-1 and responses assessed over 72 hours by a bioassay for tumour necrosis factor-alpha (TNF-alpha) and an enzyme linked immunosorbent assay (ELISA) for interleukin-6 (IL-6). Cortisol levels present in an infant at night after development of circadian rhythm (< or = 5 microg dl(-1)), did not significantly increase or decrease production of either TNF-alpha or IL-6. Concentrations of cortisol greater than 5 microg dl(-1) usually found in infants during the day or at night prior to the physiological change significantly decreased production of TNF-alpha at 12 hours and of IL-6 at 12 and 16 hours. Only cortisol levels greater than 5 microg dl(-1) significantly decreased production of the pro-inflammatory cytokines by human buffy coats stimulated with TSST-1. If the switch to the circadian rhythm pattern occurs in an infant when maternal antibodies are still present or after they have developed their own active immunity, the infant could neutralise common viruses, toxins or bacteria: however, if this switch occurs in an infant when antibody levels are low, this could be a window of vulnerability during which infants are at an increased risk of death if uncontrolled inflammatory responses are induced by infectious agents or their products.     

An infectious aetiology of sudden infant death syndrome

Due attention has been given to infectious agents and immune responses to infection in sudden infant death syndrome (SIDS). It has been acknowledged that the pathological, epidemiological and genotypic findings in SIDS infants suggest an infectious aetiology possibly being potentiated by immunoregulatory polymorphisms, however, the cause of SIDS is a mystery and remains open to debate. Consistent pathological findings are seen which display similarities to the pathogenesis of toxaemic shock and/or sepsis. The major risk factors for SIDS parallel those for increased colonization and serious bacterial infections and the natural variation in the incidence of SIDS cases is typical of an infectious disease. The roles played by viral infection, immunoregulatory genes and suspected bacterial species are discussed herein.     

Inflammatory responses in sudden infant death syndrome – past and present views

Sudden infant death syndrome (SIDS) is sudden unexpected death in infancy for which there is no explanation based on commonly accepted diagnostic criteria; however, half of the victims have had slight signs of infection prior to death. Such slight infection with fever is an important risk factor in combination with a prone sleeping position, especially in infants between 2 and 4 months of age. The purpose of this review is to summarise findings that support the theory that a significant part of cot deaths may be due to an overreaction to otherwise harmless infections. Such factors are mucosal immune stimulation, cytokines in the cerebrospinal fluid and hypoxanthine levels in vitreous humour. The review aims at explaining why we believe that a slight infection combined with a prone position, a warm environment and a vulnerable age period may trigger a vicious circle leading to death.     

The role of infection in sudden infant death syndrome

Studies on the potential role of infectious agents in sudden infant death syndrome (SIDS) have been published over the years in a variety of journals. The aim of this special issue of FEMS Immunology and Medical Microbiology is to bring together a group of the most recent studies from Europe, Australia and Canada which cover epidemiology and laboratory studies examining hypotheses relating to infection and inflammation in SIDS. The articles in this issue examine evidence for the involvement of specific micro-organisms in SIDS and the problems relating to experimental studies on infection in relation to the underlying pathology of these deaths. There is an update on the evidence for the common bacterial hypothesis proposed in 1987 examining risk factors identified in epidemiological studies, particularly how the prone sleeping position could affect bacterial colonisation or induction of toxins. Evidence for induction of inflammatory responses in SIDS infants is reviewed and the relation of these responses to mechanisms proposed as causes of death assessed. Factors found to be associated with reduction of the risk of SIDS (breast feeding and immunisation) are examined in relation to some of the toxigenic bacteria implicated in these deaths. Finally, the high incidence of SIDS in some ethnic groups is examined as a potential model to investigate the contributions of genetic, environmental and cultural differences to susceptibility of infants not only to SIDS but to serious respiratory tract infections.     

Sudden infant death syndrome, virus infections and cytokines

Many epidemiological risk factors identified for sudden infant death syndrome (SIDS) suggest a viral aetiology, e.g. exposure to cigarette smoke and winter peak, mild respiratory symptoms. Virus infections and bacterial toxins induce cytokine activity and it has been suggested that uncontrolled inflammatory mediators could be involved in some cases of SIDS. The aim of this review was to assess the evidence for virus infection in SIDS and to examine those findings in relation to individual variations in cytokine responses and various pathophysiological mechanisms proposed for SIDS such as sleep derangement, hypoxia, cardiac arrhythmia, vascular hypotonicity and hypoglycaemia.     

Accelerated heterologous adenovirus prime-boost SIV vaccine in neonatal rhesus monkeys

A pediatric human immunodeficiency virus type 1 (HIV-1) vaccine would be desirable to protect infants against HIV-1 transmission from breast-feeding. Such a vaccine would need to induce protective immunity at mucosal surfaces in neonates as soon as possible after birth. Recombinant adenovirus (rAd) vectors have been shown to elicit potent systemic and mucosal virus-specific immune responses in adult nonhuman primates and humans, but these vectors have not previously been comprehensively studied in infants. In this study, we demonstrate that a single injection of rAd26 encoding simian immunodeficiency virus mac239 (SIVmac239) Gag on the day of birth elicited detectable Gag-specific cellular immune responses in rhesus monkeys, but these responses were transient and waned quickly. In contrast, an accelerated heterologous prime-boost regimen involving administration of rAd35 at birth and rAd26 at 4 weeks of life elicited potent and durable Gag-specific cellular and humoral immune responses in neonatal rhesus monkeys, including mucosal responses that remained detectable at 1 year of age. These results suggest the potential of an accelerated heterologous rAd prime-boost regimen as a candidate HIV-1 vaccine for newborns.     

IL-10 gene polymorphisms in infectious disease and SIDS

Interleukin-10 (IL-10) is a regulatory cytokine, and its principal role in vivo is to limit inflammatory response. IL-10 has been shown to influence both the susceptibility and course of various diseases, and the different polymorphisms in the IL-10 gene promoter have been associated with disease prevalence and severity. The genes involved in the immune system are also assumed to be of importance with regard to sudden infant death syndrome (SIDS), and specific haplotypes in the IL-10 gene promoter have been reported associated both with SIDS and sudden unexpected death due to infection.     

Evidence for early local viral replication and local production of antiviral immunity upon mucosal simian-human immunodeficiency virus SHIV(89.6) infection in Macaca nemestrina

Transmission of human immunodeficiency virus type 1 (HIV-1) is largely a result of heterosexual exposure, leading many investigators to evaluate mucosal vaccines for protection against intravaginal (i.vag.) transmission in macaque models of AIDS. Relatively little is known, however, about the dynamics of viral replication and the ensuing immune response following mucosal infection. We have utilized a simian-human immunodeficiency virus (SHIV) to study the differences in viremia, CD4 T-cell percentages, and mucosal and systemic anti-SHIV humoral and cellular immune responses during primary infection of animals infected either intravenously (i.v.) or i.vag. Positive viral cocultures, peripheral blood mononuclear cell viral load peaks, and CD4 cell declines were delayed by 1 week in the i.vag. inoculated animals compared to the animals infected i.v., demonstrating delayed viral spreading to the periphery. In contrast, mucosal anti-SHIV antibody levels were greater in magnitude and arose more rapidly and mucosal CD8(+) T-cell responses were enhanced in the i.vag. group animals, whereas both the magnitudes and times of onset of systemic immune responses for the animals in the two groups did not differ. These observations demonstrate that compartmentalization of viral replication and induction of local antiviral immunity occur in the genital tract early after i.vag. but not i.v. inoculation. Induction of mucosal immunity to target this local, contained replication should be a goal in HIV vaccine development.     

Rapid virus dissemination in infant macaques after oral simian immunodeficiency virus exposure in the presence of local innate immune responses

A vaccine to protect human immunodeficiency virus (HIV)-exposed infants is an important goal in the global fight against the HIV pandemic. Two major challenges in pediatric HIV vaccine design are the competence of the neonatal/infant immune system in comparison to the adult immune system and the frequent exposure to HIV via breast-feeding. Based on the hypothesis that an effective vaccine needs to elicit antiviral immune responses directly at the site of virus entry, the pattern of virus dissemination in relation to host immune responses was determined in mucosal and lymphoid tissues of infant macaques at 1 week after multiple oral exposures to simian immunodeficiency virus (SIV). The results show that SIV disseminates systemically by 1 week. Infant macaques can respond rapidly to virus challenge and mount strong innate immune responses. However, despite systemic infection, these responses are most pronounced in tissues close to the viral entry site, with the tonsil being the primary site of virus replication and induction of immune responses. Thus, distinct anatomic compartments are characterized by unique cytokine gene expression patterns. Importantly, the early response at mucosal entry sites is dominated by the induction of proinflammatory cytokines, while cytokines with direct antiviral activity, alpha/beta interferons, are only minimally induced. In contrast, both antiviral and proinflammatory cytokines are induced in lymphoid tissues. Thus, although infant macaques can respond quickly to oral viral challenge, the locally elicited immune responses at mucosal entry sites are likely to favor immune activation and thereby virus replication and are insufficient to limit virus replication and dissemination.     

Sudden infant death syndrome and Canadian Aboriginals: bacteria and infections

Aboriginal populations in Canada, America and Australia have higher incidences of sudden infant death syndrome (SIDS) than non-Aboriginal groups. Canadian Aboriginal populations (known also as first nation, native or Indian) experience infant morbidity/mortality rates 3-7 times that of non-Aboriginals, with upper track respiratory infection and SIDS recorded as the leading causes. The aim of this investigation was to examine the home environment of Aboriginal infants, particularly during winter months when respiratory tract infections and SIDS are more common. Environmental bacteria, fungi and air particulates were examined in the residences of Aboriginal infants during visits to individual homes on an Aboriginal reserve. The physical histories of SIDS victims were gathered from medical files. Air and surfaces were sampled by agar strips which were processed by a commercial laboratory. The levels of fungi, bacteria and air particulate rates recorded in the reserve homes of Aboriginal infants registered levels considered to be detrimental to the health of the inhabitants. Such extreme levels could contribute to the high incidence of respiratory disease and SIDS experienced by Canadian Aboriginal infants.     

Low Dose Antigen Exposure for a Finite Period in Newborn Rats Prevents Induction of Mucosal Tolerance

Background

In adult rats, initial exposure to antigens by a mucosal route triggers tolerance such that any subsequent re-exposure, even by a systemic route, results in suppression of immunity. The newborn’s gut is semi-permeable for a finite period to allow maternal antibodies to enter the newborn’s circulation. We propose that antigens introduced in extreme early life can readily traverse the gut wall and therefore circumvent induction of mucosal tolerance.

Methodology/Principle Findings

Rat pups were gavaged with low-doses of ovalbumin (OVA; oral exposure group) or saline (parenteral control group) every second day for several weeks followed by an intraperitoneal (i.p.) injection at 1 month of age. When gavage was initiated the day after birth, newborn oral exposure pups responded with significantly higher anti-OVA IgA, IgM, IgG2a, and IgG1 titres in their serum and anti-OVA IgA, IgG2a and IgG1 titres in their lungs compared to negative control pups. Oral exposure alone failed to induce immunity. Pups exposed to the same treatment regimen starting at 14 days of age showed induction of mucosal tolerance after i.p. immunization. Newborn oral exposure groups subjected to secondary i.p. immunization responded with significantly increased humoral immunity in lung and sera suggesting that once antigen-specific mucosal tolerance if circumvented, it persists. Lymphocytes derived from mesenteric lymph node cells re-simulated with OVA ex vivo, from newborn oral exposure pups exposed to secondary immunization produced significantly higher IFN-γ expression and lymphocyte proliferation relative to control pups indicating prevention of tolerance in the cell-mediated immune system.

Conclusions/Significance

This work demonstrates that newborns may be uniquely qualified to prevent induction of mucosal tolerance to oral antigens. These results should be further explored to establish whether prevention of tolerance by early life oral vaccination can be exploited to prime for mucosal as well as systemic immunity and thus protect this susceptible population against infectious diseases.     

Heart Rate Variability in Sleeping Preterm Neonates Exposed to Cool and Warm Thermal Conditions

Sudden infant death syndrome (SIDS) remains the main cause of postneonatal infant death. Thermal stress is a major risk factor and makes infants more vulnerable to SIDS. Although it has been suggested that thermal stress could lead to SIDS by disrupting autonomic functions, clinical and physiopathological data on this hypothesis are scarce. We evaluated the influence of ambient temperature on autonomic nervous activity during sleep in thirty-four preterm neonates (mean ± SD gestational age: 31.4±1.5 weeks, postmenstrual age: 36.2±0.9 weeks). Heart rate variability was assessed as a function of the sleep stage at three different ambient temperatures (thermoneutrality and warm and cool thermal conditions). An elevated ambient temperature was associated with a higher basal heart rate and lower short- and long-term variability in all sleep stages, together with higher sympathetic activity and lower parasympathetic activity. Our study results showed that modification of the ambient temperature led to significant changes in autonomic nervous system control in sleeping preterm neonates. The latter changes are very similar to those observed in infants at risk of SIDS. Our findings may provide greater insight into the thermally-induced disease mechanisms related to SIDS and may help improve prevention strategies.     

Ontogeny of Toll-like receptor mediated cytokine responses of human blood mononuclear cells

Newborns and young infants suffer increased infectious morbidity and mortality as compared to older children and adults. Morbidity and mortality due to infection are highest during the first weeks of life, decreasing over several years. Furthermore, most vaccines are not administered around birth, but over the first few years of life. A more complete understanding of the ontogeny of the immune system over the first years of life is thus urgently needed. Here, we applied the most comprehensive analysis focused on the innate immune response following TLR stimulation over the first 2 years of life in the largest such longitudinal cohort studied to-date (35 subjects). We found that innate TLR responses (i) known to support Th17 adaptive immune responses (IL-23, IL-6) peaked around birth and declined over the following 2 years only to increase again by adulthood; (ii) potentially supporting antiviral defense (IFN-α) reached adult level function by 1 year of age; (iii) known to support Th1 type immunity (IL-12p70, IFN-γ) slowly rose from a low at birth but remained far below adult responses even at 2 years of age; (iv) inducing IL-10 production steadily declined from a high around birth to adult levels by 1 or 2 years of age, and; (v) leading to production of TNF-α or IL-1β varied by stimuli. Our data contradict the notion of a linear progression from an 'immature' neonatal to a 'mature' adult pattern, but instead indicate the existence of qualitative and quantitative age-specific changes in innate immune reactivity in response to TLR stimulation.