**3. Immunoglobulins and the innate immune response**

Humoral immunity of the human newborn is provided primarily by maternal immunoglobulin G (IgG) transferred transplacentally, beginning at 8 to 10 weeks of gestation and accelerating during the last trimester. In a study to evaluate the role of maternally acquired antibody to native type III polysaccharide of group B *Streptococcus* as a determinant of susceptibility for infant systemic infection the authors found a significant correlation with maternal antibody levels in 111 acutely ill infants (10). These data extended earlier observations suggesting the correlation between low levels of type-specific antibody in serum and risk for systemic infection in neonates. Premature infants, compared to fullterm infants, have lower levels of IgG at birth that further decreases during the first few weeks of life (11). The relative deficiency of humoral immunity in premature newborns might contribute to the inverse correlation of birth weight and rate of neonatal sepsis, with an 86-fold increased rate of sepsis in newborns of birth weight 600 to 999 grams compared to newborns of birth weight of more than 2500 grams (11). Ballow et al. (12) measured plasma immunoglobulin concentrations of premature infants of birth weight less than 1500 g longitudinally from birth to 10 months chronological age. During the first week of life plasma IgG levels correlated well with gestational age. At the age of three months mean plasma IgG levels were 60 mg/dl in infants of 25 to 28 weeks gestational age and 104 mg/dl in those of 29 to 32 weeks. Most infants remained hypogammaglobulinaemic at six months with 64% and 62%, respectively, of the infants having plasma IgG levels below 200 mg/dl. Plasma IgM concentrations were low in both groups during the first week of life (7.6 and 9.1 mg/dl, respectively) and rose to 41.8 and 34.7 mg/dl, respectively, by eight to ten months of life. IgA concentrations were comparable for both groups during the first week of life (1.2 and 0.6 mg/dl, respectively). After discharge Ballow et al. (12) followed 43 preterm infants until ten months chronological age and observed a significantly higher incidence of infections compared to 41 term infants (p = 0.04). In another study the level of maternal antibody required to protect neonates against early-onset disease caused by goup-B streptococci (GBS) type Ia was estimated (15). Levels of maternal serum IgG GBS Ia antibodies of 45 neonates with early onset disease case caused by GBS Ia and 319 control subjects (neonates colonized by GBS Ia but without early-onset disease) born at ≥34 weeks gestation were compared. The probability of developing infection declined with increasing maternal levels of IgG GBS Ia antibody (P <.03). Neonates whose mothers had levels of IgG GBS Ia antibody ≥5 mg/mL had an 88% lower risk (95% confidence interval, 7%–98%) of developing type-specific early-onset disease, compared with those whose mothers had levels <0.5 mg/mL (13).

82 Neonatal Bacterial Infection

asphyxia (23%) (5).

**2. Pathophysiology of neonatal sepsis** 

their stay at the NICU, prolonged artificial ventilation, intravenous feeding and antibiotic pressures. (2). The incidence is estimated to range from 1 to 5-8.1 per 1000 live births (2,4). The proportion of child deaths that occurs in the neonatal period (38% in the year 2000) is increasing, and the Millennium Development Goal for child survival cannot be met without substantial reductions in neonatal mortality. Every year an estimated 4 million babies die in the first four weeks of life (the neonatal period), and, globally, the main direct causes of neonatal death are estimated to be preterm birth (28%), severe infections (26%), and

The immune system of the neonate is immature in both humoral and cell mediated defense with prematurity further increasing the physiological inadequacies of the immune system. Sepsis spreads easily to the various organ systems in the neonates and thus, often presents as a multiorgan dysfunctions syndrome. Infection initiates a complex immune process, which includes antigen detection, T-cell activation and proliferation, and release of cytokines. Cytokines are low molecular mass proteins, which mediate cell growth, inflammation, immunity, differentiation, migration and repair. They regulate the amplitude and the duration of the inflammatory response and include interleukins-6 and -8, interferons-γ, colony-stimulating factors, tumour necrosis factor-α and others. However in the setting of overwhelming sepsis as a result to the microbial insult, these cytokines give rise to what is described as the systemic inflammatory response syndrome, where the much of the damage paradoxically results from the host defences (e.g. cytokines) analogous to a chain reaction themselves. The neutrophil functions: adhesion, diapedesis, phagocytosis and degranulation are also of prime importance in the host defence mechanisms against bacterial and fungal pathogens. The proteolytic enzymes released by the neutrophils are also damaging to the host tissue. Thus, immunoglobulins and neutrophils are responsible

for both host defence and damage in the setting of overwhelming neonatal sepsis (6).

**3. Immunoglobulins and the innate immune response** 

In 2005, definitions for paediatric infection, systemic inflammatory response syndrome, sepsis, severe sepsis, septic shock, and organ dysfunction were published that included term neonates of 0 to 7 days and newborns of 1 to 4 weeks of age (7). But one has to question why there are no criteria for the definition of sepsis and septic shock in preterm infants? The challenge of diagnosis of sepsis in the preterm infants is strongly associated with the immaturity of organ systems and transitional physiology. Suggested modifications of these definitions have recently been published (8) but still have to be proven in clinical trials (9).

Humoral immunity of the human newborn is provided primarily by maternal immunoglobulin G (IgG) transferred transplacentally, beginning at 8 to 10 weeks of gestation and accelerating during the last trimester. In a study to evaluate the role of maternally acquired antibody to native type III polysaccharide of group B *Streptococcus* as a determinant of susceptibility for infant systemic infection the authors found a significant Yang et al. (14) studied the mechanism of bacterial opsonization by intravenous immune globulin (IVIG) complement consumption and polymorphonuclear leukocyte membrane receptor mediated phagocytosis of Staphylococcus epidermidis, Klebsiella pneumoniae, and groups A and B streptococci. IGIV alone did not consume complement and showed no opsonic activity by itself for these organisms. When these bacteria were preopsonized in intravenous immune globulin, significant amounts of complement were consumed (44%- 94%) and the uptake and killing of bacteria occurred. An important finding was the fact that in vitro opsonic activity of IGIV for these organisms was significantly correlated with the amount of complement consumed by the IVIG – opsonised bacteria. The in vivo protective efficacy of IVIG also appeared to be directly associated with its ability to activate and consume complement. The higher the titers of the IVIG preparation are (higher than 200 units:ml) the more opsonic activity has been shown towards slime-producing *S*. *epidermidis* (15). Administered as a prophylactic agent to low-birth weight (lower than 1700 g) preterm neonates immediately after birth revealed significantly higher specific IgG in blood sera

compared to controls with an effect even lasting ten days after the last infusion. These results suggest that specific IgG titers might be well indicative of its opsonic activity against slime-producing *S*. *epidermidis* and might protect against bacteraemia.

Immunoglobulins in the Prevention and Treatment of Neonatal Sepsis 85

of anaphylaxis and serum sickness. Further on immune globulins obtained from pooled human plasma were used, but antibodies provided by these preparations always represented those common to the donor population, and intravenous injection of early human IgG preparations was complicated by severe allergic reactions (19). The next step was the purification of human immune globulins, and, currently there are multiple

The mortality rate of the preterm infants with septicaemia decreased from 44% in the infants receiving only antibiotics to 8% in the infants treated by IVIG together with the same antibiotic following administration of IVIG to preterm neonates (0.3 g/day in neonates below 1000 g; 0.5 g/day in neonates over 1000 g for 6 consecutive days). The IVIG preparation was well-tolerated by all newborns, and no adverse events were observed by monitoring blood gas analysis, clinical examination, monitoring of respiration, pulse and body temperature. Follow-up at an average age of 2.5 years showed no evidence of harmful

Cates et al. (21) evaluated the formation of specific and functional antibody in preterm infants born weighing less than 1500 g (mean 1088 g) and less than 32 weeks of gestational age (mean 28.8 weeks). In the presence of complement, the strain of coagulase negative staphylococcus used was opsonized by IgG antibody, and the strain of Escherichia coli by IgM. Geometric mean plasma levels of tetanus and diphtheria IgG antibody fell from birth to 4 months chronological age, but rose significantly by 9 months (approximately 2 months after the third dose of diphtheria, tetanus, pertussis vaccine). However, at 9 months they remained lower than the respective geometric mean levels in 9-month-old term infants. The preterm infants' mean plasma IgG staphylococcal opsonic activity fell from birth to 2.5 months, but by 9 months was comparable to that of term infants of the same age. Mean IgM opsonic activity for Escherichia coli was very low at birth in both preterm and term infants. It rose with chronological age, correlating with the rise in total IgM by 9 months the mean

preterm and term infants' levels of IgM opsonic activity for E. coli were comparable.

Sasidharan (22) studied serially IgG levels postnatally in 42 infants of very low birth weight with gestational ages ranging from 23 to 31 weeks (mean birth weight 971 g). Eighteen infants (43%) had IgG levels of less than 100 mg/dl by a mean postnatal age of 71 days. The lowest level was found in a 700g infant with 22 mg/dl. In sixteen cases having cord blood IgG levels determined mean IgG values was 414 mg/dl. This had dropped to a mean of 140 mg/dl by 57 days. As expected, the lowest IgG levels postnatally were a reflection of the

To proof the significance of low serum IgG and complement proteins in very low birth neonates Lassiter et al (23) measured serum IgG, C3, C4 and Factor B weekly by rate nephelometry in 15 neonates who developed proven nosocomial bacterial or candidal sepsis and 27 neonates who did not develop sepsis. In the first and second week of life the serum IgG of infected neonates was significantly lower (mean 295 and 270 mg/dl compared to 440 and 473 mg/dl, respectively. If the IgG was less than 350 mg/dl in the first week or less than 230 mg/dl in the second week, the relative risk of acquiring sepsis was greater than or equal

formulations of safe, pooled, human immunoglobulins tor the intravenous use.

effects of IVIG treatment in the neonatal period (20).

degree of prematurity and the length of postnatal age.

to 5 (CI 95% 1.7 to 11.2).

The complement-inhibitory activity of different IVIG preparations was assessed in vitro by measurement of the blocking of C1q-, C4-, and C3 deposition on solid-phase aggregated rabbit IgG by enzyme-linked immunosorbent assay (16). Results showed that IgM enrichment of IVIG preparations enhances their effect to prevent the inflammatory effects of complement activation. No IgG preparation negatively affected in vitro phagocytosis of Escherichia coli by human granulocytes.

The mechanisms and effects of IVIGs are summarized in table 1 according to the description of Ballow (17).

	- Competitive interaction of IVIG with anti-platelet antibodies for FC receptor
	- Soluble Fcγ receptors compete with membrane Fc receptors of the reticuloendothelial system
	- Enhancement of T cell suppressor function
	- Inhibit B cell function and/or antigen-processing cells via Fc receptor

**Table 1.** Mechanisms of action of intravenous immune globulins (17)

Similar to most immunoglobins, the transplacental transport of IgG from the mother to fetus begins around 32 weeks of gestational age and increases until term. Premature infants born prior to 32 weeks gestation have profound IgG deficiencies. The major function of IgG in host defense is to opsonize bacteria and neutralize viruses. Levels of postnatal IgG are often low due to insufficient production by the immature neonatal immune system and catabolism of maternal IgG. Opsonic activity is also type-specific; therefore humoral immunity transferred to the neonate will be insufficient if the mother does not have immunity to the specific pathogen (18).
