Ontogeny of Toll-Like and NOD-Like Receptor-Mediated Innate Immune Responses in Papua New Guinean Infants

Studies addressing the ontogeny of the innate immune system in early life have reported mainly on Toll-like receptor (TLR) responses in infants living in high-income countries, with little or even no information on other pattern recognition receptors or on early life innate immune responses in children living under very different environmental conditions in less-developed parts of the world. In this study, we describe whole blood innate immune responses to both Toll-like and nucleotide-binding oligomerization domain (NOD)-like receptor agonists including the widely used vaccine adjuvant ‘alum’ in a group of Papua New Guinean infants aged 1–3 (n = 18), 4–6 (n = 18), 7–12 (n = 21) and 13–18 (n = 10) months old. Depending on the ligands and cytokines studied, different age-related patterns were found: alum-induced IL-1β and CXCL8 responses were found to significantly decline with increasing age; inflammatory (IL-6, IL-1β, IFN-γ) responses to TLR2 and TLR3 agonists increased; and IL-10 responses remained constant or increased during infancy, while TNF-α responses either declined or remained the same. We report for the first time that whole blood innate immune responses to the vaccine adjuvant alum decrease with age in infancy; a finding that may imply that the adjuvant effect of alum in pediatric vaccines could be age-related. Our findings further suggest that patterns of innate immune development may vary between geographically diverse populations, which in line with the ‘hygiene hypothesis’ particularly involves persistence of innate IL-10 responses in populations experiencing higher infectious pressure

Infant characteristics.

<p>Infant characteristics.</p

Maturation of innate immune function in PNG infants.

<p>Whole blood samples from PNG infants aged 1–3 months (n = 18, <i>white bars</i>), 4–6 months (n = 18, <i>light grey bars</i>), 7–12 months (n = 21, <i>dark grey bars</i>) or 13–18 months (n = 10, <i>black bars</i>) were stimulated with TLR (LTA; PolyIC; LPS; Gardiquimod) and NLR (iE-DAP; MDP) ligands, and Alum alone or with LPS co-stimulation (denoted by ♦). Presented are the geometric means and 95% confidence intervals (pg/mL) for each age group for background-adjusted cytokine responses. Significance level is indicated where p<0.05.</p

Alum-induced chemokine production with increasing age in infancy.

<p>Whole blood samples from PNG infants aged 1–3 months (n = 10, <i>white bars</i>), 4–6 months (n = 9, <i>light grey bars</i>), 7–12 months (n = 10, <i>dark grey bars</i>) or 13–18 months (n = 9, <i>black bars</i>) were stimulated with Alum. Presented are the geometric means and 95% confidence intervals (pg/mL) for each age group for background-adjusted chemokine responses. Significance level is indicated where p<0.10.</p

Innate IL-10 responses in relation to increasing age in infancy.

<p>Presented are the geometric means and 95% confidence intervals (pg/mL) for background-adjusted IL-10 responses. Mann-Whitney U tests for significant differences in log-transformed IL-10 levels compared to the “1–3 months” age group; and Spearman rho tests for significant correlations between log-transformed IL-10 levels and ordered age groups were conducted. Significance level is indicated where p<0.05 only (<b>*</b> p = 0.046; <b>‡</b> p = 0.022).</p>♦<p>denotes LPS co-stimulation used.</p

Correlations between innate inflammatory cytokine responses and age.

<p>Spearman rho correlation coefficients for the slope/trajectory of log-transformed inflammatory cytokine responses across ordered age groups: 1–3, 4–6, 7–12 and 13–18 months were determined. Significance level is indicated; bold-faced and italicized text highlight correlations with p<0.05.</p>♦<p>denotes LPS co-stimulation used.</p

Innate TNF-α responses in relation to increasing age in infancy.

<p>Presented are the geometric means and 95% confidence intervals (pg/mL) for background-adjusted TNF-α responses. Mann-Whitney U tests for significant differences in log-transformed TNF-α levels compared to the “1–3 months” age group; and Spearman rho tests for significant correlations between log-transformed TNF-α levels and ordered age groups were conducted. Significance level is indicated where p<0.05 only (<b>*</b> p = 0.049).</p>♦<p>denotes LPS co-stimulation used.</p