Abstract
BackgroundNewborns exhibit distinct immune responses and are at high risk of infection. Neonatal immunization with BCG, the live attenuated vaccine against tuberculosis (TB), is associated with broad protection against a range of unrelated pathogens, possibly reflecting vaccine-induced training of innate immune cells (“innate memory”). However, little is known regarding the impact of age on BCG-induced innate responses.ObjectiveEstablish an age-specific human monocyte in vitro training platform to characterize and compare BCG-induced primary and memory cytokine responses and immunometabolic shifts.Design/MethodsHuman neonatal and adult CD33-selected monocytes were stimulated for 24h with RPMI (control) or BCG (Danish strain) in 10% autologous serum, washed and cultured for 5 additional days, prior to re-stimulation with the TLR4 agonist LPS for another 24h. Supernatants were collected at Day 1 (D1) to measure primary innate responses and at Day 7 (D7) to assess memory innate responses by ELISA and multiplex cytokine and chemokine assays. Lactate, a signature metabolite increased during trained immunity, was measured by colorimetric assay.ResultsCytokine production by human monocytes differed significantly by age at D1 (primary, BCG 1:750 and 1:100 vol/vol, p<0.0001) and D7 (innate memory response, BCG 1:100 vol/vol, p<0.05). Compared to RPMI control, newborn monocytes demonstrated greater TNF (1:100, 1:10 vol/vol, p<0.01) and IL-12p40 (1:100 vol/vol, p<0.05) production than adult monocytes (1:100, p<0.05). At D7, while BCG-trained adult monocytes, as previously reported, demonstrated enhanced LPS-induced TNF production, BCG-trained newborn monocytes demonstrated tolerization, as evidenced by significantly diminished subsequent LPS-induced TNF (RPMI vs. BCG 1:10, p <0.01), IL-10 and CCL5 production (p<0.05). With the exception of IL-1RA production by newborn monocytes, BCG-induced monocyte production of D1 cytokines/chemokines was inversely correlated with D7 LPS-induced TNF in both age groups (p<0.0001). Compared to BCG-trained adult monocytes, newborn monocytes demonstrated markedly impaired BCG-induced production of lactate, a metabolite implicated in immune training in adults.ConclusionsBCG-induced human monocyte primary- and memory-innate cytokine responses were age-dependent and accompanied by distinct immunometabolic shifts that impact both glycolysis and training. Our results suggest that immune ontogeny may shape innate responses to live attenuated vaccines, suggesting age-specific approaches to leverage innate training for broad protection against infection.
Highlights
As compared to other age groups, human newborns are highly susceptible to infections due in part to functionally distinct innate [1] and adaptive immunity [2]
We found that in marked contrast to increased cytokine induction by BCG-trained adult monocytes, BCG-trained newborn monocytes mounted a tolerogenic response to endotoxin
A human neonatal in vitro trained immunity platform was designed based on studies of adult monocyte BCG-induced trained immunity in vitro assays, which were used as a benchmark [25]
Summary
As compared to other age groups, human newborns are highly susceptible to infections due in part to functionally distinct innate [1] and adaptive immunity [2]. Pre-treatment with Toll-like receptor (TLR) agonists enhances subsequent responses to polymicrobial sepsis [7] and treatment with BCG results in enhanced emergency granulopoiesis [8] Evidence that such trained immunity occurs in human newborns includes: (a) critically ill preterm newborns demonstrate enhanced pathogen-specific mononuclear cell pattern recognition receptor (PRR) expression in the setting of Grampositive or Gram-negative bacteremia [9]; and (b) histologic chorioamnionitis affecting preterm infants is associated with a significantly reduced risk of late onset bacterial sepsis [10]. These observations suggest the existence of neonatal innate memory that alters responses to subsequent unrelated microbial challenges [11]. Little is known regarding the impact of age on BCG-induced innate responses
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