The role of microRNAs in the early life human innate immune response

Neonates are more susceptible to infections than adults which suggests a suboptimal host immune defense. The innate immune response is the first line of defense against pathogens and must be tightly regulated. Recently, microRNAs (miRNAs) have emerged as important regulators of many biological processes. In the innate immune response, miRNAs positively and negatively regulate signalling to promote or dampen the immune response. The role of miRNAs in the development of the innate immune response, particularly in neonatal host defense, is not well understood. In this thesis work, we comprehensively profiled miRNAs in order to decipher a potential role for miRNAs in innate immune ontogeny. To this end, we contrasted miRNA expression in unstimulated versus stimulated monocytes from adult versus cord blood (CB) donors. Six immune responsive miRNAs were selected as candidates for additional studies, three of which were novel miRNAs that have not been previously investigated in innate immunity. Over-expression of these candidates in human monocytes resulted in significant changes in cytokine and chemokine production. While this verified their functional regulatory role in innate immunity, none of these cytokines/chemokines were predicted to be regulated by the selected miRNAs leaving the intermediate targets to be determined. These six candidate miRNAs were further characterized for expression kinetics. In adults, miRNA expression was rapidly induced within the first 8 hours of TLR stimulation and returned to near basal levels by 24 hours post stimulation. In CB monocytes however, miRNA expression was only observed to gradually increase, if at all, over 24 hours and expression levels never matched that observed in adult monocytes. These age-dependent differences in miRNA expression kinetics may relate to an overall inability in CB monocytes to process miRNAs into the biologically functional form. In summary, my work has shown that there are clear differences in miRNA expression between adult and CB monocytes, and this may be a result of deficiencies in the miRNA biogenesis machinery or a delay in expression in CB monocytes. Further work can now proceed to determine the underlying molecular mechanism(s) and functional implication(s) of these striking age-dependent differences in miRNA expression in innate immunity.Medicine, Faculty ofMedicine, Department ofExperimental Medicine, Division ofGraduat