Investigating the role of olfactory receptors in innate immune memory and sepsis

Abstract

Sepsis, defined as the abnormal host response to infection causing lethal organ dysfunction, remains one of the leading causes of patient morbidity and mortality worldwide. These mortality rates are thought to be contributed to heavily by the phenomenon of ‘immune paralysis’, a state of sustained immunosuppression. It is postulated that elucidating the molecular mechanisms that govern innate immune memory could in turn translate to effective treatments. The presence of ectopic olfactory receptors (ORs) on innate immune cells has been discovered in numerous studies in the past decade, linking their activity with specific pro- and anti-inflammatory functions. This study aims to test the hypothesis that ectopic ORs are associated with the host response to infection and the induction of innate immune memory. Using publicly available datasets, over 100 ectopic ORs were identified to be differentially upregulated in sepsis. Expression data from cell-sorted blood leukocytes revealed the differential expression of multiple ORsin sepsis, with OR52B2 upregulation being singled out monocytes. Expression data for lipopolysaccharide (LPS)-treated healthy human monocyte-derived macrophages showed differential downregulation of OR8G5 and upregulation of OR52K3P. OR kinetics observed through a human endotoxemia model dataset revealed continuous upregulation of OR2B6, a suspected monocyte-specific OR, over the span most significant to the development of innate immune memory, proving an attractive target for testing. A cohort of 8 healthy volunteers (4 females, 4 males; mean age 21 years) were recruited for this study. Heparinised blood was drawn with consent from each volunteer, from which monocytes were then harvested and enriched. Enriched monocytes were split into two categories, with one being subjected to a single controlled dose of LPS, while another was subjected to two doses with an overnight rest in between. ELISA testing revealed a stronger TNF-α response upon first exposure compared to restimulation, corroborated by quantitative real-time polymerase chain reaction, which showed much stronger TNFA expression in first exposure monocytes. The observed patterns closely mimic those demonstrated in several reports of monocytes in sepsis patients, characterised by sepsis-induced immune paralysis. OR2B6 expression was not reliably detected during both first stimulation with LPS and in endotoxin tolerance (restimulation), suggesting that olfactory receptor gene expression may not be inducible by bacterial pathogen-associated molecular patterns. Although the impact of ectopic ORs on sepsis-induced immune paralysis remains largely unknown, this study provides multiple avenues through which further work may be carried out to elucidate the role of ORs in the host response during sepsis.