Intracellular pyruvate levels positively correlate with cytokine production capacity in tolerant monocytes from patients with pneumonia

Background: Community-acquired pneumonia (CAP) is responsible for a high morbidity and mortality worldwide. Monocytes are essential for pathogen recognition and the initiation of an innate immune response. Immune cells induce intracellular glycolysis upon activation to support several functions.Objective: To obtain insight in the metabolic profile of blood monocytes during CAP, with a focus on glycolysis and branching metabolic pathways, and to determine a possible association between intracellular metabolite levels and monocyte function.Methods: Monocytes were isolated from blood of patients with CAP within 24 h of hospital admission and from control subjects matched for age, sex and chronic comorbidities. Changes in glycolysis, oxidative phosphoryla-tion (OXPHOS), tricarboxylic acid (TCA) cycle and the pentose phosphate pathway were investigated through RNA sequencing and metabolomics measurements. Monocytes were stimulated ex vivo with lipopolysaccharide (LPS) to determine their capacity to produce tumor necrosis factor (TNF), interleukin (IL)-18 and IL-10. Results: 50 patients with CAP and 25 non-infectious control subjects were studied. When compared with control monocytes, monocytes from patients showed upregulation of many genes involved in glycolysis, including PKM, the gene encoding pyruvate kinase, the rate limiting enzyme for pyruvate production. Gene set enrichment analysis of OXPHOS, the TCA cycle and the pentose phosphate pathway did not reveal differences between monocytes from patients and controls. Patients' monocytes had elevated intracellular levels of pyruvate and the TCA cycle intermediate alpha-ketoglutarate. Monocytes from patients were less capable of producing cytokines upon LPS stimulation. Intracellular pyruvate (but not alpha-ketoglutarate) concentrations positively correlated with IL-18 and IL-10 levels released by patients' (but not control) monocytes upon exposure to LPS.Conclusion: These results suggest that elevated intracellular pyruvate levels may partially maintain cytokine production capacity of hyporesponsive monocytes from patients with CAP.

Automated summary

This study investigated the metabolic profile of blood monocytes in patients with community-acquired pneumonia (CAP), and found upregulation of genes involved in glycolysis. Monocytes from CAP patients had elevated levels of intracellular pyruvate and the TCA cycle intermediate alpha-ketoglutarate, and showed reduced cytokine production capacity upon LPS stimulation. Intracellular pyruvate concentration positively correlated with IL-18 and IL-10 levels released by CAP patients' monocytes. These findings suggest that elevated intracellular pyruvate levels may partially maintain cytokine production capacity in monocytes from CAP patients.