Background: Dendritic cells (DCs) play a key role in the host defence against inhaled pathogens. However, the phenotype of blood DCs in patients with acute respiratory infections is unknown. Objective: To investigate the number and the expression of function-associated molecules of blood DCs in patients with acute infectious pneumonia. Methods: Sixteen patients with acute pneumonia and 19 controls without pneumonia were included in the study. The number as well as the expression of function-associated molecules of myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) were analysed in peripheral blood using four-colour flow cytometry. Results: Elevated concentrations of procalcitonin (median: 0.55 ng/ml) and the rapid response to antibiotic treatment suggested a bacterial origin of the pneumonia in the patients. Total mDC (median: 27% of the controls) and pDC counts (median: 53% of the controls) were markedly reduced in patients with pneumonia, as compared to the controls. Percentages of blood mDCs, but not pDCs, were negatively correlated with serum concentrations of C-reactive protein. Patients with pneumonia were characterised by a significantly increased expression of Fc gamma receptors (CD32 and CD64) on mDCs and the Toll-like receptor 9 (TLR9) on pDCs. Conclusions: Circulating DCs are markedly reduced in patients with pneumonia, and characterised by an up-regulation of molecules recognising pathogen-associated molecular patterns and opsonised antigens.

Background: Dendritic cells (DCs) play a key role in the host defence against inhaled pathogens. However, the phenotype of blood DCs in patients with acute respiratory infections is unknown. Objective: To investigate the number and the expression of function-associated molecules of blood DCs in patients with acute infectious pneumonia. Methods: Sixteen patients with acute pneumonia and 19 controls without pneumonia were included in the study. The number as well as the expression of function-associated molecules of myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) were analysed in peripheral blood using four-colour flow cytometry. Results: Elevated concentrations of procalcitonin (median: 0.55 ng/ml) and the rapid response to antibiotic treatment suggested a bacterial origin of the pneumonia in the patients. Total mDC (median: 27% of the controls) and pDC counts (median: 53% of the controls) were markedly reduced in patients with pneumonia, as compared to the controls. Percentages of blood mDCs, but not pDCs, were negatively correlated with serum concentrations of C-reactive protein. Patients with pneumonia were characterised by a significantly increased expression of Fc gamma receptors (CD32 and CD64) on mDCs and the Toll-like receptor 9 (TLR9) on pDCs. Conclusions: Circulating DCs are markedly reduced in patients with pneumonia, and characterised by an up-regulation of molecules recognising pathogen-associated molecular patterns and opsonised antigens.