CLEC4C gene expression can be used to quantify circulating plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDC) are an important type I interferon producer that play an important role in the first line of host defence during viral infection. Abnormalities in pDC numbers and function have been associated with several health conditions. Quantifying pDC is important for understanding pDC related immune responses in viral infections and other diseases, however the current methods for quantifying pDC using flow cytometry have limited utility in large cohort studies involving multiple centres with limited access to flow cytometry. We reasoned that examining gene expression of the pDC marker C-type lectin domain family 4 member C (CLEC4C, also known as CD303 and BDCA2) in combination with pDC exclusive leukocyte immunoglobulin like receptor A4 (LILRA4, also known as CD85g and ILT7) might provide a more practical method that could be applied to multi-centre studies. Our results show a moderate correlation between pDC numbers measured by surface staining and CLEC4C gene expression in whole blood (rho = 0.39, P = .037, as well as a high correlation between CLEC4C gene expression in whole blood and peripheral blood mononuclear cells (rho = 0.79, P < .001). LILRA4 gene expression did not provide additional useful information. Our results indicate that measuring CLEC4C gene expression can provide an alternative method for quantifying pDC numbers in human samples.