Role of plasmacytoid dendritic cell subsets in allergic asthma

Plasmacytoid dendritic cells ( pDC s) are major type‐I interferon‐producing cells that play important roles in antiviral immunity and tolerance induction. These cells share a common DC progenitor with conventional DCs, and Fms‐like tyrosine kinase‐3 ligand is essential for their development. Several subsets of pDC s have been identified to date including CCR9 + , CD9 + , and CD2 + pDC s. Recently, three subsets of pDC s were described, namely CD8α − β − , CD8α + β − , and CD8α + β + subsets. Interestingly, CD8α + β − and CD8α + β + but not CD8α − β − pDC s were shown to have tolerogenic effects in experimentally induced allergic asthma. These tolerogenic effects were shown to be mediated by the generation of FOXP3 + regulatory T cells through retinoic acid and the induction of retinaldehyde dehydrogenase enzymes. These newly described subsets of pDC s show high potentials for novel therapeutic approaches for the treatment of allergic diseases. In this review, we will address the new progress in our understanding of pDC biology with respect to allergic disease, in particular allergic asthma.