Toll-like receptor 9 suppression in plasmacytoid dendritic cells after IgE-dependent activation is mediated by autocrine TNF-alpha.
BACKGROUND
Functional significance for the alphagamma(2) variant of the high-affinity IgE receptor (FcepsilonRI) reportedly expressed on human dendritic cell subtypes remains poorly understood. Studies show that immature plasmacytoid dendritic cells (pDCs) secrete large quantities of TNF-alpha and IL-6 when directly stimulated with anti-IgE antibody. This mode of activation, however, reduces Toll-like receptor 9 (TLR9) expression in pDCs and their ability to mount an IFN-alpha response when subsequently activated with oligodeoxynucleotide containing CpG.


OBJECTIVE
To investigate the mechanisms underlying this IgE-dependent suppression of TLR9 and innate immune responsiveness in pDCs by focusing on autocrine cytokine responses.


METHODS
pDCs were isolated from blood by using blood dendritic cell antigen 4 selection. Cytokine responses to anti-IgE antibody-dependent and/or CpG-dependent stimulation were measured by using ELISA. TLR9 expression was determined by using quantitative RT-PCR and Western blotting.


RESULTS
The time required for downregulating TLR9 expression in pDCs after anti-IgE stimulation correlated with the induction and duration of TNF-alpha secreted by these cells. Pretreatment of pDCs with recombinant TNF-alpha (but not IL-6 or IL-10) markedly suppressed TLR9 expression. Functional response to CpG (ie, IFN-alpha induction) was also inhibited with TNF-alpha pretreatment (inhibitory concentration(50) = approximately 200 pg/mL). Finally, an antibody that neutralizes TNF-alpha activity completely restored TLR9 expression during anti-IgE stimulation and significantly improved IFN-alpha secretion on subsequent activation with CpG.


CONCLUSIONS
Autocrine TNF-alpha secretion resulting from IgE/FcepsilonRI-dependent activation plays a critical role in suppressing TLR9-dependent responses in pDCs that normally promote T(H)1 activity.
Department of Medicine, Division of Allergy and Clinical Immunology, Johns Hopkins Asthma and Allergy Center, Johns Hopkins University, Baltimore, MD 21224, USA. schray@jhmi.edu
John T
JT
Schroeder
Kristin L
KL
Chichester
Anja P
AP
Bieneman
D000328:Adult; D019898:Autocrine Communication / Q000502:physiology*; D002478:Cells, Cultured; D018899:CpG Islands; D016207:Cytokines / Q000378:metabolism; D003713:Dendritic Cells / Q000378:metabolism*; D015536:Down-Regulation; D006801:Humans; D007108:Immune Tolerance; D007113:Immunity, Innate; D007073:Immunoglobulin E / Q000378:metabolism*; D007700:Kinetics; D011994:Recombinant Proteins / Q000494:pharmacology; D013997:Time Factors; D051217:Toll-Like Receptor 9 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism* / Q000494:pharmacology
D016207:Cytokines; D011994:Recombinant Proteins; C495371:TLR9 protein, human; D051217:Toll-Like Receptor 9; D014409:Tumor Necrosis Factor-alpha; D007073:Immunoglobulin E