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Adaptive Immune ResponsesExtracellular Traps Links Innate and T Lymphocyte Priming by Neutrophil
SospedraKati Tillack, Petra Breiden, Roland Martin and Mireia
published online 20 February 2012J Immunol
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The Journal of Immunology
T Lymphocyte Priming by Neutrophil Extracellular TrapsLinks Innate and Adaptive Immune Responses
Kati Tillack,* Petra Breiden,* Roland Martin,*, and Mireia Sospedra*,
Polymorphonuclear neutrophils constitute the first line of defense against infections. Among their strategies to eliminate pathogens
they release neutrophil extracellular traps (NETs), being chromatin fibers decoratedwith antimicrobial proteins. NETs trap and kill
pathogens very efficiently, thereby minimizing tissue damage. Furthermore, NETs modulate inflammatory responses by activating
plasmacytoid dendritic cells. In this study, we show that NETs released by human neutrophils can directly prime T cells by reducing
their activation threshold. NETs-mediated priming increases T cell responses to specific Ags and even to suboptimal stimuli, which
would not induce a response in resting T cells. T cell priming mediated by NETs requires NETs/cell contact and TCR signaling, but
unexpectedly we could not demonstrate a role of TLR9 in this mechanism. NETs-mediated T cell activation adds to the list of
neutrophil functions and demonstrates a novel link between innate and adaptive immune responses. The Journal of Immunology,
2012, 188: 000000.
When fighting bacterial infections, cells of the innateimmune system recognize pathogen-associated mo-lecular patterns that are not displayed by host tissue.
Innate immune cells then secrete cytokines and chemokines thatsignal danger to other cells and induce inflammation at the siteof infection. Polymorphonuclear neutrophils (PMNs) are the firstimmune cells to be recruited to inflamed tissue (1), to contain andclear infectious organisms, and to direct the extravasation of adap-tive immune cells and their activation (24). Adaptive immunecells subsequently participate in the elimination of the pathogenand set up memory for the case of reinfection.The participation of PMNs in adaptive immune responses has not
been considered relevant until recently, when it has been recog-nized that PMNs and T cells may engage in multiple interactionsand mutual activation (5). PMNs release chemokines that attractT cells to the site of inflammation (57) and also cytokines thatinfluence T differentiation (8). Several cytokines and granuleproteins secreted by PMNs such as IFN-g, TNF-a, cathepsin G,and neutrophil elastase are able to increase T cell proliferation andcytokine production (9, 10), which together enhance adaptiveimmune responses. Paradoxically, PMNs also secrete mediatorssuch as oxygen species, arginase (11), IL-10, and TGF-b that may
suppress T cell activation and proliferation. Despite this evolvingevidence, a complete picture about PMN/T cell interactions re-quires further investigation.PMNs are endowed with a variety of weapons that enable them to
efficiently contain and clear infectious organisms. These includethe engulfment and intracellular degradation of microbes (12, 13),production of reactive oxygen species and granule proteins (14),and the recently described release of extracellular chromatin fibersdecorated with antimicrobial proteins called neutrophil extracel-lular traps (NETs) (15). NETs are the most efficient means tocontain and eliminate pathogens (16), since they not only trap andkill microbes, but also prevent collateral tissue damage by local-izing toxic proteases and reducing their proteolytic activity (17).Furthermore, NETs can modulate immune responses by activatingplasmacytoid dendritic cells (pDCs), an APC population special-ized in sensing infections. NETs activate pDCs through TLR9,an intracellular receptor that recognizes viral/bacterial DNA (18).Under physiological conditions, pDCs do not respond to self-DNA, most likely because self-DNA lacks CpG motifs found inviral/bacterial DNA and also because self-DNA is rapidly de-graded in the extracellular environment and therefore fails toaccess intracellular TLR9. However, damaged cells can releasefactors such as the neutrophil antimicrobial peptide LL37 and thehigh-mobility group box protein 1, which can protect self-DNAfrom DNase degradation and deliver it to the intracellular com-partment containing TLR9 in pDCs with the consequence ofTLR9-mediated activation (1921). LL37 and high-mobilitygroup box protein 1 are also contained in NETs and hence con-fer to these structures the potential to activate pDCs via TLR9 (22,23). NETs activation of pDCs seems to play an important role inthe pathogenesis of some autoimmune diseases such as psoriasis(19) and systemic lupus erythematosus (2225), for which it hasbeen suggested that the large amounts of IFN-g produced byNETs-activated pDCs can lead to the maturation of myeloid DCs(mDCs) and exert an effect on T cell function. If such an indirectinteraction between NETs and T cells were confirmed, it wouldrepresent a new NETs-mediated mechanism of communicationbetween PMNs and T cells. Furthermore, TLR9 is expressed inT lymphocytes, and CpG-containing oligodeoxynucleotides(CpG-ODN) can modulate T cell activation (26, 27). NETs maytherefore also be able to directly activate T cells through their
*Institute for Neuroimmunology and Clinical Multiple Sclerosis Research, Center forMolecular Neurobiology, University Medical Center HamburgEppendorf, 20251Hamburg, Germany; and Department of Clinical Neuroimmunology and MultipleSclerosis Research, Neurology Clinic, University Hospital Zurich, 8091 Zurich, Swit-zerland
Received for publication November 28, 2011. Accepted for publication January 21,2012.
This work was supported by Deutsche Forschungsgemeinschaft Grant SO 1029/1-1.The Institute for Neuroimmunology and Clinical Multiple Sclerosis Research wassupported by the Gemeinnutzige Hertie Stiftung.
Address correspondence and reprint requests to Dr. Mireia Sospedra, Department ofNeuroimmunology and Multiple Sclerosis Research, Neurology Clinic, UniversityHospital Zurich, Frauenklinikstrasse 26, 8091 Zurich, Switzerland. E-mail address:Mireia.SospedraRamos@usz.ch
The online version of this article contains supplemental material.
Abbreviations used in this article: DC, dendritic cell; DPI, diphenyleneiodonium;mDC, myeloid dendritic cell; NET, neutrophil extracellular trap; ODN, oligodeox-ynucleotide; pDC, plasmacytoid dendritic cell; PFA, paraformaldehyde; PMN, poly-morphonuclear neutrophil.
Copyright 2012 by The American Association of Immunologists, Inc. 0022-1767/12/$16.00
Published February 20, 2012, doi:10.4049/jimmunol.1103414 by guest on July 16, 2018