Identification of soluble CD14 as an endogenous agonist for Toll-like receptor 2 on human astrocytes by genome-scale functional screening of glial cell derived proteins.
Human astrocytes express a limited repertoire of Toll-like receptor (TLR) family members including TLR1-4, which are expressed on the cell surface. Also, TLR3 but not TLR4 activation on astrocytes induces expression of several factors involved in neuroprotection and down-regulation of inflammation rather than in the onset of traditional pro-inflammatory reactions. The notion that astrocyte TLR may thus play a role not only in host defense but also in tissue repair responses prompted us to examine the possibility that endogenous TLR agonists could be expressed in the human central nervous system to regulate the apparently dual astrocyte functions during trauma or inflammation. As a potential source of endogenous agonists, a cDNA library derived from several human brain tumor cell lines was used. Gene pools of this library were transfected into COS-7 cells and the expression products were screened for their ability to induce TLR activation in human primary astrocytes. The screening resulted in the identification of soluble CD14. By using a panel of TLR-transfected HEK293 cells, we found that signaling by soluble CD14 was TLR2 dependent. Moreover, the CD14-triggered TLR2-mediated response in astrocytes lead to the production of CXCL8, IL-6, and IL12p40, whereas typical TLR-induced pro-inflammatory cytokines, like TNF-alpha and IL-1beta, were not produced at detectable levels. In conclusion, our data indicate that apart from its well-known ability to act as a co-receptor for TLR-dependent signaling by peptidoglycans or LPS, soluble CD14 can also act as a direct agonist for TLR2.
Bsibsi
Malika
M
Department of Biosciences, TNO Quality of Life, Leiden, The Netherlands.
Bajramovic
Jeffrey J
JJ
Van Duijvenvoorden
Eveline
E
Persoon
Carla
C
Ravid
Rivka
R
Van Noort
Johannes M
JM
Vogt
Mario H J
MH
D000328:Adult; D000595:Amino Acid Sequence; D000818:Animals; D001253:Astrocytes / Q000166:cytology / Q000276:immunology / Q000378:metabolism*; D001483:Base Sequence; D019556:COS Cells; D002478:Cells, Cultured; D002522:Chlorocebus aethiops; D018076:DNA, Complementary / Q000032:analysis; D020869:Gene Expression Profiling; D015698:Genomic Library; D006801:Humans; D053762:Interleukin-12 Subunit p40 / Q000378:metabolism; D015850:Interleukin-6 / Q000378:metabolism; D016209:Interleukin-8 / Q000378:metabolism; D007378:Interleukins / Q000378:metabolism*; D018950:Lipopolysaccharide Receptors / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D008253:Macaca mulatta; D017628:Microglia / Q000166:cytology / Q000378:metabolism; D008969:Molecular Sequence Data; D015398:Signal Transduction / Q000276:immunology / Q000502:physiology*; D051195:Toll-Like Receptor 2 / Q000819:agonists* / Q000276:immunology / Q000378:metabolism
D018076:DNA, Complementary; D053762:Interleukin-12 Subunit p40; D015850:Interleukin-6; D016209:Interleukin-8; D007378:Interleukins; D018950:Lipopolysaccharide Receptors; D051195:Toll-Like Receptor 2