Expression and Function of Toll-Like Receptors in Eosinophils: Activation by Toll-Like Receptor 7 Ligand

Abstract

We investigated the expression of a panel of Toll-like receptors (TLRs) and their functions in human eosinophils. Eosinophils constitutively expressed TLR1, TLR4, TLR7, TLR9, and TLR10 mRNAs (TLR4 greater than TLR1, TLR7, TLR9, and TLR10 greater than TLR6). In contrast, neutrophils expressed a larger variety of TLR mRNAs (TLR1, TLR2, TLR4, TLR6, TLR8 greater than TLR5, TLR9, and TLR10 greater than TLR7). Although the expression levels in eosinophils were generally less prominent compared with those in neutrophils, eosinophils expressed a higher level of TLR7. Furthermore, among various TLR ligands (S-(2,3-bis(palmitoyloxy)-(2-RS)-propyl)-N-palmitoyl-Cys-Ser-(Lys)4, poly(I:C), LPS, R-848, and CpG DNA), only R-848, a ligand of TLR7 and TLR8, regulated adhesion molecule (CD11b and L-selectin) expression, prolonged survival, and induced superoxide generation in eosinophils. Stimulation of eosinophils by R-848 led to p38 mitogen-activated protein kinase activation, and SB203580, a p38 mitogen-activated protein kinase inhibitor, almost completely attenuated R-848-induced superoxide generation. Although TLR8 mRNA expression was hardly detectable in freshly isolated eosinophils, mRNA expression of TLR8 as well as TLR7 was exclusively up-regulated by IFN-γ but not by either IL-4 or IL-5. The up-regulation of the TLRs by IFN-γ had potentially functional significance: the extent of R-848-induced modulation of adhesion molecule expression was significantly greater in cells treated with IFN-γ compared with untreated cells. Although the natural ligands for TLR7 and TLR8 have not yet been identified, our results suggest that eosinophil TLR7/8 systems represent a potentially important mechanism of a host-defensive role against viral infection and mechanism linking exacerbation of allergic inflammation and viral infection.

Toll‐like receptor‐dependent activation of several human blood cell types by protamine‐condensed mRNA

“…The cellular pattern of immunostimulation by protamine-RNA complexes indicates that TLR-1, TLR-7, or TLR-8 may be the receptors for protamine-RNA. These three receptors are the only known receptors of the IL-1 receptor family expressed on pDC, mDC, granulocytes, monocytes and NK cells [21][22][23][24][25]. At the same time, mRNA coding for TLR-1, -7 and -8 are detectable in purified human T cells [21].…”

Therapeutic anti‐tumor immunity triggered by injections of immunostimulating single‐stranded RNA

“…In mice, TLR7 is expressed by DC and TLR8 is thought to be non-functional [20]. In humans, TLR7 is expressed by pDC, B cells, monocytes [14] and eosinophils [21], whereas TLR8 is expressed mainly by monocytes [14]. By triggering different TLR expressed by non-overlapping cell populations [22], the three different families of nucleic acid PAMP (CpG DNA, dsRNA and stabilized ssRNA) give rise to different types of innate immune responses.…”

A threshold level of TLR9 mRNA predicts cellular responsiveness to CpG-ODN in haematological and non-haematological tumour cell lines

“…In humans, TLR9 is expressed mainly in B-cells and pDCs [3][4][5][6][7], but has also been reported as being expressed in activated neutrophils [8], natural killer (NK) cells [5,10], eosinophils [9], and T lymphocytes [5], in some non-immune cell types, such as pulmonary epithelial cells, airway smooth muscle cells, keratinocytes, intestinal epithelial cells [11][12][13][14], and in solid tumours such as lung and intestinal cancers [11,15].…”

“…In humans, TLR9 is expressed predominantly in B-cells and plasmacytoid dendritic cells (pDC) [3][4][5][6][7]. However, studies have reported human TLR9 (hTLR9) expression in other immune cells, such as activated neutrophils, natural killer cells, eosinophils and T lymphocytes [5,[8][9][10]. TLR9 expression has also been reported in some non-immune human cell types, such as pulmonary epithelium, keratinocytes and intestinal epithelial cells, and in lung and intestinal tumour cells [11][12][13][14][15].…”