A role for Toll-like receptors in acquired immunity: up-regulation of TLR9 by BCR triggering in naive B cells and constitutive expression in memory B cells

Abstract

Toll-like receptors (TLRs) are pattern recognition receptors that trigger innate immunity. In this study we investigated the expression of 10 TLRs in human naive and memory B-cell subsets. We report that in human naive B cells most TLRs are expressed at low to undetectable levels, but the expression of TLR9 and TLR10 is rapidly induced following B-cell-receptor ( IntroductionToll-like receptors (TLRs) are pattern recognition receptors that trigger innate immunity, providing both immediate protective responses against pathogens and instructing the adaptive immune response through the induction of dendritic cell recruitment and maturation. 1-4 TLR triggering results in nuclear factor kappa B (NF-B)-mediated activation of inflammatory genes. 5 Ten TLRs have been described in humans, and agonists have been defined for 9 of them. 6 TLRs can function as homodimers or heterodimers, increasing the repertoire of specificities. 7 TLR1, TLR2, and TLR6 are triggered by peptidoglycan and other microbial products, 8,9 TLR3 by double-stranded RNA, 10 TLR4 by lipopolysaccharide (LPS), 11 TLR5 by flagellin, 12 TLR7 and TLR8 by imidazoquinolines, 6,13 and TLR9 by unmethylated CpG DNA. [14][15][16][17][18][19][20][21] TLRs are differentially expressed in dendritic cell (DC) subsets. [22][23][24] Monocytes and myeloid DCs express TLR2 and TLR4 and respond to peptidoglycan and LPS, while plasmacytoid DCs express a reciprocal pattern-namely, TLR7 and TLR9 and respond to CpG. The induction of DC maturation by TLRs represents the functional link between innate and adaptive response. 1,25 TLR expression can be regulated; for instance, interferon-␥ (IFN-␥) up-regulates TLR4 expression in human phagocytes, enhancing their capacity to respond to LPS. 26 Mouse B cells proliferate and differentiate in response to LPS or CpG, 14,27 indicating that they express functional TLR4 and TLR9. Human B cells also respond to CpG, 19 but recent data indicate that memory B cells are more responsive than naive B cells, raising the possibility that TLR expression may be differentially regulated in human naive and memory B cells. 28 We report here that in human naive B cells TLRs are expressed at low to undetectable levels but their expression is rapidly up-regulated by B-cell-receptor (BCR) triggering. In contrast, memory B cells express several TLRs at constitutively high levels. The differential expression correlates with responsiveness to CpG DNA, an agonist of TLR9, and points to a relevant difference between human and mouse B cells. Thus, in humans TLRs are downstream of BCR and play a role in the late phase of acquired immunity. Materials and methods Cell isolation and cultureB lymphocytes were isolated from peripheral blood mononuclear cells (PBMCs) using CD19 microbeads (Miltenyi Biotec, Bergisch Gladbach, Germany) and further purified by cell sorting using a FACSVantage (Becton Dickinson, San Jose, CA). Cells were stained with directly labeled antibodies to CD27 (Pharmingen, San Diego, CA) (to identify memory B cells) and appropriate combinations...

Defective B-cell proliferation and maintenance of long-term memory in patients with chronic granulomatous disease

“…32,33 Thus a possible explanation for the loss of long-term memory in patients with CGD can be an ineffective polyclonal activation of memory B cells, which usually maintain serologic memory. 28,34,35 However, we also found numbers of CD27/IgD double-negative B cells, which were previously described as increased in elderly healthy subjects, 36 HIV-infected children, and patients undergoing kidney transplantation, 37 to be higher among patients with CGD compared with those seen in HCs. This might support the hypothesis that B cells from patients with CGD could also be unresponsive to stimuli because of premature aging.…”

“…Accordingly, restoration of the proliferative ability on combined stimulation with both BCR and TLR could be due to activation of naive B cells, in which the synergic activation of both pathways is required. 28,29 In view of this, it is possible that dysfunctional NADPH oxidase activity on naive B cells in vivo might directly affect the capability of these cells to differentiate into memory cells. In fact, patients with CGD have been reported to have lower total memory B-cell counts (CD19 1 CD27 1 ) and resting memory B-cell counts (CD19 1 CD10 2 CD21 1 CD27 1 ).…”

BAFF and LPS cooperate to induce B cells to become susceptible to CD95/Fas‐mediated cell death

“…In particular, LPS (TLR4 ligand) and CpG ODN (TLR9 ligand) can directly modulate B cell survival, proliferation, class switch recombination and plasma cell differentiation [16]. Human memory B cells but not naïve B cells proliferate in response to CpG, and this is most likely because after BCR ligation, human B cells are induced to express TLR-7 and TLR-9 [17,18]. Furthermore, the sensitivity of B cells to TLR stimulation can be regulated by DC [19], and a recent study suggested that TLR stimulation along with BCR and T cell-derived signals is a third signal required for inducing B cell maturation [20].…”

Genome-wide scan identifies a copy number variable region at 3p21.1 that influences the TLR9 expression levels in IgA nephropathy patients

“…The constitutive expression of TLRs in memory B cells provides a mechanism to generate continuously the antibody of all memory specificities, thus sustaining serologic memory. 39,40 CpG DNA motifs have a role in the mechanism by which B cells switch from IgM to IgA. 36,41,42 Coupling of antigen to CpG may be sufficient to create a strong immunogen capable of eliciting highly specific T-cell-independent antibody responses.…”