2,3,7,8-Tetrachlorodibenzo-p-dioxin–Mediated Suppression of Toll-Like Receptor Stimulated B-Lymphocyte Activation and Initiation of Plasmacytic Differentiation

Abstract

2,3,7,8-Tetrachlordibenzo-p-dioxin (TCDD) is a potent suppressor of humoral immunity, disrupting antibody production in response to both T cell-dependent and T cell-independent antigens. Among the cell types required for humoral responses, the B cell is highly, and directly, sensitive to TCDD. B cells become antibody-secreting cells via plasmacytic differentiation, a process regulated by several transcription factors, including activator protein-1, B-cell CLL/lymphoma 6 (BCL-6), and B lymphocyte-induced maturation protein 1 (Blimp-1). The overarching conceptual framework guiding experimentation is that TCDD disrupts plasmacytic differentiation by altering the expression or activity for upstream regulators of Blimp-1. Multiparametric flow cytometry was used to investigate TCDD-induced alterations in both activation marker and transcription factor expression following lipopolysaccharide (LPS) activation of purified B cells. TCDD significantly impaired LPS-activated expression of major histocompatibility complex class II, cluster of differentiation (CD)69, CD80, and CD86. Immunosuppressive concentrations of TCDD also suppressed LPS-activated Blimp-1 and phosphorylated c-Jun expression, whereas elevating BCL-6 expression. Because BCL-6 and c-Jun are directly and indirectly regulated by the kinases AKT, extracellular signal-regulated kinase (ERK), and Jun N-terminal kinase (JNK), it was hypothesized that TCDD alters toll-like receptor-activated kinase phosphorylation. TCDD at 0.03 and 0.3 nM significantly impaired phosphorylation of AKT, ERK, and JNK in CH12.LX B cells activated with LPS, CpG oligonucleotides, or resiquimod (R848). In primary B cells, R848-activated phosphorylation of AKT, ERK, and JNK was also impaired by TCDD at 30 nM. These results suggest that impairment of plasmacytic differentiation by TCDD involves altered transcription factor expression, in part, by suppressed kinase phosphorylation.

The Long Winding Road toward Understanding the Molecular Mechanisms for B-Cell Suppression by 2,3,7,8-Tetrachlorodibenzo-p-dioxin

“…Presently, the molecular mechanism responsible for modulation of AP-1 activity in B cells by TCDD treatment is intriguing but difficult to explain in the context of AhR activation. It is also noteworthy as discussed in an earlier section that altered levels of kinase activity after TCDD treatment have been demonstrated, specifically on pERK, pAKT, and pJNK as rapidly as 15 min post-B-cell activation (North et al, 2010). These early events that almost certainly occur independent of AhR-mediated transcriptional changes do not rule out the involvement of the AhR at the level of proteinprotein interaction and may represent critical events that lead to impaired AP-1 function, Blimp-1 upregulation, and altered B-cell differentiation.…”

“…More recently, the effect of TCDD on the amount of active (i.e., phosphorylated) ERK, AKT, and JNK was assessed simultaneously in individual resting and activated mouse B cells by flow cytometry in a multiparametric analysis (North et al, 2010). Each of these three kinases is critically involved in Toll-like receptor (TLR)-mediated B-cell activation (Bishop et al, 2000;Yi et al, 1996).…”

“…The TCDD-induced phosphorylation of cellular proteins in leukocytes and hepatocytes, especially those occurring within 10 min of TCDD treatment, as well as the rapid increase in intracellular calcium in hepatocytes are important as they are indicative of biological events that occur independently of AhR-mediated nuclear translocation and changes in gene transcription while potentially still involving the AhR. In addition, the differences in the profile of activity between hepatocytes and leukocytes in the signaling events affected by TCDD are indicative of cell type-specific effects.More recently, the effect of TCDD on the amount of active (i.e., phosphorylated) ERK, AKT, and JNK was assessed simultaneously in individual resting and activated mouse B cells by flow cytometry in a multiparametric analysis (North et al, 2010). Each of these three kinases is critically involved in Toll-like receptor (TLR)-mediated B-cell activation (Bishop et al, 2000;Yi et al, 1996).…”

Comparative analysis of TCDD-induced AhR-mediated gene expression in human, mouse and rat primary B cells

“…A common consequence of TCDD exposure in many species is the suppression of humoral immunity, of which the immunoglobulin M (IgM) response is highly sensitive (Dooley and Holsapple, 1988; North et al ., 2010; Lu et al ., 2011). For robust IgM antibody responses, B cells undergo rounds of activation, proliferation and plasmacytic differentiation into antibody forming cells (AFC), which synthesize and secrete large quantities of pentameric IgM.…”

“…Multiple critical transcriptional regulators control plasmacytic differentiation at the molecular level, some of which have been recently identified. Importantly, TCDD has been demonstrated to directly and indirectly deregulate mRNA and/or protein expression of AP-1, Bach-2, BCL-6, Pax5, and Blimp1, all of which are major regulators of the B cell differentiation program (Calame et al ., 2003; Igarashi et al ., 2007; De Abrew et al ., 2011; Phadnis-Moghe et al ., 2015; North et al, 2010). …”