Signaling to a CD5+ B-cell clone through surface Ig and MHC class II molecules

TFG is required for autophagy flux and to prevent endoplasmic reticulum stress in CH12 B lymphoma cells

Plasma cells depend on quality control of newly synthesized antibodies in the endoplasmic reticulum (ER) via macroautophagy/autophagy and proteasomal degradation. The cytosolic adaptor protein TFG (Trk-fused gene) regulates ER-Golgi transport, the secretory pathway and proteasome activity in non-immune cells. We show here that TFG is upregulated during lipopolysaccharide- and CpG-induced differentiation of B1 and B2 B cells into plasmablasts, with the highest expression of TFG in mature plasma cells. CRISPR-CAS9-mediated gene disruption of tfg in the B lymphoma cell line CH12 revealed increased apoptosis, which was reverted by BCL2 but even more by ectopic TFG expression. Loss of TFG disrupted ER structure, leading to an expanded ER and increased expression of ER stress genes. When compared to wild-type CH12 cells, tfg KO CH12 cells were more sensitive toward ER stress induced by tunicamycin, monensin and proteasome inhibition or by expression of an ER-bound immunoglobulin (Ig) μ heavy (µH) chain. CH12 tfg KO B cells displayed more total LC3, lower LC3-II turnover and increased numbers and size of autophagosomes. Tandem-fluorescent-LC3 revealed less accumulation of GFP-LC3 in starved and chloroquine-treated CH12 tfg KO B cells. The GFP:RFP ratio of tandem-fluorescent-LC3 was higher in tunicamycin-treated CH12 tfg KO B cells, suggesting less autophagy flux during induced ER stress. Based on these data, we suggest that TFG controls autophagy flux in CH12 B cells and propose that TFG is a survival factor that alleviates ER stress through the support of autophagy flux in activated B cells and mature plasma cells.Abbreviations: Ab, antibody; Ag, antigen; ASC, antibody-secreting cells; ATG, autophagy-related; BCR, B cell receptor; COPII, coat protein complex II; CpG, non-methylated CpG oligonucleotide; ER, endoplasmic reticulum; ERAD, ER-associated degradation; FO, follicular; GFP, green fluorescent protein; HC, heavy chain; Ig, immunoglobulin; IRES, internal ribosomal entry site; LC, light chain; MZ, marginal zone; NFKB, nuclear factor of kappa light polypeptide gene enhancer in B cells; TLR, toll-like receptor; UPR, unfolded protein response.

Aryl hydrocarbon receptor-induced activation of the human IGH hs1.2 enhancer: Mutational analysis of putative regulatory binding motifs

The human hs1.2 enhancer within the Ig heavy chain gene (IGH) is polymorphic and associated with a number of autoimmune diseases. The polymorphic region is characterized by tandem repeats of an ∼53-bp invariant sequence containing possible binding sites for several transcription factors. Our previous studies suggest the human hs1.2 enhancer is sensitive to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an environmental toxicant and high affinity ligand of the aryl hydrocarbon receptor (AhR). TCDD induced hs1.2 enhancer activity in an AhR-dependent manner and the number of invariant sequences influenced the magnitude of activity. To better understand the regulation of human hs1.2 enhancer activity, the objective of the current study was to utilize mutational analysis and luciferase reporter constructs to evaluate the contribution of putative transcription factor binding sites to overall hs1.2 enhancer activity and modulation by TCDD. Basal and LPS-induced activity of the hs1.2 enhancer appeared to be most affected by mutation of sites outside of the invariant sequence or deletion of the entire invariant sequence; whereas sites influencing the effect of TCDD were dependent on the cellular activation state (i.e. unstimulated vs. LPS stimulation) and relatively independent of the putative AhR binding site within the invariant sequence. These results suggest that AhR activation affects human hs1.2 activity through an as yet undetermined non-canonical pathway. A better understanding regarding the role of the hs1.2 enhancer in human Ig expression and how AhR ligands modulate its activity may lead to insights into overall Ig regulation and mechanisms of dysfunction.

MHC class II exacerbates demyelination in vivo independently of T cells

We have shown previously the importance of MHC class II for central nervous system remyelination; however, the function of MHC class II during cuprizone-induced demyelination has not been examined. Here, we show that I-A(beta)-/- mice exhibit significantly reduced inflammation and demyelination. RAG-1(1/1) mice are indistinguishable from controls, indicating T cells may not play a role. The role of MHC class II depends on an intact cytoplasmic tail that leads to the production of IL-1beta, TNF-alpha, and nitric oxide, and oligodendrocyte apoptosis. Thus, the function of MHC class II cytoplasmic tail appears to increase microglial proliferation and activation that exacerbates demyelination.

Interleukin-12 and -10 and gamma interferon regulate polyclonal and ligand-specific expansion of murine B-1 cells

B-1 cells (CD5+ B220+) are a self-replenishing lineage of B cells which are autoreactive and capable of producing large amounts of interleukin-10 (IL-10). In mice experimentally infected with the human helminth parasite Schistosoma mansoni, expansion of B-1 cells is seen in the peritoneal cavity just prior to egg laying. In naive mice, B-1 cell expansion can be elicited by intraperitoneal injection of saline soluble egg antigens (SEA) or the polylactosamine sugar lacto-N-fucopentaose III (LNFPIII), which contains the Lewis-X trisaccharide. In this study, we demonstrate that LNFPIII is the major stimulus in SEA responsible for expansion of B-1 cells, since SEA-induced B-1 outgrowth is blocked by multiple injections of non-cross-linked free LNFPIII. IL-10 is an autocrine growth factor for B-1 cells, and we show that B-1 outgrowth after SEA and LNFPIII administration is inhibited by injection of anti-IL-10 antibodies. Furthermore, SEA- and LNFPIII-induced expansion of B-1 cells is inhibited by in vivo administration of recombinant murine IL-12 or recombinant gamma interferon. These data suggest that activation and expansion of IL-10-producing B-1 cells are governed via cross-regulatory cytokines.

Dysregulation of IL-10 production with aging: possible linkage to the age-associated decline in DHEA and its sulfated derivative

Peripheral lymphoid cells isolated from the spleens and peritoneal cavities of aged mice were found to constitutively secrete the multifunctional cytokine interleukin (IL)-10 when cultured in vitro. B-Lymphocytes were implicated as the cell type responsible. Abnormal expression of this cytokine was also detected in vivo because high levels of mRNA for IL-10 were present in splenocytes freshly isolated from aged animals. In addition to the spontaneous secretion of IL-10, lymphoid cells from aged donors were hyperresponsive to exogenous stimulation with endotoxin, producing exaggerated quantities of both IL-10 and IL-6 in culture. Treatment of aged animals with dehydroepiandrosterone sulfate (DHEAS), a natural steroid, reversed the age-associated alterations in cytokine production, rendering the treated mice quite similar to mature adult controls. DHEAS treatment of aged mice also resulted in a lowering in the number of B1 cells present in the peritoneal cavity and also reduced the titers of circulating autoantibodies specific for phosphatidylcholine (PtC). Based on its wide range of biologic activities, a dysregulation in the mechanisms that control IL-10 production could be a major contributor to immunosenescence. The ability of DHEAS treatment to restore normal control over the expression of IL-10 may explain how this steroid enhances immunocompetence in aged animals.

Signaling via major histocompatibility complex class II molecules and antigen receptors enhances the B cell response to gp39/CD40 ligand

Activated T cells induce proliferation and differentiation of resting B cells in vitro through their CD40 molecules and lymphokine receptors. However, despite constitutive B cell expression of CD40 and lymphokine receptors, widespread nonspecific polyclonal B cell activation by activated T cells is seldom observed in vivo. The present study was designed to test the hypothesis that signals delivered via the B cell antigen (Ag) receptor (membrane immunoglobulin, mIg) and major histocompatibility complex (MHC) class II molecules enhance B cell responsiveness to CD40-mediated signals, providing specificity to the Ag-nonspecific, MHC-unrestricted CD40 signal. To test this hypothesis, both an Ag-specific mouse B cell clone CH12.LX, and freshly isolated resting splenic B cells were cultured with either soluble or membrane-bound forms of the T cell ligand for CD40 (CD40L), in the presence or absence of additional signals provided by Ag or anti-IgM, interleukin-4, and class II-specific monoclonal antibody (mAb). Differentiation of CH12.LX cells and proliferation of splenic B cells in response to both forms of CD40L was greatly enhanced by exposure to mIg-mediated signals, with greatest enhancement seen when cells were cultured with Ag prior to receiving other signals. Response to CD40L was further enhanced by concurrent culture with class II-specific, but not class I-specific mAb. Enhancement was greatest at limiting concentrations of CD40L. The ability of class II MHC-mediated signals to enhance Ag-specific B cell responsiveness to CD40-mediated signaling may selectively promote the activation of B cell clones capable of cognate interactions with helper T cells.