Autoreactive B cells in SLE, villains or innocent bystanders?

The current concepts for development of autoreactive B cells in SLE (systemic lupus erythematosus) focus on extrinsic stimuli and factors that provoke B cells into tolerance loss. Traditionally, major tolerance loss pathways are thought to be regulated by factors outside the B cell including autoantigen engagement of the B-cell receptor (BCR) with simultaneous type I interferon (IFN) produced by dendritic cells, especially plasmacytoid dendritic cells (pDCs). Later, in autoreactive follicles, B-cells encounter T-follicular helper cells (Tfh) that produce interleukin (IL)-21, IL-4 and pathogenic cytokines, IL-17 and IFN gamma (IFNɣ). This review discusses these mechanisms and also highlights recent advances pointing to the peripheral transitional B-cell stage as a major juncture where transient autocrine IFNβ expression by developing B-cells imprints a heightened susceptibility to external factors favoring differentiation into autoantibody-producing plasmablasts. Recent studies highlight transitional B-cell heterogeneity as a determinant of intrinsic resistance or susceptibility to tolerance loss through the shaping of B-cell responsiveness to cytokines and other environment factors.

Inducible Colonic M Cells Are Dependent on TNFR2 but Not Ltβr, Identifying Distinct Signalling Requirements for Constitutive Versus Inducible M Cells

BACKGROUND AND AIMS

M cells associated with organised lymphoid tissues such as intestinal Peyer's patches provide surveillance of the intestinal lumen. Inflammation or infection in the colon can induce an M cell population associated with lymphoid infiltrates; paradoxically, induction is dependent on the inflammatory cytokine tumour necrosis factor [TNF]-α. Anti-TNFα blockade is an important therapeutic in inflammatory bowel disease, so understanding the effects of TNFα signalling is important in refining therapeutics.

METHODS

To dissect pro-inflammatory signals from M cell inductive signals, we used confocal microscopy image analysis to assess requirements for specific cytokine receptor signals using TNF receptor 1 [TNFR1] and 2 [TNFR2] knockouts [ko] back-crossed to the PGRP-S-dsRed transgene; separate groups were treated with soluble lymphotoxin β receptor [sLTβR] to block LTβR signalling. All groups were treated with dextran sodium sulphate [DSS] to induce colitis.

RESULTS

Deficiency of TNFR1 or TNFR2 did not prevent DSS-induced inflammation nor induction of stromal cell expression of receptor activator of nuclear factor kappa-B ligand [RANKL], but absence of TNFR2 prevented M cell induction. LTβR blockade had no effect on M cell induction, but it appeared to reduce RANKL induction below adjacent M cells.

CONCLUSIONS

TNFR2 is required for inflammation-inducible M cells, indicating that constitutive versus inflammation-inducible M cells depend on different triggers. The inducible M cell dependence on TNFR2 suggests that this specific subset is dependent on TNFα in addition to a presumed requirement for RANKL. Since inducible M cell function will influence immune responses, selective blockade of TNFα may affect colonic inflammation.

A role for tumor necrosis factor receptor type 1 in gut-associated lymphoid tissue development: genetic evidence of synergism with lymphotoxin beta

Lymphotoxin alpha (LTalpha) signals via tumor necrosis factor receptors (TNFRs) as a homotrimer and via lymphotoxin beta receptor (LTbetaR) as a heterotrimeric LTalpha1beta2 complex. LTalpha-deficient mice lack all lymph nodes (LNs) and Peyer's patches (PPs), and yet LTbeta-deficient mice and TNFR-deficient mice have cervical and mesenteric LN. We now show that mice made deficient in both LTbeta and TNFR type 1 (TNFR1) lack all LNs, revealing redundancy or synergism between TNFR1 and LTbeta, acting presumably via LTbetaR. A complete lack of only PPs in mice heterozygous for both ltalpha and ltbeta, but not ltalpha or ltbeta alone, suggests a similar two-ligand phenomenon in PP development and may explain the incomplete lack of PPs seen in tnfr1-/- mice.