Diet and the gut microbiota may underpin numerous human diseases. A major metabolic product of commensal bacteria are short-chain fatty acids (SCFAs) that derive from fermentation of dietary fibre. Here we show that diets deficient or low in fibre exacerbate colitis development, while very high intake of dietary fibre or the SCFA acetate protects against colitis. SCFAs binding to the 'metabolite-sensing' receptors GPR43 and GPR109A in non-haematopoietic cells mediate these protective effects. The inflammasome pathway has hitherto been reported as a principal pathway promoting gut epithelial integrity. SCFAs binding to GPR43 on colonic epithelial cells stimulates K þ efflux and hyperpolarization, which lead to NLRP3 inflammasome activation. Dietary fibre also shapes gut bacterial ecology, resulting in bacterial species that are more effective for inflammasome activation. SCFAs and metabolite receptors thus explain health benefits of dietary fibre, and how metabolite signals feed through to a major pathway for gut homeostasis.

Follicular B helper T (Tfh) cells support high affinity and long-term antibody responses. Here we found that within circulating CXCR5⁺ CD4⁺ T cells in humans and mice, the CCR7(lo)PD-1(hi) subset has a partial Tfh effector phenotype, whereas CCR7(hi)PD-1(lo) cells have a resting phenotype. The circulating CCR7(lo)PD-1(hi) subset was indicative of active Tfh differentiation in lymphoid organs and correlated with clinical indices in autoimmune diseases. Thus the CCR7(lo)PD-1(hi) subset provides a biomarker to monitor protective antibody responses during infection or vaccination and pathogenic antibody responses in autoimmune diseases. Differentiation of both CCR7(hi)PD-1(lo) and CCR7(lo)PD-1(hi) subsets required ICOS and BCL6, but not SAP, suggesting that circulating CXCR5⁺ helper T cells are primarily generated before germinal centers. Upon antigen reencounter, CCR7(lo)PD-1(hi) CXCR5⁺ precursors rapidly differentiate into mature Tfh cells to promote antibody responses. Therefore, circulating CCR7(lo)PD-1(hi) CXCR5⁺ CD4⁺ T cells are generated during active Tfh differentiation and represent a new mechanism of immunological early memory.

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High expression of CXCR5 is one of the defining hallmarks of T follicular helper cells (TFH), a CD4 Th cell subset that promotes germinal center reactions and the selection and affinity maturation of B cells. CXCR5 is also expressed on 20–25% of peripheral blood human central memory CD4 T cells (TCM), although the definitive function of these cells is not fully understood. The constitutive expression of CXCR5 on TFH cells and a fraction of circulating TCM suggests that CXCR5+ TCM may represent a specialized subset of memory-type TFH cells programmed for homing to follicles and providing B cell help. To verify this assumption, we analyzed this cell population and show its specialized function in supporting humoral immune responses. Compared with their CXCR5− TCM counterparts, CXCR5+ TCM expressed high levels of the chemokine CXCL13 and efficiently induced plasma cell differentiation and Ig secretion. We found that the distinct B cell helper qualities of CXCR5+ TCM were mainly due to high ICOS expression and pronounced responsiveness to ICOS ligand costimulation together with large IL-10 secretion. Furthermore, B cell helper attributes of CXCR5+ TCM were almost exclusively acquired on cognate interaction with B cells, but not with dendritic cells. This implies that a preferential recruitment of circulating CXCR5+ TCM to CXCL13-rich B cell follicles is required for the promotion of a quick and efficient protective secondary humoral immune response. Taken together, we propose that CXCR5+ TCM represent a distinct memory cell subset specialized in supporting Ab-mediated immune responses.

B-cell receptor signaling contributes to apoptosis resistance in chronic lymphocytic leukemia (CLL), limiting the efficacy of current therapeutic approaches. In this study, we investigated the expression of spleen tyrosine kinase (SYK), a key component of the B-cell receptor signaling pathway, in CLL and its role in apoptosis. Gene expression profiling identified enhanced expression of SYK and downstream pathways in CLL compared with healthy B cells. Immunoblotting showed increased expression and phosphorylation of SYK, PLC; 2 , signal transducers and activators of transcription 3, and extracellular signal regulated kinase 1/2 in CLL compared with healthy B cells, suggesting enhanced activation of these mediators in CLL. SYK inhibitors reduced phosphorylation of SYK downstream targets and induced apoptosis in primary CLL cells. With respect to prognostic factors, SYK inhibitors exerted stronger cytotoxic effects in unmutated and ZAP70 + cases. Cytotoxic effects of SYK inhibitors also associated with SYK protein expression, potentially predicting response to therapy. Combination of fludarabine with SYK Inhibitor II or R406 increased cytotoxicity compared with fludarabine therapy alone. We observed no stroma-contact-mediated drug resistance for SYK inhibitors as described for fludarabine treatment. CD40 ligation further enhanced efficacy of SYK inhibition. Our data provide mechanistic insight into the recently observed therapeutic effects of the SYK inhibitor R406 in CLL. Combination of SYK inhibitors with fludarabine might be a novel treatment option particularly for CLL patients with poor prognosis and should be further evaluated in clinical trials. [Cancer Res 2009;69(13):5424-32]

Filamentous haemagglutinin (FHA) is the major adhesin of Bordetella pertussis, the whooping cough agent. FHA is synthesized as a 367-kDa precursor harbouring a remarkably long signal peptide with an N-terminal extension that is conserved among related virulence proteins. FHA is secreted via the two-partner secretion pathway that involves transport across the outer membrane by a cognate transporter protein. Here we have analyzed the mechanism by which FHA is targeted to, and translocated across, the inner membrane. Studies were performed both in vitro using Escherichia coli inside-out inner membrane vesicles and in vivo by pulse-chase labelling of Bordetella pertussis cells. The data collectively indicate that like classical periplasmic and outer membrane proteins, FHA requires SecA and SecB for its export through the SecYEG translocon in the inner membrane. Although short nascent chains of FHA were found to cross-link to signal recognition particle (SRP), we did not obtain indication for an SRP-dependent, co-translational membrane targeting provoked by the FHA signal sequence. Our results rule out that the extended signal peptide of FHA determines a specific mode of membrane targeting but rather suggest that it might influence the export rate at the inner membrane.

The dynamic interplay between regulatory T cells (Tregs) and effector T cells (Teffs) governs the balance between tolerance and effector immune responses. Perturbations of Treg frequency and function or imbalances in Treg/Teff levels are associated with the development of autoimmunity. The factors that mediate these changes remain poorly understood and were investigated in this study in murine autoimmune arthritis. Tregs displayed a stable phenotype in arthritic mice and were fully functional in in vitro suppression assays. However, their expansion was delayed relative to Teffs (T follicular helper cells and Th17 cells) during the early stages of autoimmune reactivity. This imbalance is likely to have led to insufficient Treg control of Teffs and induced autoimmunity. Moreover, a counterregulatory and probably IL-7–driven increase in thymic Treg production and recruitment to inflamed tissues was too slow for disease prevention. Increased Teff over Treg expansion was further aggravated by inflammation and lymphopenia. Both these conditions contribute to autoimmune pathogenesis and were accompanied by decreases in the availability of IL-2 and increases in levels of IL-21. IL-2 neutralization or supplementation was used to show that Treg expansion mainly depended on this cytokine. IL-21R−/− cells were used to demonstrate that IL-21 promoted the maintenance of Teffs. Thus, at inflammatory sites in experimental arthritis, a deficit in IL-2 hampers Treg proliferation, whereas exaggerated IL-21 levels overwhelm Treg control by supporting Teff expansion. This identifies IL-2 and IL-21 as targets for manipulation in therapies for autoimmunity.

Changed dietary habits in Western countries such as reduced fiber intake represent an important lifestyle factor contributing to the increase in inflammatory immune-mediated diseases. The mode of action of beneficial fiber effects is not fully elucidated, but short-chain fatty acids (SCFA) and gut microbiota have been implicated. The aim of this study was to explore the impact of dietary fiber on lupus pathology and to understand underlying mechanisms. Here, we show that in lupus-prone NZB/WF1 mice low fiber intake deteriorates disease progression reflected in accelerated mortality, autoantibody production and immune dysregulation. In contrast to our original assumption, microbiota suppression by antibiotics or direct SCFA feeding did not influence the course of lupus-like disease. Mechanistically, our data rather indicate that in low fiber-fed mice, an increase in white adipose tissue mass, fat-inflammation and a disrupted intestinal homeostasis go along with systemic, low-grade inflammation driving autoimmunity. The links between obesity, intestinal leakage and low-grade inflammation were confirmed in human samples, while adaptive immune activation predominantly correlated with lupus activity. We further propose that an accelerated gastro-intestinal passage along with energy dilution underlies fiber-mediated weight regulation. Thus, our data highlight the often-overlooked effects of dietary fiber on energy homeostasis and obesity prevention. Further, they provide insight into how intricately the pathologies of inflammatory immune-mediated conditions, such as obesity and autoimmunity, might be interlinked, possibly sharing common pathways.