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Krüppel-like factor 2 (KLF2) is a potent regulator of lymphocyte differentiation, activation and migration. However, its functional role in adaptive and humoral immunity remains elusive. Therefore, by using mice with a B cell-specific deletion of KLF2, we investigated plasma cell differentiation and antibody responses. We revealed that the deletion of KLF2 resulted in perturbed IgA plasma cell compartmentalization, characterized by the absence of IgA plasma cells in the bone marrow, their reductions in the spleen, the blood and the lamina propria of the colon and the small intestine, concomitant with their accumulation and retention in mesenteric lymph nodes and Peyer's patches. Most intriguingly, secretory IgA in the intestinal lumen was almost absent, dimeric serum IgA was drastically reduced and antigen-specific IgA responses to soluble Salmonellaflagellin were blunted in KLF2-deficient mice. Perturbance of IgA plasma cell localization was caused by deregulation of CCR9, Integrin chains αM, α4, β7, and sphingosine-1-phosphate receptors. Hence, KLF2 not only orchestrates the localization of IgA plasma cells by fine-tuning chemokine receptors and adhesion molecules but also controls IgA responses to Salmonellaflagellin.

Krüppel-like factor 2 (KLF2) is a potent regulator of lymphocyte differentiation, activation and migration. However, its functional role in adaptive and humoral immunity remains elusive. Therefore, by using mice with a B cell-specific deletion of KLF2, we investigated plasma cell differentiation and antibody responses. We revealed that the deletion of KLF2 resulted in perturbed IgA plasma cell compartmentalization, characterized by the absence of IgA plasma cells in the bone marrow, their reductions in the spleen, the blood and the lamina propria of the colon and the small intestine, concomitant with their accumulation and retention in mesenteric lymph nodes and Peyer’s patches. Most intriguingly, secretory IgA in the intestinal lumen was almost absent, dimeric serum IgA was drastically reduced and antigen-specific IgA responses to soluble Salmonellaflagellin were blunted in KLF2-deficient mice. Perturbance of IgA plasma cell localization was caused by deregulation of CCR9, Integrin chains αM, α4, β7, and sphingosine-1-phosphate receptors. Hence, KLF2 not only orchestrates the localization of IgA plasma cells by fine-tuning chemokine receptors and adhesion molecules but also controls IgA responses to Salmonellaflagellin.

Live attenuated vaccines are generally highly efficacious and often superior to inactivated vaccines, yet the underlying mechanisms of this remain largely unclear. Here we identify recognition of microbial viability as a potent stimulus for follicular helper T cell (TFHcell) differentiation and vaccine responses. Antigen-presenting cells (APCs) distinguished viable bacteria from dead bacteria through Toll-like receptor 8 (TLR8)-dependent detection of bacterial RNA. In contrast to dead bacteria and other TLR ligands, live bacteria, bacterial RNA and synthetic TLR8 agonists induced a specific cytokine profile in human and porcine APCs, thereby promoting TFHcell differentiation. In domestic pigs, immunization with a live bacterial vaccine induced robust TFHcell and antibody responses, but immunization with its heat-killed counterpart did not. Finally, a hypermorphic TLR8polymorphism was associated with protective immunity elicited by vaccination with bacillus Calmette-Guérin (BCG) in a human cohort. We have thus identified TLR8 as an important driver of TFHcell differentiation and a promising target for TFHcell–skewing vaccine adjuvants.

T- und B‑Lymphozyten in Milz und Lymphknoten schützen den Körper normalerweise zuverlässig vor Infektionskrankheiten. Bei rheumatoider Arthritis sind sie jedoch als gewebeinfiltrierende Lymphozyten durch die Produktion inflammatorischer löslicher Botenstoffe und autoreaktiver Antikörper wesentlich an der Gewebezerstörung im rheumatischen Gelenk beteiligt. Bislang war unklar, ob diese Lymphozyten als ausdifferenzierte Effektorzellen aus benachbarten Lymphknoten einwandern oder auch direkt vor Ort gebildet werden. Eine neue Studie zeigt nun, dass T‑ und B‑Zellen auch außerhalb lymphatischen Gewebes aktiv miteinander interagieren. Eine mit follikulären T‑Helferzellen verwandte Zellpopulation unterstützt hierbei die lokale Generierung von keimzentrumsähnlichen B‑Zellen und hochaffinen Plasmazellen. The T and B lymphocytes in secondary lymphoid organs, such as the spleen and lymph nodes, normally reliably protect our body from infectious diseases; however, in rheumatoid arthritis they infiltrate tissues and substantially contribute to tissue destruction in rheumatic joints by production of inflammatory chemokines and autoreactive antibodies. It was previously unclear whether these lymphocytes infiltrate tissues as fully differentiated effector cells from neighboring lymph nodes or whether they are locally generated. A recent study has now shown that T and B cells actively cooperate together even outside lymphoid tissue. A follicular T‑helper cell-like population promotes the local generation of germinal center-like B cells and high-affinity plasma cells.

The co-stimulators ICOS (inducible T cell co-stimulator) and CD28 are both important for T follicular helper (TFH) cells, yet their individual contributions are unclear. Here, we show that each molecule plays an exclusive role at different stages of TFH cell development. While CD28 regulated early expression of the master transcription factor Bcl-6, ICOS co-stimulation was essential to maintain the phenotype by regulating the novel TFH transcription factor Klf2 via Foxo1. Klf2 directly binds to Cxcr5, Ccr7, Psgl-1, and S1pr1, and low levels of Klf2 were essential to maintain this typical TFH homing receptor pattern. Blocking ICOS resulted in relocation of fully developed TFH cells back to the T cell zone and reversion of their phenotype to non-TFH effector cells, which ultimately resulted in breakdown of the germinal center response. Our study describes for the first time the exclusive role of ICOS and its downstream signaling in the maintenance of TFH cells by controlling their anatomical localization in the B cell follicle.

Zusammenfassung: Jüngste Publikationen zur Funktion der T-Zell-Kostimulatoren CD226 und TIGIT sowie zur Identifizierung des kostimulatorischen Liganden VISTA sind speziell für das Verständnis von Autoimmunerkrankungen von großem Interesse. Beide Systeme zeigen erstaunliche Parallelen zu den bereits gut untersuchten Kostimulatoren CD28/CTLA-4 bzw. PD-1.

Airway inflammation plays a critical role in the pathogenesis of asthma. In susceptible individuals, airway allergen exposure results in the recruitment of inflammatory cells into lung tissue, leading to a local inflammatory response. Central to the induction and regulation of this process are T lymphocytes.

T 3;cell activation, differentiation and effector functions depend on signals delivered through the antigen-specific TCR and non-clonal costimulatory receptors on the T 3;cell. Activated T 3;cells express the inducible costimulator (ICOS). We examined the co-expression of ICOS with Th cytokines in mucosal immune responses. ICOS+CD4+ Th 3;cells expressed strikingly different cytokines depending on the type of infection encountered and the cells' anatomical localization. In the Th2-dominated response to Schistosoma mansoni, ICOS expression of CD4+ cells isolated from the liver was strongly associated with the expression of IL-5, IL-10, IL-13, and T1/ST2, but not with the chemokine receptor CXCR5, a pattern consistent with Th2 effector cells. In the secondary lymphatic organs of schistosome-infected mice, ICOS expression was randomly correlated with Th2 effector-cytokines, but positively correlated with CXCR5 expression; a pattern consistent with follicular Th cells. In Th 3;cells isolated from gut or liver of mice infected with Toxoplasma gondii, ICOS expression was positively correlated with IFN-γ production. Finally, in the severe combined immunodeficiency transfer colitis model, ICOS expression was strongly positively associated with IFN-γ and IL-2. Thus, ICOS appears to costimulate distinct effector functions in different immune responses, depending on factors such as the nature of the antigen encountered and localization and chronicity of the immune response.

The studies performed to date analyzed the overall participation of the inducible costimulator (ICOS) in model diseases, but did not yield information on the nature and function of ICOS-expressing T cells in vivo. We examined ICOS+ T cells in the secondary lymphoid organs of nonmanipulated mice, in the context of an “unbiased” immune system shaped by environmental antigens. Using single cell analysis, ICOSlow cells were found to be loosely associated with the early cytokines interleukin (IL)-2, IL-3, IL-6, and interferon (IFN)-γ. ICOSmedium cells, the large majority of ICOS+ T cells in vivo, were very tightly associated with the synthesis of the T helper type 2 (Th2) cytokines IL-4, IL-5, and IL-13, and these cells exhibited potent inflammatory effects in vivo. In contrast, ICOShigh T cells were highly and selectively linked to the anti-inflammatory cytokine IL-10. Overall, these data seem to indicate that ICOS cell surface density serves as a regulatory mechanism for the release of cytokines with different immunological properties. Further in vivo functional experiments with in vitro–activated T cells strongly suggested that the ICOS+ population, although representing in vivo only around 10% of T cells bearing early or late activation markers, nevertheless encompasses virtually all effector T cells, a finding with major diagnostic and therapeutic implications.

With newly generated ICOS-ligand (ICOS-L)-specific monoclonal antibodies we determined that human Langerhans cells in situ express similar levels of ICOS-L, CD80, and CD86, compared to immature dendritic cells (DC) derived from monocytes in vitro. Maturation of DC strongly up-regulated CD80 and CD86 but did not significantly change ICOS-L levels. On coculture of "naive"CD4⁺ T cells with mature DC in the presence of superantigen, ICOS was highly up-regulated on T cells, but played only a secondary role in the CD28-dominated release of TNF-α and IFN-γ , and did not participate in the induction of IL-2. Cocultures of "effector" CD4⁺ T cells with mature DC revealed CD28 as the driving force for the secretion of IL-2, IFN-γ , IL-6, and IL-13, with no apparent contribution of ICOS. In contrast, the release of IL-10 was differentially regulated. Interaction of ICOS with ICOS-L strongly promoted IL-10 secretion, whereas the CD28/B7 pathway acted as a potent attenuator of IL-10 release. Our data thus indicate a selective regulation of IL-10 secretion by ICOS on re-activation of effector T cells with professional antigen-presenting cells (bearing CD80 and CD86) in lymphoid tissue.

Recently, we have identified the inducible co-stimulator (ICOS), an activation-dependent, T cell-specific cell surface molecule related to CD28 and CTLA-4. Detailed analysis of human ICOS presented here shows that it is a 55-60-kDa homodimer with differently N-glycosylated subunits of 27 and 29 kDa. ICOS requires both phorbol 12-myristate 13-acetate and ionomycin for full induction, and is sensitive to Cyclosporin A. ICOS is up-regulated early on all T cells, including the CD28^– subset, and continues to be expressed into later phases of T cell activation. On stimulation of T cells by antigen-presenting cells, the CD28/B7, but not the CD40 ligand/CD40 pathway is critically involved in the induction of ICOS. ICOS does not bind to B7-1 or B7-2, and CD28 does not bind to ICOS ligand; thus the CD28 and ICOS pathways do not cross-interact on the cell surface. In vivo, ICOS is expressed in the medulla of the fetal and newborn thymus, in the T cell zones of tonsils and lymph nodes, and in the apical light zones of germinal centers (predominant expression). Functionally, ICOS co-induces a variety of cytokines including IL-4, IL-5, IL-6, IFN-γ, TNF-α, GM-CSF, but not IL-2, and superinduces IL-10. Furthermore, ICOS co-stimulation prevents the apoptosis of pre-activated T cells. The human ICOS gene maps to chromosome 2q33 – 34.

Human ICOS (huICOS) is a T cell-specific molecule structurally related to CD28 and CTLA-4 with potent co-stimulatory activities on T cell proliferation, cytokine induction and T cell help for B cells. We have now cloned and characterized murine ICOS (muICOS). muICOS mRNA of 1.5 kb and 3.3 kb encodes a protein with a deduced molecular mass of 20.3 kDa, which is 71.7 % identical to huICOS. On the cell surface, muICOS is expressed as a disulfide-linked, glycosylated homodimer of 47-57 kDa, with subunits of approximately 26 kDa. With a panel of monoclonal antibodies we have determined the expression of muICOS in vitro and in vivo. Following activation of splenic T cells via CD3, muICOS became detectable at 12 h and reached a maximum of expression at around 48 h, thus exhibiting expression kinetics similar to huICOS. In vivo, muICOS was found to be substantially expressed in the thymic medulla and in the germinal centers and T cell zones of lymph nodes and Peyer's patches. Non-lymphoid tissue was ICOS negative. The muICOS gene was mapped to a region of chromosome 1 also harboring the CD28 and CTLA-4 genes. Using recombinant chimeric muICOS-Ig we determined that B7h, a recently cloned B7-like molecule, is a ligand for muICOS.