Niklas Czeloth

CCR9 is a homing receptor for plasmacytoid dendritic cells to the small intestine

Small intestine plasmacytoid dendritic cells (pDC) are poorly characterized. Here, we demonstrate that intestinal pDC show the characteristic plasma cell-like morphology, and are recognized by antibodies against B220, Ly6c, 120G8, and PDCA-1, markers that are typically expressed by pDC. Furthermore, intestinal pDC carry high levels of CCR9 and are largely absent in the intestine, but not in lung, liver, or secondary lymphoid organs of CCR9-deficient animals. Competitive adoptive transfers reveal that CCR9-deficient pDC are impaired in homing to the small intestine after i.v. transfer. In a model of cholera toxin-induced gut inflammation, pDC are recruited to the intestine in WT but not CCR9-deficient animals. Furthermore, after oral application of a Toll-like receptor (TLR) 7/8 ligand, myeloid DC of the lamina propria are rapidly mobilized in WT but not in CCR9-deficient animals. Mobilization of myeloid DC can be completely rescued by adoptively transferred WT pDC to CCR9-deficient mice before oral challenge. Together, our data reveal an essential role for CCR9 in the homing of pDC to the intestine under homeostatic and inflammatory conditions and demonstrate an important role for intestinal pDC for the rapid mobilization of lamina propria DC.

Sphingosine-1-Phosphate Mediates Migration of Mature Dendritic Cells

Sphingosine-1-phosphate (S1P) represents a potent modulator of diverse cellular activities, including lymphocyte trafficking and maintenance of lymphocyte homeostasis. The five known receptors for S1P (S1P1–5) belong to the family of G protein-coupled receptors. Upon binding S1P, they act downstream via heterotrimeric G proteins on members of the small GTPase family (Cdc42/Rac/Rho), evoking a S1P receptor-dependent activation pattern of Cdc42, Rac, and Rho, respectively. This, in turn, triggers cytoskeletal rearrangements determining cellular morphology and movement. In this study we investigated the effects of S1P on murine dendritic cells (DC). Mature DC, but not immature in vitro differentiated DC, were found to migrate to S1P, a phenomenon that correlated to the up-regulation of S1P1 and S1P3 in maturing DC. The same pattern of S1P receptor regulation could be observed in vivo on skin DC after their activation and migration into the lymph node. The migration-inducing effect of S1P could be severely hampered by application of the S1P analogon FTY720 in vitro and in vivo. A similar, yet more pronounced, block was observed upon preventing Cdc42/Rac and/or Rho activation by specific inhibitors. These results suggest that S1P-mediated signaling plays a pivotal role in the life cycle of DC.

Alloantigen-specific de novo-induced Foxp3+ Treg revert in vivo and do not protect from experimental GVHD

Induced antigen‐specific Foxp3+ T cells (iTreg) are being discussed as a promising alternative to polyclonal natural Foxp3+ T cells (nTreg) for cell‐based therapies, particularly to achieve transplantation tolerance. Using Foxp3eGFP‐reporter mice, we here establish an efficient protocol to induce and expand alloantigen‐specific iTreg from Foxp3−CD4+ T cells with cluster‐disrupted DC. These iTreg were mainly CD62L+ and showed efficient suppressive activity in vitro. However, in contrast to nTreg, adoptively transferred iTreg entirely failed to prevent lethal graft versus host disease (GVHD). Within irradiated recipients, the majority of adoptively transferred Foxp3+ iTreg, but not Foxp3+ nTreg quickly reverted to Foxp3−CD4+ T cells. We therefore suggest that therapeutic approaches to treat GVHD should rely on nTreg, whereas the use of de novo alloantigen‐induced iTreg should be handled with caution since the stability of the regulatory phenotype of the iTreg could be of major concern.

The adhesion receptor CD155 determines the magnitude of humoral immune responses against orally ingested antigens

CD155, originally known as the cellular receptor for poliovirus, is the founding member of a subfamily of immunoglobulin‐like adhesion receptors. Apart from its function in establishing adherens junctions between contacting epithelial cells, the engagement of CD155 with two recently identified ligands, CD226 and CD96, mediates immunologically relevant processes such as NK cell‐driven killing of tumor cells in humans. Here we report on the generation and immunological analysis of mice constitutively deficient of CD155. Moreover, the expression profile of CD155 on hematopoietic cells has been determined using newly established antibodies. CD155‐deficient mice develop normally without displaying an overt phenotype. However, the animals are distinguished by distinct deficits in the development of a regular humoral immune response. Whereas systemic challenges revealed no differences, orally administered antigen evoked less efficient IgG and IgA antibody responses despite of normal IgM titers when compared to wild‐type mice. Therefore, CD155 may assist in an efficient humoral immune response generated within the intestinal immune system.

Cytohesin-1 controls the activation of RhoA and modulates integrin-dependent adhesion and migration of dendritic cells.

Adhesion and motility of mammalian leukocytes are essential requirements for innate and adaptive immune defense mechanisms. We show here that the guanine nucleotide exchange factor cytohesin-1, which had previously been demonstrated to be an important component of beta-2 integrin activation in lymphocytes, regulates the activation of the small GTPase RhoA in primary dendritic cells (DCs). Cytohesin-1 and RhoA are both required for the induction of chemokine-dependent conformational changes of the integrin beta-2 subunit of DCs during adhesion under physiological flow conditions. Furthermore, use of RNAi in murine bone marrow DCs (BM-DCs) revealed that interference with cytohesin-1 signaling impairs migration of wild-type dendritic cells in complex 3D environments and in vivo. This phenotype was not observed in the complete absence of integrins. We thus demonstrate an essential role of cytohesin-1/RhoA during ameboid migration in the presence of integrins and further suggest that DCs without integrins switch to a different migration mode.

The Origin and Maturity of Dendritic Cells Determine the Pattern of Sphingosine 1-Phosphate Receptors Expressed and Required for Efficient Migration

Dendritic cells (DCs) represent the most potent inducers of adaptive immune responses. Depending on their activation phenotype, DCs drive naive T cells into distinct differentiation pathways. To achieve this, DCs are present in virtually all tissues where they sample Ag and migrate to the T cell areas of lymph nodes (LNs) and spleen. Ample evidence exists demonstrating that sphingosine 1-phosphate (S1P) is an important modulator of these processes, exerting its effects by binding to the S1P receptor S1P1 and/or S1P3. However, published data are contradictory, in part. We show in this study that the expression pattern, as well as the regulation of the S1P receptors, differs among in vitro-generated DCs experiencing different kinds and duration of stimuli. Moreover, the influence of S1P1 and S1P3 on the in vivo migration of maturing DCs depends on the origin of these cells. Thus, in vitro-generated DCs require S1P1 and S1P3 to accomplish this, whereas skin-derived DCs migrate unhindered in the absence of S1P3 but not when S1P1 signaling is blocked. Migration of lamina propria DCs to the mesenteric LNs depends on S1P1 and S1P3. In contrast, relocation of maturing spleen-resident DCs to the T cell zone is independent of S1P1 and S1P3. However, intrasplenic positioning of immature DCs to the bridging channels depends on S1P1 activity, with no noticeable contribution of S1P3. These observations reveal a tissue-dependent contribution of S1P3 to DC migration and suggest a fundamental role for S1P1 for maturing DCs migrating from periphery to draining LNs.

Sphingosine-1 Phosphate Signaling Regulates Positioning of Dendritic Cells within the Spleen

A successful execution and balance of adaptive immune responses requires a controlled positioning and navigation of dendritic cells (DC) into and inside secondary lymphoid organs. Whereas mechanisms were identified governing the migration of DC from peripheral nonlymphoid organs into their draining lymph nodes, little is known about the molecular cues controlling the proper positioning of spleen or lymph node resident DC. In this study, we show that the sphingosine-1 phosphate (S1P) receptor 1 influences the positioning of immature DC inside the murine spleen. Following treatment with FTY720 or SEW2871, drugs known to interfere with S1P1-mediated signaling, the 33D1+ DC subpopulation homogeneously redistributes from the bridging channels to the marginal zone. In contrast, the CD205+ DC subset remains associated with the T cell zone. Upon in vivo LPS treatment, the maturing DC assemble in the T cell zone. The LPS-driven redistribution occurs in the absence of CCR7 and cannot be prevented by FTY720, indicating that guiding mechanisms differ between immature and mature DC. Along with the observed DC subtype-specific S1P receptor expression pattern as well as the profound up-regulation of S1P1 and S1P3 accompanying DC maturation, these results suggest a decisive contribution of S1P signaling to intrasplenic DC motility and migration.

Heterogeneous expression of the adhesion receptor CD226 on murine NK and T cells and its function in NK-mediated killing of immature dendritic cells.

The adhesion receptor CD226 (DNAM‐1) is a member of the Ig superfamily possessing two extracellular V‐like domains. In humans, CD226 was shown to be expressed by NK as well as T cells. During T cell priming, CD226‐mediated costimulatory signals may skew the subsequent differentiation into the Th1 pathway. In addition, CD226 expressed on NK and cytotoxic T cells is engaged by its counter‐receptor CD155, present on target cells, thereby triggering their elimination. We established mAb specifically recognizing mCD226, demonstrating that CD226 is expressed by precursor and mature but not developing T cells. In contrast, NK cells are distinguished by a rather heterogeneous CD226 expression profile. In addition, expression of CD226 appears coupled to that of other NK cell receptors, as high expression of CD226 was found to correlate with decreased proportions of Ly49D and H positive NK cells. Upon injection into mice, the anti‐CD226 antibodies caused selective depletion of CD8+ T cells. Moreover, these antibodies as well as a naturally occurring CD226 splice variant lacking the outermost V‐like domain were instrumental in determining that CD226 adheres to CD155 via its first domain. In addition, antibodies were identified as capable of blocking the CD226/CD155 interaction and to prevent NK‐driven killing of immature DC. CD226 is thus the first mNK receptor identified to be essential for the elimination of this particular cell type.

Cutting edge : Egress of newly generated plasma cells from peripheral lymph nodes depends on β2 integrin

During humoral immune responses, naive B cells differentiate into Ab-secreting plasma cells within secondary lymphoid organs. Differentiating plasma cells egress from their sites of generation and redistribute to other tissues, predominantly the bone marrow and mucosal tissues. In this study, we demonstrate that within peripheral lymph nodes newly generated plasma cells localize to medullary cords which express the β2 integrin ligand ICAM-1. In β2 integrin-deficient mice plasma cells accumulate inside the lymph nodes, resulting in severely reduced plasma cell numbers in the bone marrow. Since plasma cells isolated from β2 integrin-deficient animals migrate efficiently into the bone marrow when transferred i.v., our findings provide profound evidence that β2 integrins are required for the egress of plasma cells from peripheral lymph nodes.

Intranodal Interaction with Dendritic Cells Dynamically Regulates Surface Expression of the Co-stimulatory Receptor CD226 Protein on Murine T Cells

Dendritic cells (DCs) are the most potent antigen-presenting cells of the immune system. Depending on their maturation status, they prime T cells to induce adaptive immunity or tolerance. DCs express CD155, an immunoglobulin-like receptor binding CD226 present on T and natural killer (NK) cells. CD226 represents an important co-stimulator during T cell priming but also serves as an activating receptor on cytotoxic T and NK cells. Here, we report that cells of the T and NK cell lineage of CD155−/− mice express markedly elevated protein levels of CD226 compared with wild type (WT). On heterozygous CD155+/− T cells, CD226 up-regulation is half-maximal, implying an inverse gene-dosis effect. Moreover, CD226 up-regulation is independent of antigen-driven activation because it occurs already in thymocytes and naïve peripheral T cells. In vivo, neutralizing anti-CD155 antibody elicits up-regulation of CD226 on T cells demonstrating, that the observed modulation can be triggered by interrupting CD155-CD226 contacts. Adoptive transfers of WT or CD155−/− T cells into CD155−/− or WT recipients, respectively, revealed that CD226 modulation is accomplished in trans. Analysis of bone marrow chimeras showed that regulators in trans are of hematopoietic origin. We demonstrate that DCs are capable of manipulating CD226 levels on T cells in vivo but not in vitro, suggesting that the process of T cells actively scanning antigen-presenting DCs inside secondary lymphoid organs is required for CD226 modulation. Hence, a CD226 level divergent from WT may be exploited as a sensor to detect abnormal DC/T-cell cross-talk as illustrated for T cells in mice lacking CCR7.