dsRNA induces apoptosis through an atypical death complex associating TLR3 to caspase-8.

Toll-like receptor 3 (TLR3) is a pattern-recognition receptor known to initiate an innate immune response when stimulated by double-stranded RNA (dsRNA). Components of TLR3 signaling, including TIR domain-containing adapter inducing IFN-α (TRIF), have been demonstrated to contribute to dsRNA-induced cell death through caspase-8 and receptor interacting protein (RIP)1 in various human cancer cells. We provide here a detailed analysis of the caspase-8 activating machinery triggered in response to Poly(I:C) dsRNA. Engagement of TLR3 by dsRNA in both type I and type II lung cancer cells induces the formation of an atypical caspase-8-containing complex that is devoid of classical death receptors of the TNFR superfamily, but instead is physically associated to TLR3. The recruitment of caspase-8 to TLR3 requires RIP1, and is negatively modulated by cellular inhibitor of apoptosis protein (cIAP)2-TNF receptor-associated factor (TRAF)2-TNFR-associated death domain (TRADD) ubiquitin ligase complex, which regulates RIP1 ubiquitination. Intriguingly, unlike Fas- or TRAILR-dependent death signaling, caspase-8 recruitment and activation within the TLR3 death-signaling complex appears not to be stringently dependent on Fas-associated with death domain (FADD). Our findings uncover a novel aspect of the molecular mechanisms involved during apoptosis induced by the innate immune receptor TLR3 in cancer cells.

In situ leukemic plasmacytoid dendritic cells pattern of chemokine receptors expression and in vitro migratory response.

Plasmacytoid dendritic cell (PDC) leukemia/lymphoma is a rare neoplasm presenting cutaneous lesions at the time of diagnosis, followed by dissemination to bone marrow, lymph nodes, and other lymphoid and nonlymphoid organs. Since these leukemic counterparts of human PDC are similar to normal PDC, we studied their chemokine receptor equipment and their migratory capacities. We found both in skin lesions and in invaded lymph nodes an expression by tumor cells of CXCR3, CXCR4, and CCR7, and the concomitant expression by cells in the microenvironment of their respective ligands CXCL9, CXCL12, and CCL19. Moreover, flow cytometry phenotype of leukemic PDC (LPDC) revealed an unexpected expression of CCR6. We show that fresh tumor cells are able to migrate in response to CXCR4, CCR2, CCR5, CCR6, and CCR7 ligands, and the ability of CXCR3 ligands to increase the responsiveness to CXCL12. IL-3- or virus-induced activation of LPDC leads to downregulation of CXCR3 and CXCR4, and upregulation of CCR7, associated with the loss of response to CXCL12, and the acquisition of sensitivity to CCL19. Altogether, these results suggest that the preferential accumulation of LPDC in the skin or lymph nodes could be orchestrated by CXCR3, CXCR4, CCR6, and CCR7 ligands, found in nontumoral structures of invaded organs.

IL-10 induces CCR6 expression during Langerhans cell development while IL-4 and IFN-gamma suppress it.

Immune responses are initiated by dendritic cells (DC) that form a network comprising different populations. In particular, Langerhans cells (LC) appear as a unique population of cells colonizing epithelial surfaces. We have recently shown that macrophage-inflammatory protein-3alpha/CCL20, a chemokine secreted by epithelial cells, induces the selective migration of LC among DC populations. In this study, we investigated the effects of cytokines on the expression of the CCL20 receptor, CCR6, during differentiation of LC. We found that both IL-4 and IFN-gamma blocked the expression of CCR6 and CCL20 responsiveness at different stages of LC development. The effect of IL-4 was reversible and most likely due to the transient blockade of LC differentiation. In contrast, IFN-gamma-induced CCR6 loss was irreversible and was concomitant to the induction of DC maturation. When other cytokines involved in DC and T cell differentiation were tested, we found that IL-10, unlike IL-4 and IFN-gamma, maintained CCR6 expression. The effect of IL-10 was reversible and upon IL-10 withdrawn, CCR6 was lost concomitantly to final LC differentiation. In addition, IL-10 induced the expression of CCR6 and responsiveness to CCL20 in differentiated monocytes that preserve their ability to differentiate into mature DC. Finally, TGF-beta, which induces LC differentiation, did not alter early CCR6 expression, but triggered its irreversible down-regulation, in parallel to terminal LC differentiation. Taken together, these results suggest that the recruitment of LC at epithelial surface might be suppressed during Th1 and Th2 immune responses, and amplified during regulatory immune responses involving IL-10 and TGF-beta.

Macrophage inflammatory protein 3alpha is expressed at inflamed epithelial surfaces and is the most potent chemokine known in attracting Langerhans cell precursors.

Dendritic cells (DCs) form a network comprising different populations that initiate and differentially regulate immune responses. Langerhans cells (LCs) represent a unique population of DCs colonizing epithelium, and we present here observations suggesting that macrophage inflammatory protein (MIP)-3alpha plays a central role in LC precursor recruitment into the epithelium during inflammation. (a) Among DC populations, MIP-3alpha was the most potent chemokine inducing the selective migration of in vitro-generated CD34(+) hematopoietic progenitor cell-derived LC precursors and skin LCs in accordance with the restricted MIP-3alpha receptor (CC chemokine receptor 6) expression to these cells. (b) MIP-3alpha was mainly produced by epithelial cells, and the migration of LC precursors induced by the supernatant of activated skin keratinocytes was completely blocked with an antibody against MIP-3alpha. (c) In vivo, MIP-3alpha was selectively produced at sites of inflammation as illustrated in tonsils and lesional psoriatic skin where MIP-3alpha upregulation appeared associated with an increase in LC turnover. (d) Finally, the secretion of MIP-3alpha was strongly upregulated by cells of epithelial origin after inflammatory stimuli (interleukin 1beta plus tumor necrosis factor alpha) or T cell signals. Results of this study suggest a major role of MIP-3alpha in epithelial colonization by LCs under inflammatory conditions and immune disorders, and might open new ways to control epithelial immunity.

Dendritic cell biology and regulation of dendritic cell trafficking by chemokines.

DC (dendritic cells) represent an heterogeneous family of cells which function as sentinels of the immune system. They traffic from the blood to the tissues where, while immature, they capture antigens. Then, following inflammatory stimuli, they leave the tissues and move to the draining lymphoid organs where, converted into mature DC, they prime naive T cells. The key role of DC migration in their sentinel function led to the investigation of the chemokine responsiveness of DC populations during their development and maturation. These studies have shown that immature DC respond to many CC and CXC chemokines (MIP-1 alpha, MIP-1 beta, MIP-3 alpha, MIP-5, MCP-3, MCP-4, RANTES, TECK and SDF-1) which are inducible upon inflammatory stimuli. Importantly, each immature DC population displays a unique spectrum of chemokine responsiveness. For examples, Langerhans cells migrate selectively to MIP-3 alpha (via CCR6), blood CD11c+ DC to MCP chemokines (via CCR2), monocytes derived-DC respond to MIP-1 alpha/beta (via CCR1 and CCR5), while blood CD11c- DC precursors do not respond to any of these chemokines. All these chemokines are inducible upon inflammatory stimuli, in particular MIP-3 alpha, which is only detected within inflamed epithelium, a site of antigen entry known to be infiltrated by immature DC. In contrast to immature DC, mature DC lose their responsiveness to most of these inflammatory chemokines through receptor down-regulation or desensitization, but acquire responsiveness to ELC/MIP-3 beta and SLC/6Ckine as a consequence of CCR7 up-regulation. ELC/MIP-3 beta and SLC/6Ckine are specifically expressed in the T-cell-rich areas where mature DC home to become interdigitating DC. Altogether, these observations suggest that the inflammatory chemokines secreted at the site of pathogen invasion will determine the DC subset recruited and will influence the class of the immune response initiated. In contrast, MIP-3 beta/6Ckine have a determinant role in the accumulation of antigenloaded mature DC in T cell-rich areas of the draining lymph node, as illustrated by recent observations in mice deficient for CCR7 or SLC/6Ckine. A better understanding of the regulation of DC trafficking might offer new opportunities of therapeutic interventions to suppress, stimulate or deviate the immune response.

A novel lysosome-associated membrane glycoprotein, DC-LAMP, induced upon DC maturation, is transiently expressed in MHC class II compartment.

We have identified a novel lysosome-associated membrane glycoprotein localized on chromosome 3q26.3-q27, DC-LAMP, which is homologous to CD68. DC-LAMP mRNA is present only in lymphoid organs and DC. A specific MAb detects the protein exclusively in interdigitating dendritic cells. Expression of DC-LAMP increases progressively during in vitro DC differentiation, but sharply upon activation with LPS, TNFalpha, or CD40L. Confocal microscopy confirmed the lysosomal distribution of the protein. Furthermore, DC-LAMP was found in the MHC class II compartment immediately before the translocation of MHC class II molecules to the cell surface, after which it concentrates into perinuclear lysosomes. This suggests that DC-LAMP might change the lysosome function after the transfer of peptide-MHC class II molecules to the surface of DC.

Critical role of IL-12 in dendritic cell-induced differentiation of naive B lymphocytes.

Dendritic cells (DC) are potent APCs initiating immune responses. In a previous report, we demonstrated that DC directly enhance both proliferation and differentiation of CD40-activated naive and memory B cells. The present study deciphers the molecular mechanisms involved in DC-dependent regulation of B cell responses. Herein, we have identified IL-12 as the mandatory molecule secreted by CD40-activated DC that promote the differentiation of naive B cells into plasma cells secreting high levels of IgM. In fact, IL-12 synergizes with soluble IL-6R alpha-chain (sgp80), produced by DC, to drive naive B cell differentiation. IL-12 is critical for the differentiation of naive B cells into IgM plasma cells, whereas IL-6R signaling mainly promotes Ig secretion by already differentiated B cells. The differentiation of naive B cells in cocultures of B cells, T cells, and DC is IL-12 dependent, definitely demonstrating that the role of DC in humoral responses is not confined to the activation of T cells and further extending the physiologic relevance of DC/B cell interaction. Finally, this study also identifies differential requirements for DC-dependent naive and memory B cell differentiation, the latter being IL-12 independent. Altogether these results indicate that, in addition to prime T cells toward Thl development, DC, through the production of IL-12, may also directly signal naive B cell during the initiation of the immune response.

Selective recruitment of immature and mature dendritic cells by distinct chemokines expressed in different anatomic sites.

DCs (dendritic cells) function as sentinels of the immune system. They traffic from the blood to the tissues where, while immature, they capture antigens. They then leave the tissues and move to the draining lymphoid organs where, converted into mature DC, they prime naive T cells. This suggestive link between DC traffic pattern and functions led us to investigate the chemokine responsiveness of DCs during their development and maturation. DCs were differentiated either from CD34(+) hematopoietic progenitor cells (HPCs) cultured with granulocyte/macrophage colony-stimulating factor (GM-CSF) plus tumor necrosis factor (TNF)-alpha or from monocytes cultured with GM-CSF plus interleukin 4. Immature DCs derived from CD34(+) HPCs migrate most vigorously in response to macrophage inflammatory protein (MIP)-3alpha, but also to MIP-1alpha and RANTES (regulated on activation, normal T cell expressed and secreted). Upon maturation, induced by either TNF-alpha, lipopolysaccharide, or CD40L, DCs lose their response to these three chemokines when they acquire a sustained responsiveness to a single other chemokine, MIP-3beta. CC chemokine receptor (CCR)6 and CCR7 are the only known receptors for MIP-3alpha and MIP-3beta, respectively. The observation that CCR6 mRNA expression decreases progressively as DCs mature, whereas CCR7 mRNA expression is sharply upregulated, provides a likely explanation for the changes in chemokine responsiveness. Similarly, MIP-3beta responsiveness and CCR7 expression are induced upon maturation of monocyte- derived DCs. Furthermore, the chemotactic response to MIP-3beta is also acquired by CD11c+ DCs isolated from blood after spontaneous maturation. Finally, detection by in situ hybridization of MIP-3alpha mRNA only within inflamed epithelial crypts of tonsils, and of MIP-3beta mRNA specifically in T cell-rich areas, suggests a role for MIP-3alpha/CCR6 in recruitment of immature DCs at site of injury and for MIP-3beta/CCR7 in accumulation of antigen-loaded mature DCs in T cell-rich areas.

The cytokine profile expressed by human dendritic cells is dependent on cell subtype and mode of activation.

In the present study, we have analyzed the pattern of cytokines expressed by two independent dendritic cell (DC) subpopulations generated in vitro from human cord blood CD34+ progenitors cultured with granulocyte-macrophage CSF and TNF-alpha. Molecularly, we confirmed the phenotypic differences discriminating the two subsets: E-cadherin mRNA was only detected in CD1a+-derived DC, whereas CD68 and factor XIIIa mRNAs were observed exclusively in CD14+-derived DC. Semiquantitative reverse-transcriptase PCR analysis revealed that both DC subpopulations spontaneously expressed IL-1alpha, IL-1beta, IL-6, IL-7, IL-12 (p35 and p40), IL-15, IL-18, TNF-alpha, TGF-beta, macrophage CSF, and granulocyte-macrophage CSF, but not IL-2, IL-3, IL-4, IL-5, IL-9, and IFN-gamma transcripts. Both subpopulations were shown to secrete IL-12 after CD40 triggering. Interestingly, only the CD14+-derived DC secreted IL-10 after CD40 activation, strengthening the notion that the two DC subpopulations indeed represent two independent pathways of DC development. Furthermore, both DC subpopulations expressed IL-13 mRNA and protein following activation with PMA-ionomycin, but not with CD40 ligand, in contrast to IL-12 and IL-10, revealing the existence of different pathways for DC activation. Finally, we confirmed the expression of IL-7, IL-10, and IL-13 mRNA by CD4+ CD11c+ CD3- DC isolated ex vivo from tonsillar germinal centers. Thus, CD14+-derived DC expressing IL-10 and factor XIIIa seemed more closely related to germinal center dendritic cellsGCDC than to Langerhans cells.

CD34+ hematopoietic progenitors from human cord blood differentiate along two independent dendritic cell pathways in response to granulocyte-macrophage colony-stimulating factor plus tumor necrosis factor alpha: II. Functional analysis.

In response to granulocyte-macrophage colony-stimulating factor plus tumor necrosis factor alpha, cord blood CD34+ hematopoietic progenitor cells differentiate along two unrelated dendritic cell (DC) pathways: (1) the Langerhans cells (LCs), which are characterized by the expression of CD1a, Birbeck granules, the Lag antigen, and E cadherin; and (2) CD14+ cell-derived DCs, characterized by the expression of CD1a, CD9, CD68, CD2, and factor XIIIa (Caux et al, J Exp Med 184:695, 1996). The present study investigates the functions of each population. Although the two populations are equally potent in stimulating naive CD45RA cord blood T cells through apparently identical mechanisms, each also displays specific activities. In particular CD14-derived DCs show a potent and long-lasting (from day 8 to day 13) antigen uptake activity (fluorescein isothiocyanate dextran or peroxidase) that is about 10-fold higher than that of CD1a+ cells, which is restricted to the immature stage (day 6). The antigen capture is exclusively mediated by receptors for mannose polymers. The high efficiency of antigen capture of CD14-derived cells is coregulated with the expression of nonspecific esterase activity, a tracer of lysosomial compartment. In contrast, the CD1a+ population never expresses nonspecific esterase activity. The most striking difference is the unique capacity of CD14-derived DCs to induce naive B cells to differentiate into IgM-secreting cells, in response to CD40 triggering and interleukin-2. Thus, although the two populations can allow T-cell priming, initiation of humoral responses might be preferentially regulated by the CD14-derived DCs. Altogether, those results show that different pathways of DC development might exist in vivo: (1) the LC type, which might be mainly involved in cellular immune responses, and (2) the CD14-derived DC related to dermal DCs or circulating blood DCs, which could be involved in humoral immune responses.

Human dendritic cells skew isotype switching of CD40-activated naive B cells towards IgA1 and IgA2.

Within T cell-rich areas of secondary lymphoid organs, interdigitating dendritic cells recruit antigen-specific T cells that then induce B cells to secrete Igs. This study investigates the possible role(s) of dendritic cells in the regulation of human B cell responses. In the absence of exogenous cytokines, in vitro generated dendritic cells (referred to as Dendritic Langerhans cells, D-Lc) induced surface IgA expression on approximately 10% of CD40-activated naive sIgD+ B cells. In the presence of IL-10 and TGF-beta, a combination of cytokines previously identified for its capacity to induce IgA switch, D-Lc strongly potentiated the induction of sIgA on CD40-activated naive B cells from 5% to 40-50%. D-Lc alone did not induce the secretion of IgA by CD40-activated naive B cells, which required further addition of IL-10. Furthermore, D-Lc skewed towards the IgA isotype at the expense of IgG, the Ig production of CD40-activated naive B cells cultured in the presence of IL-10 and TGF-beta. Importantly, under these culture conditions, both IgA1 and IgA2 were detected. In the presence of IL-10, secretion of IgA2 by CD40-activated naive B cells could be detected only in response to D-Lc and was further enhanced by TGF-beta. Collectively, these results suggest that in addition to activating T cells in the extrafollicular areas of secondary lymphoid organs, human D-Lc also directly modulate T cell-dependent B cell growth and differentiation, by inducing the IgA isotype switch.

Dendritic cells enhance growth and differentiation of CD40-activated B lymphocytes.

After antigen capture, dendritic cells (DC) migrate into T cell-rich areas of secondary lymphoid organs, where they induce T cell activation, that subsequently drives B cell activation. Here, we investigate whether DC, generated in vitro, can directly modulate B cell responses, using CD40L-transfected L cells as surrogate activated T cells. DC, through the production of soluble mediators, stimulated by 3- to 6-fold the proliferation and subsequent recovery of B cells. Furthermore, after CD40 ligation, DC enhanced by 30-300-fold the secretion of IgG and IgA by sIgD- B cells (essentially memory B cells). In the presence of DC, naive sIgD+ B cells produced, in response to interleukin-2, large amounts of IgM. Thus, in addition to activating naive T cells in the extrafollicular areas of secondary lymphoid organs, DC may directly modulate B cell growth and differentiation.

CD34+ hematopoietic progenitors from human cord blood differentiate along two independent dendritic cell pathways in response to GM-CSF+TNF alpha.

Human dendritic cells (DC) can now be generated in vitro in large numbers by culturing CD34+ hematopoietic progenitors in presence of GM-CSF+TNF alpha for 12 d. The present study demonstrates that cord blood CD34+ HPC indeed differentiate along two independent DC pathways. At early time points (day 5-7) during the culture, two subsets of DC precursors identified by the exclusive expression of CD1a and CD14 emerge independently. Both precursor subsets mature at day 12-14 into DC with typical morphology and phenotype (CD80, CD83, CD86, CD58, high HLA class II). CD1a+ precursors give rise to cells characterized by the expression of Birbeck granules, the Lag antigen and E-cadherin, three markers specifically expressed on Langerhans cells in the epidermis. In contrast, the CD14+ progenitors mature into CD1a+ DC lacking Birbeck granules, E-cadherin, and Lag antigen but expressing CD2, CD9, CD68, and the coagulation factor XIIIa described in dermal dendritic cells. The two mature DC were equally potent in stimulating allogeneic CD45RA+ naive T cells. Interestingly, the CD14+ precursors, but not the CD1a+ precursors, represent bipotent cells that can be induced to differentiate, in response to M-CSF, into macrophage-like cells, lacking accessory function for T cells. Altogether, these results demonstrate that different pathways of DC development exist: the Langerhans cells and the CD14(+)-derived DC related to dermal DC or circulating blood DC. The physiological relevance of these two pathways of DC development is discussed with regard to their potential in vivo counterparts.

Interleukin-3 cooperates with tumor necrosis factor alpha for the development of human dendritic/Langerhans cells from cord blood CD34+ hematopoietic progenitor cells.

We have previously shown that tumor necrosis factor (TNF)alpha strongly potentiates the granulocyte-macrophage colony-stimulating factor (GM-CSF)/interleukin (IL)-3-dependent proliferation of CD34+ hematopoietic progenitor cells (HPC) through the recruitment of early progenitors with high proliferative potential. Furthermore, the combination of GM-CSF and TNFalpha allows the generation of large numbers of dendritic/Langerhans cells (D-Lc). Herein, we analyzed whether IL-3, when combined to TNFalpha would, as does GM-CSF, allow the generation of CD1a+ D-Lc. Accordingly, cultures of cord blood CD34+ HPC with IL-3 + TNFalpha yielded 20% to 60% CD14+ cells and 11% to 17% CD1a+ cells, while IL-3 alone did not generate significant numbers of CD1a+ cells. Although the percentage of CD1a+ cells detected in IL3 + TNFalpha was lower than that observed in GM-CSF + TNFalpha (42% to 78%), the strong growth induced by IL-3 + TNFalpha generated as many CD1a+ cells as did GM-CSF + TNFalpha. The CD14+ and CD1a+ cells generated with IL-3 + TNFalpha are similar to CD14+ and CD1a+ cells generated in GM-CSF alone and GM-CSF + TNFalpha, respectively. CD1a+ cells differed from CD14+ cells by (1) dendritic morphology, (2) higher expression of CD1a, CD1c, CD4, CD40, adhesion molecules (CD11c, CD54, CD58), major histocompatibility complex (MHC) class II molecules and CD28 ligands (CD80 and CD86), (3) lack of Fc receptor FcgammaRI (CD64) and complement receptor CR1 (CD35) expression, and (4) stronger induction of allogeneic T-cell proliferation. Thus, in combination with TNFalpha, IL-3 is as potent as GM-CSF for the generation of CD1a+ D-Lc from cord blood CD34+ HPC. The dendritic cell inducing ability of IL-3 may explain why mice with inactivated GM-CSF gene display dendritic cells.

Human dendritic Langerhans cells generated in vitro from CD34+ progenitors can prime naive CD4+ T cells and process soluble antigen.

Earlier studies have concluded that fresh Langerhans cells (LC) are able to capture and process native Ags, whereas cultured LC have lost these functions while acquiring the capacity to prime naive T cells. Herein we studied the functions of human dendritic/Langerhans cells (d-Lc) generated in vitro by culturing CD34+ hemopoietic progenitor cells in the presence of granulocyte-macrophage CSF (GM-CSF) + TNF-alpha. Less than 50 d-Lc were found to strongly stimulate the proliferation of 2.5 x 10(4) allogeneic naive CD4+ T cells. Furthermore, six to 50 d-Lc induced half-maximal proliferation of naive syngeneic CD4+ cord blood T cells, in the presence of picomolar concentrations of superantigens. During the alloreaction, the CD4+ T cells were expanded up to 100-fold within two successive stimulation cycles with the same d-Lc, and the recovered T cells were specific for the d-Lc alloantigen. HLA-matched tetanus toxoid (TT)-specific T cell clones were found to proliferate in response to TT presented by CD1a+ d-Lc. Finally, electron microscopy demonstrated that CD1a+ d-Lc were able to capture an Ag (gold-labeled Igs) through receptor-mediated endocytosis. Thus, in vitro generated d-Lc can prime naive T cells and process native Ags, a property that might eventually prove useful for priming Ag-specific naive T cells for cellular immunotherapy.