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The inducible costimulator (ICOS) is the newest member of the CD28/CD152 receptor family involved in regulating T cell activation. We constructed a soluble-Ig fusion protein of the extracellular domain of human ICOS and used it as a probe to characterize expression patterns of the ICOS ligand (ICOSL). ICOSIg did not bind to CD80- or CD86-transfected Chinese hamster ovary cell lines, demonstrating that ICOSL is distinct from those ligands identified for CD28/CD152. ICOSIg showed selective binding to monocytic and B cell lines, whereas binding was undetectable on unstimulated monocytes and peripheral blood T and B cells. Expression of ICOSL was induced on monocytes after integrin-dependent plastic adhesion. Pretreatment of monocytes with mAb to the β2-integrin subunit CD18 decreased adhesion and abolished ICOSL up-regulation but had no effect on CD80/86 (CD152 ligand (CD152L)) expression. Both ICOSL and CD152L were up-regulated on monocytes by IFN-γ but by distinct signaling pathways. Unlike CD152L expression, ICOSL expression did not change when monocytes were differentiated into dendritic cells (DCs) or after DCs were induced to mature by LPS, TNF-α, or CD40 ligation. Addition of ICOSIg to allogeneic MLRs between DCs and T cells reduced T cell proliferative responses but did so less efficiently than CTLA4Ig (CD152Ig) did. Similarly, ICOSIg also blocked Ag-specific T cell proliferation to tetanus toxoid. Thus, ICOSL, like CD80/86, is expressed on activated monocytes and dendritic cells but is regulated differently and delivers distinct signals to T cells that can be specifically inhibited by ICOSIg.

Dendritic-cell (DC)-associated C-type lectin receptors (CLRs) take up antigens to present to T cells and regulate DC functions. DCAL-2 is a CLR with a cytosolic immunoreceptor tyrosine-based inhibitory motif (ITIM), which is restricted to immature DCs (iDCs), monocytes, and CD1a+ DCs. Cross-linking DCAL-2 on iDCs induced protein tyrosine phosphorylation and MAPK activation as well as receptor internalization. To test if DCAL-2 is involved in DC maturation and cytokine expression, we stimulated iDCs with anti-DCAL-2 mAb with or without LPS, zymosan, or CD40L. While anti-DCAL-2 did not induce iDCs to mature, it did up-regulate CCR7 expression and IL-6 and IL-10 production. DCAL-2 signals augmented DC maturation induced by LPS or zymosan, increasing both CCR7 and DC-LAMP expression. Of interest, DCAL-2 ligation had the opposite effects on TLR versus CD40L signaling: anti-DCAL-2 suppressed TLR-induced IL-12 expression, but significantly enhanced CD40L-induced IL-12 production. DCAL-2 ligation also suppressed the ability of TLR-matured DCs to induce IFN-gamma-secreting Th1 cells but augmented the capacity of CD40L-matured DCs to polarize naive T cells into Th1 cells. Thus, DCAL-2 may program DCs differently depending on whether DCs are signaled via TLRs or by T cells. DCAL-2 may be a potential immunotherapeutic target for modulating autoimmune diseases or for developing vaccines.

The effects of estrogen on the immune system are still largely unknown. We have investigated the effect of 17␤-estradiol (E 2 ) on human monocyte-derived immature dendritic cells (iDCs). Short-term culture in E 2 had no effect on iDC survival or the expression of cell surface markers. However, E 2 treatment significantly increased the secretion of interleukin 6 (IL-6) in iDCs and also increased secretion of osteoprotegerin (OPG) by DCs. Furthermore, E 2 significantly increased secretion of the inflammatory chemokines IL-8 and monocyte chemoattractant protein 1 (MCP-1) by iDCs, but not the production of the constitutive chemokines thymus and activation-regulated chemokine (TARC) and macrophage-derived chemokine (MDC). However, after E 2 pretreatment the lipopolysaccharide (LPS)-induced production of MCP-1, TARC, and MDC by DCs was clearly enhanced. Moreover, mature DCs pretreated with E 2 stimulated T cells better than control cells. Finally, we found that E 2 provides an essential signal for migration IntroductionDendritic cells (DCs) are key antigen-presenting cells (APCs), which recognize, capture, and process antigens, express costimulatory molecules, and then migrate to secondary lymphoid organs, where they can help initiate immune responses. 1 In addition to stimulating naive T cells and initiating primary immune responses, DCs act as effector cells in innate immunity and also play a role in maintaining peripheral tolerance. 2 DCs are also involved in T-cell polarization, for example, into T helper 1 (Th1) and Th2 cells. 2 For example, interleukin 12 (IL-12) secreted by DCs induces the production of interferon-␥ (IFN-␥) by CD4 ϩ T cells, which promotes Th1 cell differentiation and proliferation. 3 Furthermore, IL-6 derived from DCs can drive Th2 differentiation and at the same time inhibit Th1 polarization. [4][5][6] DCs can both produce and respond to chemokines, 7 which play a crucial role in leukocyte trafficking. 8 These include inflammatory, for example, IL-8 and monocyte chemoattractant protein 1 (MCP-1), as well as constitutive, eg, thymus and activation-regulated chemokine (TARC) and macrophage-derived chemokine (MDC). 7 DC-derived chemokines not only are involved in T-cell priming and Th1/Th2-mediated responses, 9 but also contribute to the pathology of certain autoimmune conditions. 10,11 In many autoimmune diseases women are more likely to be affected than men. For example, in systemic lupus erythematosus (SLE), Sjögren syndrome, autoimmune thyroid disease, and scleroderma, more than 80% of the patients affected are women. 12 Sex hormones, particularly estrogen, may contribute to the pathogenesis of some autoimmune diseases. 13 Fluctuations in estrogen levels may correlate with disease status; for example, during pregnancy circulating levels of estrogen increase notably. 14 In both multiple sclerosis and rheumatoid arthritis, disease activity decreases during the third trimester when estrogen levels are highest and flares again when the estrogen levels decrease postpartum. 13 In SLE, however, th...

We have characterized dendritic cell (DC)-associated lectin-1 (DCAL-1), a novel, type II, transmembrane, C-type lectin-like protein. DCAL-1 has restricted expression in hemopoietic cells, in particular, DCs and B cells, but T cells and monocytes do not express it. The DCAL-1 locus is within a cluster of C-type lectin-like loci on human chromosome 12p12–13 just 3′ to the CD69 locus. The consensus sequence of the DCAL-1 gene was confirmed by RACE-PCR; however, based on sequence alignment with genomic DNA and with various human expressed sequence tags, we predict that DCAL-1 has two splice variants. C-type lectins share a common sequence motif of 14 invariable and 18 highly conserved aa residues known as the carbohydrate recognition domain. DCAL-1, however, is missing three of the cysteine residues required to form the standard carbohydrate recognition domain. DCAL-1 mRNA and protein expression are increased upon the differentiation of monocytes to CD1a+ DCs. B cells also express high levels of DCAL-1 on their cell surface. Using a DCAL-1 fusion protein we identified a population of CD4+ CD45RA+ T cells that express DCAL-1 ligand. Coincubation with soluble DCAL-1 enhanced the proliferation of CD4+ T cells in response to CD3 ligation and significantly increased IL-4 secretion. In contrast, coincubation with soluble DC-specific ICAM-3-grabbing nonintegrin (CD209) fusion protein as a control had no effect on CD4+ T cell proliferation or IL-4 and IFN-γ secretion. Therefore, the function of DCAL-1 on DCs and B cells may act as a T cell costimulatory molecule, which skews CD4+ T cells toward a Th2 response by enhancing their secretion of IL-4.

Recruitment of monocytes into tissues and their differentiation into macrophages or dendritic cells (DCs) depend on the microenvironment of the inflammatory site. Although many factors affecting this process have been identified, the intracellular signaling pathways implicated are poorly understood. We found that cyclic nucleotides regulate certain steps of monocyte differentiation into DCs. Increased levels of the cyclic nucleotides, cAMP or cGMP, inhibit differentiation of CD14+/CD1alow monocytes into CD14−/CD1ahigh DCs. However, DC-specific ICAM-3-grabbing nonintegrin (CD209) up-regulation was not affected by cyclic nucleotides, indicating that DC development was not blocked at the monocyte stage. Interestingly, Ag-presenting function was increased by cyclic nucleotides, as measured by the higher expression of MHC class II, CD86, and an increased ability to stimulate CD4+ T cell proliferation in allogeneic MLRs. Although cyclic nucleotides do not completely block DC differentiation, they do block the ability of DCs to be induced to mature by LPS. Treatment during DC differentiation with either cAMP or cGMP analogues hampered LPS-induced expression of CD83, DC-LAMP, and CCR7 and the ability of DCs to migrate toward CCL19/macrophage-inflammatory protein 3β. Interestingly, the induction of a CD16+ subpopulation of cells was also observed. Thus, signals causing an increase in either cAMP or cGMP levels during monocyte recruitment to inflammatory sites may restrain the activation of acquired immunity by blocking DC development and migration to lymph nodes. At the same time, these signals promote development of an active intermediate cell type having properties between those of macrophages and DCs, which might contribute to the innate immune response in the periphery.

Dendritic-cell (DC) migration to secondary lymphoid organs is crucial for the initiation of adaptive immune responses. Although LPS up-regulates CCR7 on DCs, a second signal is required to enable them to migrate toward the chemokine CCL19 (MIP-3␤). We found that the nitric oxide (NO) donor NOR4 provides a signal allowing LPS-stimulated DCs to migrate toward CCL19. NO affects DC migration through both the initial activation of the cGMP/cGMP kinase (cGMP/cGK) pathway and a long-term effect that reduced cGK activity via negative feedback. Indeed, migration of DCs toward CCL19, unlike migration toward CXCL12 (SDF-1␣), required inhibition of cGK. LPS increased both cGK expression and cGK activity as measured by phosphorylation of the key cGK target vasodilator-stimulated phosphoprotein (VASP). Because cGK phosphorylation of VASP can disrupt focal adhesions and inhibit cell migration, LPSinduced VASP phosphorylation may prevent DCs from migrating without a second signal. Long-term NOR4 treatment inhibited the increase in cGK-dependent VASP phosphorylation, releasing this brake so that DCs can migrate. NO has been implicated in the regulation of autoimmunity through its effect on T cells. Our results suggest that NO regulation of DC migration and cytokine production may contribute to the protective effects of NO in autoimmune disorders. IntroductionDendritic cells (DCs) activate naive T cells in peripheral lymphoid tissues and play a pivotal role in initiating and instructing adaptive immune responses. 1,2 At the site of infection where they take up antigens, DCs also contribute to innate immunity: They recognize and respond to common pathogen-associated molecular patterns (PAMPs) by releasing proinflammatory products that regulate other inflammatory cells. 3 In a later phase, recognition of PAMPs triggers a complex maturation program that results in the increase in DC antigen-presenting ability and responsiveness to chemokines like CCL19/MIP-3␤, which promote DC migration from peripheral tissues to secondary lymphoid organs. 4,5 The outcome of an immune response depends not only on the pathogen that activates DCs but also on various inflammationassociated factors that modulate DC maturation and determine whether DCs polarize T-cell development into T helper 1 (Th1)-, Th2-, or regulatory T-cell subsets. 6 Prostaglandin E2 (PGE2) and other inflammatory products that act through the cAMP/cAMP kinase (cAMP/cAK) signaling pathway can affect DC maturation and drive them toward a Th2-inducing program. [7][8][9] These agents also regulate DC chemokine and chemokines receptor expression; they down-regulate receptors for inflammatory chemokines like CCR5 while increasing CCR7 or CXCR4 receptors for CCL19/ MIP-3␤ and CXCL12/SDF-1␣, respectively. [10][11][12] Interestingly, inflammatory agents are required for mature DCs to be able to migrate toward certain chemokines. Even after monocyte-derived immature DCs (iDCs) are induced to mature and up-regulate CCR7, they remain unresponsive to CCL19 and do not migrate unless they are e...

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