Interleukin 21 (IL-21) is a newly described cytokine with homology to IL-4 and IL-15. They belong to a cytokine family that uses the common ␥ chain for signaling but also have their private high-affinity receptors. Since it is well known that IL-4 modulates differentiation and activation of dendritic cells (DCs), we analyzed effects of IL-21 compared with IL-15 on DC differentiation, maturation, and function. Here we show that DCs generated with granulocyte-macrophage colony-stimulating factor (GMCSF) in the presence of IL-21 (IL-21DCs) differentiated into phenotypically and functionally altered DCs characterized by reduced major histocompatibility complex class II (MHCII) expression, high antigen uptake, and low stimulatory capacity for T-cell activation in vitro. Additionally, IL-21DCs completely failed to induce antigen (Ag)-specific T-cell mediated contact hypersensitivity. IntroductionDendritic cells (DCs) possess specialized features such as pathogen recognition, antigen (Ag) capturing and processing machinery, migratory capacity, and constitutively expressed costimulatory molecules that allow them to act as professional antigen-presenting cells (APCs). Thus, DCs display an extraordinary capacity to initiate T-cell-dependent immune responses. 1 In mediating this, DCs pass through different functional activation states. 2 Thereby DCs convert from a highly efficient antigen capture and uptake to an antigen-presenting state. The capacity of mature DCs to prime naive T cells and to promote their differentiation is critically attributed to their cytokine-secretion pattern. Vice versa, polarizing signals from the microenvironment directly shape the DC maturation. A network of cytokines modulates this sensitive system of cell communication, including interleukin 15 (IL-15), a proinflammatory cytokine that activates APCs such as DCs and macrophages. 3 IL-15 was introduced as a member of the 4-helix-bundle cytokine family able to reproduce biologic effects of This is related to the fact that the IL-15 receptor (IL-15R) complex contains the ␤ and common ␥ chain of the IL-2R complex beside its private high-affinity ␣ chain. 5 However, IL-15 was also found to have distinct functions compared with those mediated by IL-2. 6,7 In addition, IL-2 expression is nearly restricted to T cells, whereas IL-15 is hardly detectable in T cells but produced by a variety of tissues, monocytes, and DCs. 8 Therefore, IL-15 may be an important candidate for modulating DC-mediated immune responses. 9 It is established that combining granulocyte-macrophage colonystimulating factor (GMCSF) and IL-4 promotes DC generation in vitro. 10 Therefore, it was of interest to investigate other members of this cytokine family acting on DC development. It has been recently reported that IL-15 can skew monocyte differentiation into DCs with a surface phenotype of Langerhans cells 11 and supports the maturation of monocytes to DCs ex vivo. 12 Interestingly, although IL-4 and IL-15 both mobilize the common ␥ chain, IL-4 was unable to induce comparable ...

The third author's last name was misspelled. The correct spelling is Bulfone-Paus.

IL-15 stimulates the proliferation of memory phenotype CD44highCD8+ T cells and is thought to play a key role in regulating the turnover of these cells in vivo. We have investigated whether IL-15 also has the capacity to affect the life span of naive phenotype (CD44low) CD8+ T cells. We report that IL-15 promotes the survival of both CD44low and CD44high CD8+ T cells, doing so at much lower concentrations than required to induce proliferation of CD44high cells. Rescue from apoptosis was associated with the up-regulation of Bcl-2 in both cell types, whereas elevated expression of Bcl-xL was observed among CD44high but not CD44low CD8+ cells. An investigation into the role of IL-15R subunits in mediating the effects of IL-15 revealed distinct contributions of the α- and β- and γ-chains. Most strikingly, IL-15Rα was not essential for either induction of proliferation or promotion of survival by IL-15, but did greatly enhance the sensitivity of cells to low concentrations of IL-15. By contrast, the β- and γ-chains of the IL-15R were absolutely required for the proliferative and pro-survival effects of IL-15, although it was not necessary for CD44highCD8+ cells to express higher levels of IL-15Rβ than CD44low cells to proliferate in response to IL-15. These results show that IL-15 has multiple effects on CD8 T cells and possesses the potential to regulate the life span of naive as well as memory CD8+ T cells.

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Axl receptor tyrosine kinase exists as a transmembrane protein and as a soluble molecule. We show that constitutive and phorbol 12-myristate 13-acetate-induced generation of soluble Axl (sAxl) involves the activity of disintegrin-like metalloproteinase 10 (ADAM10). Spontaneous and inducible Axl cleavage was inhibited by the broad-spectrum metalloproteinase inhibitor GM6001 and by hydroxamate GW280264X, which is capable of blocking ADAM10 and ADAM17. Furthermore, murine fibroblasts deficient in ADAM10 expression exhibited a significant reduction in constitutive and inducible Axl shedding, whereas reconstitution of ADAM10 restored sAxl production, suggesting that ADAM10-mediated proteolysis constitutes a major mechanism for sAxl generation in mice. Partially overlapping 14-amino-acid stretch deletions in the membrane-proximal region of Axl dramatically affected sAxl generation, indicating that these regions are involved in regulating the access of the protease to the cleavage site. Importantly, relatively high circulating levels of sAxl are present in mouse sera in a heterocomplex with Axl ligand Gas6. Conversely, two other family members, Tyro3 and Mer, were not detected in mouse sera and conditioned medium. sAxl is constitutively released by murine primary cells such as dendritic and transformed cell lines. Upon immobilization, sAxl promoted cell migration and induced the phosphorylation of Axl and phosphatidylinositol 3-kinase. Thus, ADAM10-mediated generation of sAxl might play an important role in diverse biological processes.Receptor tyrosine kinases (RTKs) play fundamental roles in diverse cell functions, including proliferation, differentiation, survival, migration, and metabolism (16). Axl RTK (also known as Ark, Ufo, and Tyro7) is the prototype of a family of transmembrane receptors, which also includes Tyro3 (also known as Sky, Brt, Etk, Tif, Dtk, and Rse) and Mer (c-Eyk, Nyk, and Tyro12) (34,44,64). They share a distinct molecular structure characterized by two immunoglobulin-like motifs and two fibronectin type III repeats in their extracellular domain and a cytoplasmic domain that contains a conserved catalytic kinase region (34, 44). Axl, Tyro3, and Mer are variably expressed in neural, lymphoid, vascular, and reproductive tissues and in different primary cells and tumor cell lines (11,41,42). Mutant mice that lack these three receptors have a defective phagocytic clearance of apoptotic cells and impaired spermatogenesis (41) and develop a severe lymphoproliferative disorder accompanied by broad-spectrum autoimmunity (42).A common heterophilic ligand for these RTK family members is Gas6, a vitamin K-dependent protein that is widely secreted by most tissues, including the lungs, intestine, and vascular endothelium (43). Gas6 is the product of growth arrest-specific gene 6, which was initially cloned from serumstarved fibroblasts and shares about 44% sequence identity and similar domain organization with protein S, a negative regulator of blood coagulation (48). Recent studies indicate that the Gas6/A...