Induction of potent CD8 T cell cytotoxicity by specific targeting of antigen to cross-presenting dendritic cells in vivo via murine or human XCR1.

Current subunit vaccines are incapable of inducing Ag-specific CD8(+) T cell cytotoxicity needed for the defense of certain infections and for therapy of neoplastic diseases. In experimental vaccines, cytotoxic responses can be elicited by targeting of Ag into cross-presenting dendritic cells (DC), but almost all available systems use target molecules also expressed on other cells and thus lack the desired specificity. In the present work, we induced CD8(+) T cell cytotoxicity by targeting of Ag to XCR1, a chemokine receptor exclusively expressed on murine and human cross-presenting DC. Targeting of Ag with a mAb or the chemokine ligand XCL1 was highly specific, as determined with XCR1-deficient mice. When applied together with an adjuvant, both vector systems induced a potent cytotoxic response preventing the outgrowth of an inoculated aggressive tumor. By generating a transgenic mouse only expressing the human XCR1 on its cross-presenting DC, we could demonstrate that targeting of Ag using human XCL1 as vector is fully effective in vivo. The specificity and efficiency of XCR1-mediated Ag targeting to cross-presenting DC, combined with its lack of adverse effects, make this system a prime candidate for the development of therapeutic cytotoxic vaccines in humans.

Expression of XCR1 Characterizes the Batf3-Dependent Lineage of Dendritic Cells Capable of Antigen Cross-Presentation.

Cross-presentation of antigen by dendritic cells (DCs) to CD8(+) T cells is a fundamentally important mechanism in the defense against pathogens and tumors. Due to the lack of an appropriate lineage marker, cross-presenting DCs in the mouse are provisionally classified as "Batf3-IRF-8-Id2-dependent DCs" or as "CD8(+) DCs" in the spleen, and as "CD103(+)CD11b(-) DCs" in the periphery. We have now generated a mAb to XCR1, a chemokine receptor which is specifically expressed on CD8(+) DCs and a subpopulation of double negative DCs in the spleen. Using this antibody, we have determined that only XCR1(+)CD8(+) (around 80% of CD8(+) DCs) and their probable precursors, XCR1(+)CD8(-) DCs, efficiently take up cellular material and excel in antigen cross-presentation. In lymph nodes (LNs) and peripheral tissues, XCR1(+) DCs largely, but not fully, correspond to CD103(+)CD11b(-) DCs. Most importantly, we demonstrate that XCR1(+) DCs in the spleen, LNs, and peripheral tissues are dependent on the growth factor Flt3 ligand and are selectively absent in Batf3-deficient animals. These results provide evidence that expression of XCR1 throughout the body defines the Batf3-dependent lineage of DCs with a special capacity to cross-present antigen. XCR1 thus emerges as the first surface marker characterizing a DC lineage in the mouse and potentially also in the human.

The Role of XCR1 and its Ligand XCL1 in Antigen Cross-Presentation by Murine and Human Dendritic Cells.

Recently, the chemokine receptor XCR1 has been found to be exclusively expressed on a subset of dendritic cell (DC) known to be involved in antigen cross-presentation. This review aims to summarize the known biology of the XCR1 receptor and its chemokine ligand XCL1, both in the mouse and the human. Further, any involvement of this receptor-ligand pair in antigen uptake, cross-presentation, and induction of innate and adaptive cytotoxic immunity is explored. The concept of antigen delivery to DC via the XCR1 receptor is discussed as a vaccination strategy for selective induction of cytotoxic immunity against certain pathogens or tumors.

Compartmentalization of cAMP-dependent signaling by phosphodiesterase-4D is involved in the regulation of vasopressin-mediated water reabsorption in renal principal cells.

The cAMP/protein kinase A (PKA)-dependent insertion of water channel aquaporin-2 (AQP2)-bearing vesicles into the plasma membrane in renal collecting duct principal cells (AQP2 shuttle) constitutes the molecular basis of arginine vasopressin (AVP)-regulated water reabsorption. cAMP/PKA signaling systems are compartmentalized by A kinase anchoring proteins (AKAP) that tether PKA to subcellular sites and by phosphodiesterases (PDE) that terminate PKA signaling through hydrolysis of localized cAMP. In primary cultured principal cells, AVP causes focal activation of PKA. PKA and cAMP-specific phosphodiesterase-4D (PDE4D) are located on AQP2-bearing vesicles. The selective PDE4 inhibitor rolipram increases AKAP-tethered PKA activity on AQP2-bearing vesicles and enhances the AQP2 shuttle and thereby the osmotic water permeability. AKAP18delta, which is located on AQP2-bearing vesicles, directly interacts with PDE4D and PKA. In response to AVP, PDE4D and AQP2 translocate to the plasma membrane. Here PDE4D is activated through PKA phosphorylation and reduces the osmotic water permeability. Taken together, a novel, compartmentalized, and physiologically relevant cAMP-dependent signal transduction module on AQP2-bearing vesicles, comprising anchored PDE4D, AKAP18delta, and PKA, has been identified.

Spatial organisation of AKAP18 and PDE4 isoforms in renal collecting duct principal cells.

A plethora of stimuli including hormones and neurotransmitters mediate a rise of the cellular level of cAMP and thereby activation of protein kinase A (PKA). PKA phosphorylates and thereby modulates the activity of a wide range of cellular targets. It is now appreciated that different stimuli induce the activation of PKA at specific sites where the kinase phosphorylates particular substrates in close proximity. The tethering of PKA to cellular compartments is facilitated by A kinase-anchoring proteins (AKAPs). The incorporation of phosphodiesterases (PDEs) into AKAP-based signalling complexes provides gradients of cAMP that regulate PKA activity locally. An example for a process depending on compartmentalised cAMP/PKA signalling is the arginine-vasopressin (AVP)-mediated water reabsorption in renal collecting duct principal cells. Upon activation through AVP, PKA phosphorylates the water channel aquaporin-2 (AQP-2) located on intracellular vesicles. The phosphorylation triggers the redistribution of AQP2 to the plasma membrane. AKAP-anchored PKA has been shown to be involved in AQP2 shuttling. Here, AKAP18 isoforms and members of the PDE4 family of PDEs are shown to be differentially localised in renal principal cells.

Identification of a novel A-kinase anchoring protein 18 isoform and evidence for its role in the vasopressin-induced aquaporin-2 shuttle in renal principal cells.

Arginine vasopressin (AVP) increases the water permeability of renal collecting duct principal cells by inducing the fusion of vesicles containing the water channel aquaporin-2 (AQP2) with the plasma membrane (AQP2 shuttle). This event is initiated by activation of vasopressin V2 receptors, followed by an elevation of cAMP and the activation of protein kinase A (PKA). The tethering of PKA to subcellular compartments by protein kinase A anchoring proteins (AKAPs) is a prerequisite for the AQP2 shuttle. During the search for AKAP(s) involved in the shuttle, a new splice variant of AKAP18, AKAP18delta, was identified. AKAP18delta functions as an AKAP in vitro and in vivo. In the kidney, it is mainly expressed in principal cells of the inner medullary collecting duct, closely resembling the distribution of AQP2. It is present in both the soluble and particulate fractions derived from renal inner medullary tissue. Within the particulate fraction, AKAP18delta was identified on the same intracellular vesicles as AQP2 and PKA. AVP not only recruited AQP2, but also AKAP18delta to the plasma membrane. The elevation of cAMP caused the dissociation of AKAP18delta and PKA. The data suggest that AKAP18delta is involved in the AQP2 shuttle.

CD40 ligand is selectively expressed on CD4+ T cells and platelets: implications for CD40-CD40L signalling in atherosclerosis.

Atherosclerosis is a degenerative inflammatory disease of the vascular system. Endothelial cells (ECs), smooth muscle cells, and macrophages, key elements in atherosclerosis, all have the potential to express the CD40 receptor and are thus susceptible to potent pro-inflammatory signals by CD40 ligand (CD40L)-bearing cells. CD40L is a TNF-alpha-related membrane protein originally identified on activated T cells. The recent recognition of platelets as an abundant source of CD40L led to a reassessment of the involvement of CD40L in atherosclerosis. In the present report, CD40L(+) T cells were identified in the intima of atherosclerotic tissues within macrophage infiltrates and in areas of neovascularization. These CD40L(+) T cells were CD4(+), CD69(+), but negative for CD8, CD25, CD28, and ICOS. In some specimens, CD40L(+) platelets were identified in the intima and in plaque ruptures. Contrary to previous reports, CD40L was not observed on ECs, smooth muscle cells, and macrophages in atherosclerotic tissues or in vitro at the protein and mRNA levels. Functionally, flow chamber experiments demonstrated that stimulation of ECs via CD40 is sufficient to recruit neutrophils and T cells from whole blood to ECs and suggested that CD40L(+) platelets contribute significantly to the recruitment of inflammatory cells to damaged endothelium in vivo. However, due to the short half-life of platelet CD40L, the chronic CD40L-driven inflammatory component can only be sustained by activated CD4(+) T cells. Contrary to current understanding, the contribution of CD40L to chronic inflammation in atherosclerosis is thus antigen-driven and MHC-dependent. This conclusion has significant therapeutic implications.

ICOS-ligand, expressed on human endothelial cells, costimulates Th1 and Th2 cytokine secretion by memory CD4+ T cells.

Endothelial cells (EC) play a central role in inflammatory immune responses and efficiently induce effector functions in T cells, despite lacking the classical costimulatory ligands CD80 and CD86. By using the mAb HIL-131 we now demonstrate that human inducible costimulator-ligand (ICOS-L), a molecule related to CD80/CD86, is constitutively expressed on human EC in vivo. In vitro, ICOS-L expression was strongly enhanced on human umbilical vein EC and microvascular EC by the inflammatory cytokines tumor necrosis factor alpha and IL-1beta, and to a lower extent by stimulation of EC by CD40 or lipopolysaccharide. Coculture of MHC class II(+) EC with resting memory CD4(+) T cells in the presence of superantigen led to a marked up-regulation of ICOS on T cells and to the production of Th1 (IFN-gamma, IL-2) and Th2 cytokines (IL-4, IL-10, IL-13). When these cocultures were performed in the presence of the inhibitory mAb HIL-131, secretion of all cytokines was reduced by about 50-80%, indicating that ICOS-L is a major costimulator in EC-mediated T cell activation. Taken together, our data suggest an important physiological role of ICOS-L in the reactivation of effector/memory T cells on the endothelium controlling the entry of immune cells into inflamed tissue.