IntroductionThe endogenous adenine nucleotides and adenosine are normally present at low concentrations in the extracellular milieu. However, metabolically stressful conditions, including inflammation and hypoxia characteristic of asthma, solid tumors, and other pathologic conditions, result in dramatic increases in extracellular concentrations of adenosine. [1][2][3] There are also mechanisms of nonlytic secretion of adenosine during hypoxic conditions.There is growing evidence that adenosine can actively modulate differentiation and function of myeloid cells. 4 Circulating cells of myeloid lineage, including monocytes and dendritic cell (DC) precursors, migrate to tissues where they differentiate into macrophages or DCs. DCs show impressive interaction with the adjacent microenvironment, 5,6 which regulates formation of DC subtypes and their functional properties, including expression of cytokines and growth factors. Because of rapid growth, solid tumors routinely experience severe hypoxia and necrosis, which causes adenine nucleotide degradation and adenosine release. Therefore, high levels of extracellular adenosine contribute to the local tumor microenvironment and may greatly influence differentiation of DCs from monocyte/macrophages and DC precursors migrating into tumor tissue. Adenosine acts through 4 subtypes of adenosine receptors, A 1 , A 2A , A 2B , and A 3 , which are members of the G-protein-coupled family of receptors. 7,8 A 2A adenosine receptors are generally anti-inflammatory, whereas A 2B and A 3 receptors are implicated in proinflammatory action of adenosine. Adenosine receptors are expressed abundantly on monocytes, and through these receptors adenosine exerts substantial modulatory effects on monocyte function and further differentiation. A 1 receptors were shown to stimulate formation of giant multinucleated cells from monocytes, whereas A 2 receptors inhibited this process. 9 A 2B receptors were implicated in mediating the inhibitory effect of adenosine on macrophage proliferation induced by M-CSF. 10 Exogenous adenosine can prevent monocytes from differentiating into macrophages, leading them to an intermediate differentiation stage between immature DCs and monocytes. 11 Cyclic nucleotides, including cAMP, which intracellular level increases in response to stimulation of adenosine A 2 receptors, regulate certain steps of monocyte differentiation and promote their differentiation toward a CD1a low CD14 ϩ/low CD209 ϩ intermediate cell but impair differentiation into functional DCs. 12 Up-regulation of DC-specific ICAM-3-grabbing nonintegrin (CD209) was not affected by cyclic nucleotides, 12 indicating that DC development was not blocked at the monocyte stage. The expression of all 4 adenosine receptor subtypes has been reported in human monocytes and myeloid DCs. 9,13-15 However, the effects of adenosine on differentiation of myeloid DCs from monocytes, macrophages, and hematopoietic progenitor cells (HPCs) and the roles of specific adenosine receptor subtypes involved in this process hav...

Abstract-Adenosine has been reported to stimulate or inhibit the release of angiogenic factors depending on the cell type examined. To test the hypothesis that differential expression of adenosine receptor subtypes contributes to endothelial cell heterogeneity, we studied microvascular (HMEC-1) and umbilical vein (HUVEC) human endothelial cells. Based on mRNA level and stimulation of adenylate cyclase, we found that HUVECs preferentially express A 2A adenosine receptors and HMEC-1 preferentially express A 2B receptors. Neither cells expressed A 1 or A 3 receptors. The nonselective adenosine agonist 5Ј-N-ethylcarboxamidoadenosine (NECA) increased expression of interleukin-8 (IL-8), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF) in HMEC-1, but had no effect in HUVECs. In contrast, the selective A 2A agonist 2-p-(2-carboxyethyl)phenylethylamino-NECA (CGS 21680) had no effect on expression of these angiogenic factors. Key Words: adenosine receptors Ⅲ vascular endothelium Ⅲ angiogenesis Ⅲ vascular endothelial growth factor Ⅲ interleukin-8 T he purine nucleoside adenosine is an intermediate catabolite of adenine nucleotides. Adenosine serves as an autocoid in situations when oxygen supply is decreased or energy consumption is increased. Under these conditions, adenosine is released into the extracellular space and signals to restore the balance between local energy requirements and energy supply. Endothelial cells interact with adenosine mechanisms in many different ways. Endothelial cells are known to have a very active adenosine metabolism, characterized by a large capacity for uptake and release of the nucleoside, 1,2 and can be an important source of adenosine released during ischemia. 3 Conversely, adenosine may modulate endothelial function via activation of cell membrane receptors. The precise nature of the interaction between adenosine receptor subtypes and endothelial cells and their role in the regulation of endothelial function is not completely understood.Adenosine receptors belong to the G protein-coupled 7 transmembrane superfamily of cell surface receptors and include A 1 , A 2A , A 2B , and A 3 subtypes. Endothelial cells are known to express adenosine receptors, but there are conflicting reports on the presence and the role of specific adenosine receptor subtypes. For example, human umbilical vein endothelial cells (HUVECs) were reported to express either A 1 , 4 A 2A , 5,6 A 2B , 7 or A 3 6 adenosine receptors, depending on the functional end-point studied and pharmacological tools used. Coexpression of more than one adenosine receptor subtype has been reported also in endothelial cells 8,9 ; it is not clear, however, if and how coexpressed receptors interact. Furthermore, endothelial cells from different blood vessels are heterogenous, and it is possible that diverse endothelial cells show differential expression of adenosine receptor subtypes.The functional role of adenosine receptors in endothelial cells also remains unclear. Adenosine-induced vasodilation h...

Recent studies have emphasized a role of adaptive immunity, and particularly T cells, in the genesis of hypertension. We sought to determine the T cell subtypes that contribute to hypertension and renal inflammation in angiotensin II-induced hypertension. Using T cell receptor (TCR) spectratyping to examine TCR usage we demonstrated that CD8+ cells, but not CD4+ cells, in the kidney exhibited altered TCR transcript lengths in Vβ3, 8.1 and 17 families in response to angiotensin II-induced hypertension. Clonality was not observed in other organs. The hypertension caused by angiotensin II in CD4−/− and MHCII−/− mice was similar to that observed in WT mice, while CD8−/− mice and OT1xRAG-1−/− mice, which have only one TCR, exhibited a blunted hypertensive response to angiotensin II. Adoptive transfer of pan-T cells and CD8+ T cells but not CD4+/CD25− cells conferred hypertension to RAG-1−/− mice. In contrast, transfer of CD4+/CD25+ cells to wild type mice receiving angiotensin II decreased blood pressure. Mice treated with angiotensin II exhibited increased numbers of kidney CD4+ and CD8+ T cells. In response to a sodium/volume challenge, wild type and CD4−/− mice infused with angiotensin II retained water and sodium whereas CD8−/− mice did not. CD8−/− mice were also protected against angiotensin-induced endothelial dysfunction and vascular remodeling in the kidney. These data suggest that in the development of hypertension, an oligoclonal population of CD8+ cells accumulate in the kidney and likely contribute to hypertension by contributing to sodium and volume retention and vascular rarefaction.

Extracellular adenosine and purine nucleotides are elevated in many pathological situations associated with the expansion of CD11b+Gr1+ myeloid-derived suppressor cells (MDSCs). Therefore, we tested whether adenosinergic pathways play a role in MDSC expansion and functions. We found that A2B adenosine receptors on hematopoietic cells play an important role in accumulation of intratumoral CD11b+Gr1high cells in a mouse Lewis lung carcinoma (LLC) model in vivo and demonstrated that these receptors promote preferential expansion of the granulocytic CD11b+Gr1high subset of MDSCs in vitro. Flow cytometry analysis of MDSCs generated from mouse hematopoietic progenitor cells revealed that the CD11b+Gr-1high subset had the highest levels of CD73 (ecto-5′-nucleotidase) expression (ΔMFI of 118.5±16.8), followed by CD11b+Gr-1int (ΔMFI of 57.9±6.8) and CD11b+Gr-1−/low (ΔMFI of 12.4±1.0). Even lower levels of CD73 expression were found on LLC tumor cells (ΔMFI of 3.2±0.2). The high levels of CD73 expression in granulocytic CD11b+Gr-1high cells correlated with high levels of ecto-5′-nucletidase enzymatic activity. We further demonstrated that the ability of granulocytic MDSCs to suppress CD3/CD28-induced T cell proliferation is significantly facilitated in the presence of the ecto-5′-nucletidase substrate 5′-AMP. We propose that generation of adenosine by CD73 expressed at high levels on granulocytic MDSCs may promote their expansion and facilitate their immunosuppressive activity.