Development and properties of a monoclonal antibody specific for human ecto-5'-nucleotidase

Therapeutic potentials of ecto-nucleoside triphosphate diphosphohydrolase, ecto-nucleotide pyrophosphatase/phosphodiesterase, ecto-5'-nucleotidase, and alkaline phosphatase inhibitors

The modulatory role of extracellular nucleotides and adenosine in relevance to purinergic cell signaling mechanisms has long been known and is an object of much research worldwide. These extracellular nucleotides are released by a variety of cell types either innately or as a response to patho-physiological stress or injury. A variety of surface-located ecto-nucleotidases (of four major types; nucleoside triphosphate diphosphohydrolases or NTPDases, nucleotide pyrophosphatase/phosphodiesterases or NPPs, alkaline phosphatases APs or ALPs, and ecto-5'-nucleotidase or e5NT) are responsible for meticulously controlling the availability of these important signaling molecules (at their respective receptors) in extracellular environment and are therefore crucial for maintaining the integrity of normal cell functioning. Overexpression of many of these ubiquitous ecto-enzymes has been implicated in a variety of disorders including cell adhesion, activation, proliferation, apoptosis, and degenerative neurological and immunological responses. Selective inhibition of these ecto-enzymes is an area that is currently being explored with great interest and hopes remain high that development of selective ecto-nucleotidase inhibitors will prove to have many beneficial therapeutic implications. The aim of this review is to emphasize and focus on recent developments made in the field of inhibitors of ecto-nucleotidases and to highlight their structure activity relationships wherever possible. Most recent and significant advances in field of NTPDase, NPP, AP, and e5NT inhibitors is being discussed in detail in anticipation of providing prolific leads and relevant background for research groups interested in synthesis of selective ecto-nucleotidase inhibitors.

5'-nucleotidase activity in lymphocytes from patients affected by B-cell chronic lymphocytic leukemia

OBJECTIVES

The activity of membrane-bound ecto-5'-nucleotidase and soluble e-Ns and c-N-II 5'-nucleotidases was evaluated on lymphocytes from patients affected by B-cell chronic lymphocytic leukemia (B-CLL). A statistically significative decrease in ecto-5'-nucleotidase, e-Ns, and c-N-II activities was observed in peripheral blood lymphocytes and in B and T populations from affected individuals.

DESIGN AND METHODS

For the assay of ecto-5'-nucleotidase, e-Ns, and c-N-II activity we used a radioactive procedure coupled to HPLC. Since the ecto-5'-nucleotidase is identified as CD73 antigen, we performed immunofluorescence analysis using a specific monoclonal antibody. We analyzed ecto-5'-nucleotidase mRNA by RT-PCR to ascertain the possibility of an alteration in the transcription of its gene.

RESULTS

A decrease in ecto-5'-nucleotidase activity was correlated to reduction in ecto-5'-nucleotidase positive cells (CD73+) in leukemia patients. RT-PCR produced a fragment of the expected size and the specific mRNA was found expressed in both healthy subjects and leukemia patients.

CONCLUSIONS

The decrease in ecto-5'-nucleotidase activity in patients with B-CLL is not due to loss of transcription of the specific mRNA. The presence of point mutations, splicing alteration, or posttranslational modifications must be investigated. If a defect at DNA or RNA level will be detected, the molecular analysis will be considered for diagnosis of B-cell chronic lymphocytic leukemia.

Ecto-enzyme and signaling functions of lymphocyte CD73

CD73 or ecto-5'-nucleotidase (5'-NT) is a widely expressed ecto-enzyme which catalyzes the dephosphorylation of AMP and other nucleoside monophosphates. CD73 participates in purine salvage through this enzymatic activity, supplying cells with precursors for energy metabolism and nucleic acid biosynthesis. As an enzyme that produces adenosine, CD73 can also regulate adenosine receptor engagement in many tissues. However, CD73 also has functions independent of its enzyme activity. Like many glycosyl phosphatidylinositol (GPI)-anchored molecules, it transmits potent activation signals in T cells when ligated by antibodies. Less compelling evidence suggests that CD73 may function as a cell adhesion molecule. In the human immune system, CD73 is expressed on subsets of T and B cells, on germinal center follicular dendritic cells, and on thymic medullary reticular fibroblasts and epithelial cells. Many challenging areas remain to be explored before the role of CD73 in the immune system will be fully understood. These include an evaluation of the role of adenosine receptors in lymphoid development, the identification of physiological CD73 ligands, a functional assessment of the GPI anchor, and an analysis of the intricate cell-type-specific and developmental regulation of CD73 expression.

Expression of ecto-5'-nucleotidase (CD73) in normal mammary gland and in breast carcinoma

Ecto-5'-nucleotidase (ecto-5'-NT) is a phosphatidylinositol anchored membrane structure recently defined as the lymphocyte differentiation antigen CD73. Using CD73 (1E9.28.1) monoclonal antibody, normal mammary gland and breast carcinoma were immunohistochemically investigated for ecto-5'-NT expression. In normal breast epithelium, CD73 was differentially expressed in lobular, ductal and myoepithelial cells and was most frequently detected in the myoepithelial compartment. The glandular stroma contained fibrocytes, a subset of which was also CD73-positive. Among 102 unselected breast carcinoma primary lesions, only 9 contained CD73-positive tumour cells, whereas in 95 cases, stromal fibroblasts and fibrocytes showed variable degrees of CD73 expression. The extent of stromal CD73 expression correlated positively with the estrogen receptor (ER) status of the tumour (P less than 0.038). We conclude that ecto-5'-NT-expression reflects a still unknown state of activity of normal breast epithelium which is lost in the majority of carcinomas derived therefrom. It may also be indicative of some functional activity of stromal fibroblasts which is significantly enhanced in ER-positive carcinomas.

Production and characterization of monoclonal antibodies to the glycosyl phosphatidylinositol-anchored lymphocyte differentiation antigen ecto-5'-nucleotidase (CD73)

A panel of monoclonal antibodies to the 69 kDa glycosyl phosphatidylinositol anchored lymphocyte differentiation antigen ecto-5'-nucleotidase (ecto-5'-NT, CD73) was produced using highly purified human placental 5'-NT as immunogen. Antibodies 1E9.28.1 and 7G2.2.11 inhibit soluble placental 5'-NT activity and recognize lymphocyte CD73 in indirect immunofluorescence and immunoprecipitation assays. In addition, 1E9.28.1 induces vigorous T cell proliferation in the presence of submitogenic doses of phorbol myristate and F(ab')2 goat anti-mouse Ig. Both antibodies can be used to purify the three major forms of placental 5'-NT by affinity chromatography. By two-color immunofluorescence, CD73 was found to be expressed on 19 +/- 5% of CD3+, 11 +/- 4% of CD4+, 51 +/- 14% of CD8+, 25 +/- 8% of CD28+, 15 +/- 5% of CD29+, 27 +/- 7% of CD45RA+, and 70 +/- 6% of CD19+ lymphocytes. Within T cells, CD73 expression is restricted to the CD28+ subset. Thus, CD73 is found on subsets of both T and B lymphocytes, with the highest expression on B cells and CD8+ T cells. In sections of hyperplastic tonsil, CD73 expression is restricted to the small lymphocytes of the follicular mantle zone, a small subset of extrafollicular lymphocytes situated within the epithelium of the tonsillar crypt, and to follicular dendritic cells within the lower part of the "light-zone." CD73 is also detected on subsets of endothelial cells of capillaries and venules and the basal layer of non-keratinizing squamous epithelium and transitional cell type mucosa of many tissues. Given the tissue distribution of CD73, along with its glycosyl phosphatidylinositol membrane anchoring and the observation that some CD73 antibodies are mitogenic, we propose that this interesting antigen may play a role in cell activation, lymphocyte homing, and/or cell adhesion.

Qualitative and quantitative differences between Burkitt lymphoma and lymphoblastoid cell lines in the expression of membrane ecto-nucleotidases and in the response to agonists of the A2-type adenosine receptor

The activity of ecto-nucleotidases has been determined on intact cells from Burkitt lymphoma (BL) and lymphoblastoid cell lines established in vitro (LCLs). BL cells never express ecto-5'-nucleotidase (5'-N) and exhibit only low levels of ecto-ATPase activity, whereas LCL-cells usually express both enzymes to varying degrees. There is a certain correlation (r = 0.75) between 5'-N and ecto-ATPase. When cAMP formation in response to agonists of the A2-type (stimulating) adenosine receptor is measured in the same cell lines there is no correlation with the expression of ecto-nucleotidases. Within pairs of BL and LCL cell lines derived from the same donor, an inverse relationship between ecto-nucleotidases and the response to the adenosine receptor agonist N-ethyl-carboxamido-adenosine (NECA) is observed. BL cells show a good response to NECA, whereas this is low or absent in LCL cells. Treatment of cells which exhibit both 5'-N and the adenosine receptor with specific polyclonal or monoclonal antibodies against the enzyme does not impair the function of the receptor. Antisera against peptides of the membrane antigen BNLF I-MA, coded by the EBV-genome, do not co-precipitate 5'-N out of detergent extracts of LCL-cells. In both cases 5'-N cannot be closely associated with other membrane components. The differences between BL and LCL cells in ectonucleotidases and the adenosine receptor appear to be fairly stable phenotypical markers, independent of other parameters and suitable for use in distinguishing these cells in all populations.

Is ecto-5'-nucleotidase essential for stimulation of human lymphocytes? Evidence against a role of the enzyme as mitogenic lectin receptor

We studied with specific polyclonal and monoclonal antibodies against human ecto-5'-nucleotidase whether the enzyme, located on the surface of human peripheral lymphocytes, could function as a mitogenic receptor for the lectins PHA, Con A and PWM. Strong, but unspecific inhibitory effects on lymphocyte stimulation are observed with unfractionated antisera and ascitic fluids. However, when purified IgG from these sources is used, no such effects are found, while at the same time, complete inhibition of ecto-5'-nucleotidase activity is maintained. It is concluded that the enzyme does not act as a mitogenic receptor for the lectins. When purine de novo synthesis of the lymphocytes is blocked by aminopterin and purine nucleotides in the extracellular medium are given as the only purine source, lymphocyte stimulation becomes dependent on the enzymatic activity of ecto-5'-nucleotidase. This is independent of the lectin used. Under these conditions, enzyme activity on the 20-30% 5'-nucleotidase-positive cells is necessary and is sufficient to support the stimulation of the whole culture. In these cultures, anti-5'-nucleotidase-IgG completely depresses cell proliferation, showing clearly that this is the only enzyme on the lymphocyte surface that is capable of degrading extracellular nucleotides.