Neopterin : Biomarker of cell-mediated immunity and potent usage as biomarker in silicosis and other occupational diseases

Neopterin is regarded as an early biomarker of the cellular immune response. This low molecular mass compound belongs to the class of pteridine and is a metabolite of guanosine triphosphate, which is produced by the activated macrophages and dendritic cells after stimulation with γ-interferon. An international group acknowledges the fact that the levels of serum neopterin can be used as a marker of the effect of exposure to silica and other occupational diseases. The determination of neopterin is an innovative tool for monitoring diseases associated with the activation of cell-mediated immunity.


INTRODUCTION
Neopterin [D-erythro-6-(1',2',3'-trihydroxypropyl)-pterin] [1] is known to be in equilibrium with 7,8-dihydroneopterin, and the presence of a high concentration of both oxidized and reduced forms of pteridin may be associated with oxidative stress.Silica is one of the most documented contaminants of the work place.Long-term occupational exposure to silica is associated with an increased risk for respiratory diseases such as silicosis, tuberculosis, chronic bronchitis, chronic obstructive pulmonary disease and lung cancer. [2]Furthermore, a variety of immune dysfunction-related diseases have been reported in the silicotic individual.Preliminary studies indicating an enhanced level of autoantibody and several cytokines reflect an involvement of the immune system in the pathogenesis of silicosis and the resulting complication as an early and valuable marker of cellular immunity. [2]Neopterin levels can be used as a marker of the effect of exposure to silica. [3]opterin is pyrazino-pyrimidine compound of molecular weight 253 D belonging to the class of pteridines.Pteridine is a chemical compound of fused pyrimidine and pyrizine rings.A pteridine is a group of heterocyclic compounds containing a wide variety of substitutions in this structure. [4]Pterins and flavins are a class of substituted pteridines that have important biological activities. [4]It is produced by guanosine triphosphate (GTP) via γ-interferon (INF-γ) following the activation of T cells.Neopterin concentration increase in the urine or blood reflects the activation of cellular immunity and an endogenous release of INF-γ. [5]This review focuses on the clinical utility of measuring the neopterin levels in inflammatory diseases and the potential functions of neopterin as a mediator and modulator in the course of inflammatory and infectious processes.In vitro studies reveal that neopterin derivatives ex h i b i t d i s t i n c t b i o c h e m i c a l effects, most likely via interactions with reactive oxygen or nitrogen intermediates, thereby affecting the cellular redox state. [5]Neopterin enhances the cytotoxic potential of the activated macrophages (AC) and the dendritic cells (DC).In vivo, a strong correlation was obtained between the neopterin levels and the disease severity, progression and outcome of infections with inflammatory disease.The influence of neopterin derivatives on the cellular metabolism may provide an explanation for these clinical observations.

MECHANISM OF NEOPTERIN ACTIVATION
Neopterin is derived from GTP and is produced by stimulated macrophages under the influence of INF-γ of lymphocyte origin.Cleavage of GTP by GTP-cyclohydrolase I results in neopterin, yielding 7,8dihydroneopterin triphosphate, which is a joint precursor of dihydroneopterin, neopterin, tetrahydrobiopterin, a necessary cofactor of aromatic amino Human monocytes/macrophages are the unique source to produce an excess of neopterin derivatives at the rate of 5, 6, 7, 8-tetrahydrobiopterin, [6] which results as a comparably low activity of 6-pyrovoyltetrahydropterin synthase, which is the first enzyme in the conversion of 7,8-dihydroneopterin triphosphate to tetrahydrobiopterin. [7] On activation of the cellular immunity, IFN-γ induces GTP-cyclohydrolase I and also stimulates the enzyme indoleamine (2, 3)-dioxygenase (IDO) in various cells. [8,9]In tryptophan catabolism, N-formylkynurenine, the first intermediate, is formed in response to IDO within the biosynthetic pathway of nicotinamide dinucleotide.To monitor the activation status of IDO and of cellular immunity, determination of kynurenine and tryptophan concentrations has proven to be a sensitive estimate both in vivo and in vitro [10,11] [Figure 1].

OXIDATIVE STRESS AND NEOPTERIN
Recent data suggest that neopterin derivatives exhibit distinct biochemical functions.Neopterin was found to enhance the effects of toxic reactive oxygen species originating from chloramine T and hydrogen peroxide, [12,13] suggesting that neopterin derivatives are able to modulate macrophageinduced cytotoxicity by the induction of oxidative stress.In rat vascular smooth muscle cells, neopterin stimulates redoxsensitive intracellular signal transduction cascades, thereby triggering the inducible NOS (iNOS) gene expression at the messenger ribonucleic acid level with a subsequent increase in nitric oxide (NO) production. [14]In vascular muscle cells [15] and Jurkat cells, [16] the neopterin derivatives were found to activate the transcriptional nuclear factor (NF)-B.It has been shown to induce programmed cell death, which is mediated by the reactive oxygen intermediates in T-lymphoblastic cell lines and in rat alveolar cells. [15,16]In regard to these multiple biochemical functions of neopterin derivatives, it is very likely that DC might also use neopterin derivatives in the regulation of T cell response.Uniquely, in humans and primates, high concentrations of neopterin are detected during cellular immune activation produced by AM and also by DC.In all other organisms, activation of cellular immunity is accompanied by an increased production of tetrahydrobiopterin, an essential cofactor for iNOS.NO production by iNOS seems to play a role in inflammation, e.g., by acting regulatorily on NFk B, an important modulator of inflammatory gene expression, including pro-inflammatory cytokines and endothelial cell adhesion molecules. [17]The production of neopterin derivatives instead of biopterin derivatives in humans suggest that neopterin derivatives substitute regulatory and immunological functions, especially of the tetrahydrobiopterin-induced NO generation.
DC produces neopterin derivatives on stimulation, which is additionally determined by the degree of IDO-dependent tryptophan degradation.As observed, neopterin production and IDO activation were found to closely correlate in a large variety of diseases in vivo, including systemic lupus erythematosus, hepatitis immunodeficiency virus and in pregnancy. [10,18,19]

ACTIVATION OF NEOPTERIN BY INF AFFECTS THE IMMUNE SYSTEM
To activate the gene transcription by initiation of intracellular signaling via a complex mechanism, INF binds to specific cell surface receptors.The gene gets modulated by INF stimulation and effects on inhibition of viral replication, cell proliferation and immunomodulation in infected cells.An effector protein such as neopterin and 2', 5'-oligoadeylates synthetase gets stimulated by INF.Neopterin has been used in the INF studies to demonstrate its immune activation by the INF.Neopterin has been validated in a large number of studies as a marker of INF activation.The level of neopterin increases with the INF. [20]

NEUROTRANSMITTERS AND NEOPTERIN
Occupational exposure to lead affects the neuromuscular junctions and it might cause disturbances in the locomotor activity.This study was carried out to evaluate pteridine metabolism for the synthesis of neurotransmitters in urinary neopterin, biopterin, creatinine and blood dihydropteridine reductase (DHPR) activity in battery workers and the deltaaminolevulonic acid (delta-ALA) was measured.Blood and urine lead levels were detected by an atomic absorption spectrophotometer.A significant increase in the blood and urine lead levels shows traces of urinary neopterin, biopterin and delta-ALA among exposed workers. [21]DHPR activity was indifferent as compared with the control group.These studies demonstrate that an increased activity of the pteridine pathway causes accumulation of the neurotransmitters, which may be responsible for the neurological disorders. [22]

NEOPTERIN AND DIFFERENT DISEASES
It has been suggested that it is an excellent marker for  Altindag et.al. [2] Neopterin  [26] The is influenced Santelli et.al. [29] Urinary a prognostic marker.Wang.et.al. [30] Increased serum levels for neopterin and sIL-2R.Ghonaim et.al. [31] Soluble Interleukin-2 The study included 44 Significantly elevated levels of sIL-2Rα sIL-2Rα and neopterin Receptor Alpha (sIL-2Rα ) patients with active observed in TB patient with active serum level are increased and Neopterin In Patient pulmonary TB and 20 diseases (1253±492pgm/ml) as in patient with pulmonary With Pulmonary controls.Serum levels compared to controls (412±173pg/ml) TB and can be used as Tuberculosis of sIL-2Rα and neopterin and the neopterin level observed markers for disease were determined.also higher in active TB (45.8±17.1nmol/L)activity and follow up than controls (5.2±1.8nmol/L). of anti-tuberculosis treatment.Fuith et.al. [32] Neopterin, a marker of Serum and urine samples Neopterin concentration exceeded Neopterin levels provide cell mediated immune were studied for neopterin the normal range in (79%) evidences for activation activation in human and INF-γ concentration of pregnant ladies.Neopterin levels of cell mediated immunity pregnancy.
in normal pregnant women.increase with the time of pregnancy.during pregnancy.However no circulation of INF-γ was detected.Yao et.al. [33] Elevation serum neopterin This prospective study High neopterin level was found in patient High neopterin levels level: its relation to included 35 patients with sepsis and the marked elevation associated with a critical endotoxaemia and sepsis with burn size greater persists.Patient with endotoxaemia event in the development in patient with major burns.than 30% and 22 healthy had much high neopterin concentration. of severe burn sepsis.volunteers.Weiss et.al. [12] Neopterin modulates This study performed to Observations showed that 7,8-Neopterin is able both to toxicity mediated by test the ability of neopterin dihydroneopterin potentially reduces enhance and to reduce the reactive oxygen and and its reduced form to biological and chemical effects of these cytotoxicity in dependence chloride species.
modulate the effect of substances independently from the of pH value and its oxidation cytotoxic substances like pH value.In contrast, at slightly alkaline state, and it may have a hydrogen peroxide or pH (pH 7.5) neopterin enhances hydrogen pivotal role in modulation hypochlorous acid and peroxide and chloramine-T activity. of macrophage mediated N-chloramine derivatives.

Razumovitch
Influence of neopterin The study performs Neopterin induced luminance in Neutrophils respond on et.al, [34] on the generation of the influence of neopterin suspensions of neutrophils in the exposure to neopterin reactive oxygen species on the generation of presence of luminal, but not in lucigenin with additional generation (ROS) in human neutrophils ROS in neutrophils and also observed that neopterin of singlet oxygen, hydroxyl suspended in Earl's solution affect the adhesive cells.radical and nitric oxide.by monitoring of biochemiluminometer.

CONCLUSION
The most important clinical application of the determination of neopterin is as a prognostic indicator and as a followup for chronic infection, immune stimulation monitoring, differential diagnosis of acute bacterial and viral infection and also as an early indication for the complication of allograft recipient. [23,24]As the neopterin level serves as an indirect indicator for oxidative stress, the CMI marker, the levels of neopterin help in the diagnosis of different diseases. [5]Elevated levels in the serum of the silicotic patients and the correlation of baseline neopterin with IFN will help in more advances in occupational diseases. [35]In RA, an immune dysregulation alters the release of neopterin from human monocytes/ macrophages. [27]The increased formation of neopterin and degradation of tryptophan may result in a decreased T cell response along with the development of "immunodeficiency". [36]Pteridins are used as biochemical markers of immune system activation, mainly the markers of early activation and development of CMI response, which are used for the evaluation of pathologic disturbances.From the above studies, it may be concluded that elevated levels of neopterin can be used as a biomarker of CMI, silicosis and other occupational diseases.