glutathione used in the present invention includes reduced glutathione ( gsh ) and oxidized glutathione . gsh refers to a tripeptide having the γ - l - glu - l - cys - gly structure , and oxidized glutathione refers to a glutathione dipeptide in which two molecules of gsh are linked by an ss bond . the gsh and the oxidized glutathione used in the present invention may be obtained by any production process . examples of the processes for the production of gsh include the extraction method from microorganisms such as yeast ( methods in enzymology , 3 , 603 ( 1957 )), the chemical synthesis method ( bull . chem ., soc . jpn ., 53 , 2529 ( 1980 )) and the enzymatic method ( japanese published unexamined patent application no . 74595 / 86 ), and an example of the process for the production of oxidized glutathione is the method of acta biochim . pol ., 17 , 175 ( 1970 ). gsh and oxidized glutathione can also be purchased from sigma - aldrich corporation . examples of the salt of glutathione include hydrochloride , citrate , malate , α - ketoglutarate and aspartate , but the other salts or appropriate combinations of more than two salts may be used . in the present invention , the term “ immune function ” refers to the function relating to the protection from microbial infection , the rejection of cells of other individuals , and the removal of mutant cells and wasted tissue . examples of the immune function include the cell cytotoxicity by nk cells , lymphocyte proliferation , and the like . in the present invention , “ natural killer cell ( nk cell )” refers to the special lymphocyte which is not t - cell type and b - cell type and has ability of the protection from viral infection and the removal of tumor cells . in the present invention , the term “ enhancing immune function ” means , for example , enhancement of the functions of the protection from microbial infection , the rejection of cells of other individuals , and the removal of mutant cells and wasted tissue by enhancing t cell proliferation and / or by enhancing the cell cytotoxic activity of nk cells against tumors and virus - infected cells . in the present invention , the term “ natural killer cell ( nk cell ) cytotoxicity ” refers to the ability of nk cells to directly attack tumor cells , virus - infected cells , or the like to lead to their damage . in the present invention , the term “ increasing natural killer cell ( nk cell ) cytotoxicity ” means enhancement of the action of nk cells to directly attack tumor cells , virus - infected cells , or the like to lead to their damage . thus , increase in the nk cell cytotoxic action is useful as a method for treating and / or preventing tumors and viral diseases ( annual review immunity 2002 , chugai - igakusha , p . 76 - 80 ). in the present invention , the term “ lymphocyte proliferation ” refers to the proliferation of t cells by cell division caused by antigen stimulation . in the present invention , the term “ increasing lymphocyte proliferation ” means increase in the proliferation of t cells by cell division caused by antigen stimulation . this leads to increase in cytotoxic activity against intracellular and extracellular infectious microorganisms ( annual review immunity 2002 , chugai - igakusha , p . 10 - 11 ). as a result , the increase in lymphocyte proliferation is useful as a method for treating and / or preventing infectious diseases caused by the intracellular or extracellular infectious microorganisms . examples of the intracellular infectious microorganism include virus , chlamydia , rickettsia , listeria , leishmania , acidophil , salmonella typhi , and the like . examples of the extracellular infectious microorganism include protozoa , fungus , parasite , mycoplasma , enteritis coccus , escherichia coli , and the like . it has been reported that concanavalin a ( con a ) induced lymphocyte proliferation activity and nk cell cytotoxic activity are increased in a gsh concentration dependent fashion in in vitro test using murine - derived splenic lymphocytes ( yakugaku zasshi , 126 ( 10 ), pp . 849 - 857 , 2006 ). further , the lymphocyte proliferation action by administering 2 - melcaptoethanol ( 2 - me ), which has a gsh production action in vivo , has been known ( j . immunol . 146 ( 6 ), pp . 1921 - 1927 , 1991 ). furthermore , it has been reported that glutathione concentration in immune cells , pha stimulated lymphocyte proliferation , and nk cell cytotoxic activity are increased by oral administration of n - acetylcysteine ( n - ac ), which is a precursor of glutathione , to a human . in the present invention , glutathione may be administered as such , but it is usually preferred to be administered as various kinds of preparations . the preparations comprise glutathione or a salt thereof , as an active ingredient , and may further comprise any additional active ingredients . the preparations are produced according to any methods well known in the technical field of pharmaceutics by mixing the active ingredient with one or more kinds of pharmaceutically acceptable carriers . it is desirable to administer the preparation by the route that is the most effective for enhancing immune function . suitable administration routes include oral administration and parenteral administration such as intravenous administration , intraperitoneal administration or subcutaneous administration . preferred is oral administration . the preparation may be administered either in the form of oral preparations ( e . g ., tablets , powders , granules , pills , suspensions , emulsions , infusions / decoctions , capsules , syrups , liquids , elixirs , extracts , tinctures and fluid extracts ) or parenteral preparations ( e . g ., injections , drops , creams and suppositories ). preferred are oral preparations . liquid preparations suitable for oral administration such as syrups can be prepared by adding water , sugars ( e . g ., sucrose , sorbitol and fructose ), glycols ( e . g ., polyethylene glycol and propylene glycol ), oils ( e . g ., sesame oil , olive oil and soybean oil ), antiseptics ( e . g ., p - hydroxybenzoates ), preservatives ( e . g ., paraoxybenzoic acid derivatives such as methyl paraoxybenzoate , and sodium benzoate ), flavors ( e . g ., strawberry flavor and peppermint ), and the like . tablets , powders , granules or the like suitable for oral administration can be prepared by adding excipients such as sugars ( e . g ., lactose , white sugar , glucose , sucrose , mannitol and sorbitol ), starch ( e . g ., potato starch , wheat starch and corn starch ), inorganic substances ( e . g ., calcium carbonate , calcium sulfate , sodium hydrogencarbonate and sodium chloride ), crystalline cellulose and plant powders ( e . g ., licorice powder and gentian powder ), disintegrating agents such as starch , agar , gelatin powder , crystalline cellulose , carmellose sodium , carmellose calcium , calcium carbonate , sodium hydrogencarbonate and sodium alginate , lubricants such as magnesium stearate , talc , hydrogenated vegetable oil , macrogol and silicone oil , binders such as polyvinyl alcohol , hydroxypropyl cellulose , methyl cellulose , ethyl cellulose , carmellose gelatin and starch paste , surfactants such as fatty acid esters , plasticizers such as glycerin , and the like . the preparations suitable for oral administration may also comprise additives generally used in foods and drinks such as sweeteners , coloring agents , preservatives , thickening stabilizers , antioxidants , color developers , bleaching agents , fungicides , gum bases , bitter agents , enzymes , glazing agents , sour a gents , seasonings , emulsifiers , nutrient supplements , manufacture facilitating agents , flavors and spice extracts . the preparations for oral administration may be used as foods and drinks such as a health food , a functional food , a food supplement and a food for specified health uses for enhancing immune function , as such or in any of the forms such as a powder food , a sheet - shaped food , a bottled food , a canned food , a retort pouched food , a capsule food , a tablet food , a liquid food and a health drink . one or more kinds of auxiliary components selected from the above - described antiseptics , preservatives , excipients , disintegrating agents , lubricants , binders , surfactants , plasticizers , etc . for oral preparations can also be employed in these parenteral preparations . the concentration of glutathione or a salt thereof for enhancing immune function of the present invention is appropriately determined according to the kind of the preparation , the effect expected by the administration of the preparation , etc . glutathione or a salt thereof are usually contained in an amount of 0 . 1 to 100 % by weight , preferably 1 to 100 % by weight , particularly preferably 25 to 100 % by weight . when the glutathione or a salt thereof is administered according to the present invention , the dose and administration schedule vary depending on the administration route , the subject &# 39 ; s age and body weight and the like . when the subject is a human , glutathione may be administered to at a daily dose of 50 mg to 5000 mg , preferably 250 mg to 1000 mg in terms of the glutathione or a salt thereof . there is no specific restriction as to the period of administration , but the administration may be for a period of 1 day to 1 year , preferably 1 week to 6 months , most preferably 3 months . in another embodiment , the administration may be for a period of 1 day or more , preferably one week or more , further preferably three months or more . the agent according to the present invention can be used not only for humans but also for animals other than humans ( hereinafter abbreviated as nonhuman animals ). nonhuman animals include animals other than humans such as mammals , birds , reptiles , amphibians and fish . in the case of administration to a nonhuman animal , the dose varies depending upon the age and kind of the animal , and the nature or degree of severeness of the symptom . usually , the agent is administered once to several times per day in an amount to give a daily dose of 1 mg to 600 mg , preferably 2 mg to 200 mg , more preferably 4 mg to 60 mg per kg of body weight in terms of ornithine or a salt thereof and glutathione or a salt thereof . there is no specific restriction as to the period of administration , but it is usually one day to one year , preferably one week to three months . in another embodiment , the period of administration may be one day or more , preferably one week or more , further preferably three months or more . in addition , the present invention includes an oral agent for enhancing immune function , an oral agent for increasing nk cell cytotoxicity , and an oral agent for increasing lymphocyte proliferation , which are prepared for administering glutathione or a salt thereof to a human at 50 to 5000 mg / day , preferably 250 to 1000 mg / day , for one day or more , preferably one week or more , more preferably 3 months or more . the present invention is described below by examples ; however , the present invention is not limited to the following examples . the design included the recruitment of healthy subjects ( 28 - 79 yr . of age , mean = 46 . 6 years ) randomized into 3 groups ( gsh at 1000 mg / day , gsh at 250 mg / day , and placebo ). the target accrual of 48 subjects , 16 per group was based upon power calculations and results from a previous clinical trial with selenium ( el - bayoumy , k ., et al ., cancer epidemiol biomarkers prev , 2002 . 11 ( 11 ): p . 1459 - 65 ). blood samples were obtained from all subjects at baseline . subjects began supplementation according to the following schedule : glutathione capsules were provided by the sponsor , kyowa hakko usa . capsules were formulated , and subjects began supplementation according to the following schedule as follows : group a , gsh : 500 mg / capsule and cellulose : 15 mg / capsule , ( 2 pills daily ); group b , gsh : 125 mg / capsule and cellulose : 360 mg / capsule , ( 2 pills daily ); group c ( placebo ), cellulose : 470 mg / capsule , ( 2 pills daily ). eligible participants were randomly assigned to either placebo ( group c ) or high ( group a ) or low ( group b ) dose glutathione groups . supplementation continued for 6 months with biological samples collected at 1 , 3 and 6 months after baseline . at 6 months , supplementation was discontinued . a final collection of biological samples occurred after a 1 - month washout period . blood samples were collected from an antecubital vein into 3 tubes containing sodium heparin as an anticoagulant and immediately placed on ice . blood samples were collected between 9 : 00 am and 1 : 00 pm and subjects were not fasted prior to collection . tubes were mixed by gentle shaking and one 2 . 5 ml aliquot of whole blood was removed for analysis of neutrophil phagocytosis and respiratory burst ( see below ). two 0 . 5 ml aliquots of whole blood were removed and stored at − 80 ° c . for future analyses . the remaining blood was centrifuged for 10 min at 1300 × g to obtain plasma , buffy coat and red cell fractions . multiple 0 . 5 ml aliquots of plasma were placed into 1 . 5 ml cryovials and immediately frozen at − 80 ° c . packed red cells will be washed 3 × in saline and aliquoted ( 0 . 5 ml each ) into multiple cryovials and frozen at − 80 ° c . buffy coat fractions were combined and lymphocytes were isolated by ficoll - hypaque density gradient centrifugation . in brief , after addition of 3 ml of ficoll , buffy coats were centrifuged at 400 g for 30 min at 19 ° c . lymphocyte layers were removed and washed 2 times in pbs , followed by centrifugation at 250 g for 10 min . after the final wash , lymphocytes were resuspended in 5 ml pbs . cell number was assessed after addition of 40 μl trypan blue to 10 μl of cell suspension using a hemocytometer . cells were resuspended in 95 % fbs , 5 % dmso at concentrations of 2 . 5 × 10 6 cells / ml and stored in liquid nitrogen for analysis of lymphocyte proliferation or nk cell cytotoxicity ( see below ). cells were kept on ice until acid extraction as described below . glutathione — glutathione was measured in metaphosphoric ( mpa ) extracts of lymphocytes from each of the 4 time points ( 1 , 3 , 6 , and 7 months ). for lymphocytes , 400 μl of 5 % mpa was added to aliquots of packed lymphocytes containing 5 × 10 6 cells . after vigorous mixing and incubation at room temp for 15 min , samples were centrifuged at 14 , 000 g for 2 min . supernatants and pellets were stored separately at − 80 ° c . until analysis of free and gsh , respectively . gsh was administered at 250 mg / day or 1000 mg / day and glutathione concentration in lymphocytes extracted from the subjects after 1 , 3 , 6 , and 7 months from the start of administration was measured . the result is shown in fig1 . lymphocyte proliferation — lymphocyte samples from each of the 5 time points ( 0 , 1 , 3 , 6 , and 7 months ) were thawed , washed 3 times and counted to determine cell viability . cells were plated in triplicate and both 1 × 10 5 and 5 × 10 4 cells per cell in the presence and absence of the t cell mitogen phytohemagglutinin ( pha ). cell proliferation was monitored by 3h - thymidine incorporation . intracellular radioactivity was measured by liquid scintillation counting and results were expressed as cpm . the evaluation results of lymphocyte proliferation action at the three - month time point ( placebo , 250 mg / day administration group , and 1000 mg / day administration group ) are shown in fig2 . nk cell cytotoxicity — nk cell cytotoxicity was assessed using human k562 cells , labeled with sodium 51 chromate as target cell . after labeling the k562 cells , they were placed in 96 - well plates ( 1 × 10 4 cells per well ). lymphocyte samples from each of the five time points were thawed , washed 3 times , counted to determine cell viability and added in triplicate to the wells at an effector : target cell ratio of 10 : 1 . after incubation for 4 hr . at 37 ° c ., cells were collected and supernatant analyzed for radioactivity by gamma counting . results are expressed as percent of target cells lysed (% lysis ) calculated as ( cpm experimental − cpm spontaneous release )/( cpm maximum − spontaneous ). the evaluation results of nk cell cytotoxic action at the three - month time point ( placebo , 250 mg / day administration group , and 1000 mg / day administration group ) are shown in fig3 . this trial was designed to provide evidence of the efficacy of gsh and includes the assessment of both short - term ( 1 - month ) and long - term ( 6 - month ) effects . based on previous laboratory animal studies and clinical data , it was anticipated that the effects of oral gsh supplementation would be progressive and cumulative . it was previously observed a similar ˜ 6 month progressive effect of selenium on blood gsh levels in healthy adult men ( el - bayoumy , k ., et al ., cancer epidemiol biomarkers prev , 2002 . 11 ( 11 ): p . 1459 - 65 ). in addition , the study design also allowed for the evaluation of effects of withdrawal of the supplement on the outcome variables . doses were selected to optimize the likelihood that a biologically relevant response could be observed within the study time period , based upon previous clinical trial results . the primary evaluation of the effect of glutathione on key biomarkers ( nk cell cytotoxicity , and lymphoproliferation ) will involve comparison of the change from baselines to the end of the treatment , in each group using repeated measurement anova . alternatively , student &# 39 ; s t - tests and repeated measures anova will be used to compare the mean change in biomarker levels over time in each age group . 3 . effects of oral glutathione on immune function markers in blood the impact of gsh administration of several different blood - based parameters of immune function including lymphocyte proliferation , and nk cell cytotoxicity was examined . lymphocyte and nk cell assays were performed on frozen purified lymphocyte cell fractions . these later assays are dependent on the yield of living and proliferative cells obtained from frozen cell preparations . thus , all samples were carefully and quickly processed to obtain pure lymphocyte fractions which were then frozen down using a protocol designed to maintain optimal cell survival . since , in these assays it is important to analyze all samples from each time point from a given individual all together on the same day to avoid assay - to - assay variation , it was necessary that viability be maintained in samples frozen for a minimum of at least 7 months . cell samples that did not reach a minimum level of viability could not used for further analysis . unfortunately , a large number of lymphocyte samples were lost to analysis for this reason . this likely resulted from a − 80 ° c . freezer malfunction which occurred mid - study . while samples did not thaw , they did experience a period of warming temperatures of up to − 15 ° c . as a result of these lost samples , n - values for subjects which were usable pre - and post - samples was only 8 - 9 per group for the lymphocyte proliferation assay and 5 - 6 for the nk cell cytotoxicity assay . accordingly , statistical power , particularly for the subgroup analyses was negatively affected . lymphocyte total glutathione — the effects of oral gsh administration of lymphocyte levels are summarized in fig1 . values are expressed on a per million cell basis and ranged from 0 . 7 to 6 . 7 μmol / 10 6 cells . a maximum increase of about 30 % was observed after 6 months in the high dose gsh group . in the high dose group , the increase appeared to be time dependent with levels progressively increasing from 1 to 3 to 6 months of administration . thus , at the high dose ( 1 . 0 g / day ), gsh was accumulated in lymphocyte in an administration period - dependent fashion and there observed the significant difference in gsh concentration compared to that before administration ( 0 month ). therefore , a medical effect based on gsh ( such as , enhancement of immune function ) is expected to be expressed . lymphocyte proliferation — the effects of gsh administration on lymphocyte proliferation are summarized in fig2 . increases in mean proliferative capacity were observed after 3 months in a dose dependent fashion in both gsh groups . this suggested trends toward an increase in lymphocyte proliferation resulting from gsh administration . therefore , gsh or a salt thereof is useful as an agent for enhancing immune function and , preferably , useful as an agent for enhancing the functions of the protection from microbial infection , the rejection of cells of other individuals , and the removal of mutant cells and wasted tissue by enhancing t cell proliferation and / or by enhancing the cytotoxic activity of nk cells against tumors and virus - infected cells . further preferably , gsh or a salt thereof enhances the cytotoxic activity against intracellular infectious microorganisms ( such as , virus , chlamydia , rickettsia , listeria , leishmania , acidophil , salmonella typhi , or the like ) and extracellular infectious microorganisms ( such as , protozoa , fungus , parasite , mycoplasma , enteritis coccus , escherichia coli , or the like ) and is useful as an agent for treating and / or preventing infectious diseases caused by the intracellular or extracellular microorganisms . nk cell cytotoxicity — the effects of gsh administration on nk cell cytotoxicity are summarized in fig3 . increases in mean % lysis values were observed after 3 months in a dose dependent fashion in both gsh groups , but were only significant in the high gsh dose arm ( ppaired = 0 . 01 ). therefore , gsh or a salt thereof is useful as an agent for enhancing immune function and , preferably , useful as an agent for enhancing the functions of the protection from microbial infection , the rejection of cells of other individuals , and the removal of mutant cells and wasted tissue by enhancing t cell proliferation and / or by enhancing the cytotoxic activity of nk cells against tumors and virus - infected cells . preferably , gsh or a salt thereof is useful as an agent for enhancing an action of nk cells to directly attack tumor cells and virus - infected cells to lead to their damage . further preferably , gsh or a salt thereof is useful as an agent for treating and / or preventing tumors and viral diseases . overall , this was a highly successful trial which demonstrates for the first time the effectiveness of long - term gsh supplementation . the present application is based on u . s . provisional application no . 61 / 707 , 286 filed on sep . 28 , 2012 , and the contents of the patent application are incorporated herein by reference . according to the present invention , the immune function is enhanced and the natural killer cell ( nk cell ) cytotoxicity and lymphocyte proliferation are increased by glutathione or a salt thereof .
the present invention relates to a method for enhancing immune function , which comprises administering orally to a subject in need thereof effective amounts of glutathione or a salt thereof . the present invention also relates to an oral agent comprising glutathione or a salt thereof for enhancing immune function , and glutathione or a salt thereof for use in enhancing immune function .