the important in vivo function of cd4 + cd25 + regulatory t cells has been thoroughly demonstrated in rodents ( asano et al ., j . exp . med . 184 : 387 - 396 ( 1996 ); takahashi et al ., int . immunol . 10 : 1969 - 1980 ( 1998 ); thornton et al ., j . exp . med . 188 : 287 - 296 ( 1998 )). recent studies demonstrate , that cd4 + cd25 + t cells are also relevant as an immune regulator in humans ( jonuleit et al ., j . exp . med . 193 : 1285 - 1294 ( 2001 ); levings et al ., j . exp . med . 193 : 1295 - 1302 ( 2001 ); dieckmann et al ., j . exp . med . 193 : 1303 - 1310 ( 2001 ); taylor et al ., j . exp . med . 193 : 1311 - 1318 ( 2001 )). lately we and others have shown that a similar population of regulatory t cells also exists in humans ( pct / ep02 / 02671 ). these findings have been confirmed and extended by several groups up to now ( jonuleit et al ., j . exp . med . 193 : 1285 - 1294 ( 2001 ); levings et al ., j . exp . med . 193 : 1295 - 1302 ( 2001 ); dieckmann et al ., j . exp . med . 193 : 1303 - 1310 ( 2001 ); taylor et al ., j . exp . med . 193 : 1311 - 1318 ( 2001 ); nakamura et al ., j . exp . med . 194 : 629 - 644 ( 2001 ); ng et al ., blood 98 : 2736 - 2744 ( 2001 ); iellenn et al ., j . exp . med . 194 : 847 - 853 ( 2001 ); yamagiwa et al ., j . immunol . 166 : 7282 - 7289 ( 2001 ); stephens et al ., eur . j . immunol . 31 : 1247 - 1254 ( 2001 ); taams et al ., eur . j . immunol . 31 : 1122 - 1131 ( 2001 )). still numerous characteristics of cd4 + cd25 + t cells need to be explained . one important question that has to be answered is how cd4 + cd25 + t cells execute their important function in vivo , as they only constitute a small population of peripheral cd4 + t cells ( average 6 %), that need direct cell contact as well as stimulation via the tcr to suppress unwanted t cell activation . in vitro studies usually employ high ratios of cd4 + cd25 + / cd4 + cd25 − t cells , a situation that is hard to imagine at an inflammatory site in vivo . as described , coculture of cd4 + cd25 + and cd4 + cd25 − t cell leads to marked reduction of t cell proliferation ( dieckmann et al ., j . exp . med . 193 : 1303 - 1310 ( 2001 )). this effect is stable for several days ( data not shown ). although cd4 + cd25 + t cells produce sizeable quantities of il - 10 this cytokine does not seem to be responsible for the regulatory effects ( dieckmann et al ., j . exp . med . 193 : 1303 - 1310 ( 2001 )). the supernatant of cd4 + cd25 + and cd4 + cd25 − cocultures were analyzed and it was found that high levels of il - 10 are produced , peaking after 48 h . il - 10 levels in the coculture were markedly higher than il - 10 produced by cd4 + cd25 + t cells alone , suggesting that it was not only attributable to cd4 + cd25 + t cells . this was further confirmed by intracellular facs ® analysis . il - 10 is known to inhibit cytokine production from t cells ( moore et al ., science 248 : 1230 - 1234 ( 1990 )) and exert anti - inflammatory and suppressive effects on most haematopoeitic cells . it is also involved in the induction of peripheral tolerance via effects on t - cell - mediated responses ( moore et al ., science 248 : 1230 - 1234 ( 1990 )). il - 10 indirectly suppresses t - cell responses by potently inhibiting the antigen - presenting capacity of apc , including dc ( steinbrink et al ., j . immunol . 159 : 4772 - 4780 ( 1997 ), langerhans cells and macrophages ( romagnoli et al ., j . immunol . 168 : 1644 - 1648 ( 2002 )). in addition , il - 10 directly regulates t cells by inhibiting their ability to produce il - 2 , tnf - α ( de waal et al ., j . immunol . 150 : 4754 - 4765 ( 1993 )), il - 5 ( schandene et al ., j . immunol . 152 : 4368 - 4374 ( 1994 )) and to proliferate bejarano et al ., int . immunol . 4 : 1389 - 1397 ( 1992 )). it was important to rule out , that the effects seen were not only due to cd4 + cd25 + t cells in the coculture . in a set of pilot experiments we could show , that cd4 + cd25 + t cells when paraformaldehyde - fixed after polyclonal activation have similar regulatory properties as viable cd4 + cd25 + t cells . in coculture experiments , employing activated - fixed and viable cd4 + cd25 + t cells together with cd4 + cd25 − t cells , it turned out , that il - 10 production remained high , even if activated - fixed cd4 + cd25 + t cells were used . this showed , that il - 10 production was not attributable to increased production by cd4 + cd25 + but due to the anergized cd4 + cd25 − t cells . in a parallel transwell approach we showed , that direct cell contact between cd4 + cd25 + and cd4 + cd25 − t cells is necessary to prime cd4 + cd25 − t cells to become il - 10 producers . further experiments were performed to analyze , which effect this high level of il - 10 might have on t cell proliferation . indeed it was shown , that proliferation of syngeneic cd4 + t cells could be markedly decreased by anergized cd4 + cd25 − t cells . addition of anti - il - 10 abolished the suppressive effects of anergized cd4 + cd25 − t cells , while a transwell setting , permitting the free exchange of soluble factors , but no cell contact , did not change suppression . furthermore , we csfe - labeled cd4 + cd25 + t cells and separated them from cd4 + cd25 − t cells after 48 h of coculture by facs ® sorting . both populations strongly inhibited cd4 + t cell proliferation which was almost abolished in the unlabeled e . g . cd4 + cd25 − fraction by the addition of anti - il - 10 , demonstrating that il - 10 indeed is crucial for the suppressive function of energized cd4 + cd25 − t cells . this is not surprising , as other reports have shown , that activation of human cd4 + t cells in the presence of il - 10 results in a state of functional unresponsiveness without death , termed anergy ( iellenn et al ., j . exp . med . 194 : 847 - 853 ( 2001 )). cd4 + t cells with low proliferative capacity , that are generated in the presence of il - 10 have been termed type 1 t regulatory cells ( tr1 ). the cells that are generated in the presence of cd4 + cd25 + t cells show some characteristics resembling tr1 cells , especially their low proliferative capacity and the high level production of il - 10 . but in some instances they differ , as tr1 are also defined by their ability to produce tgf - β and anergized cd4 + cd25 − t cells did not produce significant amounts of tgf - β at least by the assay used . further on we clearly demonstrate , that cell contact between cd4 + cd25 + and cd4 + cd25 − t cells and not il - 10 is crucial for the induction phase of inhibitory , anergized , il - 10 - producing , cd4 + cd25 − t cells . but as coculture of anergized cd4 + cd25 − with syngeneic resting cd4 + cd25 − t cells results in anergic , il - 10 releasing cd4 + cd25 − t cells this il - 10 production may then also contribute to the generation of tr1 - like cells as described for tr1 cells . to distinguish between indirect effects via apc modulation and direct effect on t cells we used as a stimulus not only allogeneic dc but also immobilized anti - cd3 / soluble anti - cd28 as a cell free t cell stimulation system . as the effects seen were independent of the stimuli used , a direct effect on t cells is most likely . the data presented here may serve as an explanation of how cd4 + cd25 + t cells fulfill their important in vivo function . at sites of inflammation if activated they would anergize cd4 + t cells in their lose environment in an antigen - unspecific bystander effect fashion ( thornton et al ., j . immunol . 164 : 183 - 190 ( 2000 )). our findings suggest , however , that anergized cd4 + t cells ( including pathogenic ones ) in turn will produce high levels of il - 10 thereby creating an immunosuppressive environment either by indirect effect via influence on apc ( steinbrink et al ., blood 93 : 1634 - 1642 ( 1999 )) or via direct effects on other t cells thereby effectively abrogating unwanted t cell activation . the invention is further explained by the following examples , which are , however , not to be construed as to limit the invention . cfse , 5 - carboxyfluorescein diacetat succinimdyl ester ; ctla - 4 , cytotoxic t lymphocyte antigene 4 ; dc , dendritic cell ; macs ®, magnetic activated cell sorting ; tr1 , t regulatory cell 1 . rpmi 1640 ( bio whittaker ) supplemented with 1 % heat - inactivated autologous plasma , 20 μg / ml gentamicin ( merck ) and 2 mm glutamine ( bio whittaker ) was used for the generation of dendritic cells ( dc ), x - vivo - 20 ( bio whittaker ) supplemented with 1 % heat - inactivated single donor human serum , 20 μg / ml gentamicin ( merck ) and 2 mm glutamine ( bio whittaker ) for t cell culture . all cytokines used in this study were recombinant human proteins . final concentrations were : gm - csf 1 , 000 u / ml ( leukomax ™; novartis ), il - 4 800 u / ml ( sandoz ) and il - 2 ( proleukin ; chiron corp .) were used at the concentrations indicated ; for dc maturation we used a cocktail consisting of il - 1β 2 ng / ml ( sigma ); il - 6 1000 u / ml ( sandoz ); tnf - α10 ng / ml ( bender , vienna ), and pge 2 1 μg / ml ( sigma ). for immunostaining pe - and fitc - conjugated antibodies ( ab ) ( all from bd pharmingen ) against cd3 ( ucht 1 ), cd4 ( rpa - t4 ), cd5 ( ucht 2 ), cd8 ( rpa - t8 ), cd14 ( m5e2 ), cd19 ( hib 19 ), cd25 ( m - a251 ), cd28 ( cd28 . 2 ), cd45 ra ( hi 100 ), cd45 ro ( uchl 1 ), cd56 ( b159 ), cd62l ( dreg - 56 ), cd80 ( l307 . 4 ), cd83 hb15e ), cd86 ( fun - 1 ), cd95 ( dx 2 ), cd95l ( g247 - 4 ), cd122 ( mik - 132 ), cd152 ( bni3 . 1 ), cd154 ( trap1 ), hla - dr ( g46 - 6 ), and respective mouse and rat isotype controls were employed . ab used for intracellular cytokine staining were fitc - and pe - conjugated anti - il - 2 ( mq1 - 17h12 ), anti - il - 4 ( 8d4 - 8 ), anti - il - 10 ( jes3 - 19f1 ) and anti - ifn - γ ( 4s . b3 ), all from bd pharmingen . unconjugated anti - il - 10 ( jes3 - 19f1 ) ( pharmingen ) was used for neutralization experiments , anti - cd3 ( ucht1 ) and anti - cd28 ( cd28 . 2 ) for polyclonal activation of t cells . t cells were stimulated with allogeneic dc or with platebound anti - cd3 ( 10 μg / ml )+ soluble anti - cd28 ( 10 μg / ml ) in x - vivo - 20 + 1 % serum . cytokine analysis was performed at different time points by analysis of supernatants with commercially available elisa kits for human il - 10 , ifn - α ( biosource international ) and tgf - β ( bd pharmingen ). il - 2 , il - 4 , il - 5 , ifn - γ and tnf - α were measured by a cytometric bead array ( th1 / th2 cytokine cba 1 ; bd pharmingen ) according to the manufacturers instructions . for analysis of intracellular cytokine production t cells were either stimulated with pma 20 ng / ml and ca 2 + ionophore a23187 500 μg / ml ( both from sigma ) for 6 hours or with platebound anti - cd3 and soluble anti - cd28 ab for 6 hours . monensin , 2 μm ( sigma ) was added for the last 5 hours of culture . cells were collected , washed , fixed and saponine permeabilized ( fix / perm solution , bd pharmingen ) and stained with cytokine specific ab or isotype . dc were generated from buffy coats orleukapheresis products ( both obtained from the department of transfusion medicine , university of erlangen , from healthy donors after informed consent was given ) as described in romani et al , j . immunol . methods 196 : 137 - 151 ( 1996 ) and thurner et al ., j . immunol . methods 223 : 1 - 15 ( 1999 ). in brief , pbmcs were isolated by ficoll density gradient centrifugation . monocytes were isolated by plastic adherence and cultured in rpmi medium , supplemented with il - 4 and gm - csf . at day 6 , a maturation cocktail ( il - 1β , il - 6 , pge 2 and tnfα ) was added . at day 7 nonadherend cells were harvested and constituted mature dc that were & gt ; 90 % double positive for costimulatory molecules ( cd80 , cd86 ) and cd83 . cd4 + t cells were isolated from pbmc with a negative cd4 + t cell isolation kit ( miltenyi biotech ). cd4 + cd25 + t cells were isolated from the pure , untouched cd4 + t cells using cd25 microbeads ( miltenyi biotech ). purity was assessed by facs ®. for immunofluorescence staining cells were washed and stained for 20 min at 4 ° c . with optimal dilution of each ab . cells were washed again and analyzed by flow cytometry ( facs scan ® and cellquest ® software ; becton dickinson ). for analysis of intracellular cd152 cells were stained for cd4 expression , fixed and saponine permeabilized ( fix / perm solution , bd pharmingen ) and stained with cd152 specific ab or isotype . for fixation experiments cd4 + cd25 + and cd4 + cd25 − t cells were isolated and divided into three fractions . one part of each was activated with platebound anti - cd3 and soluble anti - cd28 over night . next day the stimulated parts and one resting part were fixated with 2 % paraformaldehyde for 1 h at 4 ° c . thereafter fixated cells were washed extensively and used in regulation assays together with the untreated fraction . to induce anergized cd4 + cd25 − t cells with cd4 + cd25 + t cells , both populations were isolated as described . they were either used directly or fixated as described above . 5 × 10 5 of cd4 + cd25 + and cd4 + cd25 − t cells were cultivated either with platebound anti - cd3 and soluble anti - cd28 ( 10 μg / ml each ) or with allogeneic mature dc ( 5 × 10 4 ) for 48 h in 48 well plates . thereafter cells were harvested , washed and used in proliferation experiments . to assess proliferation of the differently cultured cd4 + subtypes 10 5 sorted t cells were incubated in x - vivo - 20 with 5 × 10 3 dc in 96 - well u - bottom plates or 10 μg / ml of platebound anti - cd3 + 10 μg / ml soluble anti - cd28 in 96 - well flat - bottom plates . for assessment of regulatory properties 10 5 resting cd4 + cd25 − autologous t cells were cultured with 5 × 10 3 dc or platebound anti - cd3 and soluble anti - cd28 in 96 - well u - bottom plates . purified cd4 + cd25 + and cd4 + cd25 − t cells , anergized cd4 + cd25 − t cells or fixated cd4 + cd25 + and cd4 + cd25 − t cells were added usually at a 1 : 1 ratio , if not indicated differently . after 4 - 5 days of culture [ 3 h ] tdr ( 37 kbq / well ) was added for additional 16 h . proliferation was measured using a liquid scintillation counter . transwell experiments were performed in 24 - well plates . 10 6 cd4 + cd25 − t cells were stimulated with 5 × 10 4 dc . in addition , 10 6 cd4 + cd25 + , cd4 + cd25 − and anergized cd4 + cd25 − t cells were either added directly to the culture or were placed in transwell chambers ( millicell , 0 . 4 μm ; millipore ). after 5 days of coculture t cells were transferred to 96 - well plates ( 10 5 cells / well ) in triplicates . proliferation was measured after 16 h pulse with [ 3 h ] tdr using a liquid scintillation counter . cd4 + cd25 + and cd4 + cd25 − t cells were labeled with 0 . 5 μm of cfse ( molecular probes ) for 15 minutes at 37 ° c . reaction was stopped with ice - cold pbs buffer and cells were washed extensively . 5 × 10 5 cd4 + cd25 + cfse labeled t cells were then cultured with 5 × 10 5 unlabeled cd4 + cd25 − t cells ( and vice versa ) with platebound anti - cd3 and soluble anti - cd28 in 48 well plates . proliferation was controlled by facs ® for different time points . after 48 h cells were harvested and sorted on a facs vantage ® ( becton dickinson ). sorted cells were used for further regulation assays . coculture of cd4 + cd25 + and cd4 + cd25 − t cells yields low proliferating , il - 10 producing t cells cd4 + cd25 + and cd4 + cd25 − subpopulations were separated by magnetic cell sorting from healthy donors . separately or mixed at a 1 : 1 ratio the cells were stimulated polyclonally with platebound anti - cd3 and soluble anti - cd28 or with mature allogeneic dc . as shown before coculture of cd4 + cd25 + and cd4 + cd25 − t cells results in a constantly low proliferating t cell population ( data not shown ). the supernatant of this coculture was analyzed after various time points for different cytokines and a high level of il - 10 production was found ( peaking 48 h after onset of culture ( fig1 a )). as shown before cd4 + cd25 + t cells alone also produce sizeable amounts of il - 10 (˜ 200 pg / ml ). one might speculate , that il - 10 production was only attributable to cd4 + cd25 + t cells , but il - 10 production in the coculture was 2 - 4 times higher then production of cd4 + cd25 + t cells alone ( fig2 a & amp ; b ). intracellular facs revealed , that the number of il - 10 producing cells more then doubled ( fig1 c ). high il - 10 production after 48 h of coculture was observed regardless if polyclonal activation ( platebound anti - cd3 and soluble anti - cd28 ) or allogeneic mature dc were used ( fig1 b ). in none of the cultures increased production of tgf - β or inf - α could be observed ( data not shown ). cd4 + cd25 − t cells alone did not produce significant amounts of il - 10 ( fig1 a - c ) it is known , that cd4 + cd25 + t cells exert their regulatory function in a cell contact - dependent yet cytokine - independent manner . to further analyze their regulatory function , isolated cd4 + cd25 + t cells were divided into three parts . one was activated over night polyclonally with platebound anti - cd3 and soluble anti - cd28 and fixed thereafter with paraformaldehyde (“ activated - fixed ”), the second part was fixed with paraformaldehyde without prior activation (“ resting - fixed ”) and the third part was left untreated (“ viable ”). after this procedure the three differently treated fractions of cd4 + cd25 + t cells were used in regulation assays with syngeneic cd4 + cd25 − t cells . as shown in fig2 a activated - fixed cd4 + cd25 + t cells had a similar regulatory capacity as their normal viable counterpart . this is in sharp contrast to resting - fixed cd4 + cd25 + t cells which do not show any regulatory function at all . activated - fixed and viable cd4 + cd25 + t cells almost completely suppressed proliferation of cd4 + cd25 − t cells when a 1 : 1 ratio was used . this underlines and extends prior findings on the regulatory function , demonstrating that surface molecules , induced after activation of cd4 + cd25 + are responsible for the regulatory capacity of these cells . cd4 + cd25 + regulatory t cells induce il - 10 production in anergized cd4 + cd25 − t cells in a cell contact - dependent manner in further experiments the above mentioned findings were used to analyze the requirements for induction of il - 10 producing anergized cd4 + cd25 − t cells . cd4 + cd25 + and cd4 + cd25 − t cells were either cultured alone or at a 1 : 1 ratio with normal viable cd4 + cd25 + t cells , activated - fixed cd4 + cd25 + t cells , resting - fixed cd4 + cd25 + t cells or in a transwell setting . il - 10 production was measured 48 h after onset of culture . as shown in fig2 b a high level il - 10 production was achieved in coculture either with viable cd4 + cd25 + or activated - fixed cd4 + cd25 + t cells . in transwell experiments il - 10 production , similar to that of cd4 + cd25 + t cells alone was observed and cd4 + cd25 − t cells alone produced negligible amounts of il - 10 . cd4 + cd25 − t cells anergized by cd4 + cd25 + t cells suppress activation of syngeneic cd4 + t cells in an il - 10 dependent manner il - 10 is known as a cytokine with potent immunosuppressive function . it was therefore tempting to speculate , that the high il - 10 production of anergized cd4 + cd25 − t cells would lead to secondary suppression of other t cells . in a next set of experiments we sought to investigate this matter . as it is known , that il - 10 can act indirectly on t cells , via influence on apc , we choose a polyclonal , cell free t cell stimulus ( platebound anti - cd3 and soluble anti - cd28 ) and mature allogeneic dc , because they are known as the most powerful apc and largely resistant to il - 10 17 . cd4 + cd25 + and cd4 + cd25 − t cells alone or at a 1 : 1 ratio were stimulated with allogeneic dc ( fig3 , upper panel ) or with bound anti - cd3 and soluble anti - cd28 ( fig3 , lower panel ) for 48 h . thereafter cells were either fixed with paraformaldehyde or used viable . as expected activated cd4 + cd25 + t cells induced strong suppression of cd4 + proliferation and at a 1 : 1 ratio almost abolished it , whereas activated cd4 + cd25 − t cells did not alter proliferation of syngeneic cd4 + t cells . when the cocultured cd4 + cd25 + and cd4 + cd25 − t cells were used in regulation assays , they mediated a strong inhibition of cd4 + t cell proliferation ( fig3 ). this phenomenon was seen , regardless of the stimulus used ( fig3 , upper and lower panel ). we further added anti - il - 10 ab to the regulation assay or performed it in a transwell setting . as shown before anti - il - 10 did not alter suppressive function of pure cd4 + cd25 + t cells , whereas in a transwell setting cd4 + cd25 + t cells could not mediate suppression . the opposite was true for regulation by anergized cd4 + cd25 − t cells . addition of il - 10 antibodies almost completely abolished inhibition , whereas a transwell setting did not markedly change the regulatory function of these cells . suggesting , that inhibition is mediated dominantly by secretion of il - 10 ( fig3 ). similar effects were seen with polyclonally or allogeneic stimulated cells ( fig3 , upper and lower panel ). to further exclude that the observed effects are mediated by cd4 + cd25 + t cell directly we performed cfse labeling and facs ® sorting experiments . cd4 + cd25 − t cells were labeled with cfse and then mixed with cd4 + cd25 + unlabelled t cells at 1 : 1 ratio . this mixture was stimulated with immobilized anti - cd3 / soluble anti - cd28 for 48 h . thereafter cells were sorted by facs ® and used in regulation assays with syngeneic cd4 + cd25 − t cells . as shown in fig4 , regulation was mediated by both , the labeled and unlabeled fractions , which was abolished by addition of anti - il - 10 in the case of unlabeled ( anergized cd4 + cd25 − t cells ). not surprisingly activated cd4 + cd25 + t cells showed inhibition of t cells proliferation themselves . this could not be abolished by anti il - 10 , clearly demonstrating , that anergized cd4 + cd25 − t cells mediate suppression via il - 10 which is distinct from the mechanisms utilized by cd4 + cd25 + t cells . anergized cd4 + cd25 − t cells predominantly produce il - 10 to analyze the cytokine secretion pattern of anergized cd4 + cd25 − t cells , cd4 + cd25 + and cd4 + cd25 − t cells were sorted and stimulated alone or at a 1 : 1 mixture as described before . after 48 hours of culture supernatants were analyzed for the cytokines il - 2 , il - 4 , il - 5 , tnf - α and inf - γ by a cytometric bead array , which allows multiparameter analysis in a single sample . as shown in fig4 , anergized cd4 + cd25 − t cells similar to cd4 + cd25 + t cells do only produce very low levels of tnf - α and inf - γ and no il - 2 , - 4 or il - 5 . cd4 + cd25 − t cells on the other hand produce high levels of il - 2 , tnf - α and inf - γ and low to moderate levels of il - 4 and il - 5 , resembling a th1 phenotype . surface phenotyping with the abs mentioned in material and method did not reveal striking differences between activated cd4 + cd25 + , activated cd4 + cd25 − and cocultured cd4 + cd25 + / cd4 + cd25 − t cells after 48 h of activation ( data not shown ).