until now , no monoclonal antibodies suitable for the disclosed invention have been available . the inventors disclose their anti - human il - 2 mabs that allow the following crucial steps towards the use and commercialization of this technology in clinical applications : further sequencing and fine characterization of the anti - human il - 2 mabs . humanization of the anti - human il - 2 mabs , which is essential to avoid ( or minimize ) immunogenicity in patients . generation of different formats of anti - human il - 2 mabs , such as igg , igg1 , igg4 , fab , and single - chain fv ( scfv ). generation of a fusion protein consisting of human il - 2 and an anti - human il - 2 mab ( or a fragment of the anti - human il - 2 mab ): such a construct has the advantage of consisting of one component only , instead of two as in il - 2 bound to an anti - human il - 2 mab . the inventors have generated and characterized specific anti - human il - 2 mabs that are able to bind human il - 2 and , when tested in mice , are able to exert specific and potent stimulation of cytotoxic lymphocytes , including cd8 + t cells and natural killer ( nk ) cells . towards these ends several difficulties had to be overcome . human il - 2 shows high similarity with mouse and rat il - 2 , thus human il - 2 is able to stimulate mouse lymphocytes in vitro and in vivo . moreover , il - 2 is present at high concentrations in the primary immune organs ( such as the bone marrow ), which is the reason why il - 2 is somewhat a “ forbidden ” antigen , meaning it is very difficult to generate b cell responses leading to neutralizing antibodies against il - 2 . nevertheless , the inventors were able to elicit polyclonal anti - human il - 2 antibody responses , following immunization of c57bl / 6 mice using purified recombinant human il - 2 plus adjuvant . of the generated antibody responses , only some mabs efficiently bound to il - 2 ( socalled “ binders ”) and of those only about 0 . 35 % interacted with the presumed active site of il - 2 . finally , of these anti - human il - 2 mabs some showed the desired specific and potent in vivo activity as assessed by specialized in vivo assays in mice that are not replaceable by in vitro experiments . the inventors have developed specific screening assays that allow detection of specific antihuman il - 2 antibodies ( so - called “ binders ”) in the serum of immunized animals and in the supernatant of the b cell clones obtained after b cell hybridoma fusion . in a second step it was discriminated between standard binders and those targeting a presumed specific epitope of the human il - 2 molecule . one example of such an in vitro enzyme - linked immunosorbent assay ( elisa ) performed with different b cell clones , is shown in fig1 to 3 . after the in vitro screening of the anti - human il - 2 mabs , these mabs were characterised in vivo . to this end and in order to obtain sufficient amounts of mabs , the mabs were concentrated from the supernatant of the hybridomas , the amount was estimated using an elisa and finally the anti - human il - 2 mabs was tested in mice . the results obtained on proliferation and expansion of cd8 + t cells and nk cells is shown in fig4 to 6 . in order to characterize the binding properties of the anti - human il - 2 mabs the binding to human interleukin - 2 was tested with surface plasmon resonance binding assays . the commercially available anti - human il - 2 mab mab602 was measured as a comparison . in fig7 binding curves of mab602 ( left graph ) and nara1 ( an antibody according to this invention ; right graph ) to human interleukin - 2 at varying concentrations are shown . the dissociation constant ( k d ) as well as the rate constants k on and k off measured for mab602 and nara1 are shown in table 1 . antibodies of the invention include the antibody nara1 , which was derived , isolated and structurally characterized by its full length heavy chain according to seq id no : 5 and its full length light chain amino acid sequences according to seq id no : 6 . the corresponding variable regions , v h and v l amino acid sequences of nara1 are . seq id no : 19 ( variable heavy ) and seq id no : 20 ( variable light ). full length light and heavy chains nucleotide coding sequences of nara1 are seq id no : 3 ( heavy chain coding sequence , including leader sequence ) and seq id no : 4 ( light chain coding sequence , including leader sequence ). variable light and heavy chains nucleotide coding sequences of nara1 are seq id no : 21 ( variable heavy coding sequence ) and seq id no : 22 ( variable light coding sequence ). the cdr regions of nara1 are delineated using the kabat system ( kabat , e . a ., et al . 1991 , sequences of proteins of immunological interest , fifth edition , u . s . department of health and human services , nih publication no . 91 - 3242 , see also zhao & amp ; lu 2009 , molecular immunology 47 : 694 - 700 ). for the ease of reading , when cdr regions are delineated according to kabat definition , they are called hereafter hcdr1 , hcdr2 , hcdr3 , lcdr1 , lcdr2 , lcdr3 respectively . the cdr regions of nara1 are : hcdr1 according to seq id no : 7 , hcdr2 according to seq id no : 8 , hcdr3 according to seq id no : 9 , lcdr1 according to seq id no : 10 , lcdr2 according to seq id no : 11 , lcdr3 according to seq id no : 12 . nucleotide coding sequences for the cdr regions of nara1 are : hcdr1 coding sequence according to seq id no : 13 , hcdr2 coding sequence according to seq id no : 14 , hcdr3 coding sequence according to seq id no : 15 , lcdr1 coding sequence according to seq id no : 16 , lcdr2 coding sequence according to seq id no : 17 , lcdr3 coding sequence according to seq id no : 18 . fusion proteins are also provided according to seq id no : 23 and seq id no : 24 . seq id no : 23 is a fusion protein comprising the variable heavy chain of nara1 with its n - terminus fused to the c - terminus of hil - 2 via a gxs linker . seq id no : 24 is a fusion protein comprising the variable light chain of nara1 with its n - terminus fused to the c - terminus of hil - 2 via a gxs linker . the complex structure of a human interleukin 2 mutant ( seq id no : 2 ), called “ proleukin ”, bound to the fab fragment of antibody “ nara 1 ” ( seq id no : 5 and 6 ) was determined . the resulting numbering of residues on proleukin is given according to the numbering of wt il - 2 . as will be discussed in detail below , the differences in sequence between proleukin and wt hil - 2 are irrelevant and proleukin is a valid model for structural analysis of hil - 2 . to define the epitope , x - ray crystallography was used to solve the atomic - resolution structure of the complex mentioned above . x - ray crystallography is a technology that has become routinely and widely used to generate structural data for biomolecules including antibodies and their complexes with antigens ( adms et al , ( 2013 ) annual review biophysics 42 : 265 - 287 ; garman , ( 2014 ) science 343 : 1102 - 1108 ; joachimiak , ( 2009 ) current opinio structural biology 19 : 573 - 584 .) the antigen , proleukin , is commercially available as lyophilyzed powder together with excipients ( every 1 mg proleukin is mixed with approximately 50 mg mannitol , 0 . 18 mg sodium dodecyl sulfate , 0 . 173 mg sodium dihydrogen phosphate , and 0 . 89 mg disodium hydrogen phosphate ). before used for complex formation , proleukin was purified by reverse - phase hplc to remove the excipients . the fab fragment of nara1 ( nara1 - fab ) was generated by papain cleavage of the full - length antibody followed by protein a chromatography . briefly , 6 . 5 ml full - length nara1 ( 9 mg / ml in 50 mm citrate buffer with 90 mm sodium chloride at ph 7 . 0 ) was mixed with 5 mm dtt and 590 ug papain ( roche ). the cleavage reaction was kept at room temperature for 16 h and stopped by addition of 15 ul 56 mm e64 solution ( roche ). the cleavage solution was then diluted 10 times with 25 mm tris , 25 mm nacl , ph 8 . 0 and loaded onto a 5 ml protein a column ( ge healthcare ) equilibrate with 5 column volume of 25 mm tris , 25 mm nacl , ph 8 . 0 and fab fragment was in the loading - through fraction and fc fragment was bound to the protein a column . to form complex , proleukin powder after hplc was dissolved in h 2 o at the concentration of 5 . 5 mg / ml . 6 . 6 mg proleukin , in excess , was added to 11 . 5 mg nara1 fab fragment solution drop by drop . centrifugation was used to remove the excess proleukin that was precipitated under current condition . the complex was then purified by gel filtration with superdex 200 10 × 300 ( ge healthcare ) with running buffer of 25 mm tris , 25 mm nacl , ph 7 . 4 . proleukin / nara1 - fab complex after gel filtration was concentrated to 14 mg / ml and was screened by vapour diffusion method as sitting drops . the protein solution was mixed 1 : 1 with reservoir buffer to a total size of 0 . 4 ul . the experiments were set up with phoenix robotic system ( art robbins instruments ), stored in a rockimager hotel ( formulatrix ) at 19 ° c ., and imaged automatically . crystals were harvested 4 days after screening under condition of 20 % w / v polyethylene glycol 3350 and 0 . 2m sodium nitrate . crystals were cryo - protected with reservoir buffer containing 10 % glycerol and flashed frozen in liquid nitrogen prior to data collection . diffraction data were collected at the swiss light source ( villigen , switzerland ) at beam - line px - ii with a pilatus pixel detector using x - ray radiation wavelength of 0 . 99998 å . the dataset was processed with xds and xscale ( version dec . 6 , 2010 ) and the structure was resolved with molecular replacement method with the program phaser by using protein data bank entry “ 31nk ” as search model for il - 2 and protein data bank entry “ 3tti ” as search model for fab fragment . iterative model building and refinement were performed with the programs coot ( crystallographic object - oriented toolkit ) and autobuster ( bricogne et al ., 2011 ). all figures were generated with the program pymol ( molecular graphics system ; delano scientific : palo alto , calif . ; http :// www . pymol . org ). epitope residues are defined as those residues from proleukin that are within 4 å distance from any atom in fab fragment of nara1 and are further confirmed by ccp4 program contact and areaimol ( collaborative computational project , number 4 , version 6 . 4 . 0 ). similarly paratope residues are defined as those residues from nara1 - fab that are within 4 å distance from any atom in proleukin . the proleukin / nara1 - fab complex was solved to 1 . 95 å in space group c 1 2 1 with unit cell dimension a = 201 . 8 å , b = 36 . 2 å , c = 88 . 7 å , alpha = 90 °, beta = 102 . 9 °, gamma = 90 °. please refer to table 2 for detailed structure statistics . in each asymmetric unit , there is one complex molecule . fig1 provides the overview of the three - dimension structure of proleukin / fab - nara1 complex as obtained in example 1 . light chain of fab fragment of nara1 is designated a , heavy chain of fab fragment of nara1 is shown as b , epitope residues recognized by nara1 - fab are designated d , and proleukin is designated c and the mutation , c145s , is highlighted . fig1 provides further analysis of epitope residues . the x - axis lists the amino acid sequence and numbering according to seq id no 1 . the upper side of y - axis demonstrates the total number of atoms of nara1 - fab that are within 4 å from corresponding residue from proleukin and the lower side of y - axis demonstrates the reduced solvent - accessible area ( å 2 ) after binding to nara1 - fab . proleukin used in example 1 contains mutation of c145s . as shown in fig1 , c145s is far away from the epitope region . in addition the superposition of ca atoms between proleukin in example 1 with ca atoms from wt hil - 2 in complex with cd25 , cd122 , and cd132 ( pdb : 2b5i ) shows r . m . s . d of 0 . 447 å , which indicates that the mutation does not disturb the over - all structure . hence proleukin with c145s mutation is a valid model for structural analysis for wt hil - 2 . hil - 2 is 4 - helix bundle protein and the 4 helices are named from n - terminus to c - terminus as a , b , c , and d , respectively . the epitope recognized by nara1 - fab as shown in fig1 is a conformational epitope and spans two regions as shown in fig1 : one region ( n50 - k63 ) comprises a loop and a short helix and connects helix a and b , and the other region ( n91 - n97 ) comprises a loop and connects helix b and c . the epitope residues together with interacting paratope residues from nara1 - fab are summarized in table 3 . among all the epitope residues , arg58 as shown in fig1 is the most critical epitope residue for binding with nara1 - fab , as this residue alone has 42 interacting atoms from nara1 - fab and accounts for 17 . 7 % of total reduced solvent - accessible surface area as a consequence of binding to nara1 - fab . furthermore arg58 , as shown in fig1 , forms two strong salt - bridges with glu35 in hcdr1 and with asp100 from lcdr3 , respectively . arg58 also makes π - action interaction with the aromatic ring of try100 from lcdr3 . residues k52 , p54 , k55 , t57 , t61 , f62 , k63 , q94 , and k96 are also considered important for the binding to nara1 - fab , since they all show equal to / more than 5 interacting atoms from nara1 - fab and larger than 30 å 2 reduced solvent - accessible area as shown in fig1 . fig1 illustrates arg58 as the most critical epitope residue recognized the nara1 - fab . a represents proleukin , b represents heavy chain , and c represents light chain . the involved residues are shown as sticks . fig1 shows the overlay of proleukin / nara1 - fab complex with il - 2 / cd25 / cd122 / cd132 quaternary complex . the quaternary complex structure comes from pdb entry “ 2b5i ” with cartoon d in pale cyan representing wt hil - 2 , cartoon b in red representing cd122 , cartoon c in blue representing cd132 , and surface a in green representing cd25 . in the proleukin / nara1 - fab complex structure , cyan cartoon d overlayed with wt hil - 2 represents proleukin , cartoon e in magenta represents heavy chain , and cartoon f in yellow represents the light chain . the structure overlay of the two complexes as shown in fig1 clearly shows that nara1 - fab forms direct competition against cd25 but not against cd122 / cd132 , which is consistent with the observation that il - 2 / nara1 complex demonstrates mainly pro - t effector cell activity rather than pro - treg activity . 3 ) c helix of proleukin in complex with nara1 - fab adopts conformation that is similar to that in quaternary complex fig1 displays the overlay of c helices from il - 2_c145a ( pdb : 3ink ), superkine ( pdb : 3qb1 ), il - 2 / cd25 / cd122 / cd132 ( pdb : 2b5i ), and proleukin / nara1 - fab . the polar interface between helix c in il - 2 and cd122 plays an important role in binding between the two parts ( wang et al ( 2005 ) science 310 : 1159 - 1163 ). in 2012 levin , et al have demonstrated that superkine , an il - 2 mutant , alone has a helix c adopting confirmation similar to that in the quaternary complex and superkine showed ˜ 215 times higher binding affinity towards cd122 than wtil - 2 ( levin et al , ( 2012 ) nature 484 : 529 - 533 ). it was observed that such a conformational change in helix c is associated with conformational stabilization , which then reduces the energetic penalties for binding to cd122 . as shown in fig1 , the conformation of helix c from proleukin in complex with nara1 - fab is also similar to that observed in superkine as well as in il - 2 / cd25 / cd122 / cd132 quaternary complex , therefore it is possible that proleukin / nara1 - fab complex may demonstrate higher binding affinity towards cd122 than wt hil - 2 does . in order to map the epitope of the nara1 and mab602 antibodies , a first library of 15 - mer peptides was generated based on the sequence of human il2 . a second library of selected 15 - mer peptides was also generated based on the mutation of 3 specific residues f ( 62 ), y ( 65 ) and l ( 92 ). the latter mutations were done based on the roche / glycart il2 mutein , as disclosed in wo2012 / 107417a1 which has these 3 mutations . previous work done in lab boyman ( unpublished ) showed that the commercial mouse anti - human il2 mab 602 with analogous function as a1 has strongly reduced binding to the f42a mutant of il2 ( one of the il2 docking sites to cd25 ). accordingly , each peptide in the first library has 15 amino acids and the sequence is derived by scanning the sequence of interest ( see table 4 , reference peptides 1 to 41 ) with a step of 3 residues , starting from the n - terminus . therefore a ladder is generated and each peptide contains 12 overlapping residues with the previous peptide and 12 overlapping residues with the following peptide in the ladder . in total , 41 peptides were generated from the expressed human il2 sequence . a second library of peptides was generated by mutating f ( 62 ), y ( 65 ) and l ( 92 ) to alanine in all corresponding peptides in the first library generated as described above ( see table 4 , reference peptides no 42 to 60 ). for both libraries , the parental cysteines have been replaced by a serine ( underlined residues ) to avoid unspecific binding . both set of peptides were printed on microarray slides in triplicate , incubated with the antibodies of interest ( mab602 and nara1 ) and control antibodies . additional incubations are with unrelated antibodies from the same isotype ( mouse control igg2a / lambda and mouse control igg2a / kappa ), and secondary antibodies ( anti - mouse igg ( thermo 84545 , label dl650 ) or anti - mouse igg ( jir 115 - 175 - 072 , label cy5 )) to assess unspecific binding due to the detection antibody . the experiments are performed essentially as described in maksimov p , et al . 2012 , plos one 7 : e34212 . doi : 10 . 1371 / journal . pone . 0034212 . the determination of peptide - antibody binding was performed by replitope - analysis where the peptide microarray ( triplicate ) was incubated with the primary antibody followed by a fluorescently labelled secondary antibody directed against the fc - part of the primary one . all steps were performed on a tecan microarray processing station enabling highly reliable and reproducible washing and incubation steps . after performing the incubation steps and subsequent to the final washing steps ( to remove the unbound secondary antibodies ) the microarrays were dried using a nitrogen stream and scanned in a high resolution microarray scanning system with appropriate wavelength settings . control incubations were performed with an unrelated antibody having the same isotype to exclude false positive signals . the resulting images were analyzed und quantified using spot - recognition software genepix ( molecular devices ). for each spot , the mean signal intensity was extracted ( between 0 and 65535 arbitrary units ). for further data evaluation , the mmc2 values were determined . the mmc2 equals the mean value of all three instances on the microarray . except the coefficient of variation ( cv )— standard - deviation divided by the mean value — is larger 0 . 5 , in this case the mean of the two closest values ( mc2 ) is assigned to mmc2 . the anti - il2 ( nara1 ) antibody did not show any significant reactivity towards the immobilized peptides . only peptide 10 exhibited a weak response , however , this peptide was also weakly recognized by the mouse control antibodies . the commercial antibody mab602 ( migg2a ) provided some weak signals on peptide 22 to 26 and some strong for peptides 10 to 13 . the overlapping sequences within both set of peptides are considered as containing the binding amino acid to the target antibody ( table 5 ). one stretch is a strong binder to mab602 whereas the other is rather a weak binder to mab602 : ala mutation on specific residues f42 ( 62 ), y45 ( 65 ), l72 ( 92 ) showed that residue f42 ( 62 ) is clearly an important residue for the binding to antibody mab602 ( table 6 ). useful amino acids and nucleotide sequences for practicing the invention are found in table 7 .
the invention relates to a human interleukin - 2 specific monoclonal antibody , or antigen binding fragment thereof , the binding of which to hil - 2 inhibits binding of hil - 2 to cd25 and the antibody is characterized by any of the parameters : the variable chain of the mab comprises the amino acid sequence of seq id no 005 or seq id no 006 ; the binding to hil - 2 is characterized by a dissociation constant ≦ 7 . 5 nmol / l ; the binding to hil - 2 is characterized by an off - rate ≦ 1 × 10 − 4 s − 1 and / or the antibody displays no measurable cross - reactivity to murine il - 2 .