the present invention will now be described with reference to an scfvfc - rnase , wherein the antibody portion has an anti - cd receptor specificity , and wherein the rnase portion comprises at least in portion human pancreatic rnase . fig1 is a schematic representation of the cytotoxic activity of ab - rnase conjugates , fig2 is a schematic representation of the principal structure of scfvfc - rnase conjugates according to the present invention , fig3 shows an sds - page of purified samples of the anti - cd receptor scfvfc - rnase conjugate of the invention , fig4 shows the size exclusion chromatogram of the anti - cd receptor scfvfc - rnase conjugate of the invention , including an sds - page of fractions , fig5 shows results of an activity assay of anti - cd receptor scfvfc - rnase conjugate according to the invention , using the decrease of the in vitro transcription of luciferase mrna as an indicator , fig6 shows the results of an spr analysis of conjugates according to the invention to their target antigen , fig7 shows facs results of the binding conjugates according to the invention to their antigen specific target cells , fig8 shows microscopic pictures of cells that have been treated with conjugates according to the invention , fig9 shows the reduction of proliferation of cells carrying the specific target antigen by conjugates according to the present invention , and fig1 gives an overview of the sequence elements of one embodiment of anti - cd receptor scfvfc - rnase , wherein nucleotides and amino acids for the antigen - binding section fv are indicated by stars for vh and vl . in general , the mechanism for cytotoxic activity of ab - rnase conjugates is schematically depicted in fig1 , wherein the conjugate of antibody and rnase is also termed a targeted rnase because of the antigen specificity of the conjugate . in vitro and in vivo activity is triggered by uptake , e . g . endocytosis of the conjugate into the target cell after recognition of an accessible antigen , e . g . a membrane - bound antigen by the ab portion of the ab - rnase conjugate . following uptake into the target cell and translocation into the cytoplasm , the non - specific activity of the rnase portion provides for the cytotoxicity of the conjugate and , as a consequence , leads to a decrease of cell viability , preferably to apoptosis and / or any necrosis of target cells and target tissue . a schematic representation of the structure of the scfvfc - rnase is given in fig2 ; the dna sequence of the scfc - rnase portion forming part of the scfvfc - rnase conjugate is enclosed as seq id no . 1 . sequence seq id no 1 comprises the hinge region to which in 5 ′ a sequence providing antigen specificity , e . g . an fv section encoding sequence is to be attached at nucleotides 1 to 54 , an ch2 domain at nucleotides 55 to 385 , a ch3 domain at nucleotides 386 to 693 , a linker at nucleotides 694 to 724 , and the rnase portion at nucleotides 725 to 1092 . seq id no 2 ( dna ) gives an exemplary linker encoding sequence that can be arranged between vh and vl of an fv section forming the antigen binding fragment ; seq id no 3 gives the corresponding amino acid sequence . as an example for the conjugate according to the invention , an anti - cd receptor scfvfc - rnase conjugate ( i ) is presented , the antibody portion being specific for the membrane - bound receptor cd receptor having an extracellular domain , which antibody portion is derived from a human anti - cd receptor antibody . the rnase portion is derived from human pancreatic rnase . in the examples , a variant ( ii ) to the anti - cd receptor scfvfc - rnase conjugate ( i ) is used , which variant differs in the fv section only . the similar reactions and properties of these two fv section variants ( i and ii ) shows that it is the general structure of the conjugates according to the invention that allows for an efficient production in cell culture and provides for the effective antigen specificity and high cytotoxicity . the coding sequence comprised in seq id no . 1 was used in nucleic acid constructs , both comprising operably linked in 5 ′ the coding sequence ( i and ii , respectively ) for an fv portion specific for the cd receptor in the expression vector pcmv myc - er ( obtainable from invitrogen ), which provided operon functions and functional elements for transcription and translation . expression was transient in hek293 t - cells . after transfection of hek293 t - cells , the anti - cd receptor scfvfc - rnase conjugate was isolated from culture supernatants by absorption to a protein a sepharose column , followed by elution . the yield of pure anti - cd receptor scfvfc - rnase was in the range of 2 . 3 to 4 mg / l cell culture supernatant . an sds - page of samples after purification is shown in fig3 , wherein the conjugate construct and the anti - cd receptor scfv portion variant thereof show about the same molecular weight of approximately 75 kda . size exclusion chromatography of native anti cd receptor scfvfc - rnase was done using 80 mg purified conjugate on a calibrated superdex 200 10 / 30 gl column , indicating a molecular weight of approximately 150 kda , corresponding to a homodimer of native anti - cd receptor scfvfc - rnase with no aggregates being formed . the results of size exclusion chromatography is shown in fig4 ; the inset shows a 10 % reducing sds - page with silver staining of fractions , wherein given fraction numbers are indicated in the sec chromatogram . according to sds - page analysis , no impurities are found in the sec fractions in order to assess the rnase activity of anti - cd receptor scfvfc - rnase , sepharose a — purified preparations were added to in vitro translation reactions of luciferase mrna using the cell - free translation in rabbit reticulocyte lysate ( promega ). the reduction of luminescence by the presence of rnase activity serves as a direct indicator for the presence of rnase , either from bovine rnase used as a positive control ( 2 . 5 μm ) or by anti - cd receptor scfvfc - rnase according to the invention ( 2 . 5 μm ) or using the ab fragment anti - cd receptor scfvfc ( 2 . 5 μm ), i . e . without rnase portion as a negative control . the control for 100 % luminescence was without addition of any antibody or rnase preparations . the results , shown in fig5 , demonstrate that it is only anti - cd receptor scfvfc - rnase according to the invention as well as bovine rnase that provide for an efficient reduction of the luminescence , i . e . degradation of luciferase mrna in the in vitro translation assay . interestingly , the addition of the rnase inhibitor rnasin ( obtainable from promega ) to the reaction mixtures resulted in a slight increase in luminescence from both control and antibody preparations ( anti - cd receptor scfvfc ) as well as an expected rise in luminescence in the positive control containing bovine rnase , whereas the anti - cd receptor scfvfc - rnase according to the invention was not impaired in its activity by the rnase inhibitor . rnasin inhibits the activity of rnases , including human pancreatic rnase . accordingly , rnase activity causing the cytotoxicity of the scfvfc - rnase conjugates is essentially not impaired by rnase inhibitors , exemplified here by rnasin , indicating a stable and efficient cytotoxicity of the conjugates . as an example for the antigen - specific binding of conjugates according to the invention , the binding of anti - cd receptor scfvfc - rnase conjugates to lymphoma cells expressing the surface marker cd receptor was assayed by surface plasmon resonance ( spr ). surface plasmon resonance was assayed for both anti - cd receptor variants of anti - cd receptor scfvfc - rnase , using varying concentrations of conjugates on a cm5 chip equipped with immobilized recombinant cd receptor - fc . as a control , lysozyme was immobilized in a parallel flow cell . results are depicted in fig6 in relative units ( ru ). the affinities for the conjugate was determined to approximately 1 nm , which is contrasted by an affinity of approximately 600 nm , determined for a control construct forming a monovalent fcfv - rnase fusion protein . as an example for the binding of conjugates according to the invention to their target cells , i . e cells exhibiting the specific antigen , anti - cd receptor scfvfc - rnase in both variants ( i and ii ) after purification were incubated with cd receptor + lymphoma cells . as a negative control , cd receptor negative h - dm - yz cells were used . staining was done with 10 μg / ml anti - cd receptor scfvfc - rnase conjugates , followed by an anti - human fc - specific igg - fitc conjugate as a detection antibody . as an isotype control , mucin 1 specific igg was used . the results of facs analysis are shown in fig7 , wherein the peaks for conjugates according to the invention are labelled and the isotype control mucin 1 specific igg is depicted by the solid black line . this facs analysis demonstrates that selective binding of sc ab - rnase conjugates according to the invention to target cells having the specific antigen is obtained . as an example for an sc ab - rnase conjugate according to the invention , anti - cd receptor scfvfc - rnase conjugate was incubated with cd receptor + lymphoma cells at 25 μg / μl . after an incubation for 4 hours at 37 or 4 ° c ., respectively , cd receptor + lymphoma cells were analyzed by confocal microscopy following fixation , detecting the scfvfc - rnase conjugate with an anti - human fc - specific igg - fitc conjugate and staining of the nucleus with dapi . in fig8 a , the result of the incubation at 37 ° c . is shown , in fig8 b the incubation 4 ° c ., with the lower pictures showing overlays with brightfield . these observations demonstrate that conjugates according to the invention are localized within the nucleus , as indicated by the presence of the fitc - labelled detection antibody . at 37 ° c ., intracellular vesicles are visible in cd receptor + lymphoma cells ( fig8 a ) containing the conjugates according to the invention , whereas at 4 ° c . ( fig8 b ), membrane associated localization can be detected . in controls using cd receptor — h - dm - yz cells , no association of conjugates according to the invention was detected . using the example of cd receptor + lymphoma cells , these results show the antigen - specific detection of target cells and the target cell - specific internalization of sc ab - rnase conjugates according to the invention . as an assay for antigen - specific cytotoxicity of sc ab - rnase conjugates according to the invention , cd receptor + lymphoma cells were seeded to 104 cells per well in a 96 - well plate in triplicate and incubated for three days with varying concentrations of anti - cd receptor scfvfc - rnase in a range from 0 to 100 nm . the antibody fragment anti - cd receptor scfvfc , i . e . without rnase portion , was included as a negative control . after 3 days incubation at 37 ° c ., the cell number was determined by staining dead cells with trypan blue and counting living cells . results are shown in fig9 , demonstrating that it is only the sc ab - rnase conjugate according to the invention , which strongly inhibits proliferation . for this inhibition , an ic50 of 3 . 3 nm could be determined , whereas the proliferation of non - target control cells , namely cd receptor — h - dm - yz cells is not affected . the negative control using the antibody fragment without rnase portion proves that the inhibition of proliferation is due to the rnase portion of the conjugate according to the invention . these results could be confirmed by facs analysis using annexin v and pi staining , showing the induction of apoptosis and necrosis in tumor cells .
the present invention provides a novel antibody - rnase conjugate which is a single chain protein , providing both the specificity of its antibody portion and the rnase activity of its rnase portion , resulting in an antigen specific effectiveness against cells when applied in vivo or in vitro , wherein the rnase portion is effectively cytotoxic in at least a fraction of cells presenting the antigen , e . g . after internalization by endocytosis . in detail , the present invention provides scab - rnase conjugate having the principal structure of scfvfc - rnase . this structure could also be shown to allow the effective production of antigen - specific and cytotoxic conjugate protein in cell culture , the conjugate having a high activity with respect to antigen specificity and cyto