Patent Abstract:
the present invention refers to the field of allergy . more specifically , the invention relates to the identification of new allergens from mammals and to diagnosis and treatment of allergy towards mammals . in particular the invention relates to a recombinantly produced canine allergen , can f 4 , or bovine allergen , bos d 23k , or variants or fragments thereof sharing epitopes with the wildtype allergen , and the use thereof for the manufacture of a diagnostic composition as well as methods for in vitro diagnosis and treatment of allergy .

Detailed Description:
the example below illustrate the present invention with the isolation and use of the lipocalin - like protein can f 4 from dog . there is also a part illustrating the cross - reactivity between can f 4 and a cow allergen . the example is only illustrative and should not be considered as limiting the invention , which is defined by the scope of the appended claims . immunoblot analysis was performed on non - reduced dog dander extract separated by sds - page using a homogeneous 12 . 5 % excelgel ( ge healthcare life sciences , uppsala , sweden ) and electroblotted onto a hybond ecl nitrocellulose membrane ( ge healthcare life sciences ). as molecular weight ( mw ) markers , the lmw kit ( ge healthcare life sciences ) was used . protein blots were blocked for 1 h at room temperature using blocking buffer ( 50 mm phosphate ph 7 . 4 , 0 . 1 % ( v / v ) tween - 20 , 0 . 9 % ( w / v ) nacl , 0 . 3 % ( w / v ) dextran t10 ) and then incubated overnight with each patient &# 39 ; s serum , diluted 1 . 5 - 30 - fold in blocking buffer . after washing in blocking buffer with 0 . 5 % ( v / v ) tween - 20 , the membrane was incubated 4 hrs at room temperature with an 125 i - labelled anti - human ige antibody in blocking buffer and , after washing , bound ige was radiographically detected using a storage phosphor screen and a typhoon 9410 variable mode imager ( ge healthcare life sciences ). dog dander ( allergon , välinge , sweden ) was extracted in 20 mm mops ph 7 . 6 , 0 . 5 m nacl ( mbs ), clarified by centrifugation , filtered through a 0 . 45 μm mixed cellulose ester filter ( millipore , billerica , mass .) and applied to a superdex 75 column ( ge healthcare life sciences ) for size exclusion chromatography ( sec ). fractions containing a 16 kda band observed in immunoblot analysis were concentrated in an amicon stirred cell ( millipore ) using ym - 3 filter , desalted on a sephadex g25 superfine column ( ge healthcare life sciences ) to 20 mm tris - hcl ph 8 . 0 . the desalted preparation was then applied to a source q column ( ge healthcare life sciences ) for anion exchange chromatography ( aiec ) and eluted with a linear 0 - 0 . 5 m nacl gradient . further purification was performed by reversed phase chromatography ( rpc ) using a source 15 rpc column ( ge healthcare life . sciences ) and elution with a linear 0 - 54 % gradient of acetonitrile in water containing 0 . 05 % trifluoro acetic acid ( tfa ). fractions containing the target protein were identified by sds page and pooled . following reduction , alkylation and trypsin cleavage , peptides of the purified protein were isolated by rpc and analysed by amino acid sequencing . for evaluation of ige antibody binding by immunocap , the 16 kda protein was subjected to a final polishing step by cation exchange chromatography ( ciec ) using a sp sepharose ff column ( ge healthcare life sciences ) equilibrated in 20 mm citrate ph 4 . 0 and elution with a linear 0 - 1 m nacl gradient . cow dander ( allergon ) was extracted and fractionated by sec as described above . fractions containing a dominant 23 kda band were pooled , conditioned with nh4so4 to a final concentration of 1 m and further purified by hydrophobic interaction chromatography ( hic ) using a phenyl sepharose hp column ( ge healthcare life sciences ). the 23 kda band eluted in the flow through fraction and was desalted to 20 mm bis - tris propane ph 8 . 5 on a sephadex g25 superfine column ( ge healthcare life sciences ) and subsequently applied to a source 15q column ( ge healthcare life sciences ) equilibrated with the same buffer . elution was performed with a linear 0 - 0 . 4 m nacl gradient and fractions containing the 23 kda band were pooled . the protein concentration of the final preparation was determined from absorbance at 280 nm , using a calculated extinction coefficient of 1 . 04 per mg / ml . fractions containing a dominant 19 kda band were pooled and further purified by hic as described above . the 19 kda protein was eluted in a linear 0 - 1 m gradient of nh4so4 in 20 mm tris - hcl ph 8 . 0 and the peak fractions pooled . desalting and aiec on a source 15q column was performed as described above . the protein concentration of the final preparation was determined from absorbance at 280 nm , using a calculated extinction coefficient of 1 . 04 per mg / ml . unless otherwise specified , sds - page analysis of reduced ( 4 % β - mercaptoethanol ) and non - reduced protein samples was performed using a 10 % nupage gel ( invitrogen , carlsbad , calif .) and mark12 ( invitrogen ) as mw markers . following electrophoretic separation , proteins were visualized by coomassie brilliant blue staining . n - terminal sequence analysis of extracted protein bands were performed using a hewlett - packard g1000a instrument ( hewlett - packard , palo alto , calif .). analytical sec was performed on a superdex 75 hr 10 / 30 column ( ge healthcare life sciences ) equilibrated with mbs . mw calibration of the column was performed using the lmw gel filtration calibration kit ( ge healthcare life sciences ). for peptide mass fingerprint ( pmf ) analysis by matrix assisted laser desorption / ionization time - of - flight ( maldi - tof ) mass spectrometry ( ms ), sample preparation of rpc - purified protein in solution , including reduction , alkylation and trypsin digestion , was performed essentially as described [ 10 ] using a bruker daltonics autoflex 2 instrument ( bruker daltonics , bremen , germany ). tandem ms ( ms / ms ) analysis was performed to identify selected peptides . to identify proteins matching the pmf and ms / ms results obtained , the msdb database was searched using a mascot server ( matrixscience , london , uk ). in - gel trypsin digestion of individual protein bands from sds - page was performed essentially according to shevchenko et al . [ 11 ] sample preparation and peptide mass fingerprinting was performed as described above . total rna was prepared from a lateral segment of dog tongue using the rnaqueous kit ( ambion , austin , tex .). polyadenylated rna was isolated from total rna using the mrna purification kit ( ge healthcare life sciences ) and first strand cdna was prepared using the first - strand cdna synthesis kit ( ge healthcare life sciences ). 3 ′ race was performed according to frohman [ 12 ], using the nested forward oligonucleotide primers 5 ′- atgaagatcctactgttgtgtc - 3 ′ ( seq id no : 5 ) and 5 ′- cagctaccccttcctaatg - 3 ′ ( seq id no : 6 ), both carrying a terminal ndei restriction site for cloning . seven independent 3 ′ race clones were isolated and sequenced in their entirety whereby the can f 4 coding sequence could be defined . dna sequencing was performed using a an applied biosystems 3130 genetic analyzer ( applied biosystems , foster city , calif .). dna and amino acid sequence analyses and calculations were performed using programs of the gcg wisconsin package ( accelrys , san diego , calif ., usa ). signal peptide prediction was performed using signalp ( www . cbs . dtu . dk / services / signalp ). for the purpose of protein expression , the can f 4 coding sequence was amplified using primers 5 ′- gtcagcatatgcagctaccccttcctaatg - 3 ′ ( seq id no : 7 ) and 5 ′- actgactcgagttcatggttgggacagttgtc - 3 ′ ( seq id no : 81 and cloned between the ndei and xhoi sites of vector pet - 23a (+) ( novagen , madison , wis ., usa ). recombinant can f 4 was produced as a c - terminally hexahistidine tagged protein in e . coli bl21 , using a 3 - l bioreactor ( belach bioteknik , solna , sweden ). for rcan f 4 purification , harvested cells were resuspended in 20 mm tris - hcl ph 8 . 0 and lysed by passing the suspension through an emulsiflex c5 homogenizor ( avestin inc ., canada ) at 15 000 - 17 000 kpa . after clarification by centrifugation and filtration , the supernatant was applied to a chelating sepharose ff column ( ge healthcare life sciences ), charged with niso4 . column washing was performed with 20 mm imidazole in 20 mm tris - hcl ph 8 . 0 , 0 . 15 m nacl and the recombinant protein eluted in a linear 20 - 500 mm gradient of imidazole in the same buffer . further purification of the recombinant protein was performed by aiec in 20 mm tris - hcl ph 8 . 0 using a q sepharose ff column ( ge healthcare life sciences ). the protein was eluted using a linear 0 - 0 . 5 m nacl gradient and fractions were pooled according to sds - page results . the protein concentration of the final preparation was determined from absorbance at 280 nm , using a calculated extinction coefficient of 0 . 78 per mg / ml . the intactness of the recombinant protein was confirmed by n - terminal sequencing . sera from 37 dog allergic subjects from spain ( n = 23 ), sweden ( n = 10 ) and north america ( n = 4 ) were used in the study ( table 2 ). all patients had a doctor &# 39 ; s diagnosis of dog allergy , with symptoms such as asthma , rhinoconjunctivitis and urticaria , a positive skin prick test and a positive immunocap test ( phadia , uppsala , sweden ) for specific ige to dog dander extract . for control purposes , sera of 44 pollen allergic subjects without diagnosed or reported symptoms of dog allergy were used . all samples and clinical data were collected under the approval of the local ethics committee at each center . ige antibody binding activity of purified recombinant and natural allergens were examined using regular and experimental immunocap ™ tests ( phadia ). experimental immunocap tests were prepared as described [ 13 ]. assay specificity of experimental tests was assessed using a negative control serum , spiked with myeloma ige to a final concentration of 0 , 1000 or 3000 ku / l . an ige inhibition experiment was performed by preincubating serum samples with recombinant can f 4 at a final concentration of 100 μg / ml prior to measurement of ige antibody binding to the 23 kda cow dander protein immobilized on immunocap solid phase . results were calculated as mean values of duplicate determinations . serum samples from 10 dog allergic subjects were subjected to ige immunoblot analysis using non - reduced dog dander extract ( fig1 ). for the same sera , immunocap data on specific ige to rcan f 1 , rcan f 2 , ncan f 3 and rcan f 5 were available for comparison . the immunoblot analysis revealed ige binding to at least 8 different protein bands . of these , bands in the 23 kda region correlated with immunocap ige binding to rcan f 1 and rcan f 2 ( subjects 5 , 24 , 31 and 33 ), bands in the 28 kda region with rcan f 5 reactivity ( subjects 1 , 3 , 5 , 15 , 31 , 33 , and 36 ) and bands at about 60 kda with ncan f 3 reactivity ( subjects 9 , 15 and 33 ). immunoblot ige binding to a 16 kda band was detected with sera of 4 subjects ( 1 , 9 , 14 and 36 ) of which one ( subject 14 ) gave rise to a particularly intense signal . the fact that this serum showed no ige binding to any of rcan f 1 , rcan f 2 , ncan f 3 or rcan f 5 in immunocap analysis suggested that the 16 kda band represented a new allergen . purification of the 16 kda dog dander protein identified by immunoblotting a three step purification process comprising sec followed by aiec and rpc yielded a preparation of a 16 kda protein of 90 - 95 % purity ( fig2 ). n - terminal sequence analysis of this protein yielded no reading , suggesting that its n - terminus was blocked . to overcome this problem , the protein was reduced , alkylated and digested with trypsin to generate internal peptides . four such tryptic peptides could be separated by rpc and analysed by n - terminal sequencing , producing the sequences shown in fig3 . blast searches with these peptide sequences did not identify an exact match to any known dog protein . however , imperfect matches of peptides 1 and 4 to database entries representing odorant binding proteins from cow and swine suggested that the purified dog protein might be related to this protein family . further , a tblastn search of the dog genome database with peptide 4 , using reduced stringency settings , resulted in matches to conceptual translations of three adjacent segments in a terminal region of the x chromosome . pmf analysis of the purified protein did not result in any significant match to a known protein sequence . the ige binding activity of the 16 kda dog dander protein was assessed by immunocap immunoassay , following a final ciec polishing step to further increase the purity of the preparation . sera of 37 dog allergic individuals were analysed and the ige binding to the purified protein in immunocap correlated well with the detection of the 16 kda band in immunoblot analysis of dog dander extract , indicating that the purified protein represented the 16 kda band observed in immunoblotting . unexpected help towards identification and cloning of the 16 kda dog allergen came from a separate line of experiments in our laboratory , on epithelial allergens from other animal species . one serum , which was shared between the studies on dog and cow allergens , showed significant ige antibody binding to both dog and cow dander extract while being non - reactive in immunocap to all of rcan f 1 , rcan f 2 , ncan f 3 and rcan f 5 , raising the possibility that this serum might define a novel dog allergen . a 23 kda protein in cow dander extract was found to bind ige from this serum and could be highly enriched by sec ( fig8 , a ). the adjacent peak in the sec fractionation was found to contain the major allergen bos d 2 , a 19 kda lipocalin , as confirmed by pmf analysis ( not shown ). further purification of these proteins by hic and aiec ( not shown ) resulted in highly pure preparations ( fig8 , b ), suitable as immunoassay reagents . the 23 kda protein was identified by a significant match ( p & lt ; 0 . 05 ) of pmf data to database entry xp — 581277 ( as depicted by seq id no : 4 ), representing a bovine lipocalin annotated as “ similar to odorant binding protein ”. further , n - terminal sequence analysis of the 23 kda protein revealed the sequence eaqgdasqft , matching residues 19 - 28 of the same database entry and thus corroborating the pmf match . this protein is hereafter referred to as bos d 23k . experimental immunocap tests prepared with both bos d 23k and bos d 2 were used to assess correlation in ige binding to the 16 kda dog dander protein . sera from 37 dog allergic subjects were analysed and the results are shown in fig9 . while the 16 kda dog dander protein and bos d 2 showed no correlation in ige binding ( r = 0 . 01 ), a significant correlation was observed to bos d 23k ( r = 0 . 89 ), suggesting cross - reactivity and structural similarity between the dog allergen and bos d 23k . cloning and sequence analysis of the 16 kda dog dander protein potential sequence similarity between the 16 kda dog dander protein and bos d 23k prompted a database search aimed at identifying the dog protein or its gene sequence . a blastn search of a dog genome database with the sequence of the bovine protein ( xp — 581277 ) resulted in a match to the translation of nucleotide positions 338441 - 338307 of ace no . aaex02025758 , a 431454 bp segment of the dog genome sequence [ 14 ]. interestingly , the theoretical translation of this genomic region contained a perfect 5 - residue match of amino acid residues 9 - 13 of the reported n - terminal sequence of can f 4 [ 9 ]. further upstream ( 339006 - 338926 ), a nucleotide sequence encoding amino acids matching residue 1 - 8 of the can f 4 sequence , as well as a putative signal peptide , were found . for the purpose of cloning a cdna corresponding to the identified segment of aaex02025758 , assumed to encode can f 4 , a 3 ′ race experiment was performed . oligonucleotide primers based on the genomic sequence encoding the first part of the putative signal peptide and a stretch following the predicted cleavage site were used , together with first strand cdna prepared from poly - a rna of dog tongue as template . a distinct amplification product was obtained which was cloned , analysed and subsequently used for protein expression experiments . the complete dna sequence and amino acid translation of the cloned cdna is shown in fig4 . an open reading frame encoding 174 amino acid residues was identified , of which the first 16 residues were predicted by signalp to form a signal peptide . the mature protein deduced from the cloned cdna consisted of 158 amino acid residues , including two cysteins , and had a predicted molecular mass of 17 . 6 kda and an isoelectric point of 6 . 53 . to assess the presence of sequence diversity , seven independent 3 ′ race clones were isolated and sequenced . a few nucleotide substitutions were found among the clones ( not shown ) but no one causing variation in amino acid sequence of the protein . in all clones analyzed , the stop codon terminating the open reading frame was followed by a 213 - nucleotide untranslated segment preceding a poly - a tract . the predicted signal peptide cleavage site between residue 16 and 17 would make the glutamine residue at position 17 the first in the mature protein . consistent with our failure to obtain a sequence reading from the intact protein , an n - terminal glutamine residue may undergo pyroglutamate cyclisation which is known to cause blockage to n - terminal sequencing by edman degradation ( 15 ). further , upon re - examination of the pmf results for the native protein , a tryptic fragment with a mass of 1759 . 98 da was identified , exactly corresponding to that predicted for residue 17 - 32 of the predicted primary translation product , modified by pyroglutamate cyclisation . in addition , tryptic fragments matching amino acid residues 33 - 51 , 56 - 63 , 66 - 73 and 95 - 104 were identified . the amino acid sequence encoded by the cloned cdna further contained all four tryptic peptide sequencens obtained from the purified natural 16 kda protein ( fig3 ), with exact agreement for peptides 2 - 4 and two conservative amino acid substitutions in peptide 1 . in total , protein fragments identified by pmf and peptide sequencing represented a coverage of 58 % of the deduced amino acid sequence of can f 4 ( table 1 ). taken together , the results provide strong evidence that the purified 16 kda allergen and the protein encoded by the cloned cdna both represent dog allergen can f 4 . by aligning the cloned cdna sequence with the genomic sequence of ace . no . aaex02025758 , the coding part of the can f 4 gene was found to span a total of 5916 bp and comprise 6 exons : 339006 - 338929 ( exon 1 ), 338447 - 338307 ( exon 2 ), 335150 - 335082 ( exon 3 ), 334581 - 334468 ( exon 4 ), 333609 - 333511 ( exon 5 ) and 333114 - 333091 ( exon 6 ). the exact splice point between exons 3 and 4 could not be unambiguously deduced from the sequence and those exon boundaries may instead be 335150 - 335079 and 334578 - 334468 , respectively . the amino acid sequences deduced from the can f 4 cdna and gene segments deviated from each other at 5 positions . at position 20 of the mature can f 4 sequence , an isoleucine residue in the cdna translation corresponded to a valine residue in the gene - derived sequence , at position 30 an aspartic acid residue corresponded to a glutamic acid residue , at position 38 a methionine residue corresponded to a leucine residue , at position 51 a serine residue corresponded to a leucine residue and at position 81 a tyrosine residue corresponded to either an aspartic acid or a cysteine residue , depending on the exact splice point between exons 3 and 4 . in addition to the can f 4 gene sequence described above , database entry aaex02025758 was found to contain two other can f 4 related segments , explaining the three matches obtained in the dog genome tblastn search with the tryptic peptide 4 sequence . the three can f 4 related segments were arranged in tandem at a distance of 5 . 4 and 6 . 7 kb from each other , with the one described above located the furthest downstream . the can f 4 related segment located in the middle had an exon / intron structure similar to that of the can f 4 gene described above and its deduced amino acid sequence differed at 24 positions as compared to the can f 4 gene and at 28 positions as compared to the can f 4 cdna . interestingly , it agreed completely with the sequence of tryptic peptide 1 which deviated at two positions from the cdna - encoded amino acid sequence . in the can f 4 related segment located furthest upstream , no sequence corresponding to exon 4 could be identified . can f 4 belongs to the lipocalin superfamily and displays 38 - 39 % sequence identity to bovine ( xp — 581277 ) and porcine [ 16 ] ( np — 998961 ) odorant binding proteins . an amino acid sequence alignment of can f 4 and bos d 23k is shown in fig1 . it can be noted that , apart from a scattered pattern of similar or identical residues , the c - terminal portion of the proteins is highly conserved , as well as both cystein residues ( position 62 and 154 in can f 4 ). other conserved elements , characteristic of lipocalins [ 17 ], include a gxw motif near the n - terminus ( position 13 - 15 ), a glycine residue ( position 118 ). another motif typical of lipocalins [ 17 ], yxxxyxg , is present in bos d 23k ( position 74 - 80 ) but only partially conserved in can f 4 . interestingly , despite belonging to the same protein family , can f 4 showed only 24 % and 26 % amino acid sequence identity to can f 1 and can f 2 , respectively . recombinant can f 4 , excluding the signal peptide , was expressed as a c - terminally hexahistidine tagged protein in e . coli . the recombinant protein was purified from the soluble cell fraction by imac and aiec . to assess the aggregation state of the recombinant protein , a sample of the preparation was subjected to analytical sec , in parallel with natural purified natural can f 4 . as shown in fig5 , a , the chromatogram was dominated by a single symmetrical peak in both cases , corresponding to a mw of approximately 16 . 5 kda , as defined by the calibrators used . in sds - page ( fig5 , b ), both natural and recombinant can f 4 exhibited a shift in migration rate upon reduction , suggesting that the two cystein residues present in the protein were engaged in a disulfide bridge . it should be noted that the apparent molecular weight of can f 4 indicated by the inititial immunoblot analysis ( fig1 ) was affected by the fact the dog dander extract used for the analysis was applied to the separation gel in a non - reduced form , leading to an underestimation in relation to the size of the protein indicated by subsequent analyses of reduced samples of the purified protein . the size increment displayed by rcan f 4 in comparison to the natural protein was consistent with the addition of a c - terminal hexahistidine tag in the recombinant protein and the nearly complete retention of the initiator methionine residue . analysis of specific ige antibody binding to rcan f 4 in dog allergic subjects the immunological activity of recombinant can f 4 was assessed in comparison to the natural protein , purified from dog dander . each protein was immobilized on immunocap ™ solid phase and their ige antibody binding examined using serum samples from 37 dog allergic subjects ( fig5 , c ). the two datasets showed a very strong correlation ( r = 0 . 99 ), demonstrating that the recombinant allergen closely resembled its natural counterpart with respect to ige antibody binding determinants . the frequency and magnitude of ige antibody reactivity to can f 4 in comparison to other dog dander allergens is illustrated in fig6 . a more comprehensive dataset on the dog allergic subjects and their sensitization pattern is present in table 2 . among the 37 sera analysed , 13 ( 35 %) displayed ige antibody binding to can f 4 . one of the 13 can f 4 reactive sera showed ige binding to none of the other dog allergens tested . no correlation in ige binding could be observed between can f 4 and can f 1 or can f 2 , consistent with their highly divergent primary structure . assay specificity of the experimental can f 4 immunocap test was demonstrated by the low results obtained upon testing a negative control serum spiked with up to 3000 ku / l of myeloma ige ( table 3 ). to examine the occurrence of can f 4 specific ige antibody in atopic individuals not allergic to dog , sera of 44 pollen allergic subjects without diagnosed or reported symptoms of dog allergy were tested with all dog allergen components available and a range of pollen extracts ( table 4 ). seven of those sera ( 16 %) showed a positive response to dog dander extract , with one exception at levels of 2 ku a / l or below , one of which also showed ige antibody binding to rcan f 4 . no serum negative to dog dander showed a positive response to rcan f 4 . to study the extent of cross - reactivity between can f 4 and bos d 23k indicated by their correlation in ige antibody binding , an ige inhibition experiment was performed with three sera reactive to the bovine protein . the purified bovine protein was attached to immunocap solid phase and ige binding from sera preincubated either with rcan f 4 or buffer was measured . as can be seen in fig7 , rcan f 4 almost completely (≧ 96 %) inhibited the ige binding to the cow dander protein in all three sera , demonstrating that all determinants of ige binding to the bovine protein recognised by the sera used in the experiment were shared with rcan f 4 . as described in this application , we have isolated , cloned and characterized a ige binding protein from dog dander identified as can f 4 . prior to this work , only a 13 - residue n - terminal sequence of this allergen was known . can f 4 was found to belong to the diverse lipocalin superfamily and showed similarity to odorant binding proteins of other species , including cow and swine . can f 4 was found to cross - react with a 23 kda odorant binding protein purified from cow dander . purification of natural can f 4 resulted in a very poor yield due to both scarcity of the protein in the dander extract and low chromatographic resolution . the presence of can f 4 in several peaks in rpc and the broad can f 4 peak in the ciec step indicated some degree of heterogeneity of the protein . while several explanations for this behaviour are possible , including protein modification , partial degradation and the presence of isoforms , n - linked glycosylation is not likely as the sequence of can f 4 contained no potential site for n - glycan attachment . among the four tryptic peptide sequences obtained , one showed deviations at two positions as compared to the amino acid sequence deduced from the cdna clones and the genomic segment identified . thus , it is possible that isoform variation may have contributed to the heterogeneity observed during purification of the natural protein , even though no evidence of sequence variability was obtained in the dna sequencing of seven independent cdna clones . additionally , tryptic fragment sequencing and maldi - tof analysis together confirmed 58 % of the amino acid sequence deduced from the cdna clones , suggesting limited polymorphism . even if it cannot be exclude that at least one of the additional can f 4 related genomic segments identified is expressed and may give rise to a variant form of can f 4 , our only indication of that was the matching of two amino acid residues of tryptic fragment 1 that deviated from the cdna - encoded sequence . no other peptide sequence matched preferentially to the other can f 4 related segment and all cdna clones were clearly derived from the can f 4 gene for which the nucleotide positions are detailed in this application . regardless of possible isoform variation , the recombinant form of can f 4 produced displayed excellent agreement with the purified natural protein , both biochemically and immunologically . the two proteins eluted at exactly the same volume in analytical sec and their ige antibody binding showed a very high correlation . most importantly , no case of ige binding to the natural but not to the recombinant allergen was observed . in fact , somewhat higher ige binding was observed to the experimental tests carrying the recombinant protein but this was most likely due to a lower than optimal coupling concentration of the natural protein , forced by the poor purification yield . the importance of can f 4 as a dog dander allergen was assessed by immunocap testing of sera from 37 dog allergic subjects . in a recent study involving the same population , we reported that 49 % displayed ige antibody binding to can f 1 , 22 % to can f 2 , 16 % to can f 3 and 70 % to can f 5 . in this work , we found that 13 of the 37 subjects ( 35 %) were sensitized to can f 4 . thus , can f 4 was more commonly recognized than both rcan f 2 and ncan f 3 , which appeared as minor allergens in this study population . of the 13 can f 4 reactive sera , one showed ige binding to none of the other dog allergens tested , suggesting that can f 4 may be relevant as an independent sensitizer in dog allergy and an important addition to component - resolved diagnostics . this notion is underpinned by the uniqueness of can f 4 both in sequence and ige binding as compared to can f 1 and can f 2 . the fact that only one of 44 pollen allergic controls without dog allergy showed a weak ige antibody response to can f 4 suggested that ige recognition of this allergen does not frequently occur as a result of other aeroallergen sensitizations . in comparison to dog dander extract , cow dander extract appeared to contain much higher amounts of allergens and therefore gave more satisfactory purification results . judging from sds - page analysis of cow dander extract , the dominant proteins in the 10 - 40 kda range were bos d 2 and the 23 kda protein reported here ( bos d 23k ), seemingly being present in similar amounts . bos d 2 has been well established as a major allergen in cow dander , produced as a recombinant allergen and structurally characterized [ 18 - 22 ]. in contrast , much less is known about bos d 23k , although it may be identical to a previously reported ige - binding , 22 kda protein band revealed by immunoblot analysis of cow dander extract [ 21 - 23 ] the 154 - residue amino acid sequence of bos d 23k predicted a molecular mass of 17 . 8 kda , almost exactly the same as that of can f 4 . despite this fact , it showed significantly slower migration rate than can f 4 in sds - page . as the two proteins are related in sequence and may be assumed to have a similar fold , a difference in glycosylation would be a likely explanation for the observed electrophoretic disparity . indeed , inspection of the sequences reveals that bos d 23k contains a potential n - glycosylation site at asparagine residue 45 of the mature protein whereas none is present in the sequence of can f 4 . among known dog and cow dander allergens , serum albumin ( can f 3 and bos d 6 , respectively ) represents the only well recognised cross - reactivity to allergens of other species . the cross reactivity between can f 4 and bos d 23k thus provides a novel immunological association between dog and cow dander allergens . given their relatively low level of overall sequence identity , 37 %, the extensive cross - reactivity between the two proteins appears somewhat surprising . however , it is possible that the observed cross - reactivity is due to portions of the proteins having higher sequence similarity than the overall score . in particular , the c - terminal part of the two proteins is significantly conserved , with the 20 - residue segment between position 136 / 133 and 155 / 152 showing 75 % identity . despite the demonstrable cross - reactivity between the two proteins , the ige response was higher to can f 4 than to bos d 23k for all sera tested , implying that can f 4 rather than bos d 23k was the primary sensitizer in our study population . in conclusion , this application reports the cloning and characterization of dog allergen can f 4 . recombinant can f 4 will be important in component resolved diagnostics in dog allergy and its cross - reactivity with a highly abundant bovine dander protein raises the possibility of a linkage between allergy to dog and cow dander . 1 . custovic a , green r , taggart s c o , smith a , pickering c a c , chapman m d , et al . domestic allergens in public places ii : dog ( can f 1 ) and cockroach ( bla g 2 ) allergens in dust and mite , cat , dog and cockroach allergens in the air in public buildings . clinical and experimental allergy 1996 ; 26 : 1246 - 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