the present invention is directed against the chronic rejection of xenografts for implantation into humans . accordingly , the articular cartilage xenograft produced in accordance with the method of the invention is substantially non - immunogenic , while generally maintaining the mechanical properties of native articular cartilage . while the articular cartilage may undergo some shrinkage during processing , an articular cartilage xenograft prepared in accordance with the invention will have the general appearance of a native articular cartilage . the articular cartilage xenograft may also be cut into segments , each of which may be implanted into ajoint of a recipient as set forth below . the invention provides , in one embodiment , a method for preparing or processing a xenogeneic articular cartilage for engraftment into humans . the articular cartilage may be harvested from any non - human animal to prepare the xenografts of the invention . articular cartilage from transgenic non - human animals , or from genetically altered non - human animals may also be used as xenografts in accordance with the present invention . preferably , bovine , ovine , or porcine knee joints serve as sources of the articular cartilage used to prepare the xenografts . more preferably , immature pig , calf or lamb knee joints are the sources of the articular cartilage , since the cartilage of younger animals may be inherently more elastic and engraftable than that of older animals . most preferably , the age of the source animal is between six and eighteen months at time of slaughter . in the first step of the method of the invention , an intact articular cartilage is removed from a joint of a non - human animal . any joint may serve as the source of articular cartilage . preferably articular cartilage from a corresponding donor joint is used to make the articular cartilage xenograft of the invention . for example , articular cartilage from a femuro - tibial ( stifle ) joint is used to make an articular cartilage xenograft for implantation into a knee . similarly , articular cartilage from a donor animal &# 39 ; s hip joint is used to make an articular cartilage xenograft for a human hip joint . the joint which serves as the source of the articular cartilage should be collected from freshly killed animals and preferably immediately placed in a suitable sterile isotonic or other tissue preserving solution . harvesting of the joints should occur as soon as possible after slaughter of the animal and preferably should be performed in the cold , i . e ., in the approximate range about 5 ° c . to about 20 ° c ., to minimize enzymatic degradation of the articular cartilage tissue . the articular cartilage is harvested from the joints in the cold , under strict sterile technique . in accordance with the invention , a fine peel of articular cartilage with a small layer of subchondral bone is shaved from the donorjoint to form the xenograft . the xenograft is then washed in about ten volumes of sterile cold water to remove residual blood proteins and water soluble materials . the xenograft is then immersed in alcohol at room temperature for about five minutes , to sterilize the tissue and to remove non - collagenous materials . in a preferred form of the invention , a xenograft appears as a hyaline tissue supported on a bone substrate , having generally a spherical - shaped principal surface on the top side ( the &# 34 ; superior surface &# 34 ;), with the under surface of bone ( the &# 34 ; inferior surface &# 34 ;) being rough . after alcohol immersion , the xenograft may be directly implanted a prepared site at an articular surface of a human patient . alternatively the xenograft may be subjected to at least one of the following treatments : radiation treatment , treatment with alcohol , ozonation , one or more cycles of freezing and thawing and / or treatment with a chemical cross - linking agent ,. when more than one of these treatments is applied to the xenograft , the treatments may occur in any order . in one embodiment of the method of the invention , the xenograft may be treated by exposure to ultraviolet radiation for about fifteen minutes or gamma radiation in an amount of about 0 . 5 to 3 megarad . in another embodiment , the xenograft may be treated by again being placed in an alcohol solution . any alcohol solution may be used to perform this treatment . preferably , the xenograft is placed in a 70 % solution of isopropanol at room temperature . in still another embodiment , the xenograft may be subjected to ozonation . in a further embodiment of the method of the invention , the xenograft may be treated by freeze / thaw cycling . for example , the xenograft may be frozen using any method of freezing , so long as the xenograft is completely frozen , i . e ., no interior warm spots remain which contain unfrozen tissue . preferably , the xenograft is dipped into liquid nitrogen for about five minutes to perform this step of the method . more preferably , the xenograft is frozen slowly by placing it in a freezer . in the next step of the freeze / thaw cycling treatment , the xenograft is thawed by immersion in an isotonic saline bath at room temperature ( about 25 ° c .) for about ten minutes . no external heat or radiation source is used , in order to minimize fiber degradation . in yet a farther embodiment , the xenograft may optionally be exposed to a chemical agent to tan or crosslink the proteins within the extracellular matrix , to further diminish or reduce the immunogenic determinants present in the xenograft . any tanning or crosslinking agent may be used for this treatment , and more than one crosslinking step may be performed or more than one crosslinking agent may be used in order to ensure complete crosslinking and thus optimally reduce the immunogenicity of the xenograft . for example , aldehydes such as glutaraldehyde , formaldehyde , adipic dialdehyde , and the like , may be used to crosslink the collagen within the extracellular matrix of the xenograft in accordance wvith the method of the invention . other suitable crosslinking agents include aliphatic and aromatic diamines , carbodiimides , diisocyanates , and the like . when glutaraldehyde is used as the crosslinking agent , for example , the xenograft may be placed in a buffered solution containing about 0 . 05 to about 5 . 0 % glutaraldehyde and having a ph of about 7 . 4 . any suitable buffer may be used , such as phosphate buffered saline or trishydroxymethylaminomethane , and the like , so long as it is possible to maintain control over the ph of the solution for the duration of the crosslinking reaction , which may be from one to fourteen days , and preferably from three to five days . alternatively , the xenograft can be exposed to a crosslinking agent in a vapor form , including , but not limited to , a vaporized aldehyde crosslinking agent , such as , for example , vaporized formaldehyde . the vaporized crosslinking agent can have a concentration and a ph and the xenograft can be exposed to the vaporized crosslinking agent for a period of time suitable to permit the crosslinking reaction to occur . for example , the xenograft can be exposed to vaporized crosslinking agent having a concentration of about 0 . 05 to about 5 . 0 % and a ph of about 7 . 4 , for a period of time which can be from one to fourteen days , and preferably from three to five days . exposure to vaporized crosslinking agent can result in reduced residual chemicals in the xenograft from the crosslinking agent exposure . the crosslinking reaction should continue until the immunogenic determinants are substantially removed from the xenogeneic tissue , but the reaction should be terminated prior to significant alterations of the mechanical properties of the xenograft . when diamines are also used as crosslinking agents , the glutaraldehyde crosslinking should occur after the diamine crosslinking , so that any unreacted diarnines are capped . after the crosslinking reactions have proceeded to completion as described above , the xenograft should be rinsed to remove residual chemicals , and 0 . 01 - 0 . 05 m glycine may be added to cap any unreacted aldehyde groups which remain . in addition to or in lieu of the above treatments , the xenograft can be subjected to a cellular disruption treatment to kill the xenograft &# 39 ; s fibrochondrocytes , which precedes or follows digestion of the xenograft with glycosidases to remove surface carbohydrate moieties from the xenograft . the glycosidase digestion in turn can be followed by linkage with capping molecules to cap surface n - acetyllactosamine ends of carbohydrate chains of the xenograft . in an embodiment of this method of the invention , the xenograft is subjected to a treatment to kill the fibrochondrocytes of the articular cartilage prior to in vitro digestion of the xenograft with glycosidases . typically after surface carbohydrate moieties have been removed from nucleated cells and the extracellular matrix , nucleated , i . e ., living cells reexpress the surface carbohydrate moieties . reexpression of antigenic moieties of a xenograft can provoke continued immunogenic rejection of the xenograft . in contrast , non - nucleated , i . e ., dead cells , are unable to reexpress surface carbohydrate moieties . removal of antigenic surface carbohydrate moieties from the non - nucleated cells and extracellular matrix of a xenograft substantially permanently eliminates antigenic surface carbohydrate moieties as a source of immunogenic rejection of the xenograft . accordingly , in the above - identified embodiment , the xenograft of the present invention is subjected to freeze / thaw cycling as discussed above to disrupt , i . e ., to kill the fibrochondrocytes of the articular cartilage . alternatively , the xenograft of the present invention is treated with gamma radiation having an amount of 0 . 2 megarad up to about 3 megarad . such radiation kills the fibrochondrocytes and sterilizes the xenograft . once killed , the fibrochondrocytes are no longer able to reexpress antigenic surface carbohydrate moieties such α - gal epitopes which are factors in the immunogenic rejection of the transplanted xenografts . either before or after the fibrochondrocytes are killed , the xenograft is subjected to in vitro digestion of the xenograft with glycosidases , and specifically galactosidases , such as α - galactosidase , to enzymatically eliminate antigenic surface carbohydrate moieties . in particular , α - gal epitopes are eliminated by enzymatic treatment with α - galactosidases , as shown in the following reaction : ## str1 ## the n - acetyllactosamine residues are epitopes that are normally expressed on human and mammalian cells and thus are not immunogenic . the in vitro digestion of the xenograft with glycosidases is accomplished by various methods . for example , the xenograft can be soaked or incubated in a buffer solution containing glycosidase . in addition , the xenograft can be pierced to increase permeability , as further described below . alternatively , a buffer solution containing the glycosidase can be forced under pressure into the xenograft via a pulsatile lavage process . elimination of the α - gal epitopes from the xenograft diminishes the immune response against the xenograft . the α - gal epitope is expressed as 1 × 10 6 - 35 × 10 6 epitopes per cell , as well as on macromolecules such as proteoglycans of the extracellular matrix , in nonprimate mammals and in new world monkeys ( monkeys of south america ). u . galili et al ., man , apes , and old world monkeys differ from other mammals in the expression of igalactosyl epitopes on nucleated cells , 263 j . biol . chem . 17755 ( 1988 ). this epitope is absent in old world primates ( monkeys of asia and africa and apes ) and humans , however . id . anti - gal is produced in humans and primates as a result of an immune response to α - gal epitope carbohydrate structures on gastrointestinal bacteria . u . galili et al ., interaction between human natural anti - α - galactosyl immunoglobulin g and bacteria of the human flora , 56 infect . immun . 1730 ( 1988 ); r . m . hamadeh et al ., human natural anti - gal igg regulates alternative complement pathway activation on bacterial surfaces , 89 j . clin . invest . 1223 ( 1992 ). since nonprimate mammals produce a - gal epitopes , xenotransplantation of xenografts from these mammals into primates results in rejection because of primate anti - gal binding to these epitopes on the xenograft . the binding results in the destruction of the xenograft by complement fixation and by antibody dependent cell cytotoxicity . u . galili et al ., interaction of the natural anti - gal antibody with α - galactosyl epitopes : a major obstacle for xenotransplantation in humans , 14 immunology today 480 ( 1993 ); m . sandrin et al ., anti - pig igm antibodies in human serum react predominantly with gala1 - 3gal epitopes , 90 proc . natl . acad . sci . usa 11391 ( 1993 ); h . good et al ., identification of carbohydrate structures which bind human anti - porcine antibodies : implications for discordant grafting in man . 24 transplant . proc . 559 ( 1992 ); b . h . collins et al ., cardiac xenografts between primate species provide evidence for the importance of the α - galactosyl determinant in hyperacute rejection , 154 j . immunol . 5500 ( 1995 ). furthermore , xenotransplantation results in major activation of the immune system to produce increased amounts of high affinity anti - gal . the cartilage xenografts of the present invention are particularly well suited to in vitro enzymatic elimination of the ( x - gal epitopes , however . in contrast to organs and other tissues , the cartilage extracellular matrix undergoes extremely slow turnover . moreover , once the fibrochondrocytes are killed , these non - nucleated cells are prevented from reexpressing the α - gal epitopes , as discussed above . accordingly , the substantial elimination of α - gal epitopes from cells and the extracellular matrix , and the prevention of reexpression of cellular α - gal epitopes can diminish the immune response against the xenograft associated with anti - gal antibody binding with α - gal epitopes . following treatment with glycosidase , the remaining carbohydrate chains ( e . g ., glycosaminoglycans ) of the xenograft are optionally treated with capping molecules to cap at least a portion of the remaining carbohydrate chains . treatment with capping molecules is applicable to both glycosidase - treated and non - glycosidase - treated xenografts , however . for example , xenografts from knock out animals which may lack α - gal epitopes may be treated with capping molecules to cap carbohydrate moieties on the xenograft , thereby reducing the xenograft &# 39 ; s immunogenicity . examples of capping molecules used in the present invention include fucosyl and n - acetyl glucosamine . prior to treatment , the outer surface of the xenograft may optionally be pierced to increase permeability to agents used to render the xenograft substantially non - immunogenic . a sterile surgical needle such as an 18 gauge needle may be used to perform this piercing step , or , alternatively a comb - like apparatus containing a plurality of needles may be used . the piercing may be performed with various patterns , and with various pierce - to - pierce spacings , in order to establish a desired access to the interior of the xenograft . piercing may also be performed with a laser . in one form of the invention , one or more straight lines of punctures about three millimeters apart are established in the outer surface of the xenograft . prior to implantation , the articular cartilage xenograft of the invention may be treated with limited digestion by proteolytic enzymes such as ficin or trypsin to increase tissue flexibility or coated with anticalcification agents , antithrombotic coatings , antibiotics , growth factors , or other drugs which may enhance the incorporation of the xenograft into the recipient knee joint . the articular cartilage xenograft of the invention may be further sterilized using known methods , for example , with additional glutaraldehyde or formaldehyde treatment , ethylene oxide sterilization , propylene oxide sterilization , or the like . the xenograft may be stored frozen until required for use . the articular cartilage xenograft of the invention , or a segment thereof , may be implanted into damaged human joints by those of skill in the art using known arthroscopic surgical techniques . specific instruments for performing arthroscopic techniques are known to those of skill in the art , which ensure accurate and reproducible placement of articular cartilage implants . the underlying bone bed of the recipient joint is prepared with a bone burr to produce a cancellous bleeding bed . grafting can involve either the entire articular surface or a portion of the articular surface . the substantially non - immunogenic articular cartilage xenograft of the invention is applied to the recipient joint as a cover , which is held in place by one or more suture anchors , absorbable pins , screws , staples , and the like . a fibrin clot may also be used to hold the substantially non - immunogenic articular cartilage xenograft in place . this invention is further illustrated by the following examples which should not be construed as limiting . the contents of all references and published patents and patent applications cited throughout the application are hereby incorporated by reference . assessment of primate response to implanted porcine cartilage treated with α - galactosidase in this example , porcine cartilage implants are treated with α - galactosidase to eliminate α - galactosyl epitopes , the implants are transplanted into cynomolgus monkeys , and the primate response to the cartilage implants is assessed . porcine stifle joints are sterilely prepared and the articular cartilage and surrounding attached soft tissues surgically removed . the cartilage specimens are washed for at least five minutes with an alcohol , such as ethanol or isopropanol , to remove synovial fluid and lipid soluble contaminants . the cartilage specimens are frozen at a temperature ranging from about - 35 ° c . to about - 90 ° c ., and preferably at a temperature up to about - 70 ° c ., to disrupt , that , is to kill , the specimens &# 39 ; fibrochondrocytes . the cartilage is cut into two portions . the first portion is immersed in a buffer solution containing α - galactosidase at a predetermined concentration . the specimens are allowed to incubate in the buffer solution for a predetermined time period at a predetermined temperature . the second cartilage portion is incubated under similar conditions as the first cartilage portion in a buffer solution in the absence of α - galactosidase and serves as the control . at the end of the incubation , the cartilage is washed under conditions which allow the enzyme to diffuse out . assays are performed to confirm the complete removal of the α - gal epitopes . each cartilage sample is implanted in the supra patellar pouch of six cynomolgus monkeys . with the animals under general inhalation anesthesia , an incision of about 1 cm is made directly into the supra patellar pouch at the superior medial border of the patella extending proximally . a piece of the porcine cartilage of about 0 . 5 cm to about 1 cm in length is placed into the pouch with a single 3 - 0 nylon stitch as a marking tag . the procedure is performed under sterile surgical technique , and the wounds are closed with 3 - 0 vicryl or a suitable equivalent known to those of ordinary skill in the art . the animals are permitted unrestricted cage activity and monitored for any sign of discomfort , swelling , infection , or rejection . blood samples ( e . g ., 2 ml ) are drawn periodically ( e . g ., every two weeks ) for monitoring of antibodies . the occurrence of an immune response against the xenograft is assessed by determining anti - gal and non - anti - gal anti - cartilage antibodies ( i . e ., antibodies binding to cartilage antigens other than the α - gal epitopes ) in serum samples from the transplanted monkeys . at least two ml blood samples are drawn from the transplanted monkeys on the day of implant surgery and at periodic ( e . g ., two week ) intervals post - transplantation . the blood samples are centrifuged and the serum samples are frozen and evaluated for the anti - gal and other non - anti - gal anti - cartilage antibody activity . anti - gal activity is determined in the serum samples in elisa with α - gal - bsa as solid phase antigen , according to methods known in the prior art , such as , for example , the methods described in galili et al ., porcine and bovine cartilage transplants in cynomolgus monkey : ii . changes in anti - gal response during chronic rejection , 63 transplantation 645 - 651 ( 1997 ). assays are conducted to determine whether α - galactosidase treated xenografts induce the formation of anti - cartilage antibodies . for measuring anti - cartilage antibody activity , an elisa assay is performed according to methods known in the prior art , such as , for example , the methods described in k . r . stone et al ., porcine and bovine cartilage transplants in cynomolgus monkey : i . a model for chronic xenograft rejection , 63 transplantation 640 - 645 ( 1997 ). the cartilage is optionally explanted at one to two months post - transplantation , sectioned and stained for histological evaluation of inflammatory infiltrates . post - transplantation changes in anti - gal and other anti - cartilage antibody activities are correlated with the inflammatory histologic characteristics ( i . e ., granulocytes or mononuclear cell infiltrates ) within the explanted cartilage , one to two months post - transplantation , using methods known in the art , as , for example , the methods described in k . r . stone et al ., porcine and bovine cartilage transplants in cynomolgus monkey : i . a model for chronic xenograft rejection , 63 transplantation 640 - 645 ( 1997 ). where the cartilage is explanted , the cartilage xenograft is aseptically harvested , using anesthetic procedure , surgical exposure of the joint , removal of the implant and closure of the soft tissue . at the time of the xenograft removal , joint fluid , if present in amounts sufficient to aspirate , is collected from the stifle joints for possible immunologic testing if the gross and histopathologic evaluation of the transplants indicate good performance of the transplanted cartilage . subsequently , the animals are allowed to recover and are monitored closely until the incisions have healed and the gait is normal . the xenograft samples are collected , processed , and examined microscopically . a portion of the implant and surrounding tissue is frozen in an embedding medium for frozen tissue specimens in embedding molds for immunohistochemistry evaluation according to the methods known in the prior art . &# 34 ; tissue - tek ®&# 34 ; o . c . t . compound which includes 10 . 24 % w / w polyvinyl alcohol , 4 . 26 % w / w polyethylene glycol , and 86 . 60 % w / w nonreactive ingredients , and is manufactured by sakura fintek , torrence , calif ., is a non - limiting example of a possible embedding medium for use with the present invention . other embedding mediums known to those of ordinary skill in the art may also be used . the remaining implant and surrounding tissue is collected in 10 % neutral buffered formalin for histopathologic examination . assessment of primate response to implanted cartilage treated with α - galactosidase , fucosyl and fucosyltransferase in this example , porcine cartilage implants are treated with α - galactosidase to eliminate α - gal epitopes , as described in example 1 . the implants are further treated with fucosyl and fucosyl transferase to cap carbohydrate chains with fucosyl . fucosyltransferase facilitates the transfer of fucosyl to the xenograft . the fucosyl links to and thus caps the carbohydrate chains . capping with fucosyl interferes with the ability of the subject &# 39 ; s immune system to recognize the xenograft as foreign . the implants are transplanted into cynomolgus monkeys , and the primate response to the cartilage implants is assessed . porcine cartilage stifle joints are prepared as described in example 1 including the α - galactosidase treatment . prior to implantation into the monkeys , however , the implants are further treated with a predetermined amount of fucosyl and facosyltransferase , at specified concentrations for a predetermined time and at a predetermined temperature , to cap carbohydrate chains with fucosyl . for example , the sample is immersed in a buffer solution at predetermined concentrations of fucosyl and fucosyl transferase . the sample is incubated for a predetermined time period at a temperature . other molecules , such as n - acetyl glucosamine in combination with the corresponding glycosyltransferase , can also be used for capping the carbohydrate chains of the implants . subsequently , the samples are washed to remove the enzyme and implanted into the monkeys , and the occurrence of an immune response against the xenograft is assessed as described above in example 1 . those of skill in the art will recognize that the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof . the presently described embodiments are therefore to be considered in all respects as illustrative and not restrictive , the scope of the invention being indicated by the appended claims rather than by the foregoing description , and all variations of the invention which are encompassed within the meaning and range of equivalency of the claims are therefor intended to be embraced therein .
the invention provides an article of manufacture comprising a substantially non - immunogenic articular cartilage xenograft for implantation into humans . the invention further provides a method for preparing an articular cartilage xenograft by removing at least a portion of an articular cartilage from a non - human animal to provide a xenograft ; washing the xenograft in saline and alcohol ; subjecting the xenograft to at least one treatment selected from the group consisting of exposure to ultraviolet radiation , immersion in alcohol , ozonation , freeze / thaw cycling , and optionally to chemical crosslinking . in addition to or in lieu of the above treatments , the methods include a cellular disruption treatment and glycosidase digestion of carbohydrate moieties of the xenograft followed by treatment of carbohydrate moieties of the xenograft with capping molecules . the invention also provides articles of manufacture produced by one or more of the above - identified methods of the invention . the invention further provides an articular cartilage xenograft for implantation into a human including a portion of an articular cartilage from a non - human animal , wherein the portion includes extracellular matrix and substantially only dead cells . the matrix and dead cells have substantially no surface α - galactosyl moieties and have capping molecules linked to at least a portion of surface carbohydrate moieties . each of the xenografts of the invention is substantially non - immunogenic and has substantially the same mechanical properties as the respective native articular cartilage .