Split tolerance to a composite tissue allograft in a swine model

BACKGROUND

The antigenicity of skin is a major obstacle to expanding human composite tissue transplantation. For example, multiple rejection episodes of the skin have been noted in clinical hand transplant patients. We have previously demonstrated tolerance to vascularized musculoskeletal allografts in major histocompatibility complex (MHC)-matched miniature swine treated with 12 days of cyclosporine. This regimen did not reproducibly lead to tolerance to subsequent frozen donor skin grafts. However, such skin grafts did not have a primary vascular supply. The aim of this study was to determine if tolerance to limb allografts with a vascularized skin component could be achieved with MHC matching and a 12-day course of immunosuppression.

METHODS

Hind limb grafts harvested with a 100 cm(2) cutaneous paddle were transplanted heterotopically into six MHC-matched, minor antigen-mismatched miniature swine. All animals received a 12-day course of cyclosporine. One control animal was not immunosuppressed. Grafts were evaluated with biweekly biopsies and tissue viability determined by histologic analysis. To test for sensitization, frozen donor skin grafts were applied to all animals that survived to postoperative day 100.

RESULTS

All treated animals (n=6) were tolerant to their musculoskeletal allografts at the time of necropsy (>100 days) regardless of the status of the epidermis. One animal demonstrated tolerance to the skin for more than 180 days. The other five animals demonstrated prolonged survival of the epidermal portion of the graft. The control animal rejected the graft epidermis at 10 days postoperatively. Frozen donor skin grafts demonstrated accelerated rejection (<10 days) in three of the animals and led to simultaneous rejection of both the epidermis of the allograft and the skin graft in the long-term tolerant animal. The rejection of the skin grafts did not break tolerance to the musculoskeletal portion in any of the animals.

CONCLUSIONS

All animals exhibited indefinite survival of the musculoskeletal portion of their allografts but only prolonged survival of the epidermis. The loss of the graft skin appears to be the result of an isolated immune reaction to the skin, and, in particular, the epidermis. This observation is further substantiated by the accelerated rejection of secondarily placed frozen donor skin grafts.

Histomorphometric analysis of a cell-based model of cartilage repair

This study examined the morphology of chondrocytes in an established model of articular cartilage repair. Articular cartilage was harvested from young sheep and seeded onto pieces of devitalized sheep cartilage. The seeded pieces were stacked in pairs and wrapped in fibrin glue, and then implanted subcutaneously in the dorsum of athymic mice. Samples were harvested after 6 weeks and examined by transmission electron microscopy (TEM) or by light microscopy. TEM revealed that the cells in direct apposition to the devitalized cartilage were elongated, with an enlarged cytoplasm, and a ruffled border. TEM of cells far from the interface with scaffold tissue revealed rounded cells with large nuclei that appeared similar to normal chondrocytes. Quantitative morphometry of histologic specimens revealed that cell area, relative amount of cytoplasm, cell aspect ratio, and relative nuclear displacement were all higher in cells near the interface with the scaffold tissue, and decreased with distance from the interface. These indices of cell morphology are all consistent with an active remodeling of the scaffold at the cell-scaffold interface.

Recipient bone marrow engraftment in donor tissue after long-term tolerance to a composite tissue allograft

BACKGROUND

An important component of a composite tissue limb allograft (CTA) is the vascularized bone marrow and bone marrow stroma, which when transplanted could create immediate marrow space and engraftment. We have previously demonstrated that tolerance to musculoskeletal allografts can be achieved with a 12-day course of cyclosporine without the presence of long-term peripheral donor cell chimerism. The objective of this study was to determine the fate of the donor bone marrow after transplantation of a limb allograft in a miniature swine model.

METHODS

CTAs from donor swine were heterotopically transplanted into six MHC-matched, minor-antigen-mismatched recipients, and a 12-day course of cyclosporine was given. Previous animals transplanted without cyclosporine rejected their grafts in less than 42 days. A non-MHC-linked marker, pig allelic antigen (PAA), was used to distinguish host and donor cells. Three PAA- animals received PAA+ CTAs, and three PAA+ animals received PAA- CTAs. Bone marrow was harvested from the donor limb grafts and the recipient and analyzed by flow cytometry and histology. Thymus, spleen, and mesenteric lymph nodes were also harvested from the recipient swine and evaluated for the presence of donor cells by flow cytometry.

RESULTS

All animals receiving cyclosporine demonstrated permanent tolerance to their allografts. Donor bone marrow cells were present in all grafts at the time of transplantation and during the immediate postoperative period. By 48 weeks, donor cells were no longer detectable within the marrow space of the allograft. In long-term animals host bone marrow cells replaced donor cells in the graft marrow space. No evidence of donor cell engraftment was found in recipient animals.

CONCLUSION

This study demonstrates that in long-term tolerant recipients of musculoskeletal allografts there is no evidence of persistent donor bone marrow cells in the hematopoietic tissues of the graft or the host. Rather, the recipient's bone marrow cells and lymphocytes repopulate the donor marrow space of the graft.

Cultured chondrocytes produce injectable tissue-engineered cartilage in hydrogel polymer

The purpose of this study was to determine if chondrocytes cultured through several subcultures at very low plating density would produce new cartilage matrix after being reimplanted in vivo with or without a hydrogel polymer scaffold. Chondrocytes were initially plated in low-density monolayer culture, grown to confluence, and passaged four times. After each passage cells were suspended in purified porcine fibrinogen and injected into the subcutaneous space of nude mice while simultaneously polymerizing with thrombin to reach a final concentration of 40 million cells/cc. Controls were made by injecting fresh, uncultured cells with fibrin polymer and by injecting the cultured cells in saline (without polymer). All samples were harvested at 6 weeks. When injected in polymer, both fresh cells and cells that had undergone only one passage in culture produced cartilaginous nodules. Cultured cells did not produce cartilage, regardless of length of time spent in culture, when injected without polymer. Cartilage was also not recovered from samples with cells kept in culture for longer than one passage, even when provided with a polymer matrix. All samples harvested were subjected to histological analysis and assayed for total DNA, glycosaminoglycan (GAG), and type II collagen. There was histological evidence of cartilage in the groups that used fresh cells and cultured cells suspended in fibrin polymer that only underwent one passage. No other group contained areas that would be consistent with cartilage histologically. All experimental samples had a higher percent of DNA than native swine cartilage, and there was no statistical difference between the DNA content of the groups containing cultured or fresh cells in fibrin polymer. Whereas the GAG content of native cartilage was 8.39% of dry weight and fresh cells in fibrin polymer was 12.85%, cultured cells in fibrin polymer never exceded the 2.48% noted from first passage cells. In conclusion, this study demonstrates that porcine chondrocytes that have been cultured in monolayer for one passage will produce cartilage in vivo when suspended in fibrin polymer.

A biomechanical analysis of an engineered cell-scaffold implant for cartilage repair

This study evaluated the biomechanical and physical properties of newly formed cartilage engineered from isolated chondrocytes in combination with matrix components. Four groups of constructs were studied. Group A consisted of lyophilized articular cartilage chips mixed with a cell-fibrinogen solution and thrombin to obtain constructs made of fibrin glue, chondrocytes, and cartilage chips. Group B constructs were prepared using fibrin glue and cartilage chips without cells. Group C contained chondrocytes in fibrin glue without chips, and group D comprised constructs of fibrin glue alone. Specimens were implanted in the subcutaneous tissue of nude mice for 9 weeks. At necropsy the specimens were examined grossly, physically, biomechanically, and histologically. The original, preimplantation mass of the constructs was retained only in experimental group A. Histological analysis of specimens in experimental groups A and C demonstrated the presence of newly formed cartilaginous matrix, whereas only fibrotic tissue was observed in control groups B and D. Biomechanical analysis demonstrated higher mean values of equilibrium modulus in the experimental samples of group A with respect to all control groups. This study demonstrated that adding lyophilized cartilage chips to a fibrin glue-engineered cartilage construct maintains the biomechanical properties and the original mass after medium-/long-term in vivo transplantation.

Donor modification leads to prolonged survival of limb allografts

Chronic immunosuppression is essential for maintaining human hand transplant survival because composite tissue allografts are as susceptible to rejection as visceral organ allografts. Limb allografts comprise different types of tissues with varying antigenicities, and the immunosuppressive doses required for these allografts are as high or higher than those required for solid organ allotransplantation. In particular, bone marrow is an early target of the host immune response. This study reports on donor limb modification of the marrow compartment leading to prolonged survival of limb allografts. Chimeric limb allografts comprising a Lewis rat vascularized allograft and Brown Norway rat bone marrow were created. These chimeric limbs were transplanted into three recipients: (1) Buffalo rats (n = 12), where the chimeric limb was allogeneic for both vascular graft and bone marrow; (2) Lewis rats (n = 12), where the limb was allogeneic for marrow alone; and (3) Brown Norway rats (n = 12), where the limb was allogeneic for graft alone. This study found that Brown Norway recipients elicited significantly reduced cell-mediated and humoral immune responses in comparison with the Buffalo and Lewis recipients (p < 0.001 and p < 0.01, respectively). The Buffalo and Lewis recipients both elicited high cell-mediated and humoral responses. The Brown Norway recipients also had prolonged survival of limb tissue allograft in comparison with the other experimental groups.

Effects of cell concentration and growth period on articular and ear chondrocyte transplants for tissue engineering

This study determined the effects of chondrocyte source, cell concentration, and growth period on cartilage production when isolated porcine cells are injected subcutaneously in a nude mouse model. Chondrocytes were isolated from both ear and articular cartilage and were suspended in Ham's F-12 medium at concentrations of 10, 20, 40, and 80 million cells per cubic centimeter. Using the nude mouse model, each concentration group was injected subcutaneously in 100-microl aliquots and was allowed to incubate for 6 weeks in vivo. In addition, cells suspended at a fixed concentration of 40 million cells per cubic centimeter were injected in 100-microl aliquots and were incubated for 1, 2, 3, 4, 5, 6, 9, and 12 weeks. Each concentration or time period studied contained a total of eight mice, with four samples harvested per mouse for a final sample size of 32 constructs. All neocartilage samples were analyzed by histologic characteristics, mass, glycosaminoglycan level, and DNA content. Control groups consisted of native porcine ear and articular cartilage. Specimen mass increased with increasing concentration and incubation time. Ear neocartilage was larger than articular neocartilage at each concentration and time period. At 40 million cells per cubic centimeter, both ear and articular chondrocytes produced optimal neocartilage, without limitations in growth. Specimen mass increased with incubation time up to 6 weeks in both ear and articular samples. No significant variations in glycosaminoglycan content were found in either articular or ear neocartilage, with respect to variable chondrocyte concentration or growth period. Although articular samples demonstrated no significant trends in DNA content over time, ear specimens showed decreasing values through 6 weeks, inversely proportional to increase in specimen mass. Although both articular and ear sources of chondrocytes have been used in past tissue-engineering studies with success, this study indicates that a suspension of ear chondrocytes injected into a subcutaneous location will produce biochemical and histologic data with greater similarity to those of native cartilage. The authors believe that this phenomenon is attributable to the local environment in which isolated chondrocytes from different sources are introduced. The subcutaneous environment of native ear cartilage accommodates subcutaneously injected ear chondrocyte transplants better than articular transplants. Native structural and biochemical cues within the local environment are believed to guide the proliferation of the differentiated chondrocytes.

Tissue-Engineered Cartilage Composite With Expanded Polytetrafluoroethylene Membrane

The authors report a new approach using expanded polytetrafluoroethylene (ePTFE) membrane as pseudoperichondrium to support engineered cartilage. Swine auricular chondrocytes were isolated and mixed with fibrin glue to achieve a final concentration of 40 x 10(6) cells per milliliter. The fibrin glue-cell suspension was assembled with ePTFE and the constructs were implanted into the dorsal subcutaneous pockets of nude mice for 12 weeks. Two experimental groups were prepared in this study: (1) ePTFE placed in the central part of the specimen in group 1 and (2) ePTFE placed on the outside surfaces in group 2. All specimens were subjected to histological and gross mechanical evaluation. Histological results showed neocartilage formation in both groups. The integration between neocartilage and ePTFE forms a tight bond. Gross mechanical testing revealed that the flexibility of specimens in group 2 were similar to that of native cartilage with intact perichondrium, whereas the flexibility of specimens in group 1 were poor. From these results the authors conclude that it is possible to produce a tissue-engineered cartilage framework using ePTFE as a support material to simulate the perichondrium.

Tolerance to limb tissue allografts between swine matched for major histocompatibility complex antigens

Transplantation of limb tissue allografts would greatly expand the realm of reconstructive surgery. However, the toxicity of chronic immunosuppression has adversely tilted the risk-benefit balance for clinical transplant. In this study, a procedure was sought to achieve host tolerance to limb tissue allografts through matching of the major histocompatibility complex (MHC) antigens between donor and host swine using only a 12-day course of cyclosporine. Massachusetts General Hospital (MGH) miniature swine were used as a large animal model with defined MHC, and musculoskeletal grafts from the donor hind limb were transplanted heterotopically to the recipient femoral vessels. Allografts from MHC-mismatched donors treated with cyclosporine (n = 4) were rejected in less than 6 weeks by gross inspection and histologic sections. Allografts from MHC-matched, minor antigen mismatched donors not treated with cyclosporine (n = 4) were rejected between 9 and 12 weeks. Allografts from similarly matched donors treated with 12 days of cyclosporine (n = 7) showed no evidence of rejection until sacrifice between 25 and 47 weeks. Thus allograft tolerance was achieved between MHC-matched swine using a limited course of cyclosporine. Demonstration of limb tissue allograft survival in a large animal model without long-term immunosuppression represents an important step toward clinical transplantation.

TOLERANCE TO MUSCULOSKELETAL ALLOGRAFTS WITH TRANSIENT LYMPHOCYTE CHIMERISM IN MINIATURE SWINE1

BACKGROUND

Although transplantation of musculoskeletal allografts in humans is technically feasible, the adverse effects of long-term immunosuppression subject the patient to high risks for correcting a non-life-threatening condition. Achieving immunologic tolerance to musculoskeletal allografts, without the need for chronic immunosuppression, could expand the clinical application of limb tissue allografting. Tolerance to musculoskeletal allografts has been accomplished previously in miniature swine in our laboratory. Although stable, mixed chimerism has been suggested as the mechanism underlying long-term tolerance in a rat limb model, the mechanism of this tolerance induction has not been established. This report explores the possible relationship between hematopoietic chimerism and tolerance to musculoskeletal allografts in swine.

METHODS

Twelve miniature swine underwent vascularized musculoskeletal allograft transplantation from histocompatibility complex (MHC) matched, minor antigen-mismatched donors. Eight animals received a 12-day coprse of cyclosporine, one of which was excluded due to subtherapeutic levels. Four recipients were not immunosuppressed. Serial biopsies to assess graft viability and flow cytometry to assess chimerism were performed. Donor and third-party skin grafts were placed on recipients with surviving allografts greater than 100 days to validate tolerance.

RESULTS

Both groups developed early peripheral chimerism, but this chimerism became undetectable by postoperative day 19 in the cyclosporine group and by day 13 in the control group. Animals receiving cyclosporine developed permanent tolerance to their allografts, whereas those not receiving cyclosporine rejected their allografts in 6-9 weeks. Animals demonstrating tolerance to their bone allografts also demonstrated prolonged donor skin graft survival.

CONCLUSIONS

Induction of tolerance to musculoskeletal allografts can be achieved in the MHC matched swine. Although hematopoietic chimerism is present in the immediate postoperative period, persistent, long-term chimerism does not seem to be necessary for maintenance of such tolerance.

Strategies for tolerance induction to composite tissue allografts

The emerging field of composite tissue transplantation offers the potential to replace lost tissues from cadaveric sources. Two major obstacles currently limit the future of composite tissue allotransplantation. The first is chronic rejection, attributed to both antibody deposition and cell-mediated destruction of transplanted tissue. The second obstacle is complications associated with the chronic use of immunosuppressive agents. Our laboratory has been investigating several strategies to induce tolerance to limb tissue allografts to provide solutions to many of the current limitations in allotransplantation. Three strategies show promise in the ability to induce tolerance to organ allografts. The first involves genetic matching at the HLA loci followed by a short course of immunosuppression. The second is the application of a "mixed chimerism" regimen followed by transplantation. The third is costimulatory blockade using a short course of monoclonal antibodies, such as anti-CD40 ligand and CTLA4-Ig after transplantation. Inducing a state of tolerance to limb allografts would eliminate the need for chronic immunosuppression and may also prevent the onset of chronic rejection. The ability to induce allograft tolerance would greatly expand the indications for composite tissue transplantation.

Meniscal repair using engineered tissue

In this study, devitalized meniscal tissue pre-seeded with viable cultured chondrocytes was used to repair a bucket-handle incision in meniscal tissue transplanted to nude mice. Lamb knee menisci were devitalized by cyclic freezing and thawing. Chips measuring four by two by one-half millimeters were cut from this devitalized tissue to serve as scaffolds. These chips were then cultured either with or without viable allogeneic lamb chondrocytes. From the inner third of the devitalized meniscal tissue, rectangles were also cut approximately 8 x 6 mm. A 4 mm bucket-handle type incision was made in these blocks. The previously prepared chips either with (experimental group) or without viable chondrocytes (control group) were positioned into the incisions and secured with suture. Further control groups included blocks of devitalized menisci with incisions into which no chips were positioned and either closed with suture or left open with no suture. Specimens were transplanted to subcutaneous pouches of nude mice for 14 weeks. After 14 weeks, seven of eight experimental specimens (chips with viable chondrocytes) demonstrated bridging of the incision assessed by gross inspection and manual distraction. All the control groups were markedly different from the experimental group in that the incision remained grossly visible. Histological analysis was consistent with the differences apparent at the gross level. Only the experimental specimens (chips with viable chondrocytes) with gross bridging demonstrated obliteration of the interface between incision and scaffold. None of the control specimens revealed any cells or tissue filling the incision. Tissue engineering using scaffolds and viable cells may have an application in meniscal repair in vivo.

Injection of allogeneic bone marrow cells into the portal vein of swine in utero

The ability to safely manipulate the immune system of the developing fetus carries the hope of effective treatment strategies for certain congenital disorders that can be diagnosed during gestation. One possible intervention is the induction of specific transplantation tolerance to an adult donor who could provide tissue after birth without the need for immunosuppression. Although the introduction of allogeneic stem cells to a developing immune system has been shown to result in hematopoietic chimerism, donor-specific transplantation tolerance has not been demonstrated in a large animal model. In previous reports of in utero stem-cell transplantation, the cells were injected into the fetus by an intraperitoneal route. We sought to improve upon this technique of cell transplantation by developing a method for the safe delivery of allogeneic stem cells directly into the hepatic circulation of fetal swine. In the second phase of our study, we determined if adult allogeneic bone marrow cells delivered to the fetus by this intravascular route could result in result in hematopoietic chimerism and donor-specific transplantation tolerance. A method of successful intravascular injection was designed in which a laparotomy was performed on a sow at midgestation (50-55 days) to administer 1 cc of inoculum into the portal vein of each fetus using transuterine ultrasound guidance and a 25-gauge spinal needle. In one sow, 10 piglets were injected with saline to test safety, and 8 piglets were born. For transplantation of stem cells to the fetuses, donor bone marrow was harvested from a genetically defined miniature swine. In one sow the marrow was injected without T-cell depletion resulting in abortion. In the third sow, the marrow was depleted of T-cells to less than 0.01% using magnetic beads conjugated to anti-CD3 monoclonal antibodies. No chimerism was detected in these offspring. Only in the fourth sow where the T-cell depletion was reduced to about 1% of the cells in the inoculum did one animal demonstrate chimerism. This piglet showed reproducible blood chimerism (0.95% donor cells) detected by flow cytometry measurement of monoclonal antibodies to the donor MHC. In addition, this animal demonstrated hyporesponsiveness to donor lymphocytes in an MLR assay while reacting strongly to third-party stimulator cells. A split-thickness skin graft from the donor was accepted, and a third-party graft was rapidly rejected.

Quantitative and qualitative effects of chemical peeling on photo-aged skin: an experimental study

Chemical peel reverses the visible stigmata of photo aging in human skin. The qualitative and, in particular, the quantitative changes in the dermis that effect this transformation are unclear. This study used a recognized photo-aged animal model, the Skh:HR-1 hairless mouse, to quantify and qualify the changes that occurred in collagen and glycosaminoglycan content after chemical peel. One hundred Skh:HR-1 hairless mice were photo-aged by use of chronic ultraviolet B irradiation for 14 weeks. After irradiation the animals were randomly distributed into five groups of 20 mice each: group 1, control; group 2, 50% glycolic acid peel; group 3, 30% trichloroacetic acid peel; group 4, 50% trichloroacetic acid peel; group 5, phenol peel (Baker-Gordon formula). The respective peeling agent was applied to the dorsal skin of each animal while it was fully anesthetized. Punch biopsies were taken at several times after peel for histological and biochemical analysis. Glycosaminoglycan content was assessed at 14, 28, and 60 days using a colorimetric assay. Collagen content per unit volume increased initially 3 days after the procedure in all chemical peel groups, declining on day 7, and peaking again on day 28. Significant elevations (p < 0.04) were seen in the 30% trichloroacetic acid, 50% trichloroacetic acid, and phenol peels on days 3 and 28 in comparison with controls. This increase in collagen content was not maintained and returned to control values by 60 days. Glycosaminoglycan content per unit volume was elevated initially after peel with significant elevation (p < 0.02) in the 50% trichloroacetic acid and phenol groups on days 14 and 28. This increase in glycosaminoglycan content was not maintained beyond 28 days and declined to control values by day 60 in all groups. Histological examination demonstrated an increase in dermal thickness in the 50% trichloroacetic acid and phenol groups in comparison with controls by day 60. Under polarized light all chemical peel groups at day 60 demonstrated a reorganization of collagen in the reticular and papillary dermis. The elastotic masses that are pathognomonic of photo aging were present in the control group but were absent in the peel groups and demonstrated a reorganization of the elastic fibers in the dermis. This effect was deeper in the dermis in the deeper peel groups (50% trichloroacetic acid and phenol peel). The beneficial effects of chemical peel were due to a combination of two findings; a reorganization in dermal structural elements and an increase in dermal volume. These effects were more pronounced in the deeper peel groups.

Cell-based tissue-engineered allogeneic implant for cartilage repair

The potential for using of allogeneic cartilage chips, transplanted in a biologic polymer with articular chondrocytes, as a tool for articular cartilage repair was studied. Small lyophilized articular cartilage chips were mixed with a cell/fibrinogen solution and thrombin to obtain implantable constructs made of fibrin glue, chondrocytes, and cartilage chips. Specimens were implanted in the subcutaneous tissue on the backs of nude mice (experimental group A). Three groups of controls (groups B, C, and D) were also prepared. Group B consisted of fibrin glue and cartilage chips without chondrocytes. Group C consisted of fibrin glue and chondrocytes without cartilage chips, and group D was composed solely of fibrin glue. All samples were carefully weighed before implantation in the mice. The constructs were harvested from the animals at 6, 9, and 12 weeks, examined grossly, and weighed. The samples were then processed and stained with hematoxylin and eosin for histological examination. Gross evaluation and weight analysis of the constructs at the time of retrieval showed retention of the original mass in the samples made of fibrin glue, chondrocytes, and cartilage chips (group A) and demonstrated a cartilaginous consistency upon probing. Specimens from constructs of fibrin glue and cartilage chips without chondrocytes (control group B) retained most of their volume, but were statistically lighter than specimens from group A and were much softer and more pliable than those in group A. Samples of specimens from constructs of fibrin glue and chondrocytes (groups C) and fibrin glue alone (group D) both showed a substantial reduction of their original masses over the experimental time periods when compared to the samples in groups A and B, although specimens from group C demonstrated new cartilage matrix formation. Histological analysis of specimens in experimental group A demonstrated the presence of cartilage chips surrounded by newly formed cartilaginous matrix, while specimens of control group B showed only fibrotic tissue surrounding the devitalized cartilage pieces. Cartilaginous matrix was also observed in control group C, in which cartilage chips were absent, whereas only fibrin glue debris was observed in control group D. This study demonstrated that a composite of fibrin glue and devitalized cartilage can serve as a scaffold for chondrocyte transplantation, preserve the original phenotype of the chondrocytes, and maintain the original mass of the implant. This may represent a valid option for addressing the problem of articular cartilage repair.

Xenotransplantation model for vascularized musculoskeletal tissues in rodents

The purpose of this study was to establish a model and to define the mechanism of rejection for the transplantation of vascularized musculoskeletal xenografts between C57BL/6j (B6) mice and Lewis rats. This was accomplished by using conventional skin xenografts to determine immunologic baseline data between these species and by performing musculoskeletal grafts from the B6 mice transplanted into Lewis rats. After the transplant, the xenografts were examined histologically and the recipients were assessed for immune reaction using in vitro assays to measure both cell-mediated and humoral responses. The results obtained from the skin xenografts showed activation of both cellular and humoral immunologic responses. All musculoskeletal xenografts were rejected between 3 and 4 postoperative days. Histologically, the grafts showed extensive vascular injury manifested by thrombosis and hemorrhage, suggesting an early humoral response. Anti-donor antibody production was detected in the recipient's sera soon after rejection of the xenogeneic tissue. The cell-mediated immune response, although detectable by the in vitro assays, was less pronounced than the humoral response and corroborated the histologic findings of mild lymphocyte infiltration in the rejected tissue. These results demonstrate that humoral rejection plays a predominant role in the rejection of vascularized musculoskeletal xenotransplants between concordant species. This mouse-to-rat vascularized xenograft model will be utilized for further studies on inducing tolerance to vascularized musculoskeletal xenografts.

Neonatal induction of tolerance to skeletal tissue allografts without immunosuppression

Vascularized allogeneic skeletal tissue transplantation without the need for host immunosuppression would increase reconstructive options for treating congenital and acquired defects. Because the immune system of a fetus or neonate is immature, it may be possible to induce tolerance to allogeneic skeletal tissues by alloantigen injection during this permissive period. Within 12 hours after birth, 17 neonatal Lewis rats were injected through the superficial temporal vein with 3.5 to 5 million Brown Norway bone marrow cells in 0.1 ml normal saline. Ten weeks after the injection, peripheral blood from the Lewis rats was analyzed for the presence of Brown Norway cells to determine hemopoietic chimerism. The Lewis rats then received a heterotopic, vascularized limb tissue transplant (consisting of the knee, the distal femur, the proximal tibia, and the surrounding muscle on a femoral vascular pedicle) from Brown Norway rat donors to determine their tolerance to the allogeneic tissue. A positive control group (n = 6) consisted of syngeneic transplants from Lewis rats into naive Lewis rats to demonstrate survival of transplants. A negative control group (n = 6) consisted of Brown Norway transplants into naive Lewis rats not receiving bone marrow or other immunosuppressive treatment. The animals were assessed for transplant viability 30 days after transplantation using histologic and bone fluorochrome analysis. All the syngeneic controls (Lewis to Lewis) remained viable throughout the experiment, whereas all the Brown Norway to Lewis controls had rejected. Ten of the 17 allografts transplanted into bone marrow recipients were viable at 30 days, with profuse bleeding from the ends of the bone graft and the surrounding graft muscle. The percent of chimerism correlated with survival, with 3.31 percent (SD = 1.9) of peripheral blood, Brown Norway chimerism present in the prolonged survival groups and 0.75 percent (SD = 0.5) of Brown Norway chimerism in the rejected graft group. This study demonstrated prolonged survival of allogeneic skeletal tissue without immunosuppression after early neonatal injection of allogeneic bone marrow in a rat model.

Prolonged survival of musculoskeletal xenografts with combined cyclosporine and 15-deoxyspergualin

This study was undertaken to evaluate the feasibility of performing vascularized musculoskeletal xenografts between mice and rats using immunosuppression. Vascularized musculoskeletal grafts were harvested from the hind limb of C57BL/6J (B6) mice, transplanted heterotopically into Lewis rats, and revascularized by microanastomoses of the graft artery and the recipient femoral artery and the graft vein to the recipient femoral vein. Recipient rats were divided into four groups. Group 1 received no immunosuppression (n = 10), group 2 was treated with cyclosporine (10 mg/kg/day; n = 10), group 3 was treated with 15-deoxyspergualin (5 mg/kg/day; n = 10), and group 4 received both cyclosporine and 15-deoxyspergualin (n = 10). Graft survival was directly examined on postoperative days 4, 7, and 14. In vitro assays were performed using mixed lymphocyte reactions and anti-donor cytotoxic antibody assays to assess the recipient's immune response. Grafts were examined by histology and immunohistochemistry. All grafts in group 1 were rejected by day 4. In groups 2 and 3, all grafts were rejected by day 7. In group 4, however, 8 of 10 recipients had viable grafts on day 14. Data from mixed lymphocyte reactions showed that cell-mediated immune responses were uniformly suppressed in groups 2, 3, and 4 compared with group 1. However, anti-donor antibody production was only partly suppressed in groups 2 and 3, suggesting that graft rejection was primarily caused by circulating cytotoxic anti-donor antibodies in groups 1, 2, and 3. Histologic observations in groups 1, 2, and 3 confirmed the important role of the humoral mechanism in xenograft rejection. Furthermore, immunohistochemical results demonstrated that the small vessels in the rejected grafts showed anti-rat immunoglobulin and complement depositions. Only a combination therapy of cyclosporine and 15-deoxyspergualin attenuated the rejection of xenografts.

Adhesion of tissue-engineered cartilate to native cartilage

Reconstruction of cartilaginous defects to correct both craniofacial deformities and joint surface irregularities remains a challenging and controversial clinical problem. It has been shown that tissue-engineered cartilage can be produced in a nude mouse model. Before tissue-engineered cartilage is used clinically to fill in joint defects or to reconstruct auricular or nasal cartilaginous defects, it is important to determine whether it will integrate with or adhere to the adjacent native cartilage at the recipient site. The purpose of this study was to determine whether tissue-engineered cartilage would adhere to adjacent cartilage in vivo. Tissue-engineered cartilage was produced using a fibrin glue polymer (80 mg/cc purified porcine fibrinogen polymerized with 50 U/cc bovine thrombin) mixed with fresh swine articular chondrocytes. The polymer/chondrocyte mixture was sandwiched between two 6-mm-diameter discs of fresh articular cartilage. These constructs were surgically inserted into a subcutaneous pocket on the backs of nude mice (n = 15). The constructs were harvested 6 weeks later and assessed histologically, biomechanically, and by electron microscopy. Control samples consisted of cartilage discs held together by fibrin glue alone (no chondrocytes) (n = 10). Histologic evaluation of the experimental constructs revealed a layer of neocartilage between the two native cartilage discs. The neocartilage appeared to fill all irregularities along the surface of the cartilage discs. Safranin-O and toluidine blue staining indicated the presence of glycosaminoglycans and collagen, respectively. Control samples showed no evidence of neocartilage formation. Electron microscopy of the neocartilage revealed the formation of collagen fibers similar in appearance to the normal cartilage matrix in the adjacent native cartilage discs. The interface between the neocartilage and the native cartilage demonstrated neocartilage matrix directly adjacent to the normal cartilage matrix without any gaps or intervening capsule. The mechanical properties of the experimental constructs, as calculated from stress-strain curves, differed significantly from those of the control samples. The mean modulus for the experimental group was 0.74 +/- 0.22 MPa, which was 3.5 times greater than that of the control group (p < 0.0002). The mean tensile strength of the experimental group was 0.064 +/- 0.024 MPa, which was 62.6 times greater than that of the control group (p < 0.0002). The mean failure strain of the experimental group was 0.16 +/- 0.061 percent, which was 4.3 times greater than that of the control group (p < 0.0002). Finally, the mean fracture energy of the experimental group was 0.00049 +/- 0.00032 J, which was 15.6 times greater than that of the control group. Failure occurred in all cases at the interface between neocartilage and native cartilage. This study demonstrated that tissue-engineered cartilage produced using a fibrin-based polymer does adhere to adjacent native cartilage and can be used to join two separate pieces of cartilage in the nude mouse model. Cartilage pieces joined in this way can withstand forces significantly greater than those tolerated by cartilage samplesjoined only by fibrin glue.

A comparative study of nerve healing in adult, neonatal, and fetal rabbits

This experiment quantitatively compared the human equivalent of a nerve repair following surgical division in the fetal, adult, and early childhood period of development using a rabbit as an experimental animal model. Twelve time-dated pregnant New Zealand White rabbits at 24 days' gestation (term = 31 days) underwent hysterotomy; one hind limb was delivered through the uterine opening. The sciatic nerve was divided and repaired by primary neurorrhaphy using two 11-0 epineural sutures. Sciatic nerve repair was also performed in 10 neonatal and 10 adult New Zealand White rabbits. Following repair, each group was assessed using electromyography examination, measuring distal motor latency and amplitude at 1, 2, 3, and 4 months postrepair. There was no difference in any of the groups in distal motor latency. The amplitude rose incrementally in all groups, and the fetal group had significantly higher amplitudes (p < 0.02) at 1, 2, 3, and 4 months in comparison with the adult group. There was no statistically significant difference between fetal and neonatal nerve repairs at any of the time periods. At the completion of the study, the nerve repair sites were harvested for histologic estimation of mean myelinated fiber density and fiber diameter distribution distal and proximal to the repair site. A greater percentage of myelinated axons crossed the repair site in the fetal group (83 percent) in comparison with the adult group (63 percent) (p < 0.03). Our study also demonstrated significant increases in the number of larger myelinated fibers crossing the repair site in comparison with the neonatal and adult groups (p < 0.04). This study found that fetal nerve healing following surgical repair is superior to that found in adult animals and results in a higher number of larger myelinated fibers crossing the repair site in comparison with adult and neonatal repairs.

Skin allograft survival following intrathymic injection of donor bone marrow

BACKGROUND

Success has been reported using intrathymic injection in the preconditioning regimen to induce allograft tolerance. Although long-term stable tolerance has been achieved in numerous rodent vascularized solid organ allograft models, tolerance to skin transplants has only been achieved across minor antigenic or concordant species disparities. This study sought to induce tolerance across an allogeneic barrier in a rat model with a major genetic disparity.

MATERIALS AND METHODS

Lewis rats were injected intrathymically with 1 x 10(8) Brown-Norway (BN) bone marrow cells and intraperitoneally with 1.0 cc of rabbit anti-rat anti-lymphocyte serum (ALS). Twenty-one days later, BN skin grafts were placed on the injected animals. Control groups were included to isolate the effect of technique, thymic manipulation, strain specificity, and ALS.

RESULTS

Animals receiving both intrathymic bone marrow cells and ALS had a skin graft median survival time of 24 days versus 8 days for the control group (P = 0.003). Groups receiving anti-lymphocyte serum alone or intrathymic bone marrow cell injection alone exhibited no skin graft survival prolongation. Mixed lymphocyte reactions revealed normal responsiveness of tolerant animal lymphocytes to donor strain lymphocytes.

CONCLUSION

This protocol utilizing the intrathymic injection of donor bone marrow cells along with short-term immunosuppression with anti-lymphocyte serum produced markedly prolonged survival of skin allografts transplanted across a major histocompatibility barrier. Although tolerance was incomplete, significant prolongation has not previously been reported in genetic disparities of this degree. These results suggest that the application of this technique for central immune modulation may be beneficial for allograft tolerance induction and deserves further study in large animals models.

Biomechanical analysis of a chondrocyte-based repair model of articular cartilage

The objective of this study was to evaluate the biomechanical properties of newly formed cartilaginous tissue synthesized from isolated chondrocytes. Cartilage from articular joints of lambs was either digested in collagenase to isolated chondrocytes or cut into discs that were devitalized by multiple freeze-thaw cycles. Isolated cells were incubated in suspension culture in the presence of devitalized cartilage matrix for 3 weeks. Multiple chondrocyte/matrix constructs were assembled with fibrin glue and implanted subcutaneously in nude mice for up to 6 weeks. Testing methods were devised to quantify integration of cartilage pieces and mechanical properties of constructs. These studies showed monotonic increase with time in tensile strength, fracture strain, fracture energy, and tensile modulus to values 5-10% of normal articular cartilage by 6 weeks in vivo. Histological analysis indicated that chondrocytes grown on dead cartilage matrix produced new matrix that integrated individual cartilage pieces with mechanically functional tissue.