Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

published_or_final_version / Dentistry / Master / Master of Orthodontics

published_or_final_version / Dentistry / Master / Master of Orthodontics

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This thesis investigates the expression of a T-cell differentiation antigen, (XTLA- 1), in various strains and species of Xenopus, and demonstrates the effect of early-thymectomy, (by microcautery), on XTLA-1 expression. It further examines restoration of the T-cell dependent immune system, (particularly with respect to transplantation responses), and the extent to which tolerance to donor antigens is achieved by implantation of xenogeneic, as well as allogeneic, thymi into early- thymectomised Xenopus larvae. The means by which transplantation tolerance is maintained in intact, control Xenopus, following perimetamorphic skin grafting, is also addressed. Initial work, reported in Chapter 2, showed that XTLA-1 is expressed by the majority of thymocytes and by a proportion of splenocytes from all X.laevis, X.borealis, and hybrid clonal Xenopus,(X.laevis x X.gilli and hybrid X.laevis x X.muelleri),examined. X.tropicalis lymphocytes, however, do not express XTLA- 1. Early-thymectomy by microcautery effectively removes T-cells, as detected by XTLA-1 expression. In Chapter 3, normal adult and larval tissue distribution of XTLA-1 positive cells is described, and the XTLA-1 and X.borealis,(quinacrine fluorescence), markers are employed to demonstrate the differentiation of T-cells derived from early- thymectomised hosts within xenogeneic, (X.tropicalis), thymus implants. The effects of implantation of allogeneic and xenogeneic larval thymi into early-thymectomised hosts, in terms of T-cell responses and of induction of tolerance to thymus donor antigens, is explored in Chapter 4; X.borealis xenogeneic thymus implants are apparently as effective in these regards as are allogeneic implants, but X.tropicalis xenogeneic thymus implants do not fully restore thymus-dependent immune responses. Preliminary investigations of skin graft rejection, mixed leukocyte culture and T-cell mitogen responses of X.tropicalis, in comparison to those of other Xenopus species, are reported in Chapter 5; the results of these experiments raise the possibility that X.tropicalis splenocytes are less responsive, in mixed leukocyte culture, to xenogeneic stimulators than are splenocytes of other Xenopus species. In Chapter 6, histological examination of skin grafts, accepted by virtue of the tolerance induced by prior implantation of a thymus gland from the skin graft donor into the early-thymectomised hosts, reveals some rapid alteration in the composition of these skin grafts; infiltration of the tolerated skin grafts by host-derived lymphocytes suggests that tolerance induced by thymus implantation does not abrogate recognition of thymus donor antigens. Finally, also in Chapter 6, tolerance induced in control, intact Xenopus by perimetamorphic skin grafting is shown to be susceptible to cyclophosphamide injection, suggesting that the maintenance of this tolerance is mediated by suppressor cells.

Bone defects resulting from non-unions, fractures, significant revision joint replacements, tumour resection and osteolysis present a clinical problem. While autografts are considered the gold standard, ubiquitous use of this reparative technique is limited by graft supply and site morbidity. Recent progresses in tissue engineering using stem cells, bone enhancing molecules and gene therapy have provided more hypotheses for bone defect treatment. In vivo assessment to test these hypotheses requires animal models to mimic human conditions. Immunodeficient or nude animals have the advantage of hosting materials from human and other xenographic origins without immuno-intolerance or rejection. A thorough understanding of the biology in nude animals is vital for the further advancement of connective tissue healing and regeneration strategies. Nude mice are excellent xenographic hosts for in- vivo characterisation and provide a reproducible animal source. The immune deficiencies of nude compared to normal animals may however, influence bone healing and need to be addressed. This dissertation (a) investigated potential bone defect models in nude mice and nude rats (b) incorporated the selected bone defect model to evaluate the effect of T cell deficiency and age on bone defect healing in nude animals (c) determined the feasibility of a critical size defect (CSD) in nude mice. A distal-femur-condylar-defect (DFCD) model was successfully performed in nude mice and rats. The model was found to have some advantages as a bone defect model: (1) located at a weight-bearing skeletal site (2) no requirements for an internal or external fixator (3) does not obstruct or limit mobility (4) location is not in close proximity to any major organs such as the brain (5) easy identification of surface anatomy (6) defect size is standardised and reproducible (7) does not require lengthy and complicated surgery and (8) cost effective. This dissertation confirmed that bone healing in nude mice is similar to that of normal immunocompetent mice. Absence of T lymphocytes did not delay or inhibit bone repair. Use of older nude mice did not seem to affect the healing rate, in contrast to older normal mice, which showed delay in bone healing in the initial phase. Establishment of critical sized defects in mice at a weight-bearing location was not feasible due to the robust healing of murine. This dissertation recommends that the DFCD model could be utilized for the assessment of xenogenic materials at early time point.