Extracellular matrix proteins in organ transplantation

Epithelial tenascin predicts obliterative airway disease

BACKGROUND

Epithelial cell injury, inflammation, fibrosis and airway obliteration result in remodeling of terminal bronchi in post-transplant obliterative bronchiolitis. Tenascin as an extracellular matrix glycoprotein is expressed in several remodeling processes.

METHODS

Heterotopic bronchial allografts of pigs were studied to assess tenascin expression during development of post-transplant obliterative bronchiolitis. A total of 157 allografts or autograft controls were serially obtained 2 to 28 days after transplantation and processed for histology and immunocytochemistry for tenascin, CD4, CD8 and macrophages. Epithelial tenascin index was calculated by multiplying the percentage of positive cells by the grade of tenascin intensity (1 to 3).

RESULTS

Epithelial tenascin expression occurred during the initial ischemic damage to the respiratory epithelium. After partial recovery and before total epithelial loss and subsequent airway obliteration, tenascin expression peaked in allografts (p < 0.001). Epithelial tenascin index on Day 7 was predictive of subsequent epithelial damage, bronchial wall inflammation and the number of (CD4(+) and CD8(+)) cells, fibroproliferation, and obliteration of the bronchial lumen (R > or = 0.47, p < or = 0.01). Tenascin expression in the bronchial wall was more intense in allografts (p < 0.001), paralleling proliferation of fibroblasts and influx of inflammatory cells, and was predictive of inflammatory alterations also in the early obliterative lesions (R > or = 0.45, p < 0.05). Expression decreased during maturation of fibrosis (p < 0.05).

CONCLUSIONS

Epithelial tenascin was predictive of features observed in post-transplant obliterative bronchiolitis, demonstrating a role for tenascin in the development of obliterative bronchiolitis. Tenascin may have relevant properties in serving as a clinical marker for early obliterative bronchiolitis.

Right ventricular expression of extracellular matrix proteins, matrix-metalloproteinases, and their inhibitors over a period of 3 years after heart transplantation

Fibrillar collagens I and III, nonfibrillar collagen IV, and the glycoproteins fibronectin and laminin, are elements of the myocardial extracellular matrix (ECM). Alterations in the normal concentrations and ratios of these elements may reflect remodeling in response to physiologic stress. In the case of patients' post-heart transplantation (HTx), specific patterns of alteration may herald myocardial dysfunction. Right ventricular biopsies were taken from the same 28 HTx patients before implantation and 1 week, 2 weeks, and 1, 2, and 3 years after HTx. The above-noted five ECM proteins, six matrix metalloproteinases (MMPs) and two of their tissue inhibitors (TIMPs) were detected by immunohistochemistry and scored as cells per square millimeter or semiquantitatively. The total connective tissue fibers were detected by connective tissue stain and morphometry. Variations in these ECM components were followed in the same patient cohort over 3 years. In summary, during the first 2 weeks after HTx, a predominant increase in connective tissue occurred. Increases in MMP-8 and MMP-9 were found. By 3 years after transplantation, there was a decrease of connective tissue fibers and a significant reduction of all ECM components and an increase in MMPs and TIMPs. These findings may reflect a pattern of remodeling specific to the transplanted heart.

The allograft defines the type of rejection (acute versus chronic) in the face of an established effector immune response

Transplanted organs fail due to either acute or chronic rejection. The prevailing view is that the nature or magnitude of the recipient's immune response to donor Ags determines the type of rejection. In variance with this view, we show in this study that the status of the graft itself plays a dominant role in defining the type of rejection even in the face of an established alloimmune response. Using adoptive transfer mouse models in which the graft is exposed to a constant number of effector lymphocytes, we found that newly transplanted heart allografts are rejected acutely, while healed-in allografts undergo chronic rejection. Acute rejection of healed-in allografts was largely recapitulated by subjecting the grafts to ischemia-reperfusion injury similar to that present in newly transplanted organs. Ischemia-Reperfusion injury altered the outcome of rejection by enhancing the accumulation of effector T cells within the graft. The accumulation of effector T cells in the graft was dependent on the presence of both ischemia-reperfusion injury (inflammation) and alloantigens. These findings demonstrate that the graft plays a dominant role in shaping the outcome of rejection by controlling the trafficking of effector T cells.

Effects of commonly used immunosuppressants on graft-derived fibroblasts

In acute rejection of transplanted organs intragraft fibroblasts increase their production of hyaluronan. Hyaluronan has strong water binding capacity and an increased tissue content of hyaluronan thus contributes to the development of interstitial oedema. The present study examined the effects of commonly used immunosuppressants (prednisolone, cyclosporin, tacrolimus, mycophenolic acid and sirolimus) on fibroblast proliferation, hyaluronan production and cell surface receptor expression. Fibroblasts isolated from rejecting tissue and from normal, non-transplanted tissue were studied in parallel. All substances investigated, except tacrolimus, were found to affect fibroblasts in one way or another. The most striking effect was the almost total inhibition of fibroblast proliferation in the presence of mycophenolic acid. Cyclosporin reduced the proliferation by about 50% and prednisolone had an inhibiting effect on hyaluronan production (50% reduction). These effects were observed on fibroblasts isolated from rat cardiac allografts undergoing rejection as well as on fibroblasts obtained from normal heart tissue. In contrast, sirolimus was found to stimulate the proliferation of fibroblasts from rejecting tissue (100% increase), but not that of normal fibroblasts. The majority of the fibroblasts expressed the hyaluronan receptor CD44, with a more intense expression in cultures of fibroblasts derived at rejection. None of the immunosuppressants affected the staining pattern (number of positive cells or intensity). The inhibitory effects of prednisolone, cyclosporin and mycophenolic acid on fibroblasts may contribute to the overall beneficial effects of these drugs when used for prevention or treatment of rejection.

Antibody-induced transplantation tolerance: the role of dominant regulation

A short-treatment with nondepleting antibodies, such as those targeting CD4 or CD154 (CD40 ligand), allows long-term graft survival without the need for continuous immunosuppression. This state of immune tolerance is maintained by regulatory CD4+ T cells present within both the lymphoid tissue and the tolerated graft. The nature of such regulatory T cells, their relationship to CD4+CD25+ T cells, and their mode of action have all been the subjects of much attention recently. Here, we review recent progress on understanding the nature, specificity, and mechanisms of action of T cells mediating dominant tolerance brought about by antibody therapy.

Dominant transplantation tolerance

Long-term allograft survival in the absence of continuous immunosuppression can be induced following a short treatment of nondepleting antibodies, such as those that target CD4 or CD154 (CD40 ligand). It is now established that this may involve dominant tolerance mechanisms that are maintained by CD4+ regulatory T cells present within the lymphoid tissue and the tolerated graft. The phenotype of these cells, their relationship to CD4+CD25+ T cells, and the mechanism of action are still controversial.

Anti-adhesion antibodies efalizumab, a humanized anti-CD11a monoclonal antibody

The acquired immune response that leads to graft rejection depends on regulated adhesive interactions between T lymphocytes, endothelial cells, dendritic cells, graft tissue and the extracellular matrix to coordinate cellular trafficking and activation of antigen-reactive T lymphocytes. Inhibiting the function of molecules involved in the adhesion processes offers the potential for interfering with the allograft response. The leukocyte function associated antigen-1 molecule (LFA-1), a heterodimer of CD11a (alphaL) and CD18 (beta2) integrin subunits, is an attractive therapeutic target because it plays an important role in key steps of inflammation and tissue rejection. These include: (1) binding of leukocytes to endothelium; (2) trafficking through activated endothelium; and (3) costimulatory interactions between T lymphocytes and antigen presenting cells. Clinical experience with efalizumab, a humanized anti-CD11a monoclonal antibody (mAb), in patients with chronic plaque psoriasis has shown that anti-CD11a therapy is well tolerated and effective at reducing the severity of the disease without depleting lymphocytes. Initial results in renal transplant patients are also promising.