Cellular mechanisms of adaptation of grafts to antibody

Accommodation and related conditions in vascularized composite allografts

PURPOSE OF REVIEW

The outcome of vascularized composite allografts (VCA) often appear unrelated to the presence of donor-specific antibodies (DSA) in blood of the recipient or deposition of complement in the graft. The attenuation of injury and the absence of rejection in other types of grafts despite manifest donor-specific immunity have been explained by accommodation (acquired resistance to immune-mediated injury), adaptation (loss of graft antigen) and/or enhancement (antibody-mediated antigen blockade). Whether and how accommodation, adaptation and/or enhancement impact on the outcome of VCA is unknown. Here we consider how recent observations concerning accommodation in organ transplants might advance understanding and resolve uncertainties about the clinical course of VCA.

RECENT FINDINGS

Investigation of the mechanisms through which kidney allografts avert antibody-mediated injury and rejection provide insights potentially applicable to VCA. Interaction of DSA can facilitate replacement of donor by recipient endothelial cells, modulate or decrease synthesis of antigen, mobilize antigen that in turn blocks further immune recognition and limit the amount of bound antibody, allowing accommodation to ensue. These processes also can explain the apparent dissociation between the presence and levels of DSA in blood, deposition of C4d in grafts and antibody-mediated rejection. Over time the processes might also explain the inception of chronic graft changes.

SUMMARY

The disrupted tissue in VCA and potential for repopulation by endothelial cells of the recipient establish conditions that potentially decrease susceptibility to acute antibody-mediated rejection. These conditions include clonal suppression of donor-specific B cells, and adaptation, enhancement and accommodation. This setting also potentially highlights heretofore unrecognized interactions between these 'protective' processes.

Accommodation of grafts: implications for health and disease

Accommodation refers to the acquired resistance of a graft to immune-mediated injury. It is typically observed after antibodies that would cause rejection of a graft are removed from a recipient and then later return. In addition to being induced in this manner, accommodation can occur spontaneously, without depleting antibodies. Indeed, we postulate spontaneous accommodation may be the most common outcome of clinical organ transplantation. The paper reviews the current understanding of accommodation, emphasizing recent advances and important questions. Among the recent advances are the discoveries of potentially broader relevance of accommodation for biology and immunology and pathways by which accommodation may be achieved. To investigate these pathways and to understand how accommodation begins and how it evolves, clinical organ transplants might offer a useful and incisive model.

Cardiac xenotransplantation: future and limitations

Despite improvements in pharmacological therapies, the outlook for patients with severe cardiac disease remains poor. At present, only transplantation can 'cure' end-stage cardiac failure. However, fewer than 5% of those who need a cardiac transplant receive one in the United States each year. To address this problem, some propose using animals as a source of organs for transplantation, that is, xenotransplantation. Here, we discuss the rationale for xenotransplantation beyond overcoming the shortage of human organs, and we weigh xenotransplantation against other new technologies that might be used for the treatment of cardiac failure.

Potential applications and prospects for cardiac xenotransplantation

Despite improvements in pharmacologic therapies, the outlook for patients with severe cardiac disease remains poor. At present, the only "cure" for end-stage heart failure is transplantation. However, fewer than 5% of those who need a cardiac transplant receive one in the United States each year. As an alternative, some propose using animals as a source of organs for transplantation (i.e., xenotransplantation). In this article we review the potential applications of xenotransplantation for the treatment of cardiac disease, and weigh xenotransplantation against other new technologies that might be used. We also consider the current status of addressing the hurdles to application of xenotransplantation.

Antibody-induced albuminuria and accelerated focal glomerulosclerosis in the Thy-1.1 transgenic mouse

BACKGROUND

Podocytes play an important role in the development of proteinuria and focal glomerulosclerosis. Previously we have demonstrated that a combination of two monoclonal antibodies (mAb) against aminopeptidase A (APA), an enzyme present on podocytes, induces a massive acute albuminuria in mice. The present study examined the relationship between the acute antibody-induced albuminuria and the development of focal glomerulosclerosis in the Thy-1.1 transgenic mouse. This mouse expresses a hybrid human-mouse Thy-1.1 antigen on the podocytes, and slowly but spontaneously develops albuminuria and focal glomerulosclerosis.

METHODS

Five-week-old non-albuminuric Thy-1.1 transgenic and non-transgenic control mice were injected with anti-APA and anti-Thy-1.1 mAb or saline. Albuminuria was measured at days 1, 7, 14 and 21. At day 21 kidneys were processed for light microscopy, immunofluorescence, and electron microscopy.

RESULTS

Injection of anti-APA and anti-Thy1.1 mAb in Thy-1.1 transgenic mice induced an albuminuria at day 1 that persisted at day 21. The acute albuminuria after injection of anti-APA mAb was more prominent but transient in non-transgenic mice. In non-trangenic mice no albuminuria could be induced with anti-Thy 1.1 mAb. Light microscopy revealed normal glomeruli at day 1 in all transgenic mice, however, at day 21 advanced glomerulosclerotic lesions were seen in mice injected with either anti-APA mAb (37+/-19% of glomeruli affected) or anti-Thy-1.1 mAb (71+/-5%). Non-transgenic mice did not reveal sclerotic lesions at any time investigated. In the transgenic mice the percentage of focal glomerulosclerosis at day 21 did not correlate with albuminuria at day 21. However, we found a highly significant correlation between percentage of focal glomerulosclerosis and the time-averaged albuminuria over the three-week study period (P < 0.001).

CONCLUSION

Injection of a combination of anti-APA or anti-Thy-1.1 mAb into one mo old, non-albuminuric Thy-1.1 transgenic mice induces an acute albuminuria at day 1 that is accompanied by an accelerated focal glomerulosclerosis at day 21. We suggest that the Thy-1.1 transgenic mouse is an excellent model to study specifically the relation between podocytic injury, albuminuria and the development of focal glomerulosclerosis.

Epitope-specific antibody-induced cleavage of angiotensin-converting enzyme from the cell surface

Angiotensin I-converting enzyme (ACE; CD143, EC 3.4.15.1) is a type-1 integral membrane protein that can also be released into extracellular fluids (such as plasma, and seminal and cerebrospinal fluids) as a soluble enzyme following cleavage mediated by an unidentified protease(s), referred to as ACE secretase, in a process known as "shedding". The effects of monoclonal antibodies (mAbs) to eight different epitopes on the N-terminal domain of ACE on shedding was investigated using Chinese hamster ovary cells (CHO cells) expressing an ACE transgene and using human umbilical vein endothelial cells. Antibody-induced shedding of ACE was strongly epitope-specific: most of the antibodies increased the shedding by 20-40%, mAbs 9B9 and 3A5 increased the shedding by 270 and 410% respectively, whereas binding of mAb 3G8 decreased ACE shedding by 36%. The ACE released following mAb treatment lacked a hydrophobic transmembrane domain anchor. The antibody-induced shedding was completely inhibited at 4 degrees C and by zinc chelation using 1,10-phenanthroline, suggesting involvement of a metalloprotease in this process. A hydroxamate-based metalloprotease inhibitor (batimastat, BB-94) was 15 times more efficacious in inhibiting mAb-induced ACE shedding than basal (constitutive) ACE release. Treatment of CHO-ACE cells with BB-94 more effectively prevented elevation in antibody-dependent (but not basal) ACE release induced by 3,4-dichloroisocoumarin and iodoacetamide. These data suggest that different secretases might be responsible for ACE release under basal compared with antibody-induced shedding. Further experiments with more than 40 protease inhibitors suggest that calpains, furin and the proteasome may participate in this process.

Antibody-mediated lung endothelium targeting: in vivo model on primates

We have recently provided evidence that angiotensin-converting enzyme (ACE) is a rational target and anti-ACE monoclonal antibodies (mAbs) are suitable molecules for directing gene/drug delivery into the pulmonary endothelium of rodents. As a step towards gene therapy clinical trials using this approach, the present study evaluated the potential of anti-ACE mAbs for in vivo lung endothelium targeting in 10 species of primates. Cross-reactivity of 10 distinct mAbs directed to human ACE with ACE from baboon, macaques, cercopithecus and chimpanzee revealed that the highest binding with ACE from baboon and macaques was with mAb i2H5, from chimpanzee - mAb 9B9, and from human - 9B9 and i2H5. Thereafter, in vivo biodistribution of mAbs i2H5 and 9B9 was estimated in Macaca arctoides. MAb i2H5, which binds to macaque ACE with substantially higher affinity than mAb 9B9, also more effectively accumulates in their lungs than mAb 9B9. Immunospecificity of lung accumulation (mAb/control IgG ratio) was 37 for i2H5 and 0.5 for 9B9. Lung selectivity of i2H5 uptake (lung/blood ratio) was around 10. Therefore mAb i2H5 may be useful for in vivo lung targeting in non-human primates, whereas 9B9 may be most useful in primates that are closer to humans (chimpanzee). A combination of these two mAbs may be particularly useful for human clinical trials of gene/drug therapy for lung disorders such as pulmonary hypertension and lung metastases.

Xenotransplantation and other means of organ replacement

Exciting new technologies, such as cellular transplantation, organogenesis and xenotransplantation, are thought to be promising approaches for the treatment of human disease. The feasibility of applying these technologies, however, might be limited by biological and immunological hurdles. Here, we consider whether, and how, xenotransplantation and various other technologies might be applied in future efforts to replace or supplement the function of human organs and tissues.

In vivo antibody-mediated modulation of aminopeptidase A in mouse proximal tubular epithelial cells

Aminopeptidase A (APA) is one of the many renal hydrolases. In mouse kidney, APA is predominantly expressed on the brush borders and sparsely on the basolateral membranes of proximal tubular epithelial cells. However, when large amounts of monoclonal antibodies (MAbs) against APA were injected into mice, we observed strong binding of the MAbs to the basolateral membranes, whereas the MAbs bound only transiently to the brush borders of the proximal tubular epithelial cells. In parallel, APA itself disappeared from the brush borders by both endocytosis and shedding, whereas it was increasingly expressed on the basolateral sides. Using ultrastructural immunohistology, we found no evidence for transcellular transport of endocytosed APA to the basolateral side of the proximal tubular epithelial cells. The absence of transcellular transport was confirmed by experiments in which we used a low dose of the MAbs. Such a low dose did not result in binding of the MAbs to the brush borders and had no effect on the presence of APA in the brush borders of the proximal tubular epithelial cells. In these experiments we still could observe binding of the MAbs to the basolateral membranes in parallel with the local appearance of APA. In addition, treatment of mice with chlorpromazine, a calmodulin antagonist that interferes with cytoskeletal function, largely inhibited the MAb-induced modulation of APA. Our studies suggest that injection of MAbs to APA specifically interrupts the normal intracellular traffic of this enzyme in proximal tubular epithelial cells. This intracellular transport is dependent on the action of cytoskeletal proteins.

In vivo modulation of CD26 (dipeptidyl peptidase IV) in the mouse: effects of polyreactive and monoreactive antibodies

We previously reported that intravenous injections in rabbits or guinea pigs of divalent antibodies to purified protein or carbohydrate antigens located mainly on endothelial cells induce acute pulmonary edema, which is often lethal. Surviving animals develop resistance to the injurious effect of subsequent injection of antibodies (adaptation), associated with shedding of antigen-antibody complexes from endothelial cells. In the present study, we investigated and compared in mice the effects of 3-day multiple injections of two different rabbit antibody (IgG) preparations against antigens expressed mainly at the surface of epithelial cells. The first preparation contained antibodies to a single transmembrane protein, CD26 (dipeptidyl peptidase IV [DPP IV]) (monoreactive anti-DPP IV IgG); the second contained antibodies against multiple antigens of the renal tubular brush border (BB), including DPP IV (polyreactive anti-BB IgG). Both IgG preparations caused loss of DPP IV from the organs studied, as shown by reduction in enzyme activity in tissue homogenates and by immunofluorescence microscopy, which showed loss of DPP IV from cell surface. However, the monoreactive anti-DPP IV IgG induced considerably greater reduction than polyreactive anti-BB IgG. Loss of DPP IV from the cell surface probably occurred by shedding of immune complexes into vascular and extravascular fluids, including bile and urine. The results may have relevance to hyperacute rejection of xenografts, as from pigs to primates. Since human natural antibodies that bind to porcine cells are polyreactive, a new prophylactic strategy for hyperacute rejection might be based on down-regulation of the major xenogeneic antigen, alpha-galactosyl, by injecting donor animals with monoreactive alpha-galactosyl antibodies before transplantation.