Significance of Donor-Specific Antibodies in Acute Rejection

Significance of Donor-Specific Antibodies in Acute Rejection H.E. Feucht ABSTRACT Significant advances in recent years in the diagnosis of antibody-mediated graft rejection have led to the re-evaluation of humoral alloreactivity in organ transplantation. By introducing the “C4d-test” into the work-up of transplant biopsies, donor-specific antibodies were claimed to be directly involved in about 30% of acute rejection episodes. The diagnostic criteria for antibody-mediated rejections of renal grafts are now incorporated in the “Banff classification” as refined at a recent consensus conference. Capillary C4d is not always concordant with circulating anti-HLA-antibodies, even if these are assayed with improved techniques. Antibody absorption within the graft and antigens other than HLA, therefore, have to be considered. Effective therapy of humoral rejection is now available. Serial assessment of humoral alloreactivity also in the posttransplantation period is now mandatory to identify at-risk patients. ISTORICALLY, rejection of a transplanted organ has been attributed to cell-mediated immunity, because histopathological examination readily detects infiltrating lymphocytes in the graft interstitium or blood vessels. The components of humoral immunity—antibodies and complement factors—were not found with conventional techniques. The overwhelming concept of cell-mediated immunity in transplant rejection was further supported by the relative success of anti–T-cell directed therapy. Significant advances in the diagnosis and treatment of antibody-mediated rejection have now led to the “renaissance” of humoral immunity in organ transplantation.1,2 Few studies in the past have addressed the possible role of humoral alloreactivity in transplantation, but they consistently have led to significant clinical improvements. By introducing the “crossmatch-test” and the test for “panelreactive antibodies (PRA),” hyperacute rejection of transplanted organs was avoided, and recipients of “high immunological risk” were identified on the waiting list.1 It also appeared that transplant rejection in the presence of circulating alloantibodies had worse outcomes than rejection without antibodies, requiring alternate forms of treatment. The precise role of alloantibodies was not defined, however, because they were absent in the histological specimens. Thus, it was not clear whether preformed/circulating alloantibodies indicated enhanced immunological reactivity of the recipient or whether they acted directly on the graft.

lem.3 Antibodies against endothelial antigens, such as HLA, and most complement components are rapidly removed from the graft endothelium, escaping detection using conventional immunohistological techniques. In contrast, complement factors C4d and C3d expose a covalent thioester-bond on activation, resist rapid elimination, and represent durable markers of antibody-induced classical complement activation in blood vessels.4 Deposition of capillary C4d can be detected in grafts using several techniques, including indirect immunofluorescence and immunoperoxidase staining of cryostat and paraffin slides. Using these methods, the contribution of the humoral limb to acute and chronic transplant rejections was demonstrated in about 30% of cases. Several studies have confirmed that renal grafts with capillary C4d have lower survival rates than grafts without C4d. In two studies, C4d was the strongest independent predictor of subsequent graft loss.3

DETECTION/DIAGNOSIS OF ANTIBODY-MEDIATED GRAFT REJECTIONS In Vivo

From the Department of Organ Transplantation/Nephrology, Fachklinik Bad Heilbrunn, Bad Heilbrunn, Germany. Address reprint requests to Helmut E. Feucht, Department of Organ Transplantation/Nephrology, Fachklinik Bad Heilbrunn, Woernerweg 30, D-83670 Bad Heilbrunn, Germany. E-mail: [email protected]

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The recent introduction of the “C4d-test” into the evaluation of transplant biopsy specimens may solve this prob-

In Vitro

The serological assessment of circulating alloantibodies has likewise been improved significantly in recent years. Starting with cytotoxicity assays of low sensitivity and specificity performed with lymphocyte preparations, flow cytometry methods and enzyme-linked immunosorbent assays (ELISA) have subsequently evolved. Meanwhile, detection of alloantibod-

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0041-1345/05/$–see front matter doi:10.1016/j.transproceed.2005.09.114

Transplantation Proceedings, 37, 3693–3694 (2005)

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ies reacting with purified single HLA antigens is possible.5 With these new technologies, it seems that antibodies against HLA class I and class II determinants are equally involved in humoral rejections. However, the results obtained by in vitro tests are not always concordant with the in vivo C4d-test. Capillary C4d in a graft can be found in the absence of circulating donor-specific antibodies, and vice versa. These discrepancies are best explained by the absorption of alloantibodies in the graft and by the diversity of antigens expressed on lymphocytes/HLA molecules in vitro and on endothelial cells in vivo.3 The diagnostic criteria for antibody-mediated rejection of renal grafts are now securely based on an addendum to the commonly used “Banff classification.”6 These criteria have been refined in a recent consensus conference held in the United States.7 It is worth noting that antibody-mediated rejection can occur without signs of cellular rejection. Humoral rejection can also affect other transplants, such as hearts, lungs, and livers,8 but standardized criteria in these settings are awaited. PATHOGENESIS/PATHOPHYSIOLOGY OF ANTIBODY-MEDIATED REJECTION

It is firmly established by now that capillary deposition of C4d is caused by donor-specific antibodies, either preformed or produced de novo. Antigens include HLA-determinants, blood group antigens, MHC class I–related chain A (MICA), and possibly others.2 Ischemia could be ruled out as a possible cause of C4d deposition, but prolonged hypothermia has been shown to promote humoral rejection in experimental renal transplantation.9 In cardiac transplantation also, the use of antithymocyte globulin was associated with capillary C4d.10 An intriguing question is whether B cells infiltrating renal grafts, as revealed by DNA microarray technology,11 may contribute to the development of humoral rejection. Consequences of Antibody Binding

The leading pathogenic pathway resulting in graft dysfunction has not yet been defined. Transient antibody deposition may activate the complement system leave as may activate the complement system and induce cell-mediated cytotoxic reactions.4 Complement activation may destroy or activate endothelial cells, recruit polymorphs/monocytes, and activate the coagulation system. Experimental studies suggest that strict control of complement and coagulation

FEUCHT

cascades may also result in “accommondation.”12 In most cases, however, antibody-mediated rejection promotes vascular injury and graft vasculopathy. PERSPECTIVES

Currently used potent immunosuppression has led to an overall decrease in the incidence of humoral rejection. There are, however, still patient subgroups carrying an increased risk: presensitized patients, patients included in “tolerance” and “desensitization” protocols, and, importantly, patients with impending chronic graft failure.13 Serial assessment of humoral alloreactivity also in the posttransplantation period can identify patients at risk and is mandatory now for the selection of individual, appropriate therapy. REFERENCES 1. Terasaki PI: Humoral theory of transplantation. Am J Transplant 3:665, 2003 2. Vongwiwatana A, Tasanarong A, Hidalgo LG, et al: The role of B cells and alloantibody in the host response to human organ allografts. Immunol Rev 196:197, 2003 3. Feucht HE: Complement C4d in graft capillaries—the missing link in the recognition of humoral alloreactivity. Am J Transplant 3:646, 2003 4. Feucht HE, Opelz G: The humoral immune response towards HLA class II determinants in renal transplantation. Kidney Int 50:1464, 1996 5. Bray RA, Nickerson PW, Kerman RH, et al: Evolution of HLA antibody detection. Immunol Res 29:41, 2004 6. Racusen LC, Colvin RB, Solez K, et al: Antibody-mediated rejection criteria—an addition to the Banff ’97 classification of renal allograft rejection. Am J Transplant 3:708, 2003 7. Takemoto SK, Zeevi A, Feng S, et al: National conference to assess antibody-mediated rejection in solid organ transplantation. Am J Transplant 4:1033, 2004 8. Michaels PJ, Fishbein MC, Colvin RB: Humoral rejection of human organ transplants. Springer Semin Immunopathol 25:119, 2003 9. Gilligan BJ, Woo HM, Kosieradzki M, et al: Prolonged hypothermia causes primary nonfunction in preserved canine renal allografts due to humoral rejection. Am J Transplant 4:1266, 2004 10. Baldwin WM III, Armstrong LP, Samaniego-Picota M, et al: Antithymocyte globulin is associated with complement deposition in cardiac transplant biopsies. Human Immunol 65:1273, 2004 11. Sarwal M, Chua MS, Kambham N, et al: Molecular heterogeneity in acute renal allograft rejection identified by DNA microarray profiling. N Engl J Med 349:125, 2003 12. Williams JM, Holzknecht ZE, Plummer TB, et al: Acute vascular rejection and accommondation: divergent outcomes of the humoral response to organ transplantation. Transplantation 78: 1471, 2004 13. Terasaki PI, Ozawa M: Predicting kidney graft failure by HLA antibodies: a prospective trial. Am J Transplant 4:468, 2004