Significance of Donor-Specific Antibodies in Acute Rejection

Recipient-matching of Passenger Leukocytes Prolongs Survival of Donor Lung Allografts in Miniature Swine

BACKGROUND

Allograft rejection continues to be a vexing problem in clinical lung transplantation, and the role played by passenger leukocytes in the rejection or acceptance of an organ is unclear. We tested whether recipient-matching of donor graft passenger leukocytes would impact graft survival in a preclinical model of orthotopic left lung transplantation.

METHODS

In the experimental group (group 1), donor lungs were obtained from chimeric swine, in which the passenger leukocytes (but not the parenchyma) were major histocompatibility complex-matched to the recipients (n = 3). In the control group (group 2), both the donor parenchyma and the passenger leukocytes were major histocompatibility complex-mismatched to the recipients (n = 3).

RESULTS

Lungs harvested from swine previously rendered chimeric by hematopoietic stem cell transplantation using recipient-type cells showed a high degree of passenger leukocyte chimerism by immunohistochemistry and flow cytometry. The chimeric lungs containing passenger leukocytes matched to the lung recipient (group 1) survived on average 107 days (range, 80-156). Control lung allografts (group 2) survived on average 45 days (range, 29-64; P < 0.05).

CONCLUSIONS

Our data indicate that recipient-matching of passenger leukocytes significantly prolongs lung allograft survival.

The role of B cells in solid organ transplantation

The role of antibodies in chronic injury to organ transplants has been suggested for many years, but recently emphasized by new data. We have observed that when immunosuppressive potency decreases either by intentional weaning of maintenance agents or due to homeostatic repopulation after immune cell depletion, the threshold of B cell activation may be lowered. In human transplant recipients the result may be donor-specific antibody, C4d+ injury, and chronic rejection. This scenario has precise parallels in a rhesus monkey renal allograft model in which T cells are depleted with CD3 immunotoxin, or in a CD52-T cell transgenic mouse model using alemtuzumab to deplete T cells. Such animal models may be useful for the testing of therapeutic strategies to prevent DSA. We agree with others who suggest that weaning of immunosuppression may place transplant recipients at risk of chronic antibody-mediated rejection, and that strategies to prevent this scenario are needed if we are to improve long-term graft and patient outcomes in transplantation. We believe that animal models will play a crucial role in defining the pathophysiology of antibody-mediated rejection and in developing effective therapies to prevent graft injury. Two such animal models are described herein.

Differential effects of donor-specific alloantibody

Alloantigen exposure typically provokes an adaptive immune response that can foster rejection of transplanted organs, and these responses present the most formidable biological barrier to kidney transplantation. Although most cellular alloimmune responses can be therapeutically controlled with T-cell-specific immunosuppressants, humoral alloimmune responses remain relatively untamed. Importantly, humoral immunity, typically manifesting as allospecific antibody production, is increasingly recognized for its variable appearance after kidney transplantation. Indeed, the appearance of alloantibody can herald the onset of rapid and destructive antibody-mediated rejection or have no demonstrable acute effects. The factors determining the end result of alloantibody formation remain poorly understood. This review will discuss the breadth of alloantibody responses seen in clinical kidney transplantation and provide an overview of potential factors explaining the phenotypic variability associated with humoral alloimmunity. We propose several avenues ripe for future investigation including the influence of innate immune components and the potential influence of heterologous immune responses in determining the ultimate clinical import of an alloantibody response.

Liver transplantation: an update 2007

PURPOSE OF REVIEW

Recent attention in liver transplantation has focused on equity in organ allocation and management of posttransplant complications.

RECENT FINDINGS

Adoption of the model for end-stage liver disease for liver allocation has been successful in implementing a system based on medical urgency rather than waiting time. Refinements are being studied in improving the prediction of mortality and improving transplant benefit by balancing pretransplant mortality and posttransplant survival. Emerging literature is examining expansion of the current criteria for transplantation of hepatocellular carcinoma and the role of neoadjuvant therapy. Chronic renal dysfunction after liver transplantation is a source of considerable morbidity. Nephron-sparing immunosuppression regimens are emerging with encouraging results. Hepatitis C virus infection is difficult to differentiate histologically from rejection, although newer markers are being developed. Antiviral and immunosuppressive strategies for reducing the severity of hepatitis C virus recurrence are discussed. Alcohol relapse is common after liver transplant in alcoholic liver disease patients and can lead to worse outcomes.

SUMMARY

Organ allocation tends to evolve under the model for end-stage liver disease with a focus on maximizing transplant benefit. Hepatitis C virus, hepatocellular carcinoma, chronic renal dysfunction and alcohol relapse are major challenges, and continued research in these areas will undoubtedly lead to better outcomes for transplant recipients.

C4d: a marker for hepatic transplant rejection

BACKGROUND

Acute rejection is still a common complication of hepatic transplantation. The diagnosis, based on the histological examination of the graft, may be difficult to confirm in the setting of combined hepatitis C virus infection. The presence of C4d in the portal capillaries could facilitate differentiation between acute rejection and relapsed hepatitis C. The deposit of C4d provides evidence of activation of humoral immunity. To attempt to confirm this hypothesis, we searched for the presence of C4d in posttransplant hepatic biopsies.

METHODS

Thirty-six biopsies from 34 patients were analyzed retrospectively. The samples had been requested for one of the following reasons: suspected rejection, relapsed hepatitis C infection, or systematic check-up 1 year after the transplant.

RESULTS

C4d expression was common in biopsies classified as acute rejection (33%) and chronic rejection (100%). C4d was never detected in the event of recurrent hepatitis C infection without rejection.

CONCLUSION

These results, which are comparable to recently published data, give credence to the theory that C4d could be used as a marker for rejection following hepatic transplantation.