EXPERIMENTAL AND CLINICAL TRANSPLANTATION

Standardization of immunomodulation protocols has enabled ABO-incompatible liver transplants with outcomes similar to those of ABO-compatible liver transplants. Patients with the A2 blood group are unique because they have a diminished expression of the A antigen. Despite rare immune complications, this phenomenon of diminished expression has led to treatment of type A2 donors according to the regimen for type O blood group donors in ABO-incompatible liver transplants. Additionally, the requirement for pre-transplant recipient immunomodulation is considered minimal when considering these donors. The transplant of a type A2 donor kidney to a type B recipient is well recognized; however, for liver donation the A2-to-B transplant is rare. Here, we present a case of 48-year-old male patient with blood group type B who underwent ABO-incompatible liver transplant of a right lobe liver graft from a type A2 donor. Postoperatively, despite adequate immunosuppression and initiation of therapeutic plasma exchange, the patient developed severe and refractory antibody-mediated rejection that ultimately abated with a splenectomy. This report highlights the low but tangible risk of antibody-mediated rejection in ABO-incompatible liver transplants from type A2 donors and emphasizes the importance of serial monitoring of anti-A isohemagglutinin titers and posttransplant splenectomy to ensure that liver grafts with antibody-mediated rejection can be rescued.

Key words : A2 donor, A2-to-B blood type organ donation, ABO incompatibility, Antibody-mediated rejection, Splenectomy, Therapeutic plasma exchange

Historically, the high incidence of immune complications and the prohibitively poor outcomes have disincentivized liver transplants (LT) in patients with ABO blood group incompatibility (ABOi).^1,2 However, more recently, the urgent need to expand the donor pool has encouraged the search for a better understanding of transplant immunology, and improved immunosuppression protocols have enabled physicians to breach the ABOi barrier and achieve outcomes for ABOi LT that are comparable to ABO-compatible LT.^3-5 The liver has a privileged immunomodulatory status that further facilitates this process known as “immunotolerogenicity.”

A phenomenon peculiar to the A blood group is the intensity of phenotypic expression of the A antigen.⁶ Patients with the A2 blood group have a muted expression of this antigen, and this allows for a transplant without the need for pre-LT recipient immunomodulation.⁷ As published from several large series, type A2 donors have been considered equivalent to type O donors, in practice.^8,9 Despite this, ABOi-related immune complications are well known, and there remains a need for immunogenicity vigilance in this subgroup of recipients.

We present one such case of antibody-mediated rejection (AMR) in a blood group type B recipient who required a post-LT splenectomy to ensure that the liver allograft could be rescued. Our aim was to provide a cautionary note for treatment of this highly selective cohort of donor-recipient pairs in the niche area of ABOi LT.

A 48-year-old male patient with type B blood group presented with ethanol-related end-stage liver disease. After workup he was placed on the deceased donor wait list. During this period on the wait list, he developed COVID-19 with mild symptoms from which he had an uneventful recovery. However, his liver decompensation gradually worsened, and 12 weeks after COVID-19 infection he required hospital admission for liver decompensation. Predictably, the patient’s Model for End-Stage Liver Disease score worsened dramatically, and the need for early LT was explained to the family. The COVID-19 pandemic has caused near cessation of deceased organ donation, so the only option for this patient was to undergo a living donor LT. His nephew had A2-type blood group and volunteered to donate a part of his liver. Subsequently, the nephew underwent donor evaluation as described elsewhere10 and was deemed a suitable donor.

Preformed blood type anti-isohemagglutinin titers were tested with the conventional tube method and are expressed as the highest dilution at which macroscopic agglutination (1+) was observed in the saline phase (immunoglobulin M [IgM]) and the anti-human globulin phase (IgG). The patient’s anti-A isohemagglutinin (AAI) titers were checked preoperatively and were serially measured on a daily basis in the postoperative period. His preoperative AAI titers were 1:4 for IgM and 1:4 for IgG (Figure 1). Per protocol, we deemed it unnecessary to perform an immunomodulation priming step for the recipient of the ABOi LT. The patient underwent a living donor LT with a right lobe graft. The operation was uneventful. Intraoperatively, he was supported with type O packed red blood cells and type AB plasma products. Postoperatively, the patient was extubated on the first postoperative day (POD1), and triple immunosuppression was commenced with calcineurin inhibitors (tacrolimus), antimetabolites (mycophenolate mofetil), and corticosteroids. His peak aspartate transaminase level of 132 IU/L was on POD1. The AAI levels were monitored daily (Figure 1).

The patient showed good progress until POD5, when an upward trend in the AAI titers (1:16) was noted. Adequate immunosuppression levels were confirmed and conventional total plasma exchange (TPE) was initiated via centrifugal technology (Terumo BCT). Patient plasma volume was calculated with a derivation of the Nadler formula. One plasma volume was exchanged with 5% albumin solution and type AB fresh-frozen plasma. The inlet-to-anticoagulant ratio was held steady at 1:12 to 1:15, and the blood flow rate was held between 45 and 50 mL/min. Despite 2 cycles of TPE, the recipient’s AAI titers increased to 1:256. Additional TPE was performed with AAI immunoadsorption (Glycosorb-ABO columns, Glycorex Transplantation) according to the principle that the synthetic terminal trisaccharide A or B blood group antigen covalently binds to a Sepharose matrix in the column, thus removing blood group-specific antibodies by adsorption. Immediate preadsorption and postadsorption antibody titers were monitored. Despite the use of high-volume plasmapheresis immunoadsorption on 4 consecutive days (POD7 to POD10), the AAI titers remained alarmingly high (IgG 1:256) (Figure 1).

Subsequent to a multidisciplinary team meeting, a liver biopsy was performed that showed confluent/bridging necrosis with sinusoidal congestion, fibrin deposition, and pigment-laden macrophages (Figure 2A). The portal tracts were expanded with moderate mixed inflammatory cell infiltrate composed of lymphocytes, histiocytes, eosinophils, and polymorphonuclear neutrophils (Figure 2B). There was portal venulitis and bile ductular reaction in the portal tracts with infiltration of the ducts by polymorphonuclear neutrophils. Portal microvasculitis with intraluminal mononuclear inflammatory cells were identified (Figure 2C). Fibrin thrombi were noted in few capillaries. There was periductal fibrosis with epithelial injury (Figure 2D). Patchy ballooning and lobular bilirubinostasis were noted. No ductopenia or any viral inclusion was noted. Portal vasculature showed strong C4d expression. The final assessment indicated severe acute AMR in the allograft.

Despite serial high-volume immunoadsorption TPE, the AAI titers did not show a declining trend and AMR could not be ameliorated. We decided to perform a splenectomy to interrupt the ongoing immune process. On POD11, the recipient underwent a splenectomy and had an unremarkable recovery thereafter. Total plasma exchange was continued for 4 more cycles, and the AAI levels subsided to less than 1:8 dilution. He was discharged home on POD19 (Figure 1). The patient remains well at 3 months with a normal liver function test.

Breaching the ABOi barrier is a way to increase the donor pool, and for some patients ABOi LT may be the only option in the absence of a suitable ABO-compatible deceased or living donor. The ABO blood groups are classified according to the type of antigen converted from the H serotype precursor polysaccharide and expressed onto the surface of the cells. The A blood group is subclassified into types A1 and A2 according to the quality and quantity of cell-surface expression of the A antigen. Type A1 can be distinguished from A2 with the lectin from Dolichos biflorus, which agglutinates type A1 red blood cells but not A2 red blood cells. Furthermore, quantitatively the blood type A1 is characterized by 1 × 10⁶ A-antigen epitopes per red blood cell. In contrast, type A2 possesses only one-fifth (2.2 × 10⁵) the number of A-antigen sites as type A1. As a result of this reduction in transferase activity, the A2 phenotype has a lower expression of the A antigen and a lower immunoreactivity to AAI than the A1 phenotype.

ABO-incompatible LT has been attempted for more than half a century; however, with the availability of potent immunosuppressants and various modifi-cations to immunotherapy during the past 3 decades, the outcomes of ABOi LT have dramatically improved.^2,5,11 Numerous series from Sweden, Korea, Japan, and the United States have demonstrated the effectiveness of various immunomodulation priming protocols to successfully perform LT despite ABOi.^11-13 Splenectomy has been an integral com-ponent of these protocols in the past. However, postsplenectomy sepsis and operative morbidity have been common complications in the pre-LT setting.^4,13 Moreover, the introduction of anti-CD20 monoclonal antibody (rituximab) for B-cell depletion has meant that splenectomy is now rarely performed for this indication before LT.¹⁴

It is noteworthy that several series have recom-mended against a priming step for ABOi LT with blood group type A2 donation.¹ This is more common when AAI titers are low (<1:8) in the recipient and the risk of immune complications is negligible (especially for AMR complications). Inour case study, the donor had blood group type A2 and the recipient AAI levels (1:4) were below the threshold. Therefore, no conventional pre-LT procedures were performed for ABOi LT. However, when the AAI began to rise on POD5, immunosuppression was optimized and TPE was initiated to washout the offending AAI. It is noteworthy that, as in our patient, an increase in liver enzymes has been recognized as a late feature of immune damage and should not be considered an element for the criteria for intervention. Pharmacological B-cell depletion therapies (rituximab) have long latency periods and are an additional risk of post-LT infection.¹⁵ Hence, with an urgent need to regulate the hyperactive humoral immunity, we performed a splenectomy. Total plasma exchange was continued for 4 cycles after splenectomy to hasten the AAI decrease. Contrary to previous reports, for this case the splenectomy prevented rebound (Figure 1).

Our case report provides a note of caution for the use of type A2 organs in ABOi LT and emphasizes the importance of immunogenicity vigilance in the form of serial AAI assays. This report also highlights the fact that in the era of B-cell depletion therapies splenectomy should be considered a valuable option to facilitate rescue of the liver allograft with AMR.

Volume: 20 Issue: 1 January 2022

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