Introduction
As Human Leukocyte Antigen (HLA) antibody detection and measurement tools have improved over the last two decades, the understanding and views on desensitisation in kidney transplant candidates have increased, allowing a better understanding of who is likely to benefit from intensive desensitisation measures. Unfortunately, because desensitisation studies are retrospective single-center experiences involving diverse individuals with varying levels of baseline sensitization and a lack of common outcomes, comparing the effcacy of different procedures is challenging.
Plasmapheresis to decrease circulating HLA antibody and intravenous immunoglobulin for immunomodulatory effects and to avoid hypogammaglobinemia are included in most desensitisation procedures. This article provides a guidance about when desensitization in kidney transplantation should be considered and explores novel treatments, desensitisation endpoints, and future study techniques.
What Is Sensitization and How Is It measured?
The significance of alloantibody was made evident early in transplantation when Dr. Terasaki observed the correlation between early allograft loss and in vitro lysing of donor cells after application of recipient serum (e.g. a positive crossmatch). Other notable observations from this landmark study were that patients with prior exposure to alloimmune sensitization including females with a history of preginancy or recipients of prior transplants, had a higher incidence of immediate failure. Recently there has been interest in better understanding the contribution of non-HLA antibody to rejection and graft loss.
It also remains unclear whether the presence of non-HLA antibody leads to reduced access to transplantation, thus this review will be focused on sensitization in the context of HLA antibody only. Detecting HLA antibody in a transplant candidate’s serum and measuring sensitization to determine which HLA antigens must be avoided at transplant is a critical forrst step to avoid donor-specific alloantibody (DSA).
This is a critical point as the potential of immunologic memory response to the transplant organ has clinical relevance. A variety of techniques are available to study antigen-specific B-cell responses in the research setting, but none have been validated to be used routinely in the clinical setting.
Who Derives the Most Benefit from Desensitization in the Current Era?
Desensitization protocols are generally used for the following two reasons:
• To increase transplant candidates’ access to transplantation by decreasing HLA antibody and the number of unacceptable antigens for listing (e.g. reduction in cPRA)
• To decrease known DSA prior to a planned positive crossmatch transplant to reduce the risk of immediate graft loss from catastrophic hyperacute rejection.Given the realization that desensitization and KPD were ineffective strategies for getting many sensitized patients to transplant, deceased donor allocation schemes were revised.
In December of 2014, a new kidney allocation system came into effect in the United States that gave tremendous priority to the sensitized patient. In the system used previously, all candidates with a cPRA of >80% were given an additional 4 allocation points.
Major priority is given to candidates with a cPRA ≥ 98%.Candidates with a cPRA of 98%, 99%, and 100% get an additional 24.4, 50.09, and 202.1 additional allocation points, respectively.
Within months of the implementation of this allocation system, hundreds of transplant patients with cPRA ranging from 90-100% were fortunate to receive a deceased donor transplant after years of waiting. Although the needs for desensitization in kidney transplant have changed in the last two decades, it remains clear that a select group of transplant candidates could derive benefit from effective desensitization.
Kidney Transplant Outcomes Following Desensitization
It is key to understand the outcomes following transplant with DSA to make personalized clinical decisions weighing the risks of incompatible transplantation versus remaining on the waiting list. Despite the patient survival advantage of HLA incompatible transplant; HLA incompatible transplants are associated with reduced allograft survival, increased expense, and increased hospital readmission rates.
Understanding the immunological risk specific to a particular donor/recipient pair is a core principle in the transplantation of sensitized patients. In general, the quantity of antibody at the time of transplant correlates with risk, and the level of antibody can be semi-quantitatively determined with a combination of a variety of tests including the solid phase assay SAB mean uorescence intensity (MFI), titers, cytotoxic and flow cytometric crossmatches, and C1q antibody positivity.
The outcomes after HLA incompatible kidney transplantation with negative cytotoxic crossmatch varies and is largely based on single center retrospective studies with heterogeneous immunosuppression and desensitization protocols. A multicenter observational study of living donor transplants was performed at 22 centers in the United States. It showed that the 1 and 5 year unadjusted all-cause graft loss was 3.9% and 16.6% among patients without DSA at transplant, 3.8% and 20.2% when SAB were positive for DSA but the flow crossmatch was negative, 6.9% and 28.8% when the flow cytometric crossmatch was positive, and 19.4% and 39.9% when the cytotoxic crossmatch was positive.
Emerging Therapeutics for Desensitization
Anti-CD 20 Monoclonal Antibodies
Many desensitization regimens also include the chimetric anti-CD 20 antibody rituximab aimed to deplete B cells and minimize the memory response. Rituximab has been shown to reduce the Panel Reactive Antibody (PRA), increase the rate of transplantation, and decrease the pretransplant flow cytometric crossmatch mean channel shift; but even after treatment, nearly 50% of patients had ABMR within 30 days post-transplant.
More recently Obinutuzumab has been studied in desensitization. It has been associated with a more profound depletion of B cells and is used outside of transplant as a second line agent for hematologic malignancies refractory to rituximab. Obinutuzumab is associated with depletion of peripheral and lymph node B cells, but its effect on MFI (mean uorescence intensity), number of unacceptable antigens, and cPRA has been shown to be limited and does not appear to be clinically meaningful.
Proteasome Inhibitors
Bortezomib, a reversible proteasome inhibitor, has been shown in in vitro models to deplete bone marrow derived plasma cells. In clinical studies, this therapy led to modest reductions in alloantibody, but was not well tolerated . Similarly, the irreversible proteasome inhibitor carlzomib has been shown to deplete plasma cells and decrease HLA antibody, but its effects were transient and antibody levels returned to baseline in less than 6 months.
Anti-CD38 Monoclonal Antibodies
Daratumumab, an anti CD38 monoclonal antibody, has been studied for desensitization in a nonhuman primate model. This treatment was associated with reduced DSA and prolonged renal survival but was also followed by a rebound in DSA and a severe combined antibody and T
cell mediated rejection leading to graft loss. Daratumumab has also been used to successfully desensitize a heart transplant candidate. The cPRA dropped from 80% to 62% after daratumumab and the heart candidate received an organ with HLA that included two antigens that were considered unacceptable before treatment.
Interleukin-6 Blockade
Interleukin 6 is a cytokine that plays a key role in the acute inflammatory response. IL-6 is important for sustaining long-lived plasma cells in their niche, which is crucial to sensitization. The IL-6 inhibitor tocilizumab was utilised for desensitisation in 8 patients who were refractory to rituximab and IVIG in a short phase I/II nonrandomized research. Tocilizumab treatment was linked to a lower immunodominant DSA score based on MFI. A clinical trial using clazakizumab, a soluble IL-6 inhibitor, for chronic active ABMR is currently enrolling patients. A similar but more potent anti CD38 monoclonal antibody, isatuximab, is currently being studied in a phase 1b/2 trial to evaluate the safety, pharmacokinetics, and effcacy for desensitization in kidney transplant candidates.
Cysteine Protease
A novel agent that has shown promise in desensitization is Imlidase. This endopeptidase rapidly cleaves all IgG into F(ab’) and Fc fragments to impair the effector function from all circulating IgG. In both phase 1 & 2 desensitization trials, this agent led to a precipitous drop in DSA within hours, and therefore is a valuable tool for deceased donor positive crossmatch transplantation to avoid hyperacute rejection.
The main limitation is that this drug will likely need to be part of a combination therapy regimen because it only cleaves circulating antibody and antibody levels begin to have a brisk rebound within 3-7 days. In the future, it may be used instead of pretransplant plasmapheresis to rapidly reduce circulating DSA. Complement Inhibitors Eculizumab is a terminal complement inhibitor that does not decrease antibody but has been added to desensitization regimens to minimize the effect of a high level of DSA on the allograft. It has been shown to decrease the incidence of early active ABMR in a small single center cohort from nearly 41.2% in the historical control group compared to 7.7% in the treatment arm.
Regardless, eculizumab has not been shown to improve long term allograft survival when added to desensitization. While the long term studies of terminal complement blockade have been disappointing, there may be a role for adding eculizumab to novel high-risk desensitization protocols in the future to minimize the risk of early allograft failure. Novel Approaches to Clinical Trial Design in Desensitization Use of cPRA and Antibody Titer for Trial Endpoints Recurrent themes in the desensitization field are the small heterogeneous study populations with varied baseline sensitization, differing access to living donors, and unique desensitization endpoints.
The heterogeneous endpoints range from the change in MFI of HLA antibody based on Sensitive single Antigen Bead (SAB)results to the rate of transplantation. Of the various endpoints for desensitization, cPRA is advantageous because it can be applied to candidates with and without a living donor and eliminates the bias that occur from varied deceased donor acceptance, availability of a living donor, or access to kidney paired donation.
It is easy to measure and directly related to a candidate’s probability of receiving a kidney transplant but could be also used in heart and/or lung desensitization studies. The main drawback of cPRA is that is offen based on the MFI from undiluted serum samples, and it is well known that MFI results are impacted by inherent assay limitations and intra-laboratory variability particularly in the sensitized patient.
The number of dilutions tested truly depends on the purpose of testing. For example, if you are willing to attempt desensitization therapy if the antibody is < 1:8, you may only test a 1:8 dilution. In other cases, you may choose to start with testing one dilution (e.g. 1:64) and decide on further testing based on those results. The key is to acknowledge the limitations of the MFI and know when and how to use the titer measurement in practice.
Novel Clinical Trial Designs
Novel adaptive trial designs can be utilized to effciently study small heterogeneous populations with combination therapeutic regimens and address issues with suboptimal enrollment. These designs are most effcient if validated surrogate endpoints such as cPRA are used. The foundation of these designs is a master protocol. Examples of master protocols include umbrella, basket, and platform. Umbrella master protocols are used to study multiple targeted therapies for a single disease while a basket protocol would be used to study a single therapy for multiple diseases.
A platform master protocol is essentially an extension of the umbrella design, but multiple therapies are studied for a single disease perpetually and therapies can enter or leave the trial of the basis of predened criteria. This platform design would be ideal for desensitization because of the relatively small patient population and multiple different desensitization combinations that need rigorous study. In fact, sensitized kidney, heart, and lung candidates could be studied in the same trial if an endpoint such as cPRA was used.
Conclusion
In conclusion, candidates with the highest level of sensitization who have not benefited from Kidney Paired Donation (KPD) or policy adjustments in organ allocation still have an unmet need for desensitisation. There are numerous new therapy options available, and we are hoping that the adoption of new goals and clinical trial designs in this field may lead to effective desensitisation approaches in the future, allowing more people in need to receive transplantation.
