The goal of this clinical trial is to determine whether belimumab can improve detection of circulating HLA-specific memory B cells to support safer and more effective donor organ allocation in highly sensitized kidney transplant candidates.

The main questions it aims to answer are:

Does treatment with belimumab change the antigen specificity profile of circulating HLA-specific memory B cells compared to pre-treatment measurements?

Does a delisting strategy that incorporates mobilized memory B cells improve the probability of donor organ allocation and reduce time to transplantation?

Participants will:

Receive a short course of belimumab treatment

Provide blood samples before and during treatment to assess memory B-cell profiles

Undergo evaluation for potential adjustment of unacceptable HLA specificities (delisting) based on test results

Be followed for donor organ allocation and transplantation outcomes

Kidney transplantation is the optimal treatment for patients with end-stage kidney disease, but access is limited by antibodies targeting non-self human leukocyte antigens (HLA). Antibody-mediated rejection (AMR) occurs when HLA molecules on the donor organ correspond to preformed antibodies in the transplant recipient, which may arise from sensitizing events such as prior organ transplantation, pregnancy, or blood transfusion. To prevent AMR, pre-transplant testing identifies the HLA specificities targeted by patient antibodies, and these specificities are listed as "unacceptable HLA specificities," thereby excluding donor kidneys with incompatible HLA molecules from allocation. Although registration of unacceptable HLA specificities reduces the probability of identifying a compatible donor, this strategy is generally accepted because it mitigates the risk of AMR. However, in highly sensitized transplant candidates, defined as those with HLA antibodies against ≥85% of the donor population, the probability of identifying a suitable donor becomes extremely low. Such patients often remain on transplant waiting lists for years and face a five-year mortality rate of approximately 50% associated with maintenance dialysis. Two approaches may subsequently be considered to improve allocation probability: delisting and desensitization.

Desensitization can be achieved using immunosuppressive agents targeting antibodies and antibody-producing cells. Current desensitization strategies are associated with a high risk of AMR and do not confer a survival benefit compared to remaining on the waiting list. Consequently, delisting should be considered as an initial strategy. Delisting enables transplantation despite the presence of preformed donor-specific antibodies (DSA) that confer a relatively low risk of AMR. Ideally, HLA specificities that occur frequently in the donor pool but pose minimal individual risk are selected for delisting.

Methods for defining low-risk HLA specificities remain imperfect, and no validated international delisting protocols are currently available. After exclusion of specificities associated with a positive complement-dependent cytotoxicity assay, clinicians frequently rely on semi-quantitative mean fluorescence intensity (MFI) values obtained from the Luminex single-antigen bead (SAB) assay. However, this assay is limited by non-standardized cut-off values, a weak correlation with HLA antibody titers, and a primary reflection of antibodies produced by plasma cells. Importantly, the assay does not account for memory B cells (mBC), which can rapidly regenerate antibody production after transplantation and contribute to "rebound" AMR. This limitation partially explains the variability in transplant outcomes associated with pre-transplant DSA exhibiting low MFI values. Direct assessment of HLA-specific mBC may therefore improve pre-transplant risk stratification.

A memory B-cell assay has been developed in which peripheral blood mononuclear cells (PBMCs) are polyclonally stimulated in vitro to identify HLA-specific antibodies produced by mBC. Antibodies detected in both PBMC supernatants and serum have been associated with an increased risk of AMR, whereas specificities detected only in serum may represent low-risk candidates for delisting. However, this assay may underestimate sensitization because most memory B cells reside in secondary lymphoid tissues and are not consistently present in the circulation.

Preclinical studies indicate that memory B-cell homing is dependent on the cytokine B-cell activating factor (BAFF). Belimumab, a monoclonal antibody targeting BAFF, disrupts this homing process and promotes mobilization of memory B cells into the bloodstream. Data from seven randomized trials in patients with Systemic lupus erythematosus demonstrate a median 2.2-fold increase in circulating mBC after four weeks of belimumab therapy, accompanied by reduced expression of genes related to activation and migration. In a prior analysis of seven highly sensitized kidney transplant candidates treated with four weekly belimumab injections, a broader HLA-specific mBC profile was observed, with additional identification of potential low-risk specificities (MFI 3,000-12,000) lacking mBC reactivity. These findings suggest a role for belimumab-enhanced profiling in delisting strategies.

The present clinical trial will evaluate whether a short, four-week course of belimumab enhances the detection of HLA antibodies produced by circulating HLA-specific mBC. The hypothesis is that improved detection will facilitate identification of low-risk specificities suitable for delisting, thereby expanding access to transplantation while limiting the risk of AMR. Secondary analyses will describe transplant outcomes following delisting and will assess the effects of belimumab on serum BAFF levels, B-cell and plasma-cell subsets, HLA antibody profiles, and memory B-cell transcriptomes.

Improved detection of clinically relevant memory B cells through a belimumab-enhanced assay may enable more accurate risk stratification and safer delisting decisions, thereby facilitating transplantation in highly sensitized candidates who would otherwise remain untransplanted.

To be eligible for participation in this study, a subject must meet all of the following criteria:

Adults aged ≥18 and ≤75 years Candidate for kidney transplantation

Highly sensitized, as determined by either:

• 2% probability of being matched with a donor organ within the Eurotransplant Kidney Allocation System (ETKAS), or
• 0.5% probability of being matched with a donor organ within the Eurotransplant Acceptable Mismatch (AM) program, if eligible for inclusion in this program Provision of written informed consent for participation in this study Willingness and ability to comply with the study protocol

Female subjects are eligible if they meet one of the following criteria:

Not pregnant or breastfeeding, as confirmed by a negative pregnancy test at screening Of non-childbearing potential (i.e., status post hysterectomy, postmenopausal, bilateral oophorectomy, documented bilateral tubal ligation, or other permanent sterilization procedure) Of childbearing potential and willing to use effective contraception and agree not to become pregnant during the study

A potential subject who meets any of the following criteria will be excluded from participation:

Active pregnancy, as confirmed by a positive urine β-hCG test or a positive serum β-hCG test, adjusted for end-stage renal disease (ESRD) Significant hypogammaglobulinemia (IgG <4.0 g/L) or IgA deficiency (IgA <0.1 g/L) Receipt of any vaccination within 3 months prior to screening Enrollment in another clinical trial investigating an investigational drug or device at the time of belimumab treatment and delisting; participation in a desensitization trial after assessment of the primary outcome is permitted

Previous administration of any of the following agents within 365 days prior to screening:

BAFF inhibitors (e.g., belimumab, tabalumab) Monoclonal antibodies targeting CD20 (e.g., rituximab) Monoclonal antibodies targeting CD52 (e.g., alemtuzumab) Lymphocyte-depleting agents (e.g., rATG, ATGAM) IL-6 inhibitors or IL-6/IL-6R modulators (e.g., tocilizumab, clazakizumab) Proteasome inhibitors (e.g., bortezomib) Previous administration of high-dose corticosteroids (>50 mg prednisolone or equivalent per day) within 90 days prior to screening

Active infection at screening, defined as any of the following:

Hospitalization for treatment within 30 days prior to screening Current use of parenteral (intravenous or intramuscular) antimicrobial therapy (including antibacterial, antiviral, antifungal, or antiparasitic agents) Current serologic evidence of viral hepatitis, defined as positivity for HBsAg or HBcAb, or a positive hepatitis C antibody test without antiviral treatment Uncontrolled HIV infection, defined as CD4 count <250 cells/mm³ and/or detectable viremia History of a primary immunodeficiency, including complement deficiencies Neutrophil count <1.5 × 10⁹/L Current indication for blood product transfusion at screening, or a high likelihood of requiring transfusion during the treatment phase, in the opinion of the investigator Significant history of infections that, in the opinion of the investigator, would make participation unsuitable History of anaphylactic or severe allergic reaction to parenteral administration of human or murine proteins or monoclonal antibodies Active malignancy or a history of malignancy within the past 5 years, except for adequately treated basal cell or squamous cell carcinoma of the skin, or carcinoma in situ of the cervix with no evidence of metastatic disease for at least 3 years Evidence of psychiatric illness that, in the opinion of the investigator, would make participation unsuitable Any other abnormal laboratory value or intercurrent medical condition that, in the opinion of the investigator, would make participation unsuitable Known mental incapacity or language barriers precluding adequate understanding of the informed consent process and study procedures