Refinement of humoral immune monitoring in kidney transplantation: the role of “hidden” alloreactive memory B cells

Abstract

The advent of novel sensitive assays assessing circulating anti-human leukocyte antigen (HLA) antibodies has allowed recognizing humoral alloimmunity as the main immune-mediated mechanism responsible for allograft rejection and graft loss in kidney transplantation. However, current immune-monitoring techniques, exclusively focusing on circulating anti-HLA antibodies, may underestimate the magnitude of humoral immune response as they exclude the memory B-cell (mBC) pool. Different biological compartments are involved in the intricate mechanisms triggering humoral alloimmune responses even in absence of detectable circulating alloantibodies. Recent studies in animal models as well as in clinical kidney transplantation have shown the key role of this B-cell subset triggering allograft rejection, thus emphasizing the value of recognizing antidonor mBC both as a biomarker of allosensitization and as therapeutic targets. Therefore, considerable efforts are being made among the transplant research community to better understand the role, hierarchy, and impact of mBC in the context of organ transplantation. In this review article, we provide a deep insight into the biology of mBC as well as main evidence of their role orchestrating allograft rejection. Also, we provide a thorough description of main immune-monitoring tools aiming at tracking mBC and a rational for their potential use to refine current humoral immune-risk assessment in kidney transplantation.

A Comprehensive Phenotypic and Functional Immune Analysis Unravels Circulating Anti–Phospholipase A2 Receptor Antibody Secreting Cells in Membranous Nephropathy Patients

“…The induction and differentiation of circulating mBCs into antibody-secreting plasmablasts were performed as previously described. [23][24][25] Briefly, peripheral blood mononuclear cells were resuspended at 1.5 Â 10 6 cells/ ml concentration in Iscove's modified Dulbecco's medium (Gibco Invitrogen, Paisley, UK) containing 10% fetal bovine serum (Hyclone, Gibco) supplemented with 2 mmol/l L-glutamine (Gibco Invitrogen), 100 U/ ml penicillin, and 100 mg/ml streptomycin (Gibco Invitrogen). Cells were activated for 6 days with 500 ng/ml a-CD40 mAb (R&D Systems, Minneapolis, MN), 2.5 mg/ml toll-like receptor-9 ligand oligodeoxynucleotides 2006 CpG (Hycult Biotechnology, Uden, the Netherlands), 600 IU/ml IL-2 (Sigma-Aldrich, St. Louis, MO), 25 ng/ml IL-10 (R&D Systems), and 100 ng/ml IL-21 (Gibco Invitrogen).…”

Transfer of donor anti-HLA antibody expression to multiple transplant recipients: A potential variant of the passenger lymphocyte syndrome?

“…Antibody levels increased over subsequent weeks, despite the protocolized use of rituximab (anti‐CD20) and an absence of measureable plasma B cells. Again it was concluded that plasma cells or a subset of memory B cells without surface ‐CD20 were producing the antibodies. Recently Koshizuka et al detected donor‐derived anti‐cytomegalovirus (CMV) antibody beyond 6 months post–lung transplantation in 2 recipients and subsequently characterized the presence of donor plasma cells (via their flow cytometry CD19 − /CD138 + profile) in the recipients’ serum.…”

“…Combined with our own report, these various case series suggest that donor lymphoid tissue originating memory cells (likely plasma cells) enter and remain in the recipient circulation long term. The case report utilizing rituximab, with no circulating B cells and the 2 case reports observing antibodies within a short 7 days from transplant, argue against the theory that B cells are the bridging cell that subsequently differentiates into plasma cells . Additionally, tissue‐resident B cells are associated with local antigens, so they should not be outside the allograft in significant numbers.…”

“…The clinical relevance of demonstrating the donor's specific profile of anti‐HLA antibodies in recipient sera include the following: It confuses and interferes with the actual recipient DSA result, potentially convincing the clinician not to act, “as it's all nonspecific.” There may be less confusion if donors with sensitizing events are screened for anti‐HLA antibodies at the outset. They are potentially a source of alloreactive antibodies, because there are studies showing poorer outcomes compared to the presence of no anti‐HLA antibodies at all . This may be particularly relevant to pediatric recipients where the aim is to achieve a very long‐lasting allograft, and retransplantation may be necessary at some point. Recognizing that donor B cells can be present in the recipients making a range of antibodies (against HLA, cytomegalovirus, hepatitis C, platelets etc), could this extend to the interpretation of other serological tests, including those of CMV, Epstein Barr virus, or other antigens? …”

Mechanisms underlying human genetic diversity: consequence for antigraft antibody responses

“…There are additional factors that contribute to the development of an alloimmune response beyond genetic polymorphism. This includes both innate and adaptive immune mechanisms (e.g., antigen presentation, immunological memory, complement regulation) as well as nonimmunological factors (e.g., adherence, ischemia-reperfusion injury) [87][88][89][90][91].…”