Germinal center humoral autoimmunity independently mediates progression of allograft vasculopathy

T-B Collaboration in Autoimmunity, Infection, and Transplantation

We have attempted here to provide an up-to-date review of the collaboration between helper T cells and B cells in response to protein and glycoprotein antigens. This collaboration is essential as it not only protects from many pathogens but also contributes to a litany of autoimmune and immune-mediated diseases.

Passenger donor lymphocytes: To affinity and beyond

Graft-versus-host recognition by passenger donor lymphocytes within organ transplants can trigger host alloantibody production (Charmetant et al.).

Inverted direct allorecognition triggers early donor-specific antibody responses after transplantation

The generation of antibodies against donor-specific major histocompatibility complex (MHC) antigens, a type of donor-specific antibodies (DSAs), after transplantation requires that recipient's allospecific B cells receive help from T cells. The current dogma holds that this help is exclusively provided by the recipient's CD4+ T cells that recognize complexes of recipient's MHC II molecules and peptides derived from donor-specific MHC alloantigens, a process called indirect allorecognition. Here, we demonstrated that, after allogeneic heart transplantation, CD3ε knockout recipient mice lacking T cells generate a rapid, transient wave of switched alloantibodies, predominantly directed against MHC I molecules. This is due to the presence of donor CD4+ T cells within the graft that recognize intact recipient's MHC II molecules expressed by B cell receptor-activated allospecific B cells. Indirect evidence suggests that this inverted direct pathway is also operant in patients after transplantation. Resident memory donor CD4+ T cells were observed in perfusion liquids of human renal and lung grafts and acquired B cell helper functions upon in vitro stimulation. Furthermore, T follicular helper cells, specialized in helping B cells, were abundant in mucosa-associated lymphoid tissue of lung and intestinal grafts. In the latter, more graft-derived passenger T cells correlated with the detection of donor T cells in recipient's circulation; this, in turn, was associated with an early transient anti-MHC I DSA response and worse transplantation outcomes. We conclude that this inverted direct allorecognition is a possible explanation for the early transient anti-MHC DSA responses frequently observed after lung or intestinal transplantations.

Autoimmunity in 2019

Based on the PubMed data, we have been performing a yearly evaluation of the publications related to autoimmune diseases and immunology to ascertain the relative weight of the former in the scientific literature. It is particularly intriguing to observe that despite the numerous new avenues of immune-related mechanisms, such as cancer immunotherapy, the proportion of immunology manuscripts related to autoimmunity continues to increase and has been approaching 20% in 2019. As in the previous 13 years, we performed an arbitrary selection of the peer-reviewed articles published by the major dedicated Journals and discussed the common themes which continue to outnumber peculiarites in autoimmune diseases. The investigated areas included systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), psoriatic arthritis (PsA), autoantibodies (autoAbs), and common therapeutic avenues and novel pathogenic mechanisms for autoimmune conditions. Some examples include new pathogenetic evidence which is well represented by IL21 or P2X7 receptor (P2X7R) in SLE or the application of single-cell RNA sequencing (scRNA-seq), mass cytometry, bulk RNA sequencing (RNA-seq), and flow cytometry for the analysis of different cellular populations in RA. Cumulatively and of interest to the clinicians, a large number of findings continue to underline the importance of a strict relationship between basic and clinical science to define new pathogenetic and therapeutic developments. The therapeutic pipeline in autoimmunity continues to grow and maintain a constant flow of new molecules, as well illustrated in RA and PsA, and this is most certainly derived from the new basic evidence and the high-throughput tools applied to autoimmune diseases.

Neutralizing Antibody Responses After SARS-CoV-2 Infection in End-Stage Kidney Disease and Protection Against Reinfection

INTRODUCTION

Patients with end-stage kidney disease (ESKD) represent a vulnerable group with multiple risk factors that are associated with poor outcomes after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Despite established susceptibility to infectious complications and the importance of humoral immunity in protection against SARS-CoV-2, few studies have investigated the humoral immune response to SARS-CoV-2 within this population. Here, we evaluate the seroprevalence of SARS-CoV-2 in patients awaiting renal transplantation and determine whether seroconverted patients with ESKD have durable and functional neutralizing activity against SARS-CoV-2.

METHODS

Serum samples were obtained from 164 patients with ESKD by August 2020. Humoral immune responses were evaluated by SARS-CoV-2 spike S1 subunit and nucleoprotein semiquantitative enzyme-linked immunosorbent assay (ELISA) and SARS-CoV-2 spike pseudotype neutralization assay.

RESULTS

All patients with ESKD with reverse-transcriptase polymerase chain reaction (RT-PCR)-confirmed infection (n = 17) except for 1 individual seroconverted against SARS-CoV-2. Overall seroprevalence (anti-S1 and/or anti-N IgG) was 36% and was higher in patients on hemodialysis (44.2%). A total of 35.6% of individuals who seroconverted were asymptomatic. Seroconversion in the absence of a neutralizing antibody (nAb) titer was observed in 12 patients, all of whom were asymptomatic. Repeat measurements at a median of 93 days from baseline sampling revealed that most individuals retained detectable responses although a significant drop in S1, N and nAb titers was observed.

CONCLUSION

Patients with ESKD, including those who develop asymptomatic disease, routinely seroconvert and produce detectable nAb titers against SARS-CoV-2. Although IgG levels wane over time, the neutralizing antibodies remain detectable in most patients, suggesting some level of protection is likely maintained, particularly in those who originally develop stronger responses.

Novel strategies to target the humoral alloimmune response

Antibody-mediated rejection (ABMR) represents a major cause of late allograft loss in solid organ transplantation worldwide. This process is driven by donor-specific antibodies (DSA), which develop either de-novo or, in sensitized patients, are preformed at the time of transplantation. Effective targeting of ABMR has been hampered by a lack of robust randomized controlled trials (RCT), required for the regulatory approval of new therapeutics. In this review, we discuss the evidence behind the present "standard" of care and recent progress in the development of novel strategies targeting different aspects of the alloimmune humoral response, including naïve and memory B-cell activation, the germinal centre reaction, plasma cell survival and antibody effector functions. In particular, we focus on co-stimulation blockade and its combination with next-generation proteasome inhibitors, new depleting monoclonal antibodies (anti-CD19, anti-BCMA, anti-CD38, anti-CD138), interleukin-6 blockade, complement inhibition and DSA degradation. These treatment modalities, when used in the appropriate clinical context and combination, have the potential to finally improve long-term allograft survival.

An overview of T follicular cells in transplantation: spotlight on their clinical significance

Introduction: For late stage organ failure patients, transplantation is the best option to increase life expectancy with a superior quality of life. Unfortunately, after transplantation many patients are at risk of cellular and antibody-mediated rejection (ABMR). The latter is initiated by donor specific antibodies (DSA) which depend on the actions of B cells, T follicular helper (Tfh) cells and T follicular regulatory (Tfr) cells that are present in the germinal center of lymphoid organs.Areas covered: In this overview paper, we discuss the biology and function of Tfh and Tfr cells in lymphoid tissues, transplanted organs and their circulating counterparts. We report on their relevance to alloimmunity and on the effects of immunosuppressive drugs on these immunocompetent cell populations.Expert opinion: Growing knowledge about the actions of Tfh and Tfr allows for a better understanding of the immunological mechanisms of ABMR after organ transplantation. This understanding feeds the hypothesis that immunosuppressive drugs targeting the actions of Tfh cells have huge therapeutic potential. This new concept in the treatment of the humoral rejection response will improve graft and patient survival after organ transplantation.

Follicular helper T cells and humoral response in organ transplantation

Antibody mediated rejection has been recognized as an important contributor to long-term graft loss in most solid organ transplants. Current immunosuppressive regimes are not capable of preventing anti-HLA antibody formation and eventual damage to the graft, and there is a need to develop drugs directed against novel targets to avoid graft allorecognition. In this review we introduce follicular helper T cells (Tfh), a subtype of lymphocyte specialized in helping B cells to differentiate into plasmablasts and produce class-switched antibodies. We focus on the role of Tfh in solid organ transplantation, what is known about Tfh and the production of alloantibodies, how current immunosuppressive therapies affect Tfh and what new molecules could be used to target Tfh in transplantation, with the goal of improving graft survival.

Prolongation of allograft survival by passenger donor regulatory T cells

Tissue resident lymphocytes are present within many organs, and are presumably transferred at transplantation, but their impact on host immunity is unclear. Here, we examine whether transferred donor natural regulatory CD4 T cells (nT-regs) inhibit host alloimmunity and prolong allograft survival. Transfer of donor-strain lymphocytes was first assessed by identifying circulating donor-derived CD4 T cells in 21 consecutive human lung transplant recipients, with 3 patterns of chimerism apparent: transient, intermediate, and persistent (detectable for up to 6 weeks, 6 months, and beyond 1 year, respectively). The potential for transfer of donor nT-regs was then confirmed by analysis of leukocyte filters recovered from ex vivo normothermic perfusion circuits of human kidneys retrieved for transplantation. Finally, in a murine model of cardiac allograft vasculopathy, depletion of donor CD4 nT-regs before organ recovery resulted in markedly accelerated heart allograft rejection and augmented host effector antibody responses. Conversely, adoptive transfer or purified donor-strain nT-regs inhibited host humoral immunity and prolonged allograft survival, and more effectively so than following administration of recipient nT-regs. In summary, following transplantation, passenger donor-strain nT-regs can inhibit host adaptive immune responses and prolong allograft survival. Isolated donor-derived nT-regs may hold potential as a cellular therapy to improve transplant outcomes.

Germinal Center Alloantibody Responses Mediate Progression of Chronic Allograft Injury

Different profiles of alloantibody responses are observed in the clinic, with those that persist, often despite targeted treatment, associated with poorer long-term transplant outcomes. Although such responses would suggest an underlying germinal center (GC) response, the relationship to cellular events within the allospecific B cell population is unclear. Here we examine the contribution of germinal center (GC) humoral alloimmunity to chronic antibody mediated rejection (AMR). A murine model of chronic AMR was developed in which T cell deficient (Tcrbd-/-) C57BL/6 recipients were challenged with MHC-mismatched BALB/c heart allografts and T cell help provided by reconstituting with 103 "TCR75" CD4 T cells that recognize self-restricted allopeptide derived from the H-2Kd MHC class I alloantigen. Reconstituted recipients developed Ig-switched anti-Kd alloantibody responses that were slow to develop, but long-lived, with confocal immunofluorescence and flow cytometric characterization of responding H-2Kd-allospecific B cells confirming persistent splenic GC activity. This was associated with T follicular helper (TFH) cell differentiation of the transferred TCR75 CD4 T cells. Heart grafts developed progressive allograft vasculopathy, and were rejected chronically (MST 50 days), with explanted allografts displaying features of humoral vascular rejection. Critically, late alloantibody responses were abolished, and heart grafts survived indefinitely, in recipients reconstituted with Sh2d1a-/- TCR75 CD4 T cells that were genetically incapable of providing TFH cell function. The GC response was associated with affinity maturation of the anti-Kd alloantibody response, and its contribution to progression of allograft vasculopathy related principally to secretion of alloantibody, rather than to enhanced alloreactive T cell priming, because grafts survived long-term when B cells could present alloantigen, but not secrete alloantibody. Similarly, sera sampled at late time points from chronically-rejecting recipients induced more vigorous donor endothelial responses in vitro than sera sampled earlier after transplantation. In summary, our results suggest that chronic AMR and progression of allograft vasculopathy is dependent upon allospecific GC activity, with critical help provided by TFH cells. Clinical strategies that target the TFH cell subset may hold therapeutic potential. This work is composed of two parts, of which this is Part II. Please read also Part I: Alsughayyir et al., 2019.

Relative Frequencies of Alloantigen-Specific Helper CD4 T Cells and B Cells Determine Mode of Antibody-Mediated Allograft Rejection

Humoral alloimmunity is now recognized as a major determinant of transplant outcome. MHC glycoprotein is considered a typical T-dependent antigen, but the nature of the T cell alloresponse that underpins alloantibody generation remains poorly understood. Here, we examine how the relative frequencies of alloantigen-specific B cells and helper CD4 T cells influence the humoral alloimmune response and how this relates to antibody-mediated rejection (AMR). An MHC-mismatched murine model of cardiac AMR was developed, in which T cell help for alloantibody responses in T cell deficient (Tcrbd-/-) C57BL/6 recipients against donor H-2Kd MHC class I alloantigen was provided by adoptively transferred "TCR75" CD4 T cells that recognize processed H-2Kd allopeptide via the indirect-pathway. Transfer of large numbers (5 × 105) of TCR75 CD4 T cells was associated with rapid development of robust class-switched anti-H-2Kd humoral alloimmunity and BALB/c heart grafts were rejected promptly (MST 9 days). Grafts were not rejected in T and B cell deficient Rag2-/- recipients that were reconstituted with TCR75 CD4 T cells or in control (non-reconstituted) Tcrbd-/- recipients, suggesting that the transferred TCR75 CD4 T cells were mediating graft rejection principally by providing help for effector alloantibody responses. In support, acutely rejecting BALB/c heart grafts exhibited hallmark features of acute AMR, with widespread complement C4d deposition, whereas cellular rejection was not evident. In addition, passive transfer of immune serum from rejecting mice to Rag2-/- recipients resulted in eventual BALB/c heart allograft rejection (MST 20 days). Despite being long-lived, the alloantibody responses observed at rejection of the BALB/c heart grafts were predominantly generated by extrafollicular foci: splenic germinal center (GC) activity had not yet developed; IgG secreting cells were confined to the splenic red pulp and bridging channels; and, most convincingly, rapid graft rejection still occurred when recipients were reconstituted with similar numbers of Sh2d1a-/- TCR75 CD4 T cells that are genetically incapable of providing T follicular helper cell function for generating GC alloimmunity. Similarly, alloantibody responses generated in Tcrbd-/- recipients reconstituted with smaller number of wild-type TCR75 CD4 T cells (103), although long-lasting, did not have a discernible extrafollicular component, and grafts were rejected much more slowly (MST 50 days). By modeling antibody responses to Hen Egg Lysozyme protein, we confirm that a high ratio of antigen-specific helper T cells to B cells favors development of the extrafollicular response, whereas GC activity is favored by a relatively high ratio of B cells. In summary, a relative abundance of helper CD4 T cells favors development of strong extrafollicular alloantibody responses that mediate acute humoral rejection, without requirement for GC activity. This work is composed of two parts, of which this is Part I. Please read also Part II: Chhabra et al., 2019.

Data regarding transplant induced germinal center humoral autoimmunity

This data is related to the research article entitled “Germinal center humoral autoimmunity independently mediates progression of allograft vasculopathy” (Harper et al., 2016) [2]. The data presented here focuses on the humoral autoimmune response triggered by transferred allogeneic CD4 T cells and includes details on: (a) the recipient splenic germinal center (GC) response; (b) augmentation of humoral autoimmunity and accelerated heart allograft rejection following transplantation from donors primed against recipient; (c) flow cytometric analysis of donor and recipient CD4 T cells for signature markers of T follicular helper cell differentiation; (d) in vitro donor endothelial cell migration in response to column purified autoantibody from recipient sera; (e) analysis of development of humoral responses in recipients following adoptive transfer of donor CD4 T cells and; (f) the development of humoral autoimmunity in mixed haematopoietic chimeric mice.