B Cell Depletion at Induction—Making Bad Immunological Memory Fade?

Abstract

We evaluated the efficacy and safety of rituximab as induction therapy in renal transplant patients. In a double‐blind, placebo‐controlled study, 280 adult renal transplant patients were randomized between a single dose of rituximab (375 mg/m2) or placebo during transplant surgery. Patients were stratified according to panel‐reactive antibody (PRA) value and rank number of transplantation. Maintenance immunosuppression consisted of tacrolimus, mycophenolate mofetil and steroids. The primary endpoint was the incidence of biopsy proven acute rejection (BPAR) within 6 months after transplantation. The incidence of BPAR was comparable between rituximab‐treated (23/138, 16.7%) and placebo‐treated patients (30/142, 21.2%, p = 0.25). Immunologically high‐risk patients (PRA >6% or re‐transplant) not receiving rituximab had a significantly higher incidence of rejection (13/34, 38.2%) compared to other treatment groups (rituximab‐treated immunologically high‐risk patients, and rituximab‐ or placebo‐treated immunologically low‐risk (PRA ≤ 6% or first transplant) patients (17.9%, 16.4% and 15.7%, p = 0.004). Neutropenia (<1.5 × 109/L) occurred more frequently in rituximab‐treated patients (24.3% vs. 2.2%, p < 0.001). After 24 months, the cumulative incidence of infections and malignancies was comparable. A single dose of rituximab as induction therapy did not reduce the overall incidence of BPAR, but might be beneficial in immunologically high‐risk patients. Treatment with rituximab was safe. We evaluated the efficacy and safety of rituximab as induction therapy in renal transplant patients. In a double‐blind, placebo‐controlled study, 280 adult renal transplant patients were randomized between a single dose of rituximab (375 mg/m2) or placebo during transplant surgery. Patients were stratified according to panel‐reactive antibody (PRA) value and rank number of transplantation. Maintenance immunosuppression consisted of tacrolimus, mycophenolate mofetil and steroids. The primary endpoint was the incidence of biopsy proven acute rejection (BPAR) within 6 months after transplantation. The incidence of BPAR was comparable between rituximab‐treated (23/138, 16.7%) and placebo‐treated patients (30/142, 21.2%, p = 0.25). Immunologically high‐risk patients (PRA >6% or re‐transplant) not receiving rituximab had a significantly higher incidence of rejection (13/34, 38.2%) compared to other treatment groups (rituximab‐treated immunologically high‐risk patients, and rituximab‐ or placebo‐treated immunologically low‐risk (PRA ≤ 6% or first transplant) patients (17.9%, 16.4% and 15.7%, p = 0.004). Neutropenia (<1.5 × 109/L) occurred more frequently in rituximab‐treated patients (24.3% vs. 2.2%, p < 0.001). After 24 months, the cumulative incidence of infections and malignancies was comparable. A single dose of rituximab as induction therapy did not reduce the overall incidence of BPAR, but might be beneficial in immunologically high‐risk patients. Treatment with rituximab was safe. B cells may have a negative effect on transplant outcomes by differentiating into alloantibody-producing plasma cells, by presenting antigen to CD4 T cells, and via the production of pro-inflammatory cytokines. In particular, the damaging effects of alloantibody are well established, causing an increased frequency of acute and chronic antibody-mediated rejection (ABMR). Broadly speaking, patients can be stratified into three risk categories for antibody-mediated pathology; at lowest risk, are nonsensitized patients, of whom around 25% develop de novo alloantibodies posttransplant. At intermediate risk, are sensitized patients with preformed graft-specific memory B cells, who may not have high levels of antibody at the time of transplantation, but may rapidly develop a recall response, resulting in alloantibody formation and ABMR. At highest risk are sensitized patients with detectable donor-specific antibody (DSA), in whom pretransplant antibody reduction therapy is required for transplantation to proceed. Currently, we do not have effective treatment strategies for either acute or chronic ABMR, nor do we have any clear strategy for negating the risks associated with allo-immune memory. With evidence showing a role for B cells in ABMR, and potentially in T cell–mediated rejection (TCMR), there has been significant interest in using immunotherapeutics that specifically target B cells at induction. In theory, such a strategy might inhibit the emergence of alloreactive B cells, deplete preexisting alloreactive B cells and could limit the provision of B cell help to T cells. However, as these trials began recruiting, evidence accumulated that B cells can regulate immune responses and are important players in transplant tolerance (1.Stolp J Turka LA Woods K. B cells with immune-regulating function in transplantation.Nat Rev Neph. 2014; 10: 389-397Crossref PubMed Scopus (46) Google Scholar). This raised the possibility that indiscriminate B cell depletion might have some undesirable effects. Prior to the study by van den Hoogen et al (2.van den Hoogen MWF Kamburova EG Baas MC Rituximab as induction therapy after renal transplantation: A randomized, double-blind, placebo-controlled study of efficacy and safety.Am J Transplant. 2015; 15 (et al): 407-416Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar) two randomized controlled trials (RCT) had utilized rituximab at induction in renal transplantation; Tyden et al (3.Tyden G Ekberg H Tufveson G Mjornstedt L. A randomized, double-blind, placebo-controlled study of single dose rituximab as induction in renal transplantation: A 3-year follow-up.Transplantation. 2012; 94: e21-e22Crossref PubMed Scopus (0) Google Scholar) randomized 140 patients to receive a single dose of rituximab or placebo at the time of transplantation. There was no statistically significant difference in the rate of TCMR at 6 months (11.7% and 17.6% in rituximab and placebo groups, respectively). Three-year follow-up demonstrated similar rates of ABMR, TCMR, and de novo DSA formation, but worrying, mortality was significantly higher in rituximab-treated patients (3.Tyden G Ekberg H Tufveson G Mjornstedt L. A randomized, double-blind, placebo-controlled study of single dose rituximab as induction in renal transplantation: A 3-year follow-up.Transplantation. 2012; 94: e21-e22Crossref PubMed Scopus (0) Google Scholar). We began an RCT in which patients were treated with two doses of rituximab, one at day 0 and one at day 7, with a steroid-free maintenance regimen but were forced to halt recruitment after 13 patients, due to an excess rate of TCMR in the rituximab group (83% vs. 14% in basiliximab-treated controls) (4.Clatworthy MR Watson CJ Plotnek G B-cell-depleting induction therapy and acute cellular rejection.N Engl J Med. 2009; 360 (et al): 2683-2685Crossref PubMed Scopus (195) Google Scholar). We hypothesized that this may have been due to inadvertent depletion of regulatory B cells. The work by van den Hoogen et al used an identical regimen to Tyden et al and supports the safety of employing a regimen in which a single dose of rituximab is administered at the time of transplantation (2.van den Hoogen MWF Kamburova EG Baas MC Rituximab as induction therapy after renal transplantation: A randomized, double-blind, placebo-controlled study of efficacy and safety.Am J Transplant. 2015; 15 (et al): 407-416Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). This was a large, well-powered study that randomized 139 patients to receive rituximab (375 mg/m2) and 142 patients to receive placebo. Maintenance immunosuppression consisted of tacrolimus, mycophenolate mofetil and steroids. Exclusion criteria included patients with an HLA 0-0-0 mismatched kidney and those with panel reactive antibodies (PRA) of >85%. Overall, the incidence of biopsy proven acute rejection (BPAR) was similar in rituximab and placebo-treated patients (16.7% and 21.2% respectively, p = ns) as was ABMR (2.9% and 7.7%, respectively, p = 0.11). However, when considering patients with higher immunological risk (sensitized patients with PRA >6% or re-transplants), those receiving rituximab (n = 28) had a lower rate of rejection than those receiving placebo (n = 34, 17.9% vs. 38.2% respectively, p = 0.06). Of note, a quarter of all patients treated with rituximab developed neutropaenia (as has been described previously), but this was not associated with an excess of infection. Reassuringly, with a median follow up of 4 years, no excess mortality was observed in the rituximab group. The halving of the rate of BPAR in high-risk patients receiving rituximab is exciting and raises the question of whether this practice should be adopted in this difficult group. However, given the relatively small numbers in this subset analysis (n = 62 in total), we need more evidence, namely a trial using B cell–targeted therapy in larger numbers of sensitized, “intermediate risk” patients. These observations also suggest that a single dose of rituximab may impact on graft-specific immunological memory. Indeed, the authors noted that the higher risk group did have an increased number of circulating CD27+ memory B cells pretransplant, but unfortunately did not have the opportunity to assess how this changed posttransplant. Of note, Kamburova et al have previously demonstrated that a single dose of rituximab has little impact on lymph node resident CD27+ B cells (5.Kamburova EG Koenen HJ Borgman KJ Ten Berge IJ Joosten I Hilbrands LB. A single dose of rituximab does not deplete B cells in secondary lymphoid organs but alters phenotype and function.Am J Transplant. 2013; 13: 1503-1511Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar), therefore it may be that B cell depletion has a greater effect on the provision of help to memory T cells. This excellent trial (2.van den Hoogen MWF Kamburova EG Baas MC Rituximab as induction therapy after renal transplantation: A randomized, double-blind, placebo-controlled study of efficacy and safety.Am J Transplant. 2015; 15 (et al): 407-416Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar) does have some limitations. One might argue that the control arm did not receive current “standard of care,” since many centers would use a biologic agent at induction, particularly for higher immunological risk patients. It would be, perhaps, more relevant to assess how B cell depletion compares with CD25 blockade as an induction agent. In addition, patients were not routinely screened for DSA and protocol biopsies were not performed. Thus, we are not able to judge the effect of B cell depletion on key parameters of humoral alloimmunity, including biopsy evidence of chronic ABMR. It would also have been of interest to perform peripheral lymphocyte phenotyping posttransplant in these patients to assess the effects of rituximab on the T and B cell memory compartment. Despite these limitations, the study by van den Hoogen et al (2.van den Hoogen MWF Kamburova EG Baas MC Rituximab as induction therapy after renal transplantation: A randomized, double-blind, placebo-controlled study of efficacy and safety.Am J Transplant. 2015; 15 (et al): 407-416Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar) raises the possibility that B cell depletion at induction can blunt the impact of immunological memory and certainly provides the impetus for further trials to assess the efficacy of rituximab or other B cell–targeted therapies in higher immunological risk patients. The author of this manuscript has no conflicts of interest to disclose as described by the American Journal of Transplantation.