Combined standard and novel immunosuppressive substances affect B-lymphocyte function

Cellular Immunobiology and Molecular Mechanisms in Alloimmunity-Pathways of Immunosuppression

Current maintenance immunosuppression commonly comprises a synergistic combination of tacrolimus as calcineurin inhibitor (CNI), mycophenolic acid, and glucocorticoids. Therapy is often individualized by steroid withdrawal or addition of belatacept or inhibitors of the mechanistic target of rapamycin. This review provides a comprehensive overview of their mode of action, focusing on the cellular immune system. The main pharmacological action of CNIs is suppression of the interleukin-2 pathway that leads to inhibition of T cell activation. Mycophenolic acid inhibits the purine pathway and subsequently diminishes T and B cell proliferation but also exerts a variety of effects on almost all immune cells, including inhibition of plasma cell activity. Glucocorticoids exert complex regulation via genomic and nongenomic mechanisms, acting mainly by downregulating proinflammatory cytokine signatures and cell signaling. Belatacept is potent in inhibiting B/T cell interaction, preventing formation of antibodies; however, it lacks the potency of CNIs in preventing T cell-mediated rejections. Mechanistic target of rapamycin inhibitors have strong antiproliferative activity on all cell types interfering with multiple metabolic pathways, partly explaining poor tolerability, whereas their superior effector T cell function might explain their benefits in the case of viral infections. Over the past decades, clinical and experimental studies provided a good overview on the underlying mechanisms of immunosuppressants. However, more data are needed to delineate the interaction between innate and adaptive immunity to better achieve tolerance and control of rejection. A better and more comprehensive understanding of the mechanistic reasons for failure of immunosuppressants, including individual risk/benefit assessments, may permit improved patient stratification.

Impact of B Cell Depletion on COVID-19 in Kidney Transplant Recipients

Kidney transplant recipients are patients at high risk for coronavirus disease 2019 (COVID-19) due to being on immunosuppressive therapy. B cell depletion therapy, including rituximab, is an important strategy for ABO-incompatible transplants. However, knowledge about the effect of B cell depletion therapy on COVID-19 is lacking. Thirty kidney transplant recipients who developed COVID-19 were included in this study. To examine the impact of B cell depletion therapy, we retrospectively investigated the relationship between the background of the patients and the clinical outcome. Of the 30 patients, 13 received B cell depletion therapy. The median time between transplant and onset of COVID-19 was 6.1 years after transplantation; however, nine cases remained markedly depleted of CD19(+) cells (<4.0%). The patients were assigned to the normal (n = 21) and depletion groups (n = 9). Progression rates in the depletion and normal groups were 55.6% and 9.5%, respectively (p = 0.014). Furthermore, the survival rate was significantly lower in the depletion group (100% in the normal group vs. 66.7% in the depletion group; p = 0.021). B cell depletion therapy may have long-term effects and increase the risk of COVID-19 in kidney transplant recipients.

Anti-SARS-CoV-2 Revaccination Success in Kidney Transplant Recipients With No Initial Humoral Response Is Linked to Primary Vaccine Type

Background

While anti-SARS-CoV-2 vaccination success in kidney transplant recipients (KTR) after two doses and 1273-mRNA was associated with higher seroconversion rates compared to BNT162b2-mRNA in our “DIA-Vacc Study” (NCT04799808), it remains unclear whether this may also be the case in non-responding KTR after a third vaccination dose.

Materials and Methods

Non-responding KTR (after two mRNA vaccinations) were investigated 4.5–6 months after study enrollment at first vaccination. One hundred sixty-six of 193 received a third vaccination between 3.5 and 5 months after the initial study enrollment and were always investigated 4 weeks later, exploring humoral immune response (ELISA) and specific cellular responses (interferon-γ release assay). Sixty-seven of 193 measurements in KTR were done immediately before the third vaccination or in KTR without further vaccination at 4.5–6 months.

Results

Of 193 KTR with no initial immune response 4 weeks after the second vaccination, 106/87 were immunized twice with 1273-mRNA/BNT162b2-mRNA, respectively. Additional mRNA booster vaccination led to positive seroconversion rates of 30–50%, while 16% of the initial non-responders demonstrated a delayed seroconversion without any booster vaccination. Using logistic regression analysis, a positive IgG response after the third vaccination was 23% more likely if the primary vaccine type was 1273-mRNA compared to BNT162b2-mRNA (OR = 4.420, 95% CI [1.208–16.173], p = 0.025). Primary vaccine type, a weak anti-SpikeS1 IgG response 4 weeks after second vaccination (3.2–35.2 BAU/ml, p < 0.001) and a lack of MMF/MPA as part of the immunosuppressive treatment (trend, p = 0.06) but no other variables studied correlated with seroconversion success.

Conclusion

This observational study adds important evidence toward using 1273-mRNA as the primary mRNA vaccine type for immunosuppressed KTR.

Temporary antimetabolite treatment hold boosts SARS-CoV-2 vaccination-specific humoral and cellular immunity in kidney transplant recipients

Transplant recipients exhibit an impaired protective immunity after SARS-CoV-2 vaccination, potentially caused by mycophenolate (MPA) immunosuppression. Recent data from patients with autoimmune disorders suggest that temporary MPA hold might greatly improve booster vaccination outcomes. We applied a fourth dose of SARS-CoV-2 vaccine to 29 kidney transplant recipients during a temporary (5 weeks) MPA/azathioprine hold, who had not mounted a humoral immune response to previous vaccinations. Seroconversion until day 32 after vaccination was observed in 76% of patients, associated with acquisition of virus-neutralizing capacity. Interestingly, 21/25 (84%) calcineurin inhibitor–treated patients responded, but only 1/4 belatacept-treated patients responded. In line with humoral responses, counts and relative frequencies of spike receptor binding domain–specific (RBD-specific) B cells were markedly increased on day 7 after vaccination, with an increase in RBD-specific CD27⁺⁺CD38⁺ plasmablasts. Whereas overall proportions of spike-reactive CD4⁺ T cells remained unaltered after the fourth dose, frequencies were positively correlated with specific IgG levels. Importantly, antigen-specific proliferating Ki67⁺ and in vivo–activated programmed cell death 1–positive T cells significantly increased after revaccination during MPA hold, whereas cytokine production and memory differentiation remained unaffected. In summary, antimetabolite hold augmented all arms of immunity during booster vaccination. These data suggest further studies of antimetabolite hold in kidney transplant recipients.

Use of De Novo mTOR Inhibitors in Hypersensitized Kidney Transplant Recipients: Experience From Clinical Practice

BACKGROUND

It is commonly believed that mTOR inhibitors (mTORi) should not be used in high-immunological risk kidney transplant recipients due to a perceived increased risk of rejection. However, almost all trials that examined the association of optimal-dose mTORi with calcineurin inhibitor (CNI) have excluded hypersensitized recipients from enrollment.

METHODS

To shed light on this issue, we examined 71 consecutive patients with a baseline calculated panel reactive antibody (cPRA) ≥50% that underwent kidney transplantation from June 2013 to December 2016 in our unit. Immunosuppression was based on CNI (tacrolimus), steroids and alternatively mycophenolic acid (MPA; n = 38), or mTORi (either everolimus or sirolimus, n = 33, target trough levels 3-8 ng/mL).

RESULTS

Demographic and immunological risk profiles were similar, and almost 90% of patients in both groups received induction with lymphocyte-depleting agents. Cox-regression analysis of rejection-free survival revealed better results for mTORi versus MPA in terms of biopsy-proven acute rejection (hazard ratio [confidence interval], 0.32 [0.11-0.90], P = 0.031 at univariable analysis and 0.34 [0.11-0.95], P = 0.040 at multivariable analysis). There were no differences in 1-year renal function, Banff chronicity score at 3- and 12-month protocol biopsy and development of de novo donor-specific antibodies. Tacrolimus trough levels along the first year were not different between groups (12-mo levels were 8.72 ± 2.93 and 7.85 ± 3.07 ng/mL for MPA and mTORi group respectively, P = 0.277).

CONCLUSIONS

This single-center retrospective cohort analysis suggests that in hypersensitized kidney transplant recipients receiving tacrolimus-based immunosuppressive therapy similar clinical outcomes may be obtained using mTOR inhibitors compared to mycophenolate.

Pharmacodynamic Monitoring of mTOR Inhibitors

Pharmacodynamic (PD) monitoring may complement routine pharmacokinetic monitoring of mTOR inhibitors (mTORis) in an attempt to better guide individualized sirolimus (SRL) or everolimus (EVR) treatment after organ transplantation. This review focuses on current knowledge about PD biomarkers for personalized mTORi therapies. Different strategies have already been used in the evaluation of the pharmacodynamics of SRL and EVR as a proxy for their effects on the immune response after transplantation. These include measuring p70S6K (70 kDa ribosomal protein S6 kinase) activity, p70S6K phosphorylation (P-p70S6K), or P-S6 protein expression. Compared with Western blot and ELISA, phosphoflow cytometry can detect phosphorylated proteins and differentiate activation-induced changes of signaling molecules inside the cell from unstimulated populations of identical cells in the same sample. Alternatively, in patients receiving a combined therapy, the other PD approach is to consider biomarkers such as NFAT residual expression for calcineurin inhibitors or to evaluate nonspecific effects of the drugs such as lymphocyte proliferation, interleukin synthesis, specific peripheral blood T regulatory subsets, or lymphocyte surface antigens, which have the advantage to reflect the overall immunosuppressive status achieved. Although limited, the available data on mTOR pathway biomarkers seem promising. Before clinical implementation, the analytical methodologies must be standardized and cross-validated, and the selected biomarkers will have to demonstrate their clinical utility for SRL or EVR dose individualization in multicenter clinical trials.

Increased degradation of ATP is driven by memory regulatory T cells in kidney transplantation tolerance

Regulatory T cells were recently proposed as the central actor in operational tolerance after renal transplantation. Tolerant patients harbor increased FoxP3hi memory Treg frequency and increased demethylation in the Foxp3 Treg-specific demethylated region when compared to stable kidney recipients and exhibit greater memory Treg suppressive capacities and higher expression of the ectonucleotidase CD39. However, in this particular and unique situation the mechanisms of action of Tregs were not identified. Thus, we analyzed the ability of memory Tregs to degrade extracellular ATP in tolerant patients, healthy volunteers, and patients with stable graft function under immunosuppression and determined the role of immunosuppressive drugs on this process. The conserved proportion of memory Tregs leads to the establishment of a pro-tolerogenic balance in operationally tolerant patients. Memory Tregs in tolerant patients display normal capacity to degrade extracellular ATP/ADP. In contrast, memory Tregs from patients with stable graft function do not have this ability. Finally, in vitro, immunosuppressive drugs may favor the lower proportion of memory Tregs in stable patients, but they have no effect on CD39-dependent ATP degradation and do not explain memory Treg lack of extracellular ATP/ADP degradation ability. Thus, intrinsic active regulatory mechanisms may act long after immunosuppressive drug arrest in operationally tolerant patients and may contribute to kidney allograft tolerance via the maintenance of CD39 Treg function.

B Cell Markers of Operational Tolerance Can Discriminate Acute Kidney Allograft Rejection From Stable Graft Function

BACKGROUND

Recently, several B cell-related markers have been described to be upregulated during operational tolerance in kidney allograft recipients. Little data exist on these markers during allograft rejection.

METHODS

In this study, we investigated regulation-associated B-cell phenotypes in peripheral blood mononuclear cells (PBMCs) of kidney transplant recipients with (n=21) and without (n=22) acute rejection (AR). We also determined expression levels of the B cell-related genes, MS4A1, TCL1A, and CD79B, in PBMCs and isolated B cells. Patient samples were analyzed before transplantation at discharge and at time of AR before initiation of antirejection therapy or at matching timepoints in patients with stable graft function.

RESULTS

On transplantation, the peripheral CD19CD24CD38 transitional B cell subset strongly declined, regardless of the subsequent occurrence of AR. In contrast, the CD19CD27CD24 subset remained stable after transplantation in both patients groups. MS4A1 gene expression levels in PBMC were comparable between patient groups at all timepoints. In contrast, TCL1A expression levels increased in stable patients, but decreased in patients at the time of AR in both PBMC and isolated B cells. CD79B expression levels in stable patients were unaltered after transplantation in PBMC but showed an increase in the B cell fraction at discharge. At the time of AR, CD79B gene expression was significantly lower compared to stable patients, being most apparent in the B-cell fraction.

CONCLUSION

These results suggest that, in addition to being markers for immunologic unresponsiveness, gene expression levels of TCL1A and CD79B may also identify immune activation in the setting of kidney transplantation.