Quantification of CD4(+) T Cell Alloreactivity and Its Control by Regulatory T Cells Using Time-Lapse Microscopy and Immune Synapse Detection

Ex vivo delivery of regulatory T cells for control of alloimmune priming in the donor lung

Survival after lung transplantation (LTx) is hampered by uncontrolled inflammation and alloimmunity. Regulatory T cells (Tregs) are being studied as a cellular therapy in solid organ transplantation. Whether these systemically administered Tregs can function at the appropriate location and time is an important concern. We hypothesized that in vitro expanded, recipient-derived Tregs can be delivered to donor lungs prior to LTx via ex vivo lung perfusion (EVLP), maintaining their immunomodulatory ability.In a rat model, Wistar Kyoto (WKy) CD4⁺CD25^high Tregs were expanded in vitro prior to EVLP. Expanded Tregs were administered to Fisher 344 (F344) donor lungs during EVLP; left lungs were transplanted into WKy recipients. Treg localisation and function post-transplant were assessed. In a proof-of-concept experiment, cryopreserved expanded human CD4⁺CD25⁺CD127^low Tregs were thawed and injected into discarded human lungs during EVLP.Rat Tregs entered the lung parenchyma and retained suppressive function. Expanded Tregs had no adverse effect on donor lung physiology during EVLP; lung water as measured by wet-to-dry weight ratio was reduced by Treg therapy. The administered cells remained in the graft at 3 days post-transplant where they reduced activation of intragraft effector CD4⁺ T cells; these effects were diminished by day 7. Human Tregs entered the lung parenchyma during EVLP where they expressed key immunoregulatory molecules (CTLA4⁺, 4-1BB⁺, CD39⁺, and CD15s⁺).Pre-transplant Treg administration can inhibit alloimmunity within the lung allograft at early time points post- transplant. Our organ-directed approach has potential for clinical translation.

Lung transplantation for late-onset non-infectious chronic pulmonary complications of allogenic hematopoietic stem cell transplant

Background

Late onset non-infectious pulmonary complications (LONIPCs) following allogenic hematopoietic stem cell transplantation (allo-HSCT) confer a significant mortality risk. Lung transplantation (LTx) has the potential to provide survival benefit but the impact of prior allo-HSCT on post-LTx outcomes is not well studied.

Methods

This retrospective, single-centre cohort study assessed the post-LTx outcomes of adults with LONIPCs of allo-HSCT. Outcomes of LTx for LONIPCs were compared to propensity-score matched LTx controls (n = 38, non-HSCT) and recipients of re-LTx (n = 70) for chronic lung allograft dysfunction (CLAD).

Results

Nineteen patients underwent DLTx for LONIPCs of allo-HSCT between 2003 and 2019. Post-LTx survival was 50% at 5-years. Survival to 1-year post-LTx was similar to matched controls (p = 0.473). Survival, conditional on 1-year survival, was lower in the allo-HSCT cohort (p = 0.034). An increased risk of death due to infection was identified in the allo-HSCT cohort compared to matched controls (p = 0.003). Compared to re-LTx recipients, the allo-HSCT cohort had superior survival to 1-year post-LTx (p = 0.034) but conditional 1-year survival was similar (p = 0.145).

Conclusion

This study identifies an increased risk of post-LTx mortality in recipients with previous allo-HSCT, associated with infection. It supports the hypothesis that allo-HSCT LTx recipients are relatively more immunosuppressed than patients undergoing LTx for other indications. Optimisation of post-LTx immunosuppressive and antimicrobial strategies to account for this finding should be considered.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-021-01699-8.

CAR-Tregs as a Strategy for Inducing Graft Tolerance

Purpose of the review

The adoptive transfer of alloantigen-specific regulatory T cells (Tregs) following organ transplantation is an emerging treatment paradigm that may induce tolerance and reduce the risk for graft rejection. In particular, redirecting Treg specificity via expression of synthetic chimeric antigen receptors (CARs) has demonstrated therapeutic promise in several preclinical studies. In this review, we highlight recent progress and remaining barriers to the clinical translation of CAR-Treg therapies.

Recent findings

CAR Tregs targeting human leukocyte antigen (HLA)-A2 showed antigen-specific in vitro activation and superior in vivo protective function relative to polyclonal Tregs. Adoptively transferred anti-HLA-A2 CAR Tregs prolonged the survival of HLA-A2-positive grafts in humanized mouse models.

Summary

Donor HLA molecules are attractive candidate antigens to target with CAR Tregs in transplantation due to mismatched HLA only expressed on the transplanted organ. The feasibility of this approach has been demonstrated by several independent groups in recent years. However, substantial challenges in CAR design and preclinical modeling must be more extensively addressed prior to clinical application.

Foxp3+ Regulatory and Conventional CD4+ T Cells Display Similarly High Frequencies of Alloantigen-Reactive Cells

Foxp3⁺ regulatory T cells (Tregs) play a major role in acquired immune tolerance to allogenic transplants. Their suppressive activity is thought to require T cell receptor (TCR)-driven antigen recognition; little, however, is known about the fraction of Tregs able to recognize alloantigens within this T cell subset primarily educated against self-antigens. Performing transfer experiments of Tregs or conventional T cells (Tconv) into both lymphoreplete and lymphopenic mice, we observed a similarly high proportion of cells signaling through their TCR and proliferating in allogenic hosts. Furthermore, using an in vivo proliferation assay with limited T cell numbers infused into lymphopenic mice, we found that the overall frequency of alloreactive Tregs was similar if not higher to that of alloreactive Tconv. Overall our study highlights a noticeably high level of alloreactive Foxp3⁺ regulatory T cells accounting for their predominant role in transplantation tolerance.

Harnessing Advances in T Regulatory Cell Biology for Cellular Therapy in Transplantation

Cellular therapy with CD4FOXP3 T regulatory (Treg) cells is a promising strategy to induce tolerance after solid-organ transplantation or prevent graft-versus-host disease after transfer of hematopoietic stem cells. Treg cells currently used in clinical trials are either polyclonal, donor- or antigen-specific. Aside from variations in isolation and expansion protocols, however, most therapeutic Treg cell-based products are much alike. Ongoing basic science work has provided considerable new insight into multiple facets of Treg cell biology, including their stability, homing, and functional specialization; integrating these basic science discoveries with clinical efforts will support the development of next-generation therapeutic Treg cells with enhanced efficacy. In this review, we summarize recent advances in knowledge of how Treg cells home to lymphoid and peripheral tissues, and control antibody production and tissue repair. We also discuss newly appreciated pathways that modulate context-specific Treg cell function and stability. Strategies to improve and tailor Treg cells for cell therapy to induce transplantation tolerance are highlighted.

Measurement of T Cell Alloreactivity Using Imaging Flow Cytometry

The measurement of immunological reactivity to donor antigens in transplant recipients is likely to be crucial for the successful reduction or withdrawal of immunosuppression. The mixed leukocyte reaction (MLR), limiting dilution assays, and trans-vivo delayed-type hypersensitivity (DTH) assay have all been applied to this question, but these methods have limited predictive ability and/or significant practical limitations that reduce their usefulness.Imaging flow cytometry is a technique that combines the multiparametric quantitative powers of flow cytometry with the imaging capabilities of fluorescent microscopy. We recently made use of an imaging flow cytometry approach to define the proportion of recipient T cells capable of forming mature immune synapses with donor antigen-presenting cells (APCs). Using a well-characterized mouse heart transplant model, we have shown that the frequency of in vitro immune synapses among T-APC membrane contact events strongly predicted allograft outcome in rejection, tolerance, and a situation where transplant survival depends on induced regulatory T cells.The frequency of T-APC contacts increased with T cells from mice during acute rejection and decreased with T cells from mice rendered unresponsive to alloantigen. The addition of regulatory T cells to the in vitro system reduced prolonged T-APC contacts. Critically, this effect was also seen with human polyclonally expanded, naturally occurring regulatory T cells, which are known to control the rejection of human tissues in humanized mouse models. Further development of this approach may allow for a deeper characterization of the alloreactive T-cell compartment in transplant recipients. In the future, further development and evaluation of this method using human cells may form the basis for assays used to select patients for immunosuppression minimization, and it can be used to measure the impact of tolerogenic therapies in the clinic.

Quantification of CD4(+) T Cell Alloreactivity and Its Control by Regulatory T Cells Using Time-Lapse Microscopy and Immune Synapse Detection

Assays designed to select transplant recipients for immunosuppression withdrawal have met with limited success, perhaps because they measure events downstream of T cell-alloantigen interactions. Using in vitro time-lapse microscopy in a mouse transplant model, we investigated whether transplant outcome would result in changes in the proportion of CD4(+) T cells forming prolonged interactions with donor dendritic cells. By blocking CD4-MHC class II and CD28-B7 interactions, we defined immunologically relevant interactions as those ≥500 s. Using this threshold, T cell-dendritic cell (T-DC) interactions were examined in rejection, tolerance and T cell control mediated by regulatory T cells. The frequency of T-DC contacts ≥500 s increased with T cells from mice during acute rejection and decreased with T cells from mice rendered unresponsive to alloantigen. Regulatory T cells reduced prolonged T-DC contacts. Importantly, this effect was replicated with human polyclonally expanded naturally occurring regulatory T cells, which we have previously shown can control rejection of human tissues in humanized mouse models. Finally, in a proof-of-concept translational context, we were able to visualize differential allogeneic immune synapse formation in polyclonal CD4(+) T cells using high-throughput imaging flow cytometry.