Modeling the Potential of Treg-Based Therapies for Transplant Rejection: Effect of Dose, Timing, and Accumulation Site.

Introduction: The adoptive transfer of regulatory T cells (Tregs) has emerged as a method to promote graft tolerance. Clinical trials have demonstrated the safety of adoptive transfer and are now assessing their therapeutic efficacy. Strategies that generate large numbers of antigen specific Tregs are even more efficacious. However, the combinations of factors that influence the outcome of adoptive transfer are too numerous to be tested experimentally. Here, mathematical modeling is used to predict the most impactful treatment scenarios. Methods: We adapted our mathematical model of murine heart transplant rejection to simulate Treg adoptive transfer and to correlate therapeutic efficacy with Treg dose and timing, frequency of administration, and distribution of injected cells. Results: The model predicts that Tregs directly accumulating to the graft are more protective than Tregs localizing to draining lymph nodes. Inhibiting antigen-presenting cell maturation and effector functions at the graft site was more effective at modulating rejection than inhibition of T cell activation in lymphoid tissues. These complex dynamics define non-intuitive relationships between graft survival and timing and frequency of adoptive transfer. Conclusion: This work provides the framework for better understanding the impact of Treg adoptive transfer and will guide experimental design to improve interventions.

A short course of tofacitinib sustains the immunoregulatory effect of CTLA4-Ig in the presence of inflammatory cytokines and promotes long-term survival of murine cardiac allografts.

Costimulation blockade-based regimens are a promising strategy for management of transplant recipients. However, maintenance immunosuppression via CTLA4-Ig monotherapy is characterized by high frequency of rejection episodes. Recent evidence suggests that inflammatory cytokines contribute to alloreactive T cell activation in a CD28-independent manner, a reasonable contributor to the limited efficacy of CTLA4-Ig. In this study, we investigated the possible synergism of a combined short-term inhibition of cytokine signaling and CD28 engagement on the modulation of rejection. Our results demonstrate that the JAK/STAT inhibitor tofacitinib restored the immunomodulatory effect of CTLA4-Ig on mouse alloreactive T cells in the presence of inflammatory cytokines. Tofacitinib exposure conferred dendritic cells with a tolerogenic phenotype reducing their cytokine secretion and costimulatory molecules expression. JAK inhibition also directly affected T cell activation. In vivo, the combination of CTLA4-Ig and tofacitinib induced long-term survival of heart allografts and, importantly, it was equally effective when using grafts subjected to prolonged ischemia. Transplant survival correlated with a reduction in effector T cells and intragraft accumulation of regulatory T cells. Collectively, our studies demonstrate a powerful synergism between CTLA4-Ig and tofacitinib and suggest their combined use is a promising strategy for improved management of transplanted patients.

Multiphase Assembly of Small Molecule Microcrystalline Peptide Hydrogel Allows Immunomodulatory Combination Therapy for Long-Term Heart Transplant Survival.

Combination therapies that target multiple pathways involved in immune rejection of transplants hold promise for patients in need of restorative surgery. Herein, a noninteracting multiphase molecular assembly approach is developed to crystallize tofacitinib, a potent JAK1/3 inhibitor, within a shear-thinning self-assembled fibrillar peptide hydrogel network. The resulting microcrystalline tofacitinib hydrogel (MTH) can be syringe-injected directly to the grafting site during surgery to locally deliver the small molecule. The rate of drug delivered from MTH is largely controlled by the dissolution of the encapsulated microcrystals. A single application of MTH, in combination with systemically delivered CTLA4-Ig, a co-stimulation inhibitor, affords significant graft survival in mice receiving heterotopic heart transplants. Locoregional studies indicate that the local delivery of tofacitinib at the graft site enabled by MTH is required for the observed enhanced graft survival.

A Novel mTORC1-Dependent, Akt-Independent Pathway Differentiates the Gut Tropism of Regulatory and Conventional CD4 T Cells.

The vitamin A metabolite all-trans retinoic acid (ATRA) induces a gut-homing phenotype in activated CD4(+) conventional T cells (Tconv) by upregulating the integrin α4ß7 and the chemokine receptor CCR9. We report that, in contrast to mouse Tconv, only ∼50% of regulatory T cells (Treg) upregulate CCR9 when stimulated by physiological levels of ATRA, even though Tconv and Treg express similar levels of the retinoic acid receptor (RAR). The resulting bimodal CCR9 expression is not associated with differences in the extent of their proliferation, level of Foxp3 expression, or affiliation with naturally occurring Treg or induced Treg in the circulating Treg pool. Furthermore, we find that exposure of Treg to the mechanistic target of rapamycin (mTOR) inhibitor rapamycin suppresses upregulation of both CCR9 and α4ß7, an effect that is not evident with Tconv. This suggests that in Treg, ATRA-induced upregulation of CCR9 and α4ß7 is dependent on activation of a mTOR signaling pathway. The involvement of mTOR is independent of Akt activity, because specific inhibition of Akt, pyruvate dehydrogenase kinase-1, or its downstream target glycogen synthase kinase-3 did not prevent CCR9 expression. Additionally, Rictor (mTOR complex [mTORC]2)-deficient Treg showed unaltered ability to express CCR9, whereas Raptor (mTORC1)-deficient Treg were unable to upregulate CCR9, suggesting the selective participation of mTORC1. These findings reveal a novel difference between ATRA signaling and chemokine receptor induction in Treg versus Tconv and provide a framework via which the migratory behavior of Treg versus Tconv might be regulated differentially for therapeutic purposes.

Mechanisms of rejection in vascular composite allotransplantation.

PURPOSE OF REVIEW: For patients with devastating injuries in whom standard reconstruction is not an option, vascularized composite allotransplantation (VCA) has become a viable means of restoring form and function. However, immunological rejection continues to be a problem in VCA and has not yet been fully characterized. As the field is relatively new, much of the data on rejection and immunosuppression have been extrapolated from that of solid organ transplantation. In this review, we cover the basic mechanisms of rejection as they relate specifically to VCA with analysis of recent literature and future directions. RECENT FINDINGS: Recent clinical studies have supported previously postulated T-cell-mediated mechanism of acute rejection and have also made strides in differentiating rejection from inflammation from other skin conditions and with different treatment regimens. Antibody-mediated rejection has been described in recent cases as well as treatment of presensitized patients receiving VCAs. With more long-term grafts, chronic changes, including vasculopathy, are being reported. SUMMARY: Clinically observed types of rejection in VCA include mainly cell-mediated, antibody-mediated and chronic rejection. Advances in diagnosis and treatment of rejection have been made, but there is still much to be learned about VCA-specific rejection.

Migratory, and not lymphoid-resident, dendritic cells maintain peripheral self-tolerance and prevent autoimmunity via induction of iTreg cells.

There is evidence that dendritic cells (DCs) induce peripheral tolerance. Nevertheless, it is not known whether immature DCs in general are able to tolerize CD4(+) T cells or if this is a prerogative of specialized subtypes. Here we show that, when autoantigen presentation is extended to all conventional mouse DCs, immature lymphoid tissue resident DCs are unable to induce autoantigen-specific regulatory T (iTreg) cell conversion. In contrast, this is an exclusive prerogative of steady-state migratory DCs. Because only lymph nodes host migratory DCs, iTreg cells develop and are retained solely in lymph nodes, and not in the spleen. Mechanistically, in cutaneous lymph nodes, DC-derived CCL22 contributes to the retention of iTreg cells. The importance of the local generation of iTreg cells is emphasized by their essential role in preventing autoimmunity.

IL-27 production and STAT3-dependent upregulation of B7-H1 mediate immune regulatory functions of liver plasmacytoid dendritic cells.

Plasmacytoid dendritic cells (pDCs) are highly specialized APCs that, in addition to their well-recognized role in anti-viral immunity, also regulate immune responses. Liver-resident pDCs are considerably less immunostimulatory than those from secondary lymphoid tissues and are equipped to promote immune tolerance/regulation through various mechanisms. IL-27 is an IL-12 family cytokine that regulates the function of both APCs and T cells, although little is known about its role in pDC immunobiology. In this study, we show that mouse liver pDCs express higher levels of IL-27p28 and EBV-induced protein 3 (Ebi3) compared with those of splenic pDCs. Both populations of pDCs express the IL-27Rα/WSX-1; however, only liver pDCs significantly upregulate expression of the coregulatory molecule B7 homolog-1 (B7-H1) in response to IL-27. Inhibition of STAT3 activation completely abrogates IL-27-induced upregulation of B7-H1 expression on liver pDCs. Liver pDCs treated with IL-27 increase the percentage of CD4(+)Foxp3(+) T cells in MLR, which is dependent upon expression of B7-H1. pDCs from Ebi3-deficient mice lacking functional IL-27 show increased capacity to stimulate allogeneic T cell proliferation and IFN-γ production in MLR. Liver but not spleen pDCs suppress delayed-type hypersensitivity responses to OVA, an effect that is lost with Ebi3(-/-) and B7-H1(-/-) liver pDCs compared with wild-type liver pDCs. These data suggest that IL-27 signaling in pDCs promotes their immunoregulatory function and that IL-27 produced by pDCs contributes to their capacity to regulate immune responses in vitro and in vivo.

Selective expansion of allogeneic regulatory T cells by hepatic stellate cells: role of endotoxin and implications for allograft tolerance.

Hepatic stellate cells (HSCs) may play an important role in hepatic immune regulation by producing numerous cytokines/chemokines and expressing Ag-presenting and T cell coregulatory molecules. Due to disruption of the endothelial barrier during cold-ischemic storage and reperfusion of liver grafts, HSCs can interact directly with cells of the immune system. Endotoxin (LPS), levels of which increase in liver diseases and transplantation, stimulates the synthesis of many mediators by HSCs. We hypothesized that LPS-stimulated HSCs might promote hepatic tolerogenicity by influencing naturally occurring immunosuppressive CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs). Following their portal venous infusion, allogeneic CD4(+) T cells, including Tregs, were found closely associated with HSCs, and this association increased in LPS-treated livers. In vitro, both unstimulated and LPS-stimulated HSCs upregulated Fas (CD95) expression on conventional CD4(+) T cells and induced their apoptosis in a Fas/Fas ligand-dependent manner. By contrast, HSCs induced Treg proliferation, which required cell-cell contact and was MHC class II-dependent. This effect was augmented when HSCs were pretreated with LPS. LPS increased the expression of MHC class II, CD80, and CD86 and stimulated the production of IL-1α, IL-1ß, IL-6, IL-10 and TNF-α by HSCs. Interestingly, production of IL-1α, IL-1ß, IL-6, and TNF-α was strongly inhibited, but that of IL-10 enhanced in LPS-pretreated HSC/Treg cocultures. Adoptively transferred allogeneic HSCs migrated to the secondary lymphoid tissues and induced Treg expansion in lymph nodes. These data implicate endotoxin-stimulated HSCs as important immune regulators in liver transplantation by inducing selective expansion of tolerance-promoting Tregs and reducing inflammation and alloimmunity.

T cell responsiveness to IL-10 defines the immunomodulatory effect of costimulation blockade via anti-CD154 and impacts transplant survival.

Costimulation blockade (CoB)-based immunotherapy is a promising alternative to immunosuppression for transplant recipients; however, the current limited understanding of the factors that impact its efficacy restrains its clinical applicability. In this context, pro- and anti-inflammatory cytokines are being recognized as having an impact on T cell activation beyond effector differentiation. This study aims at elucidating the impact of direct IL-10 signaling in T cells on CoB outcomes. We used a full-mismatch skin transplantation model where recipients had a T cell-restricted expression of a dominant negative IL-10 receptor (10R-DN), alongside anti-CD154 as CoB therapy. Unlike wild-type recipients, 10R-DN mice failed to benefit from CoB. This accelerated graft rejection correlated with increased accumulation of T cells producing TNF-α, IFN-γ, and IL-17. In vitro experiments indicated that while lack of IL-10 signaling did not change the ability of anti-CD154 to modulate alloreactive T cell proliferation, the absence of this pathway heightened TH1 effector cell differentiation. Furthermore, deficiency of IL-10 signaling in T cells impaired Treg induction, a hallmark of anti-CD154 therapy. Overall, these findings unveil an important and novel role of IL-10 signaling in T cells that defines the success of CoB therapies and identifies a target pathway for obtaining robust immunoregulation.

Design and characterization of lipid nanocarriers for oral delivery of immunotherapeutic peptides.

The use of therapeutic proteins and peptides is of great interest for the treatment of many diseases, and advances in nanotechnology offer a path toward their stable delivery via preferred routes of administration. In this study, we sought to design and formulate a nanostructured lipid carrier (NLC) containing a nominal antigen (insulin peptide) for oral delivery. We utilized the design of experiments (DOE) statistical method to determine the dependencies of formulation variables on physicochemical particle characteristics including particle size, polydispersity (PDI), melting point, and latent heat of melting. The particles were determined to be non-toxic in vitro, readily taken up by primary immune cells, and found to accumulate in regional lymph nodes following oral administration. We believe that this platform technology could be broadly useful for the treatment of autoimmune diseases by supporting the development of oral delivery-based antigen specific immunotherapies.

Tregs integrate native and CAR-mediated costimulatory signals for control of allograft rejection.

Tregs expressing chimeric antigen receptors (CAR-Tregs) are a promising tool to promote transplant tolerance. The relationship between CAR structure and Treg function was studied in xenogeneic, immunodeficient mice, revealing advantages of CD28-encoding CARs. However, these models could underrepresent interactions between CAR-Tregs, antigen-presenting cells (APCs), and donor-specific Abs. We generated Tregs expressing HLA-A2-specific CARs with different costimulatory domains and compared their function in vitro and in vivo using an immunocompetent model of transplantation. In vitro, the CD28-encoding CAR had superior antigen-specific suppression, proliferation, and cytokine production. In contrast, in vivo, Tregs expressing CARs encoding CD28, ICOS, programmed cell death 1, and GITR, but not 4-1BB or OX40, all extended skin allograft survival. To reconcile in vitro and in vivo data, we analyzed effects of a CAR encoding CD3ζ but no costimulatory domain. These data revealed that exogenous costimulation from APCs can compensate for the lack of a CAR-encoded CD28 domain. Thus, Tregs expressing a CAR with or without CD28 are functionally equivalent in vivo, mediating similar extension of skin allograft survival and controlling the generation of anti-HLA-A2 alloantibodies. This study reveals a dimension of CAR-Treg biology and has important implications for the design of CARs for clinical use in Tregs.

Bioengineered particles expand myelin-specific regulatory T cells and reverse autoreactivity in a mouse model of multiple sclerosis.

Multiple sclerosis (MS) is an autoimmune disease characterized by autoreactive immune cells damaging myelinated nerves, impairing brain function. Treatments aim for tolerance induction to reeducate the immune system to recognize myelin as "self" rather than "foreign." As peripheral immune tolerance is primarily mediated by regulatory T cells (Tregs), we developed a therapy to support Treg expansion and activity in vivo. To target, engage, and activate myelin-specific Tregs, we designed a biodegradable microparticle (MP) loaded with rapamycin and functionalized with a biased interleukin-2 (IL-2) fusion protein and a major histocompatibility complex (MHC) class II loaded with a myelin peptide. These tolerogenic MPs (Tol-MPs) were validated in vitro and then evaluated in a mouse model of MS, experimental autoimmune encephalomyelitis (EAE). Tol-MPs promoted sustained disease reversal in 100% of mice and full recovery in 38% of mice with symptomatic EAE. Tol-MPs are a promising platform for treatment of autoimmune diseases.

Mammalian target of rapamycin inhibition and alloantigen-specific regulatory T cells synergize to promote long-term graft survival in immunocompetent recipients.

Minimization of immunosuppression and donor-specific tolerance to MHC-mismatched organ grafts are important clinical goals. The therapeutic potential of regulatory T cells (Tregs) has been demonstrated, but conditions for optimizing their in vivo function posttransplant in nonlymphocyte-depleted hosts remain undefined. In this study, we address mechanisms through which inhibition of the mammalian target of rapamycin (Rapa) synergizes with alloantigen-specific Treg (AAsTreg) to permit long-term, donor-specific heart graft survival in immunocompetent hosts. Crucially, immature allogeneic dendritic cells allowed AAsTreg selection in vitro, with minimal expansion of unwanted (Th17) cells. The rendered Treg potently inhibited T cell proliferation in an Ag-specific manner. However, these AAsTreg remained unable to control T cells stimulated by allogeneic mature dendritic cells, a phenomenon dependent on the release of proinflammatory cytokines. In vivo, Rapa administration reduced danger-associated IL-6 production, T cell proliferation, and graft infiltration. Based on these observations, AAsTreg were administered posttransplant (day 7) in combination with a short course of Rapa and rendered >80% long-term (>150 d) graft survival, a result superior to that achieved with polyclonal Treg. Moreover, graft protection was alloantigen-specific. Significantly, long-term graft survival was associated with alloreactive T cell anergy. These findings delineate combination of transient mammalian target of Rapa inhibition with appropriate AAsTreg selection as an effective approach to promote long-term organ graft survival.

Targeting inflammation and immune activation to improve CTLA4-Ig-based modulation of transplant rejection.

For the last few decades, Calcineurin inhibitors (CNI)-based therapy has been the pillar of immunosuppression for prevention of organ transplant rejection. However, despite exerting effective control of acute rejection in the first year post-transplant, prolonged CNI use is associated with significant side effects and is not well suited for long term allograft survival. The implementation of Costimulation Blockade (CoB) therapies, based on the interruption of T cell costimulatory signals as strategy to control allo-responses, has proven potential for better management of transplant recipients compared to CNI-based therapies. The use of the biologic cytotoxic T-lymphocyte associated protein 4 (CTLA4)-Ig is the most successful approach to date in this arena. Following evaluation of the BENEFIT trials, Belatacept, a high-affinity version of CTLA4-Ig, has been FDA approved for use in kidney transplant recipients. Despite its benefits, the use of CTLA4-Ig as a monotherapy has proved to be insufficient to induce long-term allograft acceptance in several settings. Multiple studies have demonstrated that events that induce an acute inflammatory response with the consequent release of proinflammatory cytokines, and an abundance of allograft-reactive memory cells in the recipient, can prevent the induction of or break established immunomodulation induced with CoB regimens. This review highlights advances in our understanding of the factors and mechanisms that limit CoB regimens efficacy. We also discuss recent successes in experimentally designing complementary therapies that favor CTLA4-Ig effect, affording a better control of transplant rejection and supporting their clinical applicability.

Engineered human cytokine/antibody fusion proteins expand regulatory T cells and confer autoimmune disease protection.

Low-dose human interleukin-2 (hIL-2) treatment is used clinically to treat autoimmune disorders due to the cytokine's preferential expansion of immunosuppressive regulatory T cells (Tregs). However, off-target immune cell activation and short serum half-life limit the clinical potential of IL-2 treatment. Recent work showed that complexes comprising hIL-2 and the anti-hIL-2 antibody F5111 overcome these limitations by preferentially stimulating Tregs over immune effector cells. Although promising, therapeutic translation of this approach is complicated by the need to optimize dosing ratios and by the instability of the cytokine/antibody complex. We leverage structural insights to engineer a single-chain hIL-2/F5111 antibody fusion protein, termed F5111 immunocytokine (IC), which potently and selectively activates and expands Tregs. F5111 IC confers protection in mouse models of colitis and checkpoint inhibitor-induced diabetes mellitus. These results provide a roadmap for IC design and establish a Treg-biased immunotherapy that could be clinically translated for autoimmune disease treatment.

Jakinibs of All Trades: Inhibiting Cytokine Signaling in Immune-Mediated Pathologies.

Over the last 25 years, inhibition of Janus kinases (JAKs) has been pursued as a modality for treating various immune and inflammatory disorders. While the clinical development of JAK inhibitors (jakinibs) began with the investigation of their use in allogeneic transplantation, their widest successful application came in autoimmune and allergic diseases. Multiple molecules have now been approved for diseases ranging from rheumatoid and juvenile arthritis to ulcerative colitis, atopic dermatitis, graft-versus-host-disease (GVHD) and other inflammatory pathologies in 80 countries around the world. Moreover, two jakinibs have also shown surprising efficacy in the treatment of hospitalized coronavirus disease-19 (COVID-19) patients, indicating additional roles for jakinibs in infectious diseases, cytokine storms and other hyperinflammatory syndromes. Jakinibs, as a class of pharmaceutics, continue to expand in clinical applications and with the development of more selective JAK-targeting and organ-selective delivery. Importantly, jakinib safety and pharmacokinetics have been investigated alongside clinical development, further cementing the potential benefits and limits of jakinib use. This review covers jakinibs that are approved or are under late phase investigation, focusing on clinical applications, pharmacokinetic and safety profiles, and future opportunities and challenges.

Vascularized composite allotransplantation combined with costimulation blockade induces mixed chimerism and reveals intrinsic tolerogenic potential.

Vascularized composite allotransplantation (VCA) has become a valid therapeutic option to restore form and function after devastating tissue loss. However, the need for high-dose multidrug immunosuppression to maintain allograft survival is still hampering more widespread application of VCA. In this study, we investigated the immunoregulatory potential of costimulation blockade (CoB; CTLA4-Ig and anti-CD154 mAb) combined with nonmyeoablative total body irradiation (TBI) to promote allograft survival of VCA in a fully MHC-mismatched mouse model of orthotopic hind limb transplantation. Compared with untreated controls (median survival time [MST] 8 days) and CTLA4-Ig treatment alone (MST 17 days), CoB treatment increased graft survival (MST 82 days), and the addition of nonmyeloablative TBI led to indefinite graft survival (MST > 210 days). Our analysis suggests that VCA-derived BM induced mixed chimerism in animals treated with CoB and TBI + CoB, promoting gradual deletion of alloreactive T cells as the underlying mechanism of long-term allograft survival. Acceptance of donor-matched secondary skin grafts, decreased ex vivo T cell responsiveness, and increased graft-infiltrating Tregs further indicated donor-specific tolerance induced by TBI + CoB. In summary, our data suggest that vascularized BM-containing VCAs are immunologically favorable grafts promoting chimerism induction and long-term allograft survival in the context of CoB.

A computational model for microcirculation including Fahraeus-Lindqvist effect, plasma skimming and fluid exchange with the tissue interstitium.

We present a two-phase model for microcirculation that describes the interaction of plasma with red blood cells. The model takes into account of typical effects characterizing the microcirculation, such as the Fahraeus-Lindqvist effect and plasma skimming. Besides these features, the model describes the interaction of capillaries with the surrounding tissue. More precisely, the model accounts for the interaction of capillary transmural flow with the surrounding interstitial pressure. Furthermore, the capillaries are represented as one-dimensional channels with arbitrary, possibly curved configuration. The latter two features rely on the unique ability of the model to account for variations of flow rate and pressure along the axis of the capillary, according to a local differential formulation of mass and momentum conservation. Indeed, the model stands on a solid mathematical foundation, which is also addressed in this work. In particular, we present the model derivation, the variational formulation, and its approximation using the finite element method. Finally, we conclude the work with a comparative computational study of the importance of the Fahraeus-Lindqvist, plasma skimming, and capillary leakage effects on the distribution of flow in a microvascular network.

High PD-L1/CD86 ratio on plasmacytoid dendritic cells correlates with elevated T-regulatory cells in liver transplant tolerance.

BACKGROUND: Both dendritic cells (DC) and T-regulatory cells (Treg) have been implicated in regulation of alloimmune responses and transplant tolerance. METHODS: We analyzed B7 coregulatory molecule expression on circulating DC subset precursors, together with CD4+CD25(hi) Foxp3+ Treg by rare event, flow cytometric analysis in operationally tolerant pediatric liver transplant recipients (TOL), those undergoing prospective immunosuppressive drug weaning (PW) or maintenance immunosuppression (MI), and normal healthy individuals (controls). RESULTS: Use of DC subset-specific monoclonal antibodies confirmed elevated precursor plasmacytoid DC/myeloid DC ratios in TOL and PW compared with MI. In addition, Treg frequencies were higher in TOL than in PW and MI, but not controls. While there was no difference in levels of costimulatory and coinhibitory molecules on precursor myeloid DC between the groups, the programmed death ligand-1 (PD-L1=B7-H1):CD86 (B7-2) ratio on precursor plasmacytoid DC was significantly higher in TOL than MI and correlated with the Treg frequency. There was no relation between prednisone or tacrolimus dose or tacrolimus trough level and either the PD-L1/CD86 ratio on plasmacytoid DC or the Treg frequency. Moreover, clinically relevant concentrations of dexamethasone or tacrolimus did not affect these values in short-term culture. CONCLUSION: These novel findings suggest a possible functional relationship between the enhanced incidence of precursor plasmacytoid DC, their comparatively high relative expression of the coinhibitory molecule PD-L1, and the elevated frequency of Treg in operational liver transplant tolerance.