Bechara Mfarrej

A novel role of CD4 Th17 cells in mediating cardiac allograft rejection and vasculopathy

T-bet plays a crucial role in Th1 development. We investigated the role of T-bet in the development of allograft rejection in an established MHC class II–mismatched (bm12 into B6) model of chronic allograft vasculopathy (CAV). Intriguingly, and in contrast to IFN-γ−/− mice that are protected from CAV, T-bet−/− recipients develop markedly accelerated allograft rejection accompanied by early severe vascular inflammation and vasculopathy, and infiltration by predominantly IL-17–producing CD4 T cells. Concurrently, T-bet−/− mice exhibit a T helper type 1 (Th1)–deficient environment characterized by profound IFN-γ deficiency, a Th2 switch characterized by increased production of interleukin (IL) 4, IL-5, IL-10, and IL-13 cytokines, as well as increased production of the proinflammatory cytokines IL-6, IL-12p40, and IL-17. Neutralization of IL-17 inhibits accelerated allograft rejection and vasculopathy in T-bet−/− mice. Interestingly, CD4 but not CD8 T cell deficiency in T-bet−/− mice affords dramatic protection from vasculopathy and facilitates long-term graft acceptance. This is the first study establishing that in the absence of Th1-mediated alloimmune responses, CD4 Th17 cells mediate an aggressive proinflammatory response culminating in severe accelerated allograft rejection and vasculopathy. These results have important implications for the development of novel therapies to target this intractable problem in clinical solid organ transplantation.

Deleterious Effect of CTLA4-Ig on a Treg-Dependent Transplant Model

Blockade of the B7:CD28 costimulatory pathway has emerged as a promising therapy to prevent allograft rejection. However, results from the belatacept phase III clinical trial demonstrated a higher rejection rate when compared to cyclosporine, raising concern about potential deleterious effects of this agent. In this study, we investigated the consequences of B7:CD28 blockade by hCTLA4Ig on regulator T cell (Treg) generation in different major histocompatibility complex (MHC) mismatch transplant models. Administration of hCTLA4Ig significantly decreased the amount of Tregs in B6 WT animals and this effect was predominant in thymus‐induced Tregs (Helios+). Although hCTLA4Ig prevented rejection in a fully allogeneic mismatch model, it accelerated rejection in a MHC class‐II mismatch model (MST = 26, p < 0.0001), in which long‐term allograft survival is dependent on Tregs. This accelerated rejection was associated with a marked reduction in thymus‐induced Tregs and led to a higher effector/regulatory T‐cell ratio in secondary lymphoid organs and in the allograft. This study confirms the importance of the B7:CD28 pathway in Treg homeostasis in an in vivo transplant model and suggests that hCTLA4Ig therapy may be deleterious in circumstances where engraftment is dependent on Tregs.

The Link between the PDL1 Costimulatory Pathway and Th17 in Fetomaternal Tolerance

Fetomaternal tolerance has been shown to depend both on regulatory T cells (Tregs) and negative signals from the PD1–PDL1 costimulatory pathway. More recently, IL-17–producing T cells (Th17) have been recognized as a barrier in inducing tolerance in transplantation. In this study, we investigate the mechanisms of PDL1-mediated regulation of fetomaternal tolerance using an alloantigen-specific CD4+ TCR transgenic mouse model system (ABM-tg mouse). PDL1 blockade led to an increase in embryo resorption and a reduction in litter size. This was associated with a decrease in Tregs, leading to a lower Treg/effector T cell ratio. Moreover, PDL1 blockade inhibited Ag-specific alloreactive T cell apoptosis and induced apoptosis of Tregs and a shift toward higher frequency of Th17 cells, breaking fetomaternal tolerance. These Th17 cells arose predominantly from CD4+Foxp3− cells, rather than from conversion of Tregs. Locally in the placenta, similar decrease in regulatory and apoptotic markers was observed by real-time PCR. Neutralization of IL-17 abrogated the anti-PDL1 effect on fetal survival rate and restored Treg numbers. Finally, the adoptive transfer of Tregs was also able to improve fetal survival in the setting of PDL1 blockade. This is to our knowledge the first report using an alloantigen-specific model that establishes a link between PDL1, Th17 cells, and fetomaternal tolerance.

Polylactide-cyclosporin A nanoparticles for targeted immunosuppression

Polymeric nanoparticles (NPs), prepared via coprecipitation of drugs and polymers, are promising drug delivery vehicles for treating diseases with improved efficacy and reduced toxicity. Here, we report an unprecedented strategy for preparing polylactide‐cyclosporine A (PLA‐CsA) NPs (termed CsA‐NPs) through CsA‐initiated ring‐opening polymerization of lactide (LA) followed by nanoprecipitation. The resulting CsA‐NPs have sub‐100 nm sizes and narrow particle size distributions, and release CsA in a sustained manner without a “burst”‐release effect. Both free CsA and CsA‐NPs displayed comparable suppression of T‐cell proliferation and production of inflammatory cytokines in various T‐cell assays in a dose‐dependent manner. The IC50 values for CsA and CsA‐NPs were 27.5 and 72.0 ng/ml, respectively. As lymph nodes are the main loci for T‐cell activation, we coupled dendritic cells (DCs) with CsA‐NPs and successfully delivered CsA selectively to the lymph nodes. Our studies indicated that CsA‐NPs could be internalized in the DCs with a sustained release of CsA to the culture medium, suppressing alloreactive T‐cell proliferation. Allogeneic DCs loaded with CsA‐NPs were able to migrate to the draining lymph nodes where the T‐cell priming was significantly reduced without any systemic release. This innovative nanoparticulate CsA delivery technology constitutes a strong basis for future targeted delivery of immunosuppressive drugs with improved efficiency and reduced toxicity.—Azzi, J., Tang, L., Tong, R., El Haddad, N., Akiyoshi, T., Mfarrej, B., Moore, R., Yang, S., Jurewicz, M., Ichimura, T., Lindeman, N., Cheng, J., Abdi, R. Polylactide‐cyclosporin A nanoparticles for targeted immunosuppression. FASEB J. 24, 3927–3938 (2010). www.fasebj.org

TIM-3: A Novel Regulatory Molecule of Alloimmune Activation

T cell Ig domain and mucin domain (TIM)-3 has previously been established as a central regulator of Th1 responses and immune tolerance. In this study, we examined its functions in allograft rejection in a murine model of vascularized cardiac transplantation. TIM-3 was constitutively expressed on dendritic cells and natural regulatory T cells (Tregs) but only detected on CD4+FoxP3− and CD8+ T cells in acutely rejecting graft recipients. A blocking anti–TIM-3 mAb accelerated allograft rejection only in the presence of host CD4+ T cells. Accelerated rejection was accompanied by increased frequencies of alloreactive IFN-γ–, IL-6–, and IL-17–producing splenocytes, enhanced CD8+ cytotoxicity against alloantigen, increased alloantibody production, and a decline in peripheral and intragraft Treg/effector T cell ratio. Enhanced IL-6 production by CD4+ T cells after TIM-3 blockade plays a central role in acceleration of rejection. Using an established alloreactivity TCR transgenic model, blockade of TIM-3 increased allospecific effector T cells, enhanced Th1 and Th17 polarization, and resulted in a decreased frequency of overall number of allospecific Tregs. The latter is due to inhibition in induction of adaptive Tregs rather than prevention of expansion of allospecific natural Tregs. In vitro, targeting TIM-3 did not inhibit nTreg-mediated suppression of Th1 alloreactive cells but increased IL-17 production by effector T cells. In summary, TIM-3 is a key regulatory molecule of alloimmunity through its ability to broadly modulate CD4+ T cell differentiation, thus recalibrating the effector and regulatory arms of the alloimmune response.

Critical role of proinflammatory cytokine IL-6 in allograft rejection and tolerance.

The proinflammatory cytokine IL‐6 plays an important role in controlling T‐cell differentiation, especially the development of Th17 and regulatory T cells. To determine the function of IL‐6 in regulating allograft rejection and tolerance, BALB/c cardiac grafts were transplanted into wild‐type or IL‐6‐deficient C57BL/6 mice. We observed that production of IL‐6 and IFN‐γ was upregulated during allograft rejection in untreated wild‐type mice. In IL‐6‐deficient mice, IFN‐γ production was greater than that observed in wild‐type controls, suggesting that IL‐6 production affects Th1/Th2 balance during allograft rejection. CD28‐B7 blockade by CTLA4‐Ig inhibited IFN‐γ production in C57BL/6 recipients, but had no effect on the production of IL‐6. Although wild‐type C57BL/6 recipients treated with CTLA4‐Ig rejected fully MHC‐mismatched BALB/c heart transplants, treatment of IL‐6‐deficient mice with CTLA4‐Ig resulted in graft acceptance. Allograft acceptance appeared to result from the combined effect of costimulatory molecule blockade and IL‐6‐deficiency, which limited the differentiation of effector cells and promoted the migration of regulatory T cells into the grafts. These data suggest that the blockade of IL‐6, or its signaling pathway, when combined with strategies that inhibit Th1 responses, has a synergistic effect on the promotion of allograft acceptance. Thus, targeting the effects of IL‐6 production may represent an important part of costimulation blockade‐based strategies to promote allograft acceptance and tolerance.

TIM-3 Regulates Innate Immune Cells To Induce Fetomaternal Tolerance

TIM-3 is constitutively expressed on subsets of macrophages and dendritic cells. Its expression on other cells of the innate immune system and its role in fetomaternal tolerance has not yet been explored. In this study, we investigate the role of TIM-3–expressing innate immune cells in the regulation of tolerance at the fetomaternal interface (FMI) using an allogeneic mouse model of pregnancy. Blockade of TIM-3 results in accumulation of inflammatory granulocytes and macrophages at the uteroplacental interface and upregulation of proinflammatory cytokines. Furthermore, TIM-3 blockade inhibits the phagocytic potential of uterine macrophages resulting in a build up of apoptotic bodies at the uteroplacental interface that elicits a local immune response. In response to inflammatory cytokines, Ly-6ChiGneg monocytic myeloid–derived suppressor cells expressing inducible NO synthase and arginase 1 are induced. However, these suppressive cells fail to downregulate the inflammatory cascade induced by inflammatory granulocytes (Ly-6CintGhi) and apoptotic cells; the increased production of IFN-γ and TNF-α by inflammatory granulocytes leads to abrogation of tolerance at the FMI and fetal rejection. These data highlight the interplay between cells of the innate immune system at the FMI and their influence on successful pregnancy in mice.

Mesenchymal stem cells express serine protease inhibitor to evade the host immune response.

Clinical trials using mesenchymal stem cells (MSCs) have been initiated worldwide. An improved understanding of the mechanisms by which allogeneic MSCs evade host immune responses is paramount to regulating their survival after administration. This study has focused on the novel role of serine protease inhibitor (SPI) in the escape of MSCs from host immunosurveillance through the inhibition of granzyme B (GrB). Our data indicate bone marrow-derived murine MSCs express SPI6 constitutively. MSCs from mice deficient for SPI6 (SPI6(-/-)) exhibited a 4-fold higher death rate by primed allogeneic cytotoxic T cells than did wild-type MSCs. A GrB inhibitor rescued SPI6(-/-) MSCs from cytotoxic T-cell killing. Transduction of wild-type MSCs with MigR1-SPI6 also protected MSCs from cytotoxic T cell-mediated death in vitro. In addition, SPI6(-/-) MSCs displayed a shorter lifespan than wild-type MSCs when injected into an allogeneic host. We conclude that SPI6 protects MSCs from GrB-mediated killing and plays a pivotal role in their survival in vivo. Our data could serve as a basis for future SPI-based strategies to regulate the survival and function of MSCs after administration and to enhance the efficacy of MSC-based therapy for diseases.

Monocyte-secreted inflammatory cytokines are associated with transplant glomerulopathy in renal allograft recipients.

Background. Although there is ample evidence about the role of adaptive immunity in the development of chronic allograft dysfunction, little is known about the contribution of innate immunity to this process. Herein, we studied the relationship between inflammation, chronic biopsy scores, and anti-human leukocyte antigen (HLA) circulating alloantibodies in a cohort of 57 patients recruited at our center. Methods. Available biopsies (n=27) were graded for chronic lesion scores according to Banff criteria. The production of cytokines by peripheral blood mononuclear cells after 48 hr of culture under resting conditions was quantified by Luminex. Tumor necrosis factor (TNF)-&agr; secretion assay and depletion studies were used to identify the source of these cytokines. Results. There was a high correlation between the levels of interleukin (IL)-1&bgr;, IL-6, and TNF-&agr; (r>0.8, P<0.001 for all correlations). The levels of these cytokines were associated with transplant glomerulopathy (IL-1&bgr;, P=0.019; IL-6, P=0.015; and TNF-&agr;, P=0.006) but not with other chronic lesions or anti-HLA circulating alloantibodies. TNF-&agr; was predominantly secreted by monocytes (percent of TNF-&agr; secreting cells: 20.4±4.8 vs. 1.2±0.5 vs. 1.4±0.6 vs. 1.7±0.5 for CD14+, CD4+, CD8+, and CD19+ cells, respectively; all P<0.01 vs. CD14+). The levels of all three proinflammatory cytokines were significantly reduced after monocyte depletion. Intriguingly, cytokine levels increased after ex vivo depletion of regulatory T cells (all P<0.001). Conclusions. Taken together, these data suggest that in vivo–activated monocytes in peripheral blood spontaneously secrete proinflammatory cytokines in renal allograft recipients with transplant glomerulopathy and seem to be under the regulation of functional regulatory T cells in this setting.

Derivation and Validation of a Cytokine-Based Assay to Screen for Acute Rejection in Renal Transplant Recipients

BACKGROUND AND OBJECTIVES Acute rejection remains a problem in renal transplantation. This study sought to determine the utility of a noninvasive cytokine assay in screening of acute rejection. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS In this observational cross-sectional study, 64 patients from two centers were recruited upon admission for allograft biopsy to investigate acute graft dysfunction. Blood was collected before biopsy and assayed for a panel of 21 cytokines secreted by PBMCs. Patients were classified as acute rejectors or nonrejectors according to a classification rule derived from an initial set of 32 patients (training cohort) and subsequently validated in the remaining patients (validation cohort). RESULTS Although six cytokines (IL-1β, IL-6, TNF-α, IL-4, GM-CSF, and monocyte chemoattractant protein-1) distinguished acute rejectors in the training cohort, logistic regression modeling identified a single cytokine, IL-6, as the best predictor. In the validation cohort, IL-6 was consistently the most accurate cytokine (area under the receiver-operating characteristic curve, 0.85; P=0.006), whereas the application of a prespecified cutoff level, as determined from the training cohort, resulted in a sensitivity and specificity of 92% and 63%, respectively. Secondary analyses revealed a strong association between IL-6 levels and acute rejection after multivariate adjustment for clinical characteristics (P<0.001). CONCLUSIONS In this pilot study, the measurement of a single cytokine can exclude acute rejection with a sensitivity of 92% in renal transplant recipients presenting with acute graft dysfunction. Prospective studies are needed to determine the utility of this simple assay, particularly for low-risk or remote patients.