Absence of TSC1 Accelerates CD8+ T cell-mediated Acute Cardiac Allograft Rejection

Tuberous sclerosis complex (TSC) is an autosomal dominant disease caused by inactivating mutations in TSC1 or TSC2.Patients with TSC often require organ transplantation after organ failure. TSC1 serves as an important control node in immune cell development and responses; however, its effect on T cells in transplant immunity has not yet been explored. Here, we characterized the effect of TSC1 deficiency in T cells on acute allograft rejection using a mouse cardiac transplantation model. We observed compromised allograft survival in mice with TSC1-deficient T cells. Notably, the allografts in mice transferred with TSC1-deficient CD8⁺T cells showed accelerated acute allograft rejection. TSC1 deficiency triggered the increased accumulation of CD8⁺ T cells in allografts due to augmented infiltration caused by increased CXCR3 expression levels and elevated in-situ proliferation of TSC1-deficient CD8⁺ T cells. Compared to CD8⁺ T cells from wild-type (WT) mice, TSC1-deficient CD8⁺ T cells exhibited enhanced cell proliferation and increased expression levels of interferon-γ and granzyme B after alloantigen stimulation. Rapamycin, an inhibitor of mammalian target of rapamycin (mTOR), is used to treat patients with TSC and prevent rejection after solid-organ transplantation. Although rapamycin induced most cardiac allografts to survive beyond 100 d in WT mice, rapamycin-treated cardiac allografts in TSC1-deficient mice were rejected within 60 d. These results suggest that TSC1-deficient recipients may be more resistant to rapamycin-mediated immunosuppression during organ transplantation. Collectively, TSC1 significantly accelerates acute allograft rejection by enhancing the alloreactivity of CD8⁺ T cells, making them more resistant to mTOR inhibitor-mediated immunosuppression.

Nucleic acid scavenging microfiber mesh inhibits trauma-induced inflammation and thrombosis

Trauma patients produce a host of danger signals and high levels of damage-associated molecular patterns (DAMPs) after cellular injury and tissue damage. These DAMPs are directly and indirectly involved in the pathogenesis of various inflammatory and thrombotic complications in patients with severe injuries. No effective therapeutic agents for the removal of DAMPs from blood or tissue fluid have been developed. Herein, we demonstrated that nucleic acid binding polymers, e.g., polyethylenimine (PEI) and polyamidoamine dendrimers, immobilized onto electrospun microfiber mesh can effectively capture various DAMPs, such as extracellular DNAs and high mobility group box 1 (HMGB1). Furthermore, treatment with PEI-immobilized microfiber mesh abrogated the ability of DAMPs, released from dead and dying cells in culture or found in patients following traumatic injury, to activate innate immune responses and coagulation in vitro and in vivo. Nucleic acid scavenging microfiber meshes represent an effective strategy to combat inflammation and thrombosis in trauma.

Rapamycin Interferes With Postdepletion Regulatory T Cell Homeostasis and Enhances DSA Formation Corrected by CTLA4-Ig

Previously, we demonstrated that alemtuzumab induction with rapamycin as sole maintenance therapy is associated with an increased incidence of humoral rejection in human kidney transplant patients. To investigate the role of rapamycin in posttransplant humoral responses after T cell depletion, fully MHC mismatched hearts were transplanted into hCD52Tg mice, followed by alemtuzumab treatment with or without a short course of rapamycin. While untreated hCD52Tg recipients acutely rejected B6 hearts (n = 12), hCD52Tg recipients treated with alemtuzumab alone or in conjunction with rapamycin showed a lack of acute rejection (MST > 100). However, additional rapamycin showed a reduced beating quality over time and increased incidence of vasculopathy. Furthermore, rapamycin supplementation showed an increased serum donor-specific antibodies (DSA) level compared to alemtuzumab alone at postoperation days 50 and 100. Surprisingly, additional rapamycin treatment significantly reduced CD4(+) CD25(+) FoxP3(+) T reg cell numbers during treatment. On the contrary, ICOS(+) PD-1(+) CD4 follicular helper T cells in the lymph nodes were significantly increased. Interestingly, CTLA4-Ig supplementation in conjunction with rapamycin corrected rapamycin-induced accelerated posttransplant humoral response by directly modulating Tfh cells but not Treg cells. This suggests that rapamycin after T cell depletion could affect Treg cells leading to an increase of Tfh cells and DSA production that can be reversed by CTLA4-Ig.

The role of B cells in solid organ transplantation

The role of antibodies in chronic injury to organ transplants has been suggested for many years, but recently emphasized by new data. We have observed that when immunosuppressive potency decreases either by intentional weaning of maintenance agents or due to homeostatic repopulation after immune cell depletion, the threshold of B cell activation may be lowered. In human transplant recipients the result may be donor-specific antibody, C4d+ injury, and chronic rejection. This scenario has precise parallels in a rhesus monkey renal allograft model in which T cells are depleted with CD3 immunotoxin, or in a CD52-T cell transgenic mouse model using alemtuzumab to deplete T cells. Such animal models may be useful for the testing of therapeutic strategies to prevent DSA. We agree with others who suggest that weaning of immunosuppression may place transplant recipients at risk of chronic antibody-mediated rejection, and that strategies to prevent this scenario are needed if we are to improve long-term graft and patient outcomes in transplantation. We believe that animal models will play a crucial role in defining the pathophysiology of antibody-mediated rejection and in developing effective therapies to prevent graft injury. Two such animal models are described herein.

Primary vascularization of the graft determines the immunodominance of murine minor H antigens during organ transplantation

Grafts can be rejected even when matched for MHC because of differences in the minor histocompatibility Ags (mH-Ags). H4- and H60-derived epitopes are known as immunodominant mH-Ags in H2(b)-compatible BALB.B to C57BL/6 transplantation settings. Although multiple explanations have been provided to explain immunodominance of Ags, the role of vascularization of the graft is yet to be determined. In this study, we used heart (vascularized) and skin (nonvascularized) transplantations to determine the role of primary vascularization of the graft. A higher IFN-γ response toward H60 peptide occurs in heart recipients. In contrast, a higher IFN-γ response was generated against H4 peptide in skin transplant recipients. Peptide-loaded tetramer staining revealed a distinct antigenic hierarchy between heart and skin transplantation: H60-specific CD8(+) T cells were the most abundant after heart transplantation, whereas H4-specific CD8(+) T cells were more abundant after skin graft. Neither the tissue-specific distribution of mH-Ags nor the draining lymph node-derived dendritic cells correlated with the observed immunodominance. Interestingly, non-primarily vascularized cardiac allografts mimicked skin grafts in the observed immunodominance, and H60 immunodominance was observed in primarily vascularized skin grafts. However, T cell depletion from the BALB.B donor prior to cardiac allograft induces H4 immunodominance in vascularized cardiac allograft. Collectively, our data suggest that immediate transmigration of donor T cells via primary vascularization is responsible for the immunodominance of H60 mH-Ag in organ and tissue transplantation.

Memory T cells migrate to and reject vascularized cardiac allografts independent of the chemokine receptor CXCR3

BACKGROUND

Memory T cells migrate to and reject transplanted organs without the need for priming in secondary lymphoid tissues, but the mechanisms by which they do so are not known. Here, we tested whether CXCR3, implicated in the homing of effector T cells to sites of infection, is critical for memory T-cell migration to vascularized allografts.

METHODS

CD4 and CD8 memory T cells were sorted from alloimmunized CXCR3 and wildtype B6 mice and cotransferred to congenic B6 recipients of BALB/c heart allografts. Graft-infiltrating T cells were quantitated 20 and 72 hr later by flow cytometry. Migration and allograft survival were also studied in splenectomized alymphoplastic (aly/aly) recipients, which lack secondary lymphoid tissues.

RESULTS

We found that polyclonal and antigen-specific memory T cells express high levels of CXCR3. No difference in migration of wildtype versus CXCR3 CD4 and CD8 memory T cells to allografts could be detected in wildtype or aly/aly hosts. In the latter, wildtype and CXCR3 memory T cells precipitated acute rejection at similar rates. Blocking CCR5, a chemokine receptor also upregulated on memory T cells, did not delay graft rejection mediated by CXCR3 memory T cells.

CONCLUSIONS

CXCR3 is not critical for the migration of memory T cells to vascularized organ allografts. Blocking CXCR3 or CXCR3 and CCR5 does not delay acute rejection mediated by memory T cells. These findings suggest that the mechanisms of memory T cell-homing to transplanted organs may be distinct from those required for their migration to sites of infection.

Coronary artery disease from isolated non-H2-determined incompatibilities in transplanted mouse hearts

BACKGROUND

Most studies of vascular disease in transplanted organs have used combinations involving disparities determined by genes of the major histocompatibility complex (MHC). This report describes examples of coronary vascular disease occurring in transplanted mouse hearts involving isolated, non-H2-determined incompatibilities.

METHODS

Mice, incompatible in respect of HY, H4, or H60, were selected. For H60, the incompatibility depended on breeding congenic pairs or the introduction of H60 by transgenic methods because the latter method results in more widespread expression. Transplant survival was determined, and the appearance and prevalence of coronary artery vasculopathy (CAV) was established by appropriate histologic methods.

RESULTS

Advanced changes of CAV were found at 56 days in transplants involving incompatibilities confined to HY or H4. In both combinations, skin grafts were also rejected. H60 incompatibility does not result in skin graft rejection and only a minority of heart transplants shows evidence of CAV. If heart transplants are preceded by skin grafts bearing both H60 and HY incompatibilities to promote "help" in generating immunity, H60 incompatible hearts develop advanced CAV. Heart transplants in all non-MHC categories ostensibly survive in excellent condition throughout this period despite their CAV.

CONCLUSIONS

CAV can develop as a consequence of non-MHC incompatibilities alone and even when antigens are sparsely expressed on cardiac tissue. Presensitization leads to much more severe vascular disease. Human leukocyte antigen compatible kidney transplants may also develop vascular disease and patients manifesting reactivity to MHC antigens should also be more prone to develop vascular disease because of undetectable non-MHC incompatibilities.

High pretransplant serum levels of CXCL9 are associated with increased risk of acute rejection and graft failure in kidney graft recipients

Several clinical and experimental models have underlined the role of the CXCR3-binding chemokines in the immune-mediated kidney diseases. This study aimed to investigate the predictive value of measuring pretransplant CXCL9 levels for acute rejection (AR) onset and kidney transplantation outcome. Pretransplantation serum levels of CXCL9 were tested retrospectively in 252 kidney graft recipients, whose stratification in two groups according to CXCL9 levels (<272.1 pg/ml vs. >272.1 pg/ml) showed highly significant differences in 5-year survival rates (97.7% vs. 73.3%; P < 0.001). Multivariate analysis demonstrated that among the analysed variables, CXCL9 [relative risk (RR) 11.708] and AR (RR 3.604) had the highest predictive power of graft loss. Accordingly, patients with AR (254.4 + or - 22.1; P < 0.05) and, even more, those with anti-thymoglobulin (ATG)-treated AR also showed increased pretransplant serum CXCL9 levels (319.3 + or - 28.1, P < 0.001). Moreover, CXCL9 expression and distribution were investigated in tissue specimens obtained from 10 patients affected by AR, and wide CXCL9 expression was detected not only in infiltrating inflammatory cells but also in vascular and tubular structures. Measurement of pretransplant serum CXCL9 levels might represent the tracking of a clinically useful parameter to identify subjects at high risk of AR and graft failure. These findings might be used for the individualization of immunosuppressive therapies.

CXCR3 antagonism impairs the development of donor-reactive, IFN-gamma-producing effectors and prolongs allograft survival

BACKGROUND

Current immunosuppression regimens are toxic to transplant recipients and, in many cases, acute rejection episodes occur because of escape of donor-reactive lymphocytes from the immunosuppression. T cells are the mediators of acute, cell-mediated graft damage and are hypothesized to use the CXCR3 chemokine axis for migration into the allograft. This study investigates the effect of CXCR3 blockade using a nonpeptide, small molecule inhibitor, AMG1237845, in murine cardiac allograft survival.

METHODS

C57BL/6 (H-2) mice received vascularized cardiac allografts from A/J (H-2) donors and were treated with the CXCR3 antagonist. Histologic and flow cytometric analyses were used to measure infiltration of leukocytes, and quantitative reverse-transcriptase polymerase chain reaction and interferon-gamma ELISPOT assays were used to measure donor-specific reactivity.

RESULTS

CXCR3 antagonism modestly prolonged allograft survival compared with vehicle treatment, but at time-matched intervals posttransplant, neutrophil, CD8, and CD4 T cell infiltration was indistinguishable. Although proliferation of donor-reactive naïve T cells was unaffected by CXCR3 antagonism, the frequency of interferon-gamma-producing cells in the recipient spleen was significantly reduced by AMG1237845 treatment. CXCR3 blockade for 30 days synergized with short-term, low-dose anti-CD154 monoclonal antibodies to prolong survival past 50 days in 75% of grafts and past 80 days in 25% of the cases.

CONCLUSIONS

These results indicate that in synergy with co-stimulation blockade, CXCR3 is a viable therapeutic target to prevent acute graft rejection.

Chemokines and transplant vasculopathy

Transplant vasculopathy (TV) remains the leading cause of late death among heart transplant recipients. Transplant vasculopathy is characterized by progressive neointimal proliferation, leading to ischemic failure of the allograft. Multiple experimental and clinical studies have shown that injury to the graft at various stages of transplantation can be a risk factor for development of transplant vasculopathy. The hallmark of cardiac allograft injury is the infiltration of leukocytes. Recruitment of leukocytes requires intercellular communication between infiltrating cells, endothelium, parenchymal cells, and components of extracellular matrix. These events are mediated via the generation of adhesion molecules, cytokines, and chemokines. The chemokines, by virtue of their specific cell receptor expression, can selectively mediate the local recruitment/activation of distinct leukocytes/cells, allowing for migration across the endothelium and beyond the vascular compartment. This report provides a comprehensive review of the chemokines that participate in the development of transplant vasculopathy.

The chemokine receptor Cxcr3 is not essential for acute cardiac allograft rejection in mice and rats

Chemokine receptors have gained attention as potential targets for novel therapeutic strategies. We investigated the mechanisms of allograft rejection in chemokine receptor Cxcr3-deficient mice using a model of acute heart allograft rejection in the strain combination BALB/c to C57BL/6. Allograft survival was minimally prolonged in Cxcr3-deficient mice compared to wild-type (wt) animals (8 vs. 7 days) and treatment with a subtherapeutic dose of cyclosporine A (CsA) led to similar survival in Cxcr3-deficient and wt recipients (13 vs. 12 days). At rejection grafts were histologically indistinguishable. Microarray analysis revealed that besides Cxcr3 only few genes were differentially expressed in grafts or in spleens from transplanted or untransplanted animals. Transcript analysis by quantitative RT-PCR of selected cytokines, chemokines, or chemokine receptors or serum levels of selected cytokines and chemokines showed similar levels between the two groups. Furthermore, in a rat heart allograft transplantation model treatment with a small molecule CXCR3 antagonist did not prolong survival despite full blockade of Cxcr3 in vivo. In summary, Cxcr3 deficiency or pharmacologic blockade does not diminish graft infiltration, tempo and severity of rejection. Thus, Cxcr3 does not appear to play a pivotal role in the allograft rejection models described here.

Unaltered graft survival and intragraft lymphocytes infiltration in the cardiac allograft of Cxcr3-/- mouse recipients

Previous studies showed that absence of chemokine receptor Cxcr3 or its blockade prolong mouse cardiac allograft survival. We evaluated the effect of the CXCR3 receptor antagonist MRL-957 on cardiac allograft survival, and also examined the impact of anti-CXCR3 mAb in human CXCR3 knock-in mice. We found only a moderate increase in graft survival (10.5 and 16.6 days, p < 0.05) using either the antagonist or the antibody, respectively, compared to control (8.7 days). We re-evaluated cardiac allograft survival with two different lines of Cxcr3(-/-) mice. Interestingly, in our hands, neither of the independently derived Cxcr3(-/-) lines showed remarkable prolongation, with mean graft survival of 9.5 and 10.8 days, respectively. There was no difference in the number of infiltrating mononuclear cells, expansion of splenic T cells or IFN-gamma production of alloreactive T cells. Mechanistically, an increased other chemokine receptor fraction in the graft infiltrating CD8 T cells in Cxcr3(-/-) recipients compared to wild-type recipients suggested compensatory T-cell trafficking in the absence of Cxcr3. We conclude Cxcr3 may contribute to, but does not govern, leukocyte trafficking in this transplant model.