Objectives: Cardiopulmonary bypass is associated with severe immune dysfunctions. Particularly, a cardiopulmonary bypass–related long-lasting immunosuppressive state predisposes patients to a higher risk of postoperative complications, such as persistent bacterial infections. This study was conducted to elucidate mechanisms of post-cardiopulmonary bypass immunosuppression. Design: In vitro studies with human peripheral blood mononuclear cells. Setting: Cardiosurgical ICU, University Research Laboratory. Patients: Seventy-one patients undergoing cardiac surgery with cardiopulmonary bypass (enrolled May 2017 to August 2018). Interventions: Peripheral blood mononuclear cells before and after cardiopulmonary bypass were analyzed for the expression of immunomodulatory cell markers by real-time quantitative reverse transcription polymerase chain reaction. T cell effector functions were determined by enzyme-linked immunosorbent assay, carboxyfluorescein succinimidyl ester staining, and cytotoxicity assays. Expression of cell surface markers was assessed by flow cytometry. CD15⁺ cells were depleted by microbead separation. Serum arginine was measured by mass spectrometry. Patient peripheral blood mononuclear cells were incubated in different arginine concentrations, and T cell functions were tested. Measurements and Main Results: After cardiopulmonary bypass, peripheral blood mononuclear cells exhibited significantly reduced levels of costimulatory receptors (inducible T-cell costimulator, interleukin 7 receptor), whereas inhibitory receptors (programmed cell death protein 1 and programmed cell death 1 ligand 1) were induced. T cell effector functions (interferon γ secretion, proliferation, and CD8⁺-specific cell lysis) were markedly repressed. In 66 of 71 patients, a not yet described cell population was found, which could be characterized as myeloid-derived suppressor cells. Myeloid-derived suppressor cells are known to impair immune cell functions by expression of the arginine-degrading enzyme arginase-1. Accordingly, we found dramatically increased arginase-1 levels in post-cardiopulmonary bypass peripheral blood mononuclear cells, whereas serum arginine levels were significantly reduced. Depletion of myeloid-derived suppressor cells from post-cardiopulmonary bypass peripheral blood mononuclear cells remarkably improved T cell effector function in vitro. Additionally, in vitro supplementation of arginine enhanced T cell immunocompetence. Conclusions: Cardiopulmonary bypass strongly impairs the adaptive immune system by triggering the accumulation of myeloid-derived suppressor cells. These myeloid-derived suppressor cells induce an immunosuppressive T cell phenotype by increasing serum arginine breakdown. Supplementation with L-arginine may be an effective measure to counteract the onset of immunoparalysis in the setting of cardiopulmonary bypass.

Introduction: Inflammatory responses and coagulation disorders are a relevant challenge for successful cardiac xenotransplantation on its way to the clinic. To cope with this, an effective and clinically practicable anti‐inflammatory and anti‐coagulatory regimen is needed. The inflammatory and coagulatory response can be reduced by genetic engineering of the organ‐source pigs. Furthermore, there are several therapeutic strategies to prevent or reduce inflammatory responses and coagulation disorders following xenotransplantation. However, it is still unclear, which combination of drugs should be used in the clinical setting.To elucidate this, we present data from pig‐to‐baboon orthotopic cardiac xenotransplantation experiments using a combination of several anti‐inflammatory drugs.Methods: Genetically modified piglets (GGTA1‐KO, hCD46/hTBM transgenic) were used for orthotopic cardiac xenotransplantation into captive‐bred baboons (n = 14). All animals received an anti‐inflammatory drug therapy including a C1 esterase inhibitor, an IL‐6 receptor antagonist, a TNF‐α inhibitor, and an IL‐1 receptor antagonist. As an additive medication, acetylsalicylic acid and unfractionated heparin were administered. The immunosuppressive regimen was based on CD40/CD40L co‐stimulation blockade. During the experiments, leukocyte counts, levels of C‐reactive protein (CRP) as well as systemic cytokine and chemokine levels and coagulation parameters were assessed at multiple timepoints. Four animals were excluded from further data analyses due to porcine cytomegalovirus/porcine roseolovirus (PCMV/PRV) infections (n = 2) or technical failures (n = 2).Results: Leukocyte counts showed a relevant perioperative decrease, CRP levels an increase. In the postoperative period, leukocyte counts remained consistently within normal ranges, CRP levels showed three further peaks after about 35, 50, and 80 postoperative days. Analyses of cytokines and chemokines revealed different patterns. Some cytokines, like IL‐8, increased about 2‐fold in the perioperative period, but then decreased to levels comparable to the preoperative values or even lower. Other cytokines, such as IL‐12/IL‐23, decreased in the perioperative period and stayed at these levels. Besides perioperative decreases, there were no relevant alterations observed in coagulation parameters. In summary, all parameters showed an unremarkable course with regard to inflammatory responses and coagulation disorders following cardiac xenotransplantation and thus showed the effectiveness of our approach.Conclusion: Our preclinical experience with the anti‐inflammatory drug therapy proved that controlling of inflammation and coagulation disorders in xenotransplantation is possible and well‐practicable under the condition that transmission of pathogens, especially of PCMV/PRV to the recipient is prevented because PCMV/PRV also induces inflammation and coagulation disorders. Our anti‐inflammatory regimen should also be applicable and effective in the clinical setting of cardiac xenotransplantation.

Xenotransplantation, like allotransplantation, is usually associated with microchimerism, i.e., the presence of cells from the donor in the recipient. Microchimerism was reported in first xenotransplantation trials in humans, as well as in most preclinical trials in nonhuman primates (for review, see Denner, Viruses 2023, 15, 190). When using pigs as xenotransplantation donors, their cells contain porcine endogenous retroviruses (PERVs) in their genome. This makes it difficult to discriminate between microchimerism and PERV infection of the recipient. Here, we demonstrate the appropriate virological methods to be used for the identification of microchimerism, first by screening for porcine cellular genes, and then how to detect infection of the host. Using porcine short interspersed nuclear sequences (SINEs), which have hundreds of thousands of copies in the pig genome, significantly increased the sensitivity of the screening for pig cells. Second, absence of PERV RNA demonstrated an absence of viral genomic RNA or expression as mRNA. Lastly, absence of antibodies against PERV proteins conclusively demonstrated an absence of a PERV infection. When applying these methods for analyzing baboons after pig heart transplantation, microchimerism could be demonstrated and infection excluded in all animals. These methods can be used in future clinical trials.

Cytotoxic T cell (CTL) dysfunction is a hallmark of immune paralysis after major surgery, increasing susceptibility to severe nosocomial infections and contributing to mortality in critically ill patients. The mechanisms remain poorly understood. We demonstrate that reactive oxygen species (ROS) released by myeloid-derived suppressor cells (MDSC) transiently emerging after surgery, drive perioperative CTL immunoparalysis. These ROS damage CTL mitochondria, triggering secondary mitochondrial ROS amplification and overwhelming antioxidant defenses. The resulting oxidative cascade impairs oxidative phosphorylation and suppresses CTL effector function. Concurrently, stress-induced mitochondrial hyperfusion disrupts fission-dependent translocation to the immunological synapse, exacerbating bioenergetic failure. MitoTEMPO, a mitochondria-targeted antioxidant, partially mitigates these effects, highlighting mitochondrial stabilization as a potential strategy to prevent perioperative immune dysfunction.

The blockade of the CD40/CD40L immune checkpoint is considered essential for cardiac xenotransplantation. However, it is still unclear which single antibody directed against CD40 or CD40L (CD154), or which combination of antibodies, is better at preventing organ rejection. For example, the high doses of antibody administered in previous experiments might not be feasible for the treatment of humans, while thrombotic side effects were described for first-generation anti-CD40L antibodies. To address these issues, we conducted six orthotopic pig-to-baboon cardiac xenotransplantation experiments, combining a chimeric anti-CD40 antibody with an investigational long-acting PASylated anti-CD40L Fab fragment. The combination therapy effectively resulted in animal survival with a rate comparable to a previous study that utilized anti-CD40 monotherapy. Importantly, no incidence of thromboembolic events associated with the administration of the anti-CD40L PAS-Fab was observed. Two experiments failed early because of technical reasons, two were terminated deliberately after 90 days with the baboons in excellent condition and two were extended to 120 and 170 days, respectively. Unexpectedly, and despite the absence of any clinical signs, histopathology revealed fungal infections in all four recipients. This study provides, for the first time, insights into a combination therapy with anti-CD40/anti-CD40L antibodies to block this immune checkpoint.

Xenotransplantation is a promising approach to reduce organ shortage, while genetic modification of donor pigs has significantly decreased the immunogenic burden of xenotransplants, organ rejection is still a hurdle. Genetically modified pig organs are used in xenotransplantation research, and the first clinical pig‐to‐human heart transplantation was performed in 2022. However, the impact of genetic modification has not been investigated on a cellular level yet. Endothelial cells (EC) and their sugar‐rich surface known as the glycocalyx are the first barrier encountering the recipient's immune system, making them a target for rejection. We have previously shown that wild type venous but not arterial EC were protected against heparan sulfate (HS) shedding after activation with human serum or human tumor necrosis factor alpha (TNF𝛼). Using a 2D microfluidic system we investigated the glycocalyx dynamics of genetically modified porcine arterial and venous EC (Gal𝛼1,3 Gal knock‐out, transgenic for human CD46 and thrombomodulin, GTKO/hCD46/hTM) after activation with human serum or human TNF𝛼. Interestingly, we observed that GTKO/hCD46/hTM arterial cells, additionally to venous cells, do not shed HS. Unscathed HS on GTKO/hCD46/hTM EC correlated with reduced complement deposition, suggesting that protection against complement activation contributes to maintaining an intact glycocalyx layer on arterial EC. This protection was lost on GTKO/hCD46/hTM cells after simultaneous perfusion with human serum and human TNF𝛼. HS shedding on arterial cells and increased complement deposition on both arterial and venous cells was observed. These findings suggest that GTKO/hCD46/hTM EC revert to a proinflammatory phenotype in an inflammatory xenotransplantation setting, potentially favoring transplant rejection.