Future directions for xenotransplantation in lungs

PURPOSE OF REVIEW

Advancements in preclinical xenotransplant studies have opened doors for clinical heart and kidney xenotransplantation. This review assesses recent progress in lung xenotransplantation research and its potential clinical implications.

RECENT FINDINGS

The efficacy of the humanized von Willebrand factor in reducing platelet sequestration in ex-vivo and in-vivo lung xenotransplant models was showcased. Combining human tissue factor pathway inhibitor and CD47 expression with selectin and integrin inhibition delayed neutrophil and platelet sequestration. Enhanced expression of human complement regulatory proteins and thrombomodulin in genetically engineered pig lungs improved graft survival by reducing platelet activation and modulating coagulation disruptions. Knocking out the CMAH gene decreased antibody-mediated inflammation and coagulation activation, enhancing compatibility for human transplantation. Furthermore, CMAH gene knockout in pigs attenuated sialoadhesin-dependent binding of human erythrocytes to porcine macrophages, mitigating erythrocyte sequestration and anemia. Meanwhile, in-vivo experiments demonstrated extended survival of xenografts for up to 31 days with multiple genetic modifications and comprehensive treatment strategies.

SUMMARY

Experiments have uncovered vital insights for successful xenotransplantation, driving further research into immunosuppressive therapy and genetically modified pigs. This will ultimately pave the way for clinical trials designed to improve outcomes for patients with end-stage lung disease.

Pigs in Transplantation Research and Their Potential as Sources of Organs in Clinical Xenotransplantation

The pig has long been used as a research animal and has now gained importance as a potential source of organs for clinical xenotransplantation. When an organ from a wild-type (i. e., genetically unmodified) pig is transplanted into an immunosuppressed nonhuman primate, a vigorous host immune response causes hyperacute rejection (within minutes or hours). This response has been largely overcome by 1) extensive gene editing of the organ-source pig and 2) the administration to the recipient of novel immunosuppressive therapy based on blockade of the CD40/CD154 T cell costimulation pathway. Gene editing has consisted of 1) deletion of expression of the 3 known carbohydrate xenoantigens against which humans have natural (preformed) antibodies and 2) the introduction of human 'protective' genes. The combination of gene editing and novel immunosuppressive therapy has extended life-supporting pig kidney graft survival to greater than 1 y and of pig heart survival to up to 9 mo. This review briefly describes the techniques of gene editing, the potential risks of transfer of porcine endogenous retroviruses with the organ, and the need for breeding and housing of donor pigs under biosecure conditions.

Incidence of serum antibodies to xenoantigens on triple-knockout pig cells in different human groups

BACKGROUND

Xenoantigens other than Gal, Neu5Gc, and Sda may be playing a role in pig graft rejection. We investigated the incidence of antibodies to unknown pig xenoantigen in different human groups.

METHODS

We collected blood from TKO/hCD55 pigs (n = 3), and isolated PBMCs and RBCs. Serum samples were collected from (i) healthy human volunteers (n = 43), (ii) patients with end-stage renal disease (ESRD) (n = 87), (iii) the same patients after kidney allotransplantation (n = 50), and (iv) renal allotransplant recipients experiencing T cell-mediated rejection (allo-TCMR, n = 10). The sera were initially incubated with TKO/hCD55 pRBCs (1 × 108 cells) for 1 h to absorb anti-pig antibodies (except against SLA and possibly other antigens not expressed on pRBCs) and then the serum (absorbed or unabsorbed) was tested for antibody binding and complement-dependent cytotoxicity (CDC) to TKO/hCD55 pig PBMCs.

RESULTS

A significant reduction in IgM/IgG binding and CDC was observed in the absorbed sera. Serum obtained before and after renal allotransplantation showed no significant difference in IgM or IgG binding to, or in CDC of, TKO/hCD55 pig cells. IgM antibodies (but rarely IgG) against unknown xenoantigens expressed on TKO/hCD55 PBMCs, possibly against swine leukocyte antigens, were documented in healthy humans, patients with ESRD, and those with renal allografts undergoing acute T cell rejection. IgM (but not CDC) was higher in patients experiencing allo-TCMR.

CONCLUSION

Human sera contain IgM antibodies against unknown pig xenoantigens expressed on TKO/hCD55 pPBMCs. Although not confirmed in the present study, the targets for these antibodies may include swine leukocyte antigens.

In vitro and in vivo immune assessments of genetically-engineered pig skin grafts in New World (squirrel) monkeys

Half a million patients in the USA alone require treatment for burns annually. Following an extensive burn, it may not be possible to provide sufficient autografts in a single setting. Genetic manipulations (GM) of pigs offer the possibility of reducing primate humoral and cellular rejection of pig skin xenografts and thus extending graft survival. We compared the survival of skin grafts from pigs with 9-GM with that of autografts and allografts in squirrel monkeys. Monitoring for rejection was by (1) macroscopic examination, (2) histopathological examination of skin biopsies, and (3) measurement of anti-monkey and anti-pig IgM and IgG antibodies. Autografts (n = 5) survived throughout the 28 days of follow-up without histopathological features of rejection. Median survival of allografts (n = 6) was 14 days and of pig xenografts (n = 12) 21 days. Allotransplantation was associated with an increase in anti-monkey IgM, but the anticipated subsequent rise in IgG had not yet occurred at the time of euthanasia. Pig grafts were associated with increases in anti-pig IgM and IgG. In all cases, histopathologic features of rejection were similar. 9-GM pig skin xenografts survive at least as long as monkey skin allografts (and trended to survive longer), suggesting that they are a realistic clinical option for the temporary treatment of burns. Although monkeys with pig skin grafts developed anti-pig IgM and IgG antibodies, these did not cross-react with monkey antigens, indicating that a primary 9-GM pig skin graft would not be detrimental to a subsequent monkey skin allograft.

Future prospects for the clinical transfusion of pig red blood cells

Transfusion of allogeneic human red blood cell (hRBCs) is limited by supply and compatibility between individual donors and recipients. In situations where the blood supply is constrained or when no compatible RBCs are available, patients suffer. As a result, alternatives to hRBCs that complement existing RBC transfusion strategies are needed. Pig RBCs (pRBCs) could provide an alternative because of their abundant supply, and functional similarities to hRBCs. The ability to genetically modify pigs to limit pRBC immunogenicity and augment expression of human 'protective' proteins has provided major boosts to this research and opens up new therapeutic avenues. Although deletion of expression of xenoantigens has been achieved in genetically-engineered pigs, novel genetic methods are needed to introduce human 'protective' transgenes into pRBCs at the high levels required to prevent hemolysis and extend RBC survival in vivo. This review addresses recent progress and examines future prospects for clinical xenogeneic pRBC transfusion.

Antibody-mediated rejection in xenotransplantation: Can it be prevented or reversed?

Antibody-mediated rejection (AMR) is the commonest cause of failure of a pig graft after transplantation into an immunosuppressed nonhuman primate (NHP). The incidence of AMR compared to acute cellular rejection is much higher in xenotransplantation (46% vs. 7%) than in allotransplantation (3% vs. 63%) in NHPs. Although AMR in an allograft can often be reversed, to our knowledge there is no report of its successful reversal in a pig xenograft. As there is less experience in preventing or reversing AMR in models of xenotransplantation, the results of studies in patients with allografts provide more information. These include (i) depletion or neutralization of serum anti-donor antibodies, (ii) inhibition of complement activation, (iii) therapies targeting B or plasma cells, and (iv) anti-inflammatory therapy. Depletion or neutralization of anti-pig antibody, for example, by plasmapheresis, is effective in depleting antibodies, but they recover within days. IgG-degrading enzymes do not deplete IgM. Despite the expression of human complement-regulatory proteins on the pig graft, inhibition of systemic complement activation may be necessary, particularly if AMR is to be reversed. Potential therapies include (i) inhibition of complement activation (e.g., by IVIg, C1 INH, or an anti-C5 antibody), but some complement inhibitors are not effective in NHPs, for example, eculizumab. Possible B cell-targeted therapies include (i) B cell depletion, (ii) plasma cell depletion, (iii) modulation of B cell activation, and (iv) enhancing the generation of regulatory B and/or T cells. Among anti-inflammatory agents, anti-IL6R mAb and TNF blockers are increasingly being tested in xenotransplantation models, but with no definitive evidence that they reverse AMR. Increasing attention should be directed toward testing combinations of the above therapies. We suggest that treatment with a systemic complement inhibitor is likely to be most effective, possibly combined with anti-inflammatory agents (if these are not already being administered). Ultimately, it may require further genetic engineering of the organ-source pig to resolve the problem entirely, for example, knockout or knockdown of SLA, and/or expression of PD-L1, HLA E, and/or HLA-G.

What Have We Learned From In Vitro Studies About Pig-to-primate Organ Transplantation?

Natural preformed and de novo antibodies against pig antigens are a major cause of pig xenograft rejection in nonhuman primates (NHPs). In vivo studies in pig-to-NHP models are time consuming. In vitro assays, for example, antibody binding to pig cells, complement-dependent cytotoxicity assays, provide valuable information quickly and inexpensively. Using in vitro assays for several years, it has been documented that (1) during the first year of life, humans and NHPs develop anti-wild-type pig antibodies, but humans develop no or minimal antibody to triple-knockout (TKO) pig cells. (2) Some adult humans have no or minimal antibodies to TKO pig cells and are therefore unlikely to rapidly reject a TKO organ, particularly if the organ also expresses human "protective" proteins. (3) There is good correlation between immunoglobulin (Ig)M (but no t IgG) binding and complement injury. (4) All Old World NHPs develop antibodies to TKO pig cells and are not optimal recipients of TKO organs. (5) galactosyltransferase gene-knockout/β4GalNT2KO pigs are preferred for Old World NHPs. (6) Humans develop anti-pig IgE and IgA antibodies against pig cells, but their role remains uncertain. (7) In a small percentage of allosensitized humans, antibodies that cross-react with swine leukocyte antigens may be detrimental to a pig organ xenograft. (8) Prior sensitization to pig antigens is unlikely to be detrimental to a subsequent allograft. (9) Deletion of expression of Gal and Neu5Gc is associated with a reduction in the T-cell response to pig cells. All of these valuable observations have largely predicted the results of in vivo studies.

The case for the therapeutic use of mechanistic/mammalian target of rapamycin (mTOR) inhibitors in xenotransplantation

The mechanistic/mammalian target of rapamycin (mTOR) is one of the systems that are necessary to maintain cell homeostasis, such as survival, proliferation, and differentiation. mTOR inhibitors (mTOR-Is) are utilized as immunosuppressants and anti-cancer drugs. In organ allotransplantation, current regimens infrequently include an mTOR-I, which are positioned more commonly as alternative immunosuppressants. In clinical allotransplantation, long-term efficacy has been established, but there is a significant incidence of adverse events, for example, inhibition of wound healing, buccal ulceration, anemia, hyperglycemia, dyslipidemia, and thrombocytopenia, some of which are dose-dependent. mTOR-Is have properties that may be especially beneficial in xenotransplantation. These include suppression of T cell proliferation, increases in the number of T regulatory cells, inhibition of pig graft growth, and anti-inflammatory, anti-viral, and anti-cancer effects. We here review the potential benefits and risks of mTOR-Is in xenotransplantation and suggest that the benefits exceed the adverse effects.

Multicenter registry of the Watchman left atrial appendage closure device for patients with atrial fibrillation in Japan: The TERMINATOR registry

Funding Acknowledgements

Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): Boston Scientific Japan

Background

Transcatheter left atrial appendage closure (LAAC) provides an alternative to oral anticoagulation for thromboembolic risk reduction in patients with nonvalvular atrial fibrillation (AF). A meta-analysis of previous two randomized trials reported improved rates of hemorrhagic stroke, cardiovascular/unexplained death, and nonprocedural bleeding compared to warfarin (1). Recently, the next-generation LAAC device, the Watchman FLX system, became available, and showed a low incidence of adverse events and a high incidence of anatomic closure (2). This transcatheter stroke prevention has already been approved in Asian countries. However, there is little data of LAAC in Asian population.

Purpose

This study sought to assess efficacy and safety of LAAC for patients with nonvalvular AF in Asia.

Methods

The TERMINATOR (Transcatheter Modification of Left Atrial Appendage by Obliteration with Device) registry is a multicenter nonrandomized study in Japan. This enrolled patients who underwent LAAC in 23 Japanese institutions. The LAAC was indicated for patients with nonvalvular atrial fibrillation in whom oral anticoagulation is required, but who have a risk of bleeding (history of BARC type 3 bleeding or HAS-BLED score ≥3 points). Baseline patient and procedural characteristics and clinical outcomes were evaluated.

Results

A total of 729 patients were enrolled between September 2019 and November 2021. The mean age was 74.9±8.8 years and the mean CHA2DS2-VASc score was 4.7±1.5. The Watchman generation 2.5 and FLX system were used in 469 (64.3%) and 260 patients (35.7%), respectively. Procedural success was achieved in 722 patients (99.0%). In-hospital adverse events were as follows; 6 tamponades (0.8%), 3 pericardial effusion (0.4%), 2 device embolization (0.3%), no stroke (0%), and no death (0%). During follow-up, device-related thrombus and all-cause death were reported in 16 (2.2%) and 23 patients (3.2%), respectively.

Conclusions

LAAC with the Watchman system provides compatible efficacy and safety outcomes in Asian population.

The respective relevance of sensitization to alloantigens and xenoantigens in pig organ xenotransplantation

BACKGROUND

Antibody-mediated rejection is a major cause of graft injury and contributes to failure of pig xenografts in nonhuman primates (NHPs). Most 'natural' or elicited antibodies found in humans and NHPs are directed against pig glycan antigens, but antibodies binding to swine leukocyte antigens (SLA) have also been detected. Of clinical importance is (i) whether the presence of high levels of antibodies directed towards human leukocyte antigens (HLA) (i.e., high panel-reactive antibodies) would be detrimental to the outcome of a pig organ xenograft; and (ii) whether, in the event of sensitization to pig antigens, a subsequent allotransplant would be at increased risk of graft failure due to elicited anti-pig antibodies that cross-react with human HLA or other antigens.

SUMMARY

A literature review of pig-to-primate studies indicates that relatively few highly-HLA-sensitized humans have antibodies that cross-react with pigs, predicting that most would not be at increased risk of rejecting an organ xenograft. Furthermore, the existing evidence indicates that sensitization to pig antigens will probably not elicit increased alloantibody titers; if so, 'bridging' with a pig organ could be carried out without increased risk of subsequent antibody-mediated allograft failure.

KEY MESSAGE

These issues have important implications for the design and conduct of clinical xenotransplantation trials.

Observations on hydronephrosis after pig kidney transplantation in baboons

We have seen hydronephrosis (obstructive nephropathy) at necropsy in 3 of 11 (21%) genetically-engineered pig kidneys that functioned in baboons for >36 days, even when the clinical and histopathological features of rejection were minimal. We briefly report one such case and illustrate the macroscopic and microscopic appearances of such a kidney and ureter. The causes of the observed changes remain uncertain. In our small experience, there seems to be no correlation between the development of hydronephrosis and (i) the surgical technique, (ii) the genotype of the pig, (iii) the length of the pig ureter, or (iv) the immunosuppressive and anti-inflammatory therapy administered. We suggest that the distal ureteric thickening may be the result of an inflammatory response. In two cases, we resolved the problem by carrying out a secondary side-to-side anastomosis between the proximal pig ureter and the baboon bladder.

Ten years survival benefit of CABG or PCI based on individual prediction

Background

To compare the observed and individual predicted mortalities according to the SYNTAX score II 2020 (SSII-2020) in the all-comers SYNTAX population, and retrospectively assess the appropriateness of revascularization with percutaneous coronary intervention (PCI) and coronary artery bypass grafting (CABG) in patients with three vessel disease (3VD) and/or left main disease (LMCAD).

Methods

Internal and external validation of the SSII-2020 to predict 10-year all-cause death was performed in the respective randomized and registry SYNTAX populations. Differences in individual predicted mortalities following CABG or PCI were ranked and displayed with the observed mortalities. The proportions of screened patients deriving a survival benefit from CABG or PCI were determined retrospectively.

Results

A total of 2602 participants (as-treated population) were included in the randomized and registry cohorts. In the randomized cohort, all-cause mortality at 10 years, as an average treatment effect, was 23.8% (199/865) with CABG and 28.6% (249/901) with PCI, with a differential survival benefit of 4.6% (95% CI: 0.58% to 8.7%, log-rank p value=0.023). In the CABG and PCI registries, mortalities were 27.8% (167/644) and 55.4% (99/192), respectively. Calibration and discrimination of the SSII-2020 was helpful in CABG and PCI patients in the randomized and registry cohorts. In the PCI registry, the SSII-2020 underestimated mortality since specific comorbidities that entail high mortality are not included in the formula (C-index: 0.72, intercept: 0.38, slope: 0.66), whilst in the CABG registry, it predicted mortality with a helpful calibration and discrimination (C-index: 0.70, intercept: 0.00, slope: 0.76). The proportions of patients with a predicted survival benefit following CABG and PCI were respectively 78.3% (1383/1766) and 21.7% (383/1766) in the randomized cohort, and 82.4% (2143/2602) and 17.7% (459/2602) in the whole SYNTAX trial population.

Conclusion

In the randomized and registry cohort of this all-comers population with 3VD and/or LMCAD, there was reasonable agreement between the individual predicted and observed mortalities after CABG or PCI, such that the predicted 10-year survival benefit might be helpful in determining the appropriateness of each modality of revascularization.

Funding Acknowledgement

Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): The SYNTAX Extended Survival study was supported by the German Foundation of Heart Research (Frankfurt am Main, Germany). The SYNTAX trial, during 0-5 years follow-up, was funded by Boston Scientific Corporation (Marlborough, MA, USA). Both sponsors had no role in the study design, data collection, data analyses, and interpretation of the study data, nor were involved in the decision to publish the final manuscript. The principal investigators and authors had complete scientific freedom.

Will previous palliative surgery for congenital heart disease be detrimental to subsequent pig heart xenotransplantation?

INTRODUCTION

Pig heart xenotransplantation might act as a bridge in infants with complex congenital heart disease (CHD) until a deceased human donor heart becomes available. Infants develop antibodies to wild-type (WT, i.e., genetically-unmodified) pig cells, but rarely to cells in which expression of the 3 known carbohydrate xenoantigens has been deleted by genetic engineering (triple-knockout [TKO] pigs). Our objective was to test sera from children who had undergone palliative surgery for complex CHD (and who potentially might need a pig heart transplant) to determine whether they had serum cytotoxic antibodies against TKO pig cells.

METHODS

Sera were obtained from children with CHD undergoing Glenn or Fontan operation (n = 14) and healthy adults (n = 8, as controls). All of the children had complex CHD and had undergone some form of cardiac surgery. Seven had received human blood transfusions and 3 bovine pericardial patch grafts. IgM and IgG binding to WT and TKO pig red blood cells (RBCs) and peripheral blood mononuclear cells (PBMCs) were measured by flow cytometry, and killing of PBMCs by a complement-dependent cytotoxicity assay.

RESULTS

Almost all children and adults demonstrated relatively high IgM/IgG binding to WT RBCs, but minimal binding to TKO RBCs (p < 0.0001 vs WT), although IgG binding was greater in children than adults (p < 0.01). All sera showed IgM/IgG binding to WT PBMCs, but this was much lower to TKO PBMCs (p < 0.0001 vs WT) and was greater in children than in adults (p < 0.05). Binding to both WT and TKO PBMCs was greater than to RBCs. Mean serum cytotoxicity to WT PBMCs was 90% in both children and adults, whereas to TKO PBMCs it was only 20% and < 5%, respectively. The sera from 6/14 (43%) children were cytotoxic to TKO PBMCs, but no adult sera were cytotoxic.

CONCLUSIONS

Although no children had high levels of antibodies to TKO RBCs, 13/14 demonstrated antibodies to TKO PBMCs, in 6 of these showed mild cytotoxicity. As no adults had cytotoxic antibodies to TKO PBMCs, the higher incidence in children may possibly be associated with their exposure to previous cardiac surgery and biological products. However, the numbers were too small to determine the influence of such past exposures. Before considering pig heart xenotransplantation for children with CHD, testing for antibody binding may be warranted.

The potential of genetically engineered pig heart transplantation in infants with complex congenital heart disease

Despite advances in surgical and medical techniques, complex congenital heart disease in neonates and infants continues to be associated with significant mortality and morbidity. More than 500 infants in the USA are placed on the cardiac transplantation wait-list annually. However, there remains a critical shortage of deceased human donor organs for transplantation with a median wait-time of 4 months. Hence, infant mortality on the heart transplant wait-list in the USA is higher than for any other solid organ transplant group. Orthotopic transplantation of a pig heart as a bridge to allotransplantation might offer the best prospect of long-term survival of these patients. In recent years, there have been several advances in genetic engineering of pigs to mitigate the vigorous antibody-mediated rejection of a pig heart transplanted into a nonhuman primate. In this review, we briefly highlight (i) the history of clinical heart xenotransplantation, (ii) current advances and techniques of genetically engineering pigs, (iii) the current status of pig orthotopic cardiac graft survival in nonhuman primates, and (iv) progress toward pursuing clinical trials of cardiac xenotransplantation. Ultimately, we argue that pig heart xenotransplantation should initially be used as a bridge to cardiac allotransplantation in neonates and infants.

Cardiac and Pulmonary Histopathology in Baboons Following Genetically-Engineered Pig Orthotopic Heart Transplantation

BACKGROUND Although improving, survival after pig orthotopic heart transplantation (OHTx) in baboons has been mixed and largely poor. The causes for the high incidence of early failure remain uncertain. MATERIAL AND METHODS We have carried out pig OHTx in 4 baboons. Two died or were euthanized within hours, and 2 survived for 3 and 8 months, respectively. There was evidence of a significant 'cytokine storm' in the immediate post-OHTx period with the elevations in IL-6 correlating closely with the final outcome. RESULTS All 4 baboons demonstrated features suggestive of respiratory dysfunction, including increased airway resistance, hypoxia, and tachypnea. Histopathological observations of pulmonary infiltration by neutrophils and, notably, eosinophils within vessels and in the perivascular and peribronchiolar space, with minimal cardiac pathology, suggested a role for early lung acute inflammation. In one, features suggestive of transfusion-related acute lung injury were present. The 2 longer-term survivors died of (i) a cardiac dysrhythmia with cellular infiltration around the conducting tissue (at 3 months), and (ii) mixed cellular and antibody-mediated rejection (at 8 months). CONCLUSIONS These initial findings indicate a potential role of acute lung injury early after OHTx. If this response can be prevented, increased survival may result, providing an opportunity to evaluate the factors affecting long-term survival.

The first clinical pig heart transplant: Was IVIg or pig cytomegalovirus detrimental to the outcome?

The clinical course of the first patient to receive a gene-edited pig heart transplant was recently reported by the University of Maryland team. Although the pig heart functioned well for >40 days, serum anti-pig antibodies then increased, and the patient sadly died after 60 days. Because of his debilitated pre-transplant state, the patient never thrived despite excellent graft function for several weeks, and the cause of his demise continues to be uncertain. A few days before an increase in anti-pig antibodies was observed, the patient had received intravenous human immunoglobulin (IVIg), and whether this played a role in his cardiac deterioration has been discussed. Furthermore, mcfDNA testing indicated an increase in pig cytomegalovirus (CMV), and its possible role in the development of cardiac dysfunction has also been considered. On the basis of the limited data provided in the publication and on our previous investigations into whether IVIg contains anti-TKO pig antibodies and therefore might be deleterious to TKO pig organ xenografts, we suggest that the steady rise in anti-pig antibody titer was more consistent with the failure of the immunosuppressive regimen to prevent elicited anti-TKO pig antibody production, rather than from the passive transfusion of IVIg or the presence of pig CMV in the graft. Although the outcome of the Maryland experience was disappointing, valuable lessons were learned. Our attention was drawn to the potential risks of heart transplantation in a "deconditioned" patient, the administration of IVIg, the transmission of pig CMV, and of the difficulties in interpreting myocardial biopsy findings.

T and B lymphocyte dynamics after genetically-modified pig-to-baboon kidney xenotransplantation with an anti-CD40mAb-based immunosuppressive regimen

BACKGROUND

The aim was to monitor recovery of T/B lymphocytes in baboons after depletion by anti-thymocyte globulin (ATG) and anti-CD20mAb (Rituximab), followed by pig kidney transplantation and maintenance therapy with an anti-CD40mAb-based regimen.

METHODS

In baboons (n = 14), induction was with ATG and anti-CD20mAb, and maintenance with (i) anti-CD40mAb, (ii) rapamycin, and (iii) methylprednisolone. Follow-up was for 6 months, or until rejection or other complication developed. Baboon blood was collected at intervals to measure T/B cells and subsets by flow cytometry. In a separate study in baboons receiving the same immunosuppressive regimen (n = 10), the populations of T/B lymphocytes in PBMCs, lymph nodes, and spleen were examined.

RESULTS

After induction therapy, the total lymphocyte count and the absolute numbers of CD3⁺, CD4⁺, and CD8⁺T cells fell by >80%, and no CD22⁺B cells remained (all p < 0.001). T cell numbers began to recover early, but no CD22⁺B cells were present in the blood for 2 months. Recovery of both T and B cells remained at <30% of baseline (p < 0.001), even if rejection developed. At 6 months, effector memory CD8⁺T cells had increased more than other T cell subsets, but a greater percentage of B cells were naïve. In contrast to blood and spleen, T and B cells were not depleted in lymph nodes.

CONCLUSIONS

ATG and anti-CD20mAb effectively decreased T and B lymphocytes in the blood and, in the presence of anti-CD40mAb maintenance therapy, recovery of these cells was inhibited. The recovery of effector memory CD8⁺T cells may be detrimental to long-term graft survival.

Profound thrombocytopenia associated with administration of multiple anti-inflammatory agents in baboons

Congestion, granular platelet debris both within macrophage and extracellularly, and neutrophil infiltration in the spleen of a baboon that was euthanized with profound thrombocytopenia.