Research

Overview

The Lei-Markmann Laboratory Group at the Center for Transplant Science and Innovation, part of the Perelman School of Medicine at the University of Pennsylvania and the Penn Transplant Institute is a dynamic and multifaceted research team. The lab focuses on advancing transplant science through several key areas of investigation. One major area of research explores regulatory T and B cell-mediated transplant tolerance, aiming to protect grafts by investigating both small animal and nonhuman primate models. The lab is also at the forefront of xenotransplantation, investigating pig-to-human liver transplants as a potential solution to the shortage of donor liver allografts. In parallel, the team utilizes cutting-edge technologies to process human organs, generating primary islets and hepatocytes that are used in clinical treatments for conditions such as Type 1 diabetes(T1D), chronic pancreatitis, and end-stage liver disease. Pushing the boundaries of translational science, the lab is actively developing and evaluating novel cell types, including immunomodulatory cells, induced pluripotent stem cells (iPSCs), and related derivatives in nonhuman primate models. These efforts support preclinical studies aimed at improving allogeneic islet and stem cell-derived beta cell transplantation approaches for the treatment of T1D.

Regulatory B cell-mediated transplant tolerance

While regulatory T cells have been evaluated in both relevant preclinical non-human primate and early-phase clinical trials for their capacity to curtail alloimmunity and achieve allograft survival without chronic immunosuppression by our team as well as others, producing therapeutically relevant regulatory B cells (Bregs) remains a fertile area for progress and translation to clinical application. The delineation of the Breg function and its mechanisms of suppression remains to be clarified.

Several years ago, our group first described a transplant tolerance protocol that was B cell dependent. We determined that a short course of anti-CD45RB antibody treatment prolongs heart allografts indefinitely, but not in the absence of B cells. Since then, we and others have demonstrated that additional tolerance induction protocols are B cell-dependent, with the recovery and adoptive transfer of B cells from tolerant mice being sufficient to induce transplantation tolerance in naïve hosts.

To better understand the role and translational potential of regulatory B cells as a cell-based therapeutic, we have investigated methods of expanding naïve B cells ex vivo through Toll-like receptors (TLRs) mediated activation and generating B cells with regulatory properties. In vitro and in vivo, these Bregs are capable of suppressing T cell proliferation, prolonging graft survival, and inducing regulatory T cells. Phenotypically, our Bregs display upregulated markers of B cell activation, including CD80, CD86, MHC class II and CD38. In addition, greater expression of TIM1, IL-10, CD25, PD-L1 and LAP (a surrogate for the anti-inflammatory cytokine, TGF-β) is observed. Through single cell RNA-sequencing, we have substantiated an important role for TGF-β-mediated signaling that may underlie the immunobiology of Bregs we term Bregs-TLR9/4.

Backed by compelling evidence from our lab highlighting the unique advantages of antigen-specific Bregs, we are focused on advancing the understanding of Breg differentiation and suppression. With extensive experience generating Bregs ex vivo, identifying key immunomodulatory pathways, and studying tolerance in diverse transplant models, the Lei-Markmann Lab is actively developing and optimizing innovative, Breg-based approaches for clinical translation.

Pig-to-primate liver xenotransplantation

According to the U.S. Organ Procurement and Transplantation Network (OPTN), the United States performed a record 48,149 organ transplants in 2024, a 3.3% increase from the previous year. Despite this progress, as of April 1, 2025, 104,813 candidates remain on the organ transplant waiting list. Based on the most recent data from 2023, approximately 11,129 patients, about 30 per day, were removed from the list due to death or becoming too ill for transplant. With organ demand continuing to outpace supply, the only possible solution with the potential to address the inadequate organ supply in the near term appears to be the use of animal organs, also known as xenotransplantation.  The Lei-Markmann Lab is working to understand the immunological and physiological barriers to liver and kidney xenotransplantation and determining the needed genetic modifications and immunosuppressive regimens necessary to achieve durable graft survival.

In recent years, we have witnessed breakthroughs in xenotransplantation based on the use of organs from genetically modified pigs targeting antibody and complement-activation pathways. The focus of our research is on liver ex-vivo perfusion and orthotopic liver xenotransplantation. Transplantation of livers from GTKO miniature swine into baboon recipients performed by our team and others has shown the longest survival without evidence of rejection, inflammation or thrombotic microangiopathy (TMA) at 29 days. True long-term survival of liver xenotransplants has yet to be achieved and will require tackling key concerns such as identifying the most effective immunosuppression regimen, overcoming coagulation dysregulation and management of systemic inflammation and graft ischemia reperfusion injury (IRI). Our team aims to evaluate the impact of CRISPR-Cas9-induced gene panels on the incidence and severity of initial xenograft dysfunction.

Nonhuman primate models for testing next-gen cell therapies in T1D

Replacement of the beta-cell mass with cadaveric whole pancreas or allogeneic islet transplantation represents the most effective treatment alternative for improving metabolic control and quality of life in T1D patients. However, the limited number of donors and the need for lifelong, nonspecific immunosuppression to control rejection limit the broad application of islet transplantation for T1D.  To overcome the shortage of islet material, successful protocols have been established to generate stem cell-derived islets (SC-islets) from human stem cells. While SC-islets could offer an unlimited cell resource, their broad application faces the same challenges of allograft rejection as primary islets, which require immunosuppression to protect the graft. To overcome this, genetic engineering of stem cells to generate SC-islets that evade immune recognition has emerged as a promising strategy. This approach could lead to a universal, off-the-shelf cell source capable of treating millions of patients with insulin-dependent diabetes. Our team is applying gene engineering technologies to develop the next generation of novel cell types in non-human primate models, aiming to advance allogeneic islet and SC-islet transplantation strategies for the treatment of T1D.

Advancing islet and hepatocyte therapies for diabetes and liver disease

Islet transplantation is a cutting-edge therapy for patients with severe T1D and for those with surgical diabetes following total pancreatectomy. The quality of islets is critical to achieving successful clinical outcomes, yet the isolation process is technically complex, variable in outcome. Over the past two decades, our team has been deeply involved in refining and advancing human islet isolation techniques, with a combined experience of over 1,000 isolations from human, non-human primate, and pig pancreases, supporting both clinical transplantation and research. In recent years, we have also led efforts to develop and standardize human hepatocyte manufacturing protocols for clinical transplantation, aiming to treat patients with end-stage liver diseases who are not eligible for whole-organ transplants.