Genetic modifications designed for xenotransplantation attenuate sialoadhesin-dependent binding of human erythrocytes to porcine macrophages

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.

Sialic acid in the regulation of blood cell production, differentiation and turnover

Sialic acid is a unique sugar moiety that resides in the distal and most accessible position of the glycans on mammalian cell surface and extracellular glycoproteins and glycolipids. The potential for sialic acid to obscure underlying structures has long been postulated, but the means by which such structural changes directly affect biological processes continues to be elucidated. Here, we appraise the growing body of literature detailing the importance of sialic acid for the generation, differentiation, function and death of haematopoietic cells. We conclude that sialylation is a critical post-translational modification utilized in haematopoiesis to meet the dynamic needs of the organism by enforcing rapid changes in availability of lineage-specific cell types. Though long thought to be generated only cell-autonomously within the intracellular ER-Golgi secretory apparatus, emerging data also demonstrate previously unexpected diversity in the mechanisms of sialylation. Emphasis is afforded to the mechanism of extrinsic sialylation, whereby extracellular enzymes remodel cell surface and extracellular glycans, supported by charged sugar donor molecules from activated platelets.

A Monocytic Barrier to the Humanization of Immunodeficient Mice

Mice with severe immunodeficiencies have become very important tools for studying foreign cells in an in vivo environment. Xenotransplants can be used to model cells from many species, although most often, mice are humanized through the transplantation of human cells or tissues to meet the needs of medical research. The development of immunodeficient mice is reviewed leading up to the current state-of-the-art strains, such as the NOD-scid-gamma (NSG) mouse. NSG mice are excellent hosts for human hematopoietic stem cell transplants or immune reconstitution through transfusion of human peripheral blood mononuclear cells. However, barriers to full hematopoietic engraftment still remain; notably, the survival of human cells in the circulation is brief, which limits overall hematological and immune reconstitution. Reports have indicated a critical role for monocytic cells - monocytes, macrophages, and dendritic cells - in the clearance of xenogeneic cells from circulation. Various aspects of the NOD genetic background that affect monocytic cell growth, maturation, and function that are favorable to human cell transplantation are discussed. Important receptors, such as SIRPα, that form a part of the innate immune system and enable the recognition and phagocytosis of foreign cells by monocytic cells are reviewed. The development of humanized mouse models has taken decades of work in creating more immunodeficient mice, genetic modification of these mice to express human genes, and refinement of transplant techniques to optimize engraftment. Future advances may focus on the monocytic cells of the host to find ways for further engraftment and survival of xenogeneic cells.