Acute Rejection of Myofibers in Nonhuman Primates: Key Histopathologic Features

Human MuStem cells are competent to fuse with nonhuman primate myofibers in a clinically relevant transplantation context: A proof-of-concept study

We previously reported that human muscle-derived stem cells (hMuStem cells) contribute to tissue repair after local administration into injured skeletal muscle or infarcted heart in immunodeficient rodent models. However, extrapolation of these findings to a clinical context is problematic owing to the considerable differences often seen between in vivo findings in humans versus rodents. Therefore, we investigated whether the muscle regenerative behavior of hMuStem cells is maintained in a clinically relevant transplantation context. Human MuStem cells were intramuscularly administered by high-density microinjection matrices into nonhuman primates receiving tacrolimus-based immunosuppression thereby reproducing the protocol that has so far produced the best results in clinical trials of cell therapy in myopathies. Four and 9 weeks after administration, histological analysis of cell injection sites revealed large numbers of hMuStem cell-derived nuclei in all cases. Most graft-derived nuclei were distributed in small myofiber groups in which no signs of a specific immune response were observed. Importantly, hMuStem cells contributed to simian tissue repair by fusing mainly with host myofibers, demonstrating their capacity for myofiber regeneration in this model. Together, these findings obtained in a valid preclinical model provide new insights supporting the potential of hMuStem cells in future cell therapies for muscle diseases.

Human Muscle Precursor Cells Form Human-Derived Myofibers in Skeletal Muscles of Nonhuman Primates: A Potential New Preclinical Setting to Test Myogenic Cells of Human Origin for Cell Therapy of Myopathies

This study aimed to verify if human myogenic cells could participate in muscle regeneration in macaques. This experimental setting would grant researchers a model that could better evaluate the effects of cell therapies in myopathies with a better translation to human patients. Human muscle precursor cells (MPCs) were cultured in vitro and transduced with ß-galactosidase. The cells were subsequently injected into 1-cm3 muscle regions of 6 macaques immunosuppressed with tacrolimus and dexamethasone. Allogeneic ß-galactosidase+ MPCs were injected in other regions as positive controls. Some cell-grafted regions were electroporated to induce extensive muscle regeneration. MPC-grafted regions were sampled 1 month later and analyzed by histology. There were ß-galactosidase+ myofibers in both the regions grafted with human and macaque MPCs. Electroporation increased the engraftment of human MPCs in the same way as in macaque allografts. The histological analysis (hematoxylin and eosin, CD8, and CD4 immunodetection) demonstrated an absence of cellular rejection in most MPC-grafted regions, as well as minimal lymphocytic infiltration in the regions transplanted with human MPCs in the individual with the lowest tacrolimus levels. Circulating de novo anti-donor antibodies were not detected. In conclusion, we report the successful engraftment of human myogenic cells in macaques, which was possible using tacrolimus-based immunosuppression.

Sarcolemmal Complement Membrane Attack Complex Deposits During Acute Rejection of Myofibers in Nonhuman Primates

We have previously studied in nonhuman primates several aspects of the acute rejection of myofibers, including the histological characteristics, the mechanisms of myofiber elimination by the T cells, and the development of anti-donor antibodies. Here, we report the participation of the complement membrane attack complex (MAC) in this context. We used muscle sections of macaques from experiments of allogeneic muscle precursor cell transplantation with confirmed rejection of the graft-derived myofibers. Sections were stained with hematoxylin and eosin, alizarin red and for immunodetection of MAC, CD8, CD4, C3, C4d, and immunoglobulins. The prominent finding was the presence of sarcolemmal MAC (sMAC) deposits in biopsies with ongoing acute rejection or with recent acute rejection. The numbers of sMAC-positive myofibers were variable, being higher when there was an intense lymphocyte infiltration. Few sMAC-positive myofibers were necrotic or had evidence of sarcolemma permeation. The immunodetection of C3, C4d, and immunoglobulins did not provide significant elements. In conclusion, sMAC deposits were related to myofiber rejection. The fact that the vast majority of sMAC-positive myofibers had no signs of necrosis or sarcolemmal permeation suggests that MAC would not be harmful to myofibers by itself.

De Novo Circulating Antidonor's Cell Antibodies During Induced Acute Rejection of Allogeneic Myofibers in Myogenic Cell Transplantation: A Study in Nonhuman Primates

Background

Transplantation of myogenic cells has potential applications in the treatment of muscle pathologies. Excluding purely autologous cell transplantation, graft viability depends on an adequate control of acute rejection (AR). To contribute in understanding AR in this context, we analyzed whether de novo circulating antibodies against donor’s cells are detected during induced AR of graft-derived myofibers in nonhuman primates.

Methods

We allotransplanted satellite cell-derived myoblasts in macaques immunosuppressed with tacrolimus. To induce AR of graft-derived myofibers, we administered tacrolimus for 4 weeks to allow complete myofiber formation, and then we stopped tacrolimus administration. Cell-grafted sites were biopsied at tacrolimus withdrawal and then every 2 weeks and analyzed by histology until AR completion. Blood samples were taken before immunosuppression, at tacrolimus withdrawal and then every 2 weeks to detect antibodies against the donor’s cells by flow cytometry.

Results

There was an increase of antibodies against the donor’s cells related to AR in all monkeys. This increase was variable in intensity, and preceded, coincided or followed the histological evidence of AR (focal accumulations of lymphocytes) and/or the loss of myofibers of donor origin, and remained until the end of the follow-up (up to 8 weeks after tacrolimus withdrawal).

Conclusions

Flow cytometry detection of de novo circulating antibodies against the donor’s cells was consistently associated with AR. A clear increase in this antibody detection indicated current or recent AR. Smaller increases in comparison to the preimmunosuppression values were not associated with AR.

The Process of Engraftment of Myogenic Cells in Skeletal Muscles of Primates: Understanding Clinical Observations and Setting Directions in Cell Transplantation Research

We studied in macaques the evolution of the intramuscular transplantation of muscle precursor cells between the time of administration and the time at which the graft is considered stable. Satellite cell–derived myoblasts labeled with ß-galactosidase were transplanted into 1 cm³ muscle regions following cell culture and transplantation protocols similar to our last clinical trials. These regions were biopsied 1 h, 1, 3, 7 d, and 3 wk later and analyzed by histology. We observed that the cell suspension leaks from the muscle bundles during injection toward the epimysium and perimysium, where most cells accumulate after transplantation. We observed evidence of necrosis, apoptosis, and mitosis in the accumulations of grafted cells, and of potential migration to participate in myofiber regeneration in the surrounding muscle bundles. After 3 wk, the compact accumulations of grafted cells left only some graft-derived myotubes and small myofibers in the perimysium. Hybrid myofibers were abundant in the muscle fascicles at 3 wk posttransplantation, and they most likely occur by grafted myoblasts that migrated from the peripheral accumulations than by the few remaining within the fascicles immediately after injection. These observations explain the findings in clinical trials of myoblast transplantation and provide information for the future research in cell therapy in myology.

Cell Therapy in Myology: Dynamics of Muscle Precursor Cell Death after Intramuscular Administration in Non-human Primates

Cell therapy could be useful for the treatment of myopathies. A problem observed in mice, with different results and interpretations, is a significant death among the transplanted cells. We analyzed this problem in non-human primates, the animal model more similar to humans. Autologous or allogeneic myoblasts (with or without a reporter gene) were proliferated in vitro, labeled with [¹⁴C]thymidine, and intramuscularly injected in macaques. Some monkeys were immunosuppressed for long-term follow-up. Cell-grafted regions were biopsied at different intervals and analyzed by radiolabel quantification and histology. Most radiolabel was lost during the first week after injection, regardless of whether the cells were allogeneic or autologous, the culture conditions, and the use or not of immunosuppression. There was no significant difference between 1 hr and 1 day post-transplantation, a significant decrease between days 1 and 3 (45% to 83%), a significant decrease between days 3 and 7 (80% to 92%), and no significant differences between 7 days and 3 weeks. Our results confirmed in non-human primates a progressive and significant death of the grafted myoblasts during the first week after administration, relatively similar to some observations in mice but with different kinetics.

Strategies for skeletal muscle tissue engineering: seed vs. soil

The most commonly used tissue engineering approach includes the ex vivo combination of site-appropriate cell(s) and scaffold material(s) to create three-dimensional constructs for tissue replacement or reconstruction. These three-dimensional combinations are typically subjected to a period of culture and conditioning (i.e., self-assembly and maturation) to promote the development of ex vivo constructs which closely mimic native target tissue. This cell-based approach is challenged by the host response to the engineered tissue construct following surgical implantation. As an alternative to the cell-based approach, acellular biologic scaffolds attract endogenous cells and remodel into partially functional mimics of native tissue upon implantation. The present review examines cell-types (i.e., seed), scaffold materials (i.e., soil), and challenges associated with functional tissue engineering. Skeletal muscle is used as the target tissue prototype but the discussed principles will largely apply to most body systems.

First study of intra-arterial delivery of myogenic mononuclear cells to skeletal muscles in primates

The main challenge of cell transplantation as a treatment of myopathies is the large amount of tissue to treat. Intravascular delivery of cells may be an ideal route if proven to be effective and safe. Given the importance of nonhuman primates for preclinical research in transplantation, we tested the intra-arterial injection of β-galactosidase (β-Gal)-labeled myoblasts in macaques. Cells were injected into one of the femoral arteries in seven monkeys. Some muscle sites were damaged concomitantly in three monkeys. Various organs and muscles were sampled 1 h, 1 day, 12 days, 3 weeks, and 5 weeks after transplantation. Samples were analyzed by histology. Most β-Gal(+) cells were observed in the capillaries and arterioles of muscles and other tissues of the leg homolateral to the cell injection. Groups of necrotic myofibers in the proximity of an arteriole plugged by a β-Gal(+) embolus were interpreted as microinfarcts. Scarce β-Gal(+) cells were observed in the lungs 1 h and 1 day posttransplantation. No β-Gal(+) cells were observed in other organs or muscles. β-Gal(+) myofibers were observed 12 days, 3 weeks, and 5 weeks after transplantation in muscles of the leg after the cell injection, in sites that were damaged at the time of cell injection. In conclusion, most intra-arterially injected myoblasts were retained in vessels of the leg homolateral to the cell injection site, and they fused with myofibers in regions in which there was a process of myofiber regeneration. This manuscript is published as part of the International Association of Neurorestoratology (IANR) special issue of Cell Transplantation.

Intramuscular transplantation of myogenic cells in primates: importance of needle size, cell number, and injection volume

The aim of this study was to quantitatively define the main measurable technical parameters for the intramuscular transplantation of myogenic cells in primates. Myoblasts transduced with the gene coding for β-galactosidase were injected into the skeletal muscles of 15 monkeys. The following parameters were studied: needle size, number of cells per injection, and volume of cell suspension per injection. Monkeys were immunosuppressed with tacrolimus. The cell-injected sites were biopsied 1 or 2 months later. Biopsies were examined histologically to assess the myoblast engraftment and the muscle structure. The conclusions were as follows: (1) Needles should be thin enough to avoid important tissue damage and allow muscle regeneration as satisfactory as possible. Among those tested, 27G should be the choice if the length is consistent with depth of injection. (2) At least 100,000 cells should be delivered per centimeter of needle trajectory. (3) The smallest volumes of cell suspension per injection should be used. In this study, 1 µl/cm of injection trajectory was sufficient. In principle, these parameters apply to muscles in which no damage occurred other than the injections.

Necrosis, sarcolemmal damage and apoptotic events in myofibers rejected by CD8+ lymphocytes: Observations in nonhuman primates

To detect the mechanisms of death in allogeneic myofibers rejected by the immune system, myoblasts were allotransplanted in muscles of macaques immunosuppressed with tacrolimus. Immunosuppression was stopped 1month later to induce a massive rejection of allogeneic myofibers. Grafted sites were biopsied at 2-week intervals and analyzed by histology. The loss of allogeneic myofibers was rapid and concomitant with an intense infiltration of CD8+ lymphocytes. Several necrotic myofibers were observed in the lymphocyte accumulations by intracellular complement immunodetection. Dystrophin and spectrin immunodetection showed sarcolemmal damage in myofibers surrounded and invaded by CD8+ lymphocytes. Active caspase-3 was immunodetected in some myofibers surrounded by CD8+ lymphocytes. This is the first evidence that the collapse of myofibers attacked by T lymphocytes occurs by necrosis possibly due to damage of the sarcolemma. Caspase 3 is activated at least in some myofibers, but there was no evidence of a complete classical process of apoptosis.