Complement Deposition and Cell Death after Myoblast Transplantation

The Immune System in Duchenne Muscular Dystrophy Pathogenesis

Growing evidence demonstrates the crosstalk between the immune system and the skeletal muscle in inflammatory muscle diseases and dystrophic conditions such as Duchenne Muscular Dystrophy (DMD), as well as during normal muscle regeneration. The rising of inflammation and the consequent activation of the immune system are hallmarks of DMD: several efforts identified the immune cells that invade skeletal muscle as CD4+ and CD8+ T cells, Tregs, macrophages, eosinophils and natural killer T cells. The severity of muscle injury and inflammation dictates the impairment of muscle regeneration and the successive replacement of myofibers with connective and adipose tissue. Since immune system activation was traditionally considered as a consequence of muscular wasting, we recently demonstrated a defect in central tolerance caused by thymus alteration and the presence of autoreactive T-lymphocytes in DMD. Although the study of innate and adaptive immune responses and their complex relationship in DMD attracted the interest of many researchers in the last years, the results are so far barely exhaustive and sometimes contradictory. In this review, we describe the most recent improvements in the knowledge of immune system involvement in DMD pathogenesis, leading to new opportunities from a clinical point-of-view.

Differentiation of Murine Embryonic Stem Cells in Skeletal Muscles of Mice

Possible myogenic differentiation of SSEA-1- and OCT-4-positive murine embryonic stem cells (ESCs) and embryoid bodies (EBs) was studied in vitro and in vivo. In vitro, ESC- or EB-derived ESCs (EBs/ESCs) showed only traces of Pax 3 and 7 expression by immunocytochemistry and Pax 3 expression by immunoblot. By RT-PCR, myogenic determinant molecules (myf5, myoD, and myogenin) were expressed by EBs/ESCs but not by ESCs. However, in such cultures, very rare contracting myotubes were still present. Suspensions of LacZ-labeled ESCs or EBs were injected into anterior tibialis muscles (ATM) of different cohorts of mice for the study of their survival and possible myogenic differentiation. The different cohorts of mice included isogenic adult 129/Sv, nonisogenic CD1 and mdx, as well as mdx immunosuppressed with 2.5 mg/kg daily injections of tacrolimus. Ten to 90 days postinjections, the injected ATM of nonisogenic mice did not contain cells positive for LacZ, SSEA-1, OCT-4, or embryonic myosin heavy chain. The ATM of intact mdx mice contained very rare examples of muscle fibers positive for dystrophin and/or embryonic myosin heavy chain. In the ATM of the isogenic normal and the immunosuppressed mdx mice, as expected, large teratomas developed containing the usual diverse cell types. In some teratomas of immunosuppressed mdx mice, small pockets of muscle fibers expressed dystrophin and myosin heavy chain. Our studies indicated that in muscles of animals nonisogenic with the used ESCs, only very rare ESCs survived with myogenic differentiation. These studies also indicated that ESCs will not undergo significant, selective, and preferential myogenic differentiation in vitro or in vivo in any of the models studied. It is probable that this strain of murine ESC requires some experimentally induced alteration of its gene expression profile to secure significant myogenicity and suppress tumorogenicity.

Comparative analysis of genetically engineered immunodeficient mouse strains as recipients for human myoblast transplantation

The development of an optimized animal model for the in vivo analysis of human muscle cells remains an important goal in the search of therapy for muscular dystrophy. Here we examined the efficiency of human myoblast xenografts in three distinct immunodeficient mouse models. We found that different conditioning regimes used to provoke host muscle regeneration (i.e., cardiotoxin versus cryodamage) had a marked impact on xenograft success. Tibialis anterior muscle of Rag2-, Rag-/gammac-, and Rag-/gammac-/C5- mice was treated by cardiotoxin or cryodamage, submitted to enzymatic digestion, and analyzed by cytofluorometry to quantitate inflammatory cells. Human myoblasts were injected into pretreated muscles from immunodeficient recipients and the cell engraftment evaluated by immunocytochemistry, 4-8 weeks after transplantation. Donor cell differentiation and dispersion within the host muscles was also investigated. Host regeneration in cardiotoxin-treated mice was accompanied by a higher inflammatory cell infiltration when compared to that induced by cryodamage. Accordingly, when compared to the cardiotoxin group, more human myogenic cells were found after cryodamage. When the distinct immunodeficient mice were compared, we found that the alymphoid strain lacking the complement component C5 (Rag-/gammac-/C5- mice) was the most efficient host for human muscle xenografts, when compared with C5(+)Rag-/gammac- mice or Rag- mice. Our results demonstrate that cryolesion-conditioned muscles of Rag-/gammac-/C5- mice provide the best environment for long-term in vivo human myoblast differentiation, opening the way for a novel approach to study the pathophysiology of human muscle disorders.

Skeletal myoblast transplant in heart failure

Despite recent advances in the prevention and treatment of ischemic heart disease (IHD), treatment of patients with heart failure secondary to myocardial infarction remains a therapeutic challenge. Heart transplantation has emerged as a viable option but is fraught with problems of supply. Mechanical assist devices are extremely expensive and dynamic cardiomyoplasty has shown only limited success in the clinical setting. Recent insights into the pathogenesis of myocardial diseases and the progress made in the field of molecular biology have resulted in the development of new strategies at molecular as well as cellular levels for cardiac muscle repair. One such strategy is to augment ventricular function by means of cellular cardiomyoplasty through intracardiac cell grafting using adult and fetal cardiomyocytes, stem cells, and autologous skeletal myoblasts.

Dynamics of the early immune cellular reactions after myogenic cell transplantation

The role of immune cells in the early donor cell death/survival following myoblast transplantation is confusing, one of the reasons being the lack of data about the immune reactions following cell transplantation. We used outbred mice as hosts for transplantation of primary cultured muscle cells and T-antigen-immortalized myoblasts. The host muscles were analyzed 1 h to 7 days after cell injection. No net loss of the donor primary cultured cell population was observed in this period. The immune cellular reaction in this case was: 1) a brief (<48 h) neutrophil invasion; 2) macrophage infiltration from days 1 to 7; 3) a specific response involving CTL and few NK cells (days 6 and 7), preceded by a low CD4+ cell infiltration starting at day 3. In contrast, donor-immortalized myoblasts completely disappeared during the 7-day follow-up. In this case, an intense infiltration of CTL and macrophages, with moderate CD4+ infiltration and lower amounts of NK cells, was observed starting at day 2. The nonspecific immune response at days 0 and 1 was similar for both types of donor cells. The present observations set a basis to interpret the role of immune cells on the early death/survival of donor cells following myoblast transplantation.

Tubulyzine, a novel tri-substituted triazine, prevents the early cell death of transplanted myogenic cells and improves transplantation success

Myoblast transplantation (MT) is a potential therapeutic approach for several muscular dystrophies. A major limiting factor is that only a low percentage of the transplanted myoblasts survives the procedure. Recent advances regarding how and when the myoblasts die indicate that events preceding actual tissue implantation and during the first days after the transplantation are crucial. Myoseverin, a recently identified tri-substituted purine, was shown to induce in vitro the fission of multinucleated myotubes and affect the expression of a variety of growth factors, and immunomodulation, extracellular matrix-remodeling, and stress response genes. Since the effects of myoseverin are consistent with the activation of pathways involved in wound healing and tissue regeneration, we have investigated whether pretreatment and co-injection of myoblasts with Tubulyzine (microtubule lysing triazine), an optimized myoseverin-like molecule recently identified from a triazine library, could reduce myoblast cell death following their transplantation and consequently improves the success of myoblast transplantation. In vitro, using annexin-V labeling, we showed that Tubulyzine (5 microM) prevents normal myoblasts from apoptosis induced by staurosporine (1 microM). In vivo, the pretreatment and co-injection of immortal and normal myoblasts with Tubulyzine reduced significantly cell death (assessed by the radio-labeled thymidine of donor DNA) and increased survival of myoblasts transplanted in Tibialis anterior (TA) muscles of mdx mice, thus giving rise to more hybrid myofibers compared to transplanted untreated cells. Our results suggest that Tubulyzine can be used as an in vivo survival factor to improve the myoblast-mediated gene transfer approach.

Progress in myoblast transplantation: a potential treatment of dystrophies

Myoblast transplantation (MT) consists of injecting normal or genetically modified myogenic cells into muscles, where they are expected to fuse and form mature fibers. As an experimental approach to treat severe genetic muscle diseases, MT was tested in dystrophic patients at the beginning of the 1990s. Although these early clinical trials were unsuccessful, MT has progressed through the research on animal models. Many factors that may condition the success of MT were identified in the last years. The present review updates our knowledge on MT and describes the different problems that have limited its success. Factors that were first underestimated, like the specific immune response after MT, are presently well characterized. Destruction of the hybrid fibers by activated T-lymphocytes and production of antibodies against the transplanted myoblasts take place after MT and are responsible for the graft rejection. The choice of the immunosuppression seems to be very important, and FK506 is the best agent known to allow the best results after MT. Under FK506 immunosuppression, very efficient MT were obtained both in mice and monkeys. Moreover, in dystrophic mice it was demonstrated that MT ameliorates some phenotypical characteristics of the disease. The improvement of the survival of the transplanted cells and the increase of their migration into the injected tissue are presently under investigation. Some of the present research is directed also to bypass the immunosuppression by using the patient's own cells for MT. In this sense, efforts are conducted to introduce the normal gene into the patient's myoblasts before MT and to improve the ability of these cells to proliferate in vitro. Micros. Res. Tech. 48:213-222, 2000.