Journal of immunology and regenerative medicine最新文献

Objectives

Some vertebrates in the animal kingdom including salamanders and teleost fish have the astonishing ability to fully regenerate many appendages and organs throughout their lifespan. In contrast, most mammals exhibit limited regenerative capabilities that decline with age. Over the last decade, cells in both the innate and adaptive immune system have emerged as key players that direct successful appendage and organ regenerative outcomes. Furthermore, recent studies have highlighted the importance of communication between damaged tissues and the immune system for orchestration of a pro-regenerative response. Understanding the differences in immune response to tissue injury between regenerative and non-regenerative organisms may therefore help inform efforts to stimulate regenerative abilities in humans.

Key findings

This review summarizes present knowledge of the known roles of both the innate and adaptive immune system in the regeneration of organs and appendages in highly regenerative species, with a particular focus on macrophages and T-cells. Furthermore, recent studies showing the importance of communication between the immune system and stem or progenitor cells during tissue homeostasis and regeneration are highlighted. Finally, as cells in the immune system have highly plastic phenotypes depending on their micro-environment, this review sheds light on the possibility that intrinsic differences in tissue damage responses may be the main driver of divergent immune responses in regenerative vs. non-regenerative systems.

Conclusions

Altogether, insights from these studies illustrate the need to fully examine not only the immune cells, but also the micro-environment to which they are exposed, as there may be both important cues from non-immune cells and intrinsic differences in how damaged tissues respond to injury between regenerative and non-regenerative organisms.

Common to all types of injuries, inflammation is the initial stage of healing. The final outcome of healing, however, can be quite distinct after different types of injuries. Some tissues, such as bone, excel at regenerating damaged structures while other tissues, for example skin, heal via deposition of excessive, unpatterned collagen. How does the initial inflammatory environment pave the way for regeneration or fibrosis in mammals? In this review, we look at three wound healing paradigms in mammals: fibrosis (i.e. scar formation of skin), tissue-specific regeneration, (i.e. fracture healing), and epimorphic regeneration (i.e. blastema-mediated skeletal regeneration). We discuss the roles that innate immune cells, specifically macrophages, play in each type of repair response in an attempt to synthesize where similarities and differences have been observed across wound healing models. By juxtaposing the roles of macrophages in regeneration and fibrotic healing in this way, we aim to gain insight into how the initial tissue environment sets the stage for the healing outcome. Finally, we discuss how regenerative medicine could capitalize on aspects of the immune response to promote regeneration over fibrotic healing.

Central nervous system damage in mammals leads to neuronal cell death, axonal degeneration, and formation of a glial scar resulting in functional and behavioral defects. Other vertebrates, like fish and salamanders, have retained the ability to functionally regenerate after central nervous system injury. To date research from many research organisms has led to a more concise understanding of the response of local neural cells to injury. However, it has become clear that non-neural cells of the immune system play an important role in determining the tissue response to injury. In this review we briefly consider the mammalian response to injury compared to organisms with the natural ability to regenerate. We then discuss similarities and differences in how cells of the innate and adaptive immune system respond and contribute to tissue repair in various species.

While tail regeneration is observed in a number of vertebrate groups, including teleost fish such as the zebrafish, urodeles such as the axolotl, and anurans such as Xenopus tadpoles, mammalian and avian amniote vertebrates have lost this capacity. Among the amniotes, squamate reptiles such as lizards retain the ability to regrow their tails and also display the capacity to autotomize, or self-amputate, these structures as a predator evasion response. The regenerated tail is a biomechanically functional structure consisting of regrown and repatterned tissues including spinal cord, peripheral nerves, cartilage, skeletal muscle, vasculature, and skin. The green anole lizard, Anolis carolinensis, was the first reptile with a sequenced and annotated genome, thus allowing transcriptomic analyses. Furthermore, anoles exhibit a high degree of conservation of both innate and adaptive immune pathways with mammals. In histological analyses of tail regeneration in the green anole, we observed early cellular infiltration of the tail stump followed by a second phase of epithelial formation of the wound surface. These events preceded the period of rapid tail outgrowth, which typically starts at 10 days post autotomy. To identify genes activated during the initial phase of tail regeneration, we carried out whole transcriptome sequencing at 0.5, 1, 2, 3, 4, and 5 days post-autotomy. We identified that 5315 genes were differentially expressed between any of these time points and clustered into two major groups with elevated expression either in a first phase (0.5–1 DPA) or in a later second phase (3–5 DPA), with a marked shift in expression at 2 DPA. Genes with elevated expression in the first phase included those regulating the immune system, T cell receptor signaling, and the p38 MAPK signaling pathway. Genes upregulated in the second phase included those regulating cell proliferation, developmental growth, and Wnt/Hippo signaling pathways. Identifying the immunomodulatory events that set the stage for regenerative cell proliferation and outgrowth in an amniote model may help guide post-injury treatments as part of regenerative medical therapies.

Introduction

Oral tolerance is an immunological phenomenon defined by specific inhibition of immune responses to proteins contacted by the oral route. However, parenteral re-exposure to orally-tolerated proteins has systemic effects that reduce inflammation to unrelated agents injected soon afterward. Chronic skin wounds are major complications for diabetic patients, which may be related with pro-inflammatory conditions in the wound bed, as well as with impaired angiogenesis. We used a mouse-model of streptozotocin-induced diabetes to test whether injection of a regular dietary protein (zein) concomitantly with skin lesions reduces wound bed inflammation and improves wound healing in diabetic mice.

Methods

C57BL/6 mice fed a standard chow containing zein (corn protein) were turned diabetic by streptozotocin injection. Two full skin thickness excisional wounds were created on the dorsum of anaesthetized mice. Experimental groups received one i.p. injection of 10 μg zein in adjuvant, 10 min before or 6 h after wounding and were sacrificed 7 and 40 days thereafter. Skin samples were processed and examined macroscopically and microscopically.

Results

Intraperitoneal injection of zein either before or after skin injuries, reduced the number of leukocytes (CD45⁺ cells) and of myofibroblasts, increased the number of alternatively activated (M2) macrophages, increased the expression transforming-growth factor (TGF)-β3 and rescued angiogenesis in the wound bed of diabetic mice. Zein treatment reduced the scar area and improved the organization of collagen fibers in the neodermis.

Conclusion

Parenteral re-exposure to a protein previously contacted by the oral route concomitantly with or 6 h after skin injuries reduces wound inflammation and improves wound healing in diabetic mice.

Indoleamine 2,3-dioxygenase (IDO), an intracellular enzyme responsible for catalyzing the rate limiting step of tryptophan catabolism, plays a critical role in immune cell suppression and tolerance. Indoleamine 2,3-dioxygenase-mediated depletion of the essential amino acid tryptophan increases susceptibility of T cells to apoptosis, while kynurenine and its downstream metabolites, such as 3-hydroxyanthranilic acid and quinolinic acid, have a direct cytotoxic effect on conventional effector T cells. Additionally, IDO-expressing antigen presenting cells (APCs) induce proliferation of regulatory T cells. When expressed by an APC, the immunosuppressive effects of IDO may act directly on the APC as well as indirectly upon local T cells. One approach to elicit immune tolerance or reduce inflammation therefore is to promote expression of IDO. However, this approach is constrained by several factors including the potential for deleterious biologic effects of conventional IDO-inducing agents such as interferon gamma (IFNγ), and the potential limitations of constitutive gene transfection. Alternatively, direct action of recombinant IDO enzyme supplied exogenously as a potential therapeutic in the extracellular space has not been investigated previously, and is the focus of this work. Results indicate exogenous recombinant human IDO supplementation influences murine dendritic cell (DC) maturation and ability to suppress antigen specific T cell proliferation. Following treatment, DCs were refractory to maturation by LPS as defined by co-stimulatory molecule expression (CD80 and CD86) and major histocompatibility complex II (MHC-II) expression. Dendritic cells exhibited skewing toward an anti-inflammatory cytokine release profile, with reduced secretion of IL-12p70 and maintained basal level of secreted IL-10. Notably, IDO-treated DCs suppressed proliferation of ovalbumin (OVA) antigen-specific CD4⁺ and CD8⁺ T cells in the presence of cognate antigen presentation in a manner dependent on active enzyme, as introduction of IDO inhibitor 1-methyl-tryptophan, restored T cell proliferation. Defined media experiments indicate a cumulative role for both tryptophan depletion and kynurenine presence, in the suppressive programming of DCs. In sum, we report that exogenously supplied IDO maintains immunoregulatory function on DCs, suggesting that IDO may have potential as a therapeutic protein for suppressive programming with application toward inflammation and tolerance.

Objectives

It has been well established that the survival and long-term outcome for patients suffering a myocardial infarction in part depends on the resulting immune response to injury. These processes are complex, and a clear path to useful immunotherapies for the treatment of cardiovascular damage in humans remains elusive. Mammals hold a great potential for repair of cardiac tissue during fetal and early neonatal life, an ability that is lost in the adult, coinciding with a maturation of the immune system. Unlike mammals, the axolotl and zebrafish, which are popular model organisms in regenerative medicine, successfully recover functionally and anatomically following infarction injury. In this review, we present an in-depth comparative look at the immune response to cardiac infarction damage in adult and fetal/early neonatal mammals as well as axolotls and zebrafish, with an emphasis on the role of macrophages. This current knowledge is instrumental for transferring new findings in regenerative animal models to the development of novel immune-modulating treatments. These could improve the rate of survival and quality of life after injury for the millions of people suffering from a myocardial infarction every year.

Key findings

The regenerative process in axolotls and zebrafish has been found to rely on the actions of key immune cells. Macrophages in particular are essential to cardiac regeneration in axolotls and zebrafish as well as mammalian fetuses and neonates. There is great interest in the heterogeneity of macrophage populations, as mammalian embryonic macrophages appear to be facilitators of regeneration, while monocyte-derived macrophages in adults chiefly promote fibrosis. Monocyte derived macrophages also exist in a spectrum of phenotypes grossly divided into pro-inflammatory M1 and immune-resolving M2 cells, with divergent roles following tissue damage. The phenotypes of axolotl macrophages remain uncharacterized, but early studies suggest that the macrophages recruited to the infarction site are primarily similar to embryonic or M2-type macrophages.

Conclusions

Findings in animal models as well as humans, indicates that the inflammatory response and especially the action of macrophages should be examined further, which requires a detailed understanding of these processes in models both capable and incapable of cardiac regeneration. Immunotherapies aimed at improving outcomes in mammals, should not eliminate the inflammatory response, but rather modulate it to resemble that of competent regenerators.

Stem cell-based therapies are promising solutions to tackle several conditions. In particular, mesenchymal stem cells (MSCs) own very unique characteristics that have often been described as a “cure-all”. In addition to their differentiation capacity, MSCs have demonstrated both trophic and immunomodulatory suppression of inflammation. These properties have been exploited to develop cellular therapies for different inflammatory-related pathologies. In this review, we describe how to utilize the immunosuppressive potential of MSCs for regenerative purposes and cancer therapy. Moreover, we report innovative approaches from tissue engineering and nanomedicine that demonstrate MSCs’ potential to improve their clinical translation.

A promising approach to rescue cardiac function after a myocardial infarction (MI) is to apply an engineered heart tissue (EHT) onto the infarcted area. After the onset of MI, a dynamic inflammatory environment develops comprising of the temporal recruitment of macrophages (Mϕs), and their interactions with the cells of the damaged myocardium. There is limited knowledge about the interactions between this inflammatory environment and the cells that could potentially be used to create an EHT, such as pluripotent stem cell derived-cardiomyocytes. In the present study, a cell-based system was used to study the in vitro interactions between lipopolysaccharide (LPS) and interferon-gamma (IFNγ)-activated Mϕs, and interleukin 4 (IL4) and interleukin 13 (IL13)-activated Mϕs and human embryonic stem cell-derived cardiomyocytes (hESC-CMs). Using a co-culture system, gene expression profiles of key markers of both the Mϕs and the hESC-CMs were obtained, as well as the protein secretion. Additionally, the effects of Mϕ polarizing cytokines on hESC-CMs with or without the presence of Mϕs were studied. Mϕs co-cultured with hESC-CMs showed no significant changes in their gene expression profile after two days in culture. hESC-CMs, however, were noted to have an overall decrease in expression of cardiac-related genes upon exposure to both Mϕ subtypes in co-culture. Gene expression of Bone morphogenetic protein-2 (BMP2), Bone morphogenetic protein-4 (BMP4) and GATA-binding protein-4 (GATA4) were also affected by Mϕ exposure and by inflammatory signals such as LPS and IFNγ. This study represents an important step towards the design of advanced in vitro testing platforms to further study the effect of Mϕs and inflammatory signals on EHTs in vitro.