Cyclophosphamide-induced leukopenia and suppression of limb regeneration in the adult newt, notophthalmus viridescens

Distinct toll-like receptor signaling in the salamander response to tissue damage

BACKGROUND

Efficient wound healing or pathogen clearance both rely on balanced inflammatory responses. Inflammation is essential for effective innate immune-cell recruitment; however, excessive inflammation will result in local tissue destruction, pathogen egress, and ineffective pathogen clearance. Sterile and nonsterile inflammation operate with competing functional priorities but share common receptors and overlapping signal transduction pathways. In regenerative organisms such as the salamander, whole limbs can be replaced after amputation while exposed to a nonsterile environment. In mammals, exposure to sterile-injury Damage Associated Molecular Patterns (DAMPS) alters innate immune-cell responsiveness to secondary Pathogen Associated Molecular Pattern (PAMP) exposure.

RESULTS

Using new phospho-flow cytometry techniques to measure signaling in individual cell subsets we compared mouse to salamander inflammation. These studies demonstrated evolutionarily conserved responses to PAMP ligands through toll-like receptors (TLRs) but identified key differences in response to DAMP ligands. Co-exposure of macrophages to DAMPs/PAMPs suppressed MAPK signaling in mammals, but not salamanders, which activate sustained MAPK stimulation in the presence of endogenous DAMPS.

CONCLUSIONS

These results reveal an alternative signal transduction network compatible with regeneration that may ultimately lead to the promotion of enhanced tissue repair in mammals.

Complex Tissue Regeneration in Mammals Is Associated With Reduced Inflammatory Cytokines and an Influx of T Cells

While mammals tend to repair injuries, other adult vertebrates like salamanders and fish regenerate damaged tissue. One prominent hypothesis offered to explain an inability to regenerate complex tissue in mammals is a bias during healing toward strong adaptive immunity and inflammatory responses. Here we directly test this hypothesis by characterizing part of the immune response during regeneration in spiny mice (Acomys cahirinus and Acomys percivali) vs. fibrotic repair in Mus musculus. By directly quantifying cytokines during tissue healing, we found that fibrotic repair was associated with a greater release of pro-inflammatory cytokines (i.e., IL-6, CCL2, and CXCL1) during acute inflammation in the wound microenvironment. However, reducing inflammation via COX-2 inhibition was not sufficient to reduce fibrosis or induce a regenerative response, suggesting that inflammatory strength does not control how an injury heals. Although regeneration was associated with lower concentrations of many inflammatory markers, we measured a comparatively larger influx of T cells into regenerating ear tissue and detected a local increase in the T cell associated cytokines IL-12 and IL-17 during the proliferative phase of regeneration. Taken together, our data demonstrate that a strong adaptive immune response is not antagonistic to regeneration and that other mechanisms likely explain the distribution of regenerative ability in vertebrates.

Complex Tissue Regeneration in Mammals Is Associated With Reduced Inflammatory Cytokines and an Influx of T Cells

While mammals tend to repair injuries, other adult vertebrates like salamanders and fish regenerate damaged tissue. One prominent hypothesis offered to explain an inability to regenerate complex tissue in mammals is a bias during healing toward strong adaptive immunity and inflammatory responses. Here we directly test this hypothesis by characterizing part of the immune response during regeneration in spiny mice (Acomys cahirinus and Acomys percivali) vs. fibrotic repair in Mus musculus. By directly quantifying cytokines during tissue healing, we found that fibrotic repair was associated with a greater release of pro-inflammatory cytokines (i.e., IL-6, CCL2, and CXCL1) during acute inflammation in the wound microenvironment. However, reducing inflammation via COX-2 inhibition was not sufficient to reduce fibrosis or induce a regenerative response, suggesting that inflammatory strength does not control how an injury heals. Although regeneration was associated with lower concentrations of many inflammatory markers, we measured a comparatively larger influx of T cells into regenerating ear tissue and detected a local increase in the T cell associated cytokines IL-12 and IL-17 during the proliferative phase of regeneration. Taken together, our data demonstrate that a strong adaptive immune response is not antagonistic to regeneration and that other mechanisms likely explain the distribution of regenerative ability in vertebrates.

Macrophages are required for adult salamander limb regeneration

The failure to replace damaged body parts in adult mammals results from a muted growth response and fibrotic scarring. Although infiltrating immune cells play a major role in determining the variable outcome of mammalian wound repair, little is known about the modulation of immune cell signaling in efficiently regenerating species such as the salamander, which can regrow complete body structures as adults. Here we present a comprehensive analysis of immune signaling during limb regeneration in axolotl, an aquatic salamander, and reveal a temporally defined requirement for macrophage infiltration in the regenerative process. Although many features of mammalian cytokine/chemokine signaling are retained in the axolotl, they are more dynamically deployed, with simultaneous induction of inflammatory and anti-inflammatory markers within the first 24 h after limb amputation. Systemic macrophage depletion during this period resulted in wound closure but permanent failure of limb regeneration, associated with extensive fibrosis and disregulation of extracellular matrix component gene expression. Full limb regenerative capacity of failed stumps was restored by reamputation once endogenous macrophage populations had been replenished. Promotion of a regeneration-permissive environment by identification of macrophage-derived therapeutic molecules may therefore aid in the regeneration of damaged body parts in adult mammals.

Stress and changes in the blood of newts, Notophthalmus viridescens, during early regeneration

During the summers of 1984 and 1985, adult red-spotted newts, Notophthalmus viridescens, were maintained in the laboratory at 23 degrees +/- 0.5 degrees C under natural photoperiods. From each of the experimental animals, the right forelimb was amputated just proximal to the elbow. Control newts were not manipulated surgically. Eight, 15, and 22 days after the time of amputation, equal numbers of regenerating and control animals were sacrificed, and blood smears of each individual were prepared with Wright's stain. Mean differential counts of leukocytes of the two groups of newts indicated that the relative number of neutrophils increased and the relative number of lymphocytes decreased in the regenerating animals as compared to their controls (Fig. 1 and Fig. 2). Earlier studies had shown that lymphopenia and neutrophilia occur in red-spotted newts treated with hydrocortisone or with ACTH or subjected to environmental stress (Bennett and Daigle 1983). Consequently, it is suggested that amputation and/or early regeneration may stimulate the increased production of hormones associated with stress in vertebrates, which may, in turn, influence regeneration, itself, and that the detailed study of the distribution of leukocytes in Notophthalmus viridescens may provide an assay with which to study the regulation of regeneration in this species.