Immune modulation by MANF promotes tissue repair and regenerative success in the retina

INTRODUCTION Regenerative therapies based on cell replacement hold promise for the treatment of a range of age-related degenerative diseases but are limited by unfavorable microenvironments in degenerating tissues. A promising strategy to improve success is to harness endogenous repair mechanisms that promote tissue integrity and function. Innate immune cells are central to such repair mechanisms because they coordinate local and systemic responses to tissue injury by secreting inflammatory and anti-inflammatory signals in a context-dependent manner. A proper balance between these opposing phenotypes of innate immune cells is essential for efficient tissue repair, and immune modulation may be an effective way to promote repair and enhance regenerative therapies. Here, we identified a new evolutionarily conserved immune modulatory function for mesencephalic astrocyte-derived neurotrophic factor (MANF) that biases immune cells toward an anti-inflammatory phenotype, thereby promoting tissue repair in both vertebrates and invertebrates and enhancing retinal regenerative therapy. RATIONALE In Drosophila , interactions between damaged tissues and hemocytes are essential for tissue repair. We used this model to identify immune cell–derived factors with immune modulatory activity that promote tissue repair after retinal injury. The identification of MANF as such a factor prompted us to test its role in mammalian retinal repair and ask whether its immune modulatory activity helped cell replacement therapies in degenerating retinas. RESULTS Using a combination of transcriptome analysis and genetic studies, we identified MANF as a hemocyte-derived factor that is induced by platelet-derived growth factor (PDGF)– and vascular endothelial growth factor (VEGF)–related factor 1 (Pvf-1)/PDGF- and VEGF-receptor related (PvR) signaling. MANF was necessary and sufficient to promote retinal repair after ultraviolet-light–induced retinal injury in Drosophila. MANF also had an autocrine immune-modulatory function in fly hemocytes, which was necessary for its tissue repair–promoting activity. This regulation and function of MANF was evolutionarily conserved: Mouse photoreceptors expressed PDGF-A (a Pvf-1 homolog) in response to damage signals, which promoted MANF expression in innate immune cells. This PDGF-A/MANF signaling cascade was required to limit photoreceptor apoptosis in the retina. Exogenously supplied recombinant MANF protected photoreceptors in several paradigms of retinal injury and degeneration. As in flies, this prorepair function was associated with alternative activation of macrophages and microglia in the retina. Ablation of CD11b + immune cells and deletion of Cx3Cr1, a chemokine receptor required for MANF-induced alternative activation, prevented MANF-induced repair. Thus, the protective effects of MANF in retinal injury rely on its immune modulatory activity. Finally, MANF supplementation to photoreceptors transplanted into congenitally blind mice increased integration efficiency and accelerated and improved visual function recovery. CONCLUSION Combining genetic studies in invertebrates and vertebrates has rapidly identified factors with promising therapeutic potential. Immune modulation is a promising strategy to optimize regenerative therapies. With its conserved immune modulatory function, MANF is a particularly promising molecule that is likely to be useful for the treatment of inflammatory conditions in many different disease contexts. MANF in retinal repair. In Drosophila (left) or mouse (right), the damaged retina secretes Pvf-1/PDGF-A, which acts on innate immune cells. MANF derived from innate immune cells (and other sources) promotes phenotypic changes in immune cells as part of a mechanism required for tissue repair. Therapeutically, MANF supplementation can delay retinal degeneration and improve the success of cell-replacement regenerative therapies in the retina.