Dissecting the damaging versus regenerative roles of CNS macrophages: implications for the use of immunomodulatory therapeutics

Characterization of Recruited Mononuclear Phagocytes following Corneal Chemical Injury

Mononuclear phagocytes (MP) have central importance in innate immunity, inflammation, and fibrosis. Recruited MPs, such as macrophages, are plastic cells and can switch from an inflammatory to a restorative phenotype during the healing process. However, the role of the MPs in corneal wound healing is not completely understood. The purpose of this study is to characterize the kinetics of recruited MPs and evaluate the role of macrophage metalloelastase (MMP12) in the healing process, using an in vivo corneal chemical injury model. Unwounded and wounded corneas of wild-type (WT) and Mmp12^-/- mice were collected at 1, 3, and 6 days after chemical injury and processed for flow cytometry analysis. Corneal MP phenotype significantly changed over time with recruited Ly6C^high (proinflammatory) cells being most abundant at 1 day post-injury. Ly6C^int cells were highly expressed at 3 days post-injury and Ly6C^neg (patrolling) cells became the predominant cell type at 6 days post-injury. CD11c⁺ dendritic cells were abundant in corneas from Mmp12^-/- mice at 6 days post-injury. These findings show the temporal phenotypic plasticity of recruited MPs and provide valuable insight into the role of the MPs in the corneal repair response, which may help guide the future development of MP-targeted therapies.

Transferrin Enhances Microglial Phagocytic Capacity

Transferrin (Tf) is a glycoprotein playing a critical role in iron homeostasis and transport and distribution throughout the body and within tissues and cells. This molecule has been shown to accelerate the process of myelination and remyelination in the central nervous system (CNS) in vivo and induce oligodendroglial cell maturation in vitro. While the mechanisms involved in oligodendroglial precursor cell (OPC) differentiation have not been fully elucidated yet, our group has previously described the first molecular events taking place in OPC in response to extracellular Tf. Here, we show the effect of Tf on the different glial cell populations. We demonstrate that, after a CNS demyelinating injury, Tf can be incorporated by all glial cells-i.e., microglia, astrocytes, and OPC-and that, acting on microglial cells in vitro, Tf increases microglial proliferation rates and phagocytic capacity. It may be then speculated that the in vivo correlation of this process could generate a favorable microenvironment for OPC maturation and remyelination.

Minireview: Emerging Concepts in Islet Macrophage Biology in Type 2 Diabetes

Chronic systemic inflammation is a hallmark feature of obesity and type 2 diabetes. Both resident and recruited islet macrophages contribute to the proinflammatory milieu of the diabetic islet. However, macrophages also appear to be critical for β-cell formation during development and support β-cell replication in experimental models of pancreas regeneration. In light of these findings, perhaps macrophages in the islet need to be viewed more as a fulcrum where deleterious inflammatory activation is balanced with beneficial tissue repair processes. Undoubtedly, defining the factors that contribute to the ontogeny, heterogeneity, and functionality of macrophages in normal, diseased, and regenerating islets will be necessary to determine whether that fulcrum can be moved to preserve functional β-cell mass in persons with diabetes. The intent of this review is to introduce the reader to emerging concepts of islet macrophage biology that may challenge the perception that macrophage accumulation in islets is merely a pathological feature of type 2 diabetes.

The evolution of our understanding of macrophages and translation of findings toward the clinic

'There is at bottom only one genuinely scientific treatment for all diseases, and that is to stimulate the phagocytes,' so declaimed Sir Ralph Bloomfield Bonington in The Doctor's Dilemma, Act 1, by George Bernard Shaw (1906). More often nowadays, the need is to calm the phagocytes, given their role in inflammation and tissue damage. In spite of the growth of cellular and molecular information gained from studies in macrophage cell culture, mouse models and, to a lesser extent, human investigations, and the importance of macrophages in pathogenesis in a wide range of chronic disease processes, there is still a substantial shortfall in terms of clinical applications. In this review, we summarize concepts derived from macrophage studies and suggest possible properties suitable for diagnosis, prognosis and selective targeting of macrophage pathogenic functions.