Involvement of splenic marginal zone macrophages in the recognition of systemically administered phosphatidylserine-coated liposomes in mice

Autoimmune diseases present a significant clinical problem, highlighting the need for the development of novel or improved therapeutic methods. One of the factors that causes autoimmune diseases is a defect in the clearance of apoptotic cells by phagocytes. Thus, improved apoptotic cell processing has been considered as a strategy to treat autoimmune diseases. However, therapeutic strategies focusing on apoptotic cell clearance have not been approved till date. We have reported that liposomes composed of phosphatidylserine (PS liposomes) exhibit anti-inflammatory or immunosuppressive effects in macrophages. A PS liposome display PS on its surface, which plays a crucial role in the phagocytosis of apoptotic cells by marginal zone macrophages (MZMs), a key player in the clearance of apoptotic cells, by recognizing PS exposed on the surface of apoptotic cells. Therefore, we hypothesized that PS liposomes could be used as "antigen delivery vesicles" to act as a substitute for apoptotic cells in the treatment of autoimmune diseases. In this study, we showed that systemically administered PS liposomes accumulated in the marginal zone of the spleen due to recognition of surface-displayed PS by MZMs because it was observed that liposomes without PS did not accumulate in the marginal zone. In conclusion, PS liposomes may be useful vehicles to function as active agents and/or antigens against autoimmune diseases.

Age-related changes in expression and signaling of TAM receptor inflammatory regulators in monocytes

The multifactorial immune deterioration in aging--termed “inflamm-aging”--is comprised of a state of low-grade, chronic inflammation and complex dysregulation of responses to immune stimulation. The TAM family (Tyro 3, Axl, and Mer) of receptor tyrosine kinases are negative regulators of Toll like receptor-mediated immune responses that broadly inhibit cytokine receptor cascades to inhibit inflammation. Here we demonstrate elevated expression of TAM receptors in monocytes of older adults, and an age-dependent difference in signaling mediator AKT resulting in dysregulated responses to signaling though Mer. Our results may be especially significant in tissue, where levels of Mer are highest, and may present avenues for modulation of chronic tissue inflammation noted in aging.

Therapeutic applications of reconstituted HDL: When structure meets function

Reconstituted forms of HDL (rHDL) are under development for infusion as a therapeutic approach to attenuate atherosclerotic vascular disease and to reduce cardiovascular risk following acute coronary syndrome and ischemic stroke. Currently available rHDL formulations developed for clinical use contain apolipoprotein A-I (apoA-I) and one of the major lipid components of HDL, either phosphatidylcholine or sphingomyelin. Recent data have established that quantitatively minor molecular constituents of HDL particles can strongly influence their anti-atherogenic functionality. Novel rHDL formulations displaying enhanced biological activities, including cellular cholesterol efflux, may therefore offer promising prospects for the development of HDL-based, anti-atherosclerotic therapies. Indeed, recent structural and functional data identify phosphatidylserine as a bioactive component of HDL; the content of phosphatidylserine in HDL particles displays positive correlations with all metrics of their functionality. This review summarizes current knowledge of structure-function relationships in rHDL formulations, with a focus on phosphatidylserine and other negatively-charged phospholipids. Mechanisms potentially underlying the atheroprotective role of these lipids are discussed and their potential for the development of HDL-based therapies highlighted.

Dichotomous roles for externalized cardiolipin in extracellular signaling: Promotion of phagocytosis and attenuation of innate immunity

Among the distinct molecular signatures present in the mitochondrion is the tetra-acylated anionic phospholipid cardiolipin, a lipid also present in primordial, single-cell bacterial ancestors of mitochondria and multiple bacterial species today. Cardiolipin is normally localized to the inner mitochondrial membrane; however, when cardiolipin becomes externalized to the surface of dysregulated mitochondria, it promotes inflammasome activation and stimulates the elimination of damaged or nonfunctional mitochondria by mitophagy. Given the immunogenicity of mitochondrial and bacterial membranes that are released during sterile and pathogen-induced trauma, we hypothesized that cardiolipins might function as "eat me" signals for professional phagocytes. In experiments with macrophage cell lines and primary macrophages, we found that membranes with mitochondrial or bacterial cardiolipins on their surface were engulfed through phagocytosis, which depended on the scavenger receptor CD36. Distinct from this process, the copresentation of cardiolipin with the Toll-like receptor 4 (TLR4) agonist lipopolysaccharide dampened TLR4-stimulated production of cytokines. These data suggest that externalized, extracellular cardiolipins play a dual role in host-host and host-pathogen interactions by promoting phagocytosis and attenuating inflammatory immune responses.