Lipid-Laden Multilocular Cells in the Aging Thymus Are Phenotypically Heterogeneous

Abstract

Intrathymic lipid-laden multilocular cells (LLMC) are known to express pro-inflammatory factors that might regulate functional activity of the thymus. However, the phenotype of age-associated intrathymic LLMC is still controversial. In this study, we evaluated LLMC density in the aging thymus and better characterized their distribution, ultrastructure and phenotype. Our results show an increased density of LLMC in the thymus from 03 to 24 months of age. Morphologically, intrathymic LLMC exhibit fibroblastoid fusiform, globular or stellate shapes and can be found in the subcapsular region as well as deeper in the parenchyma, including the perivascular area. Some parenchymal LLMC were like telocytes accumulating lipids. We identified lipid droplets with different electrondensities, lipofuscin granules and autolipophagosome-like structures, indicating heterogeneous lipid content in these cells. Autophagosome formation in intrathymic LLMC was confirmed by positive staining for beclin-1 and perilipin (PLIN), marker for lipid droplet-associated proteins. We also found LLMC in close apposition to thymic stromal cells, endothelial cells, mast cells and lymphocytes. Phenotypically, we identified intrathymic LLMC as preadipocytes (PLIN+PPARγ2+), brown adipocytes (PLIN+UCP1+), macrophages (PLIN+Iba-1+) or pericytes (PLIN+NG2+) but not epithelial cells (PLIN- panCK+). These data indicate that intrathymic LLMC are already present in the young thymus and their density significantly increases with age. We also suggest that LLMC, which are morphologically distinct, establish direct contact with lymphocytes and interact with stromal cells. Finally, we evidence that intrathymic LLMC correspond to not only one but to distinct cell types accumulating lipids.

Age-associated alterations in the levels of cytotoxic lipid molecular species and oxidative stress in the murine thymus are reduced by growth hormone treatment

“…We found an increase of TG levels, lipid droplets and adipose cells in the thymus with advancing age. The latter findings are consistent with previous reports that unilocular and multilocular lipid-laden cells progressively infiltrate the thymus during aging (Steinman et al 1985; de Mello-Coelho et al 2010; Langhi et al 2015). Our analysis of lymphocytes isolated from young and middle-age mice showed a trend to decrease their TG levels with age, therefore establishing that the accumulation of TG in the thymus may occur in other cell types, including adipocytes.…”

“…There is a progressive increase of cells containing lipid droplets in the aging thymus, which is associated with increased apoptosis of thymocytes (Sempowsky et al , 2000; Langhi et al , 2015). Our data support previous studies showing increased T cell death in the thymus with age progress (Aspinall et al , 1997), which can explain the age-associated loss of clear cortico-medullary region demarcation in the thymic lobules (Aw et al , 2008) while apoptotic thymocytes are cleared by macrophages (Platt et al , 2007).…”

“…These results indicate that the age-related increase in the levels of TG and free cholesterol in the thymus are not the result of the accumulation of these lipids in thymocytes. Our data revealing altered cholesterol metabolism in thymocytes and, possibly, in other thymic stromal cells, which include macrophages (Youm et al 2012), adipocytes or intrathymic lipid-laden cells (Langhi et al ., 2015), suggest that increased accumulation of lipids could contribute to maintenance of a basal inflammatory state and increased oxidative stress that adversely affect thymocytes during aging.…”

“…The expression of these cytokines increases with age and promote atrophy of the mouse thymus following in vivo injection (Sempowsky et al , 2000). Intrathymic adipocytes and lipid-laden cells, which increase with aging, express LIF (Sempowsky et al , 2000; de Mello Coelho et al , 2004; Langhi et al , 2015). Adipose tissue cells also can secrete pro-inflammatory factors, such as IL-1, IL-6 and TNF-α (Rajala and Scherer, 2003).…”

“…In this context, the redistribution and accumulation of various species of lipids and lipid droplets occurs not only in blood vessels but also in lymphohematopoietic organs, including the thymus (Steinmann et al 1985; Flores et al ., 1999; Mello-Coelho et al . 2010; Langhi et al . 2015).…”