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Sweat out the fat with TSLP! Thymic stromal lymphopoietin (TSLP) is a cytokine that can promote immune responses that characterize allergic diseases. Choa et al. found that mice engineered to produce elevated TSLP displayed selective white adipose tissue loss that protected them from obesity, insulin resistance, and steatohepatitis (see the Perspective by Schneider). Protection was not mediated by eosinophils, regulatory T cells, or innate lymphoid cells. Rather, TLSP induced the migration of conventional T cells to sebaceous glands in the skin. Once there, these T cells promoted white adipose tissue loss by the hypersecretion of sebum, a lipid-rich substance that augments the skin's barrier function. This mechanism, which likely evolved to enhance cutaneous antimicrobial defenses, could be possibly targeted in future treatments for obesity. Science, abd2893, this issue p. eabd2893; see also abg9079, p. 487 T cells can promote abnormally high levels of sebum secretion from the skin, which can lead to the loss of adipose tissue. INTRODUCTION Obesity and its associated complications are serious global concerns. Despite growing public health initiatives, obesity rates continue to rise. Thus, there is a critical need to identify pathways that affect adiposity. Recent studies indicate that the immune system can regulate adipose tissue and its metabolic function. Type 2 immune cells, such as type 2 innate lymphoid cells (ILC2s) and eosinophils, increase the metabolic rate, whereas regulatory T cells (Treg cells) promote insulin sensitivity. RATIONALE Thymic stromal lymphopoietin (TSLP) is an epithelial cell cytokine that is expressed at barrier sites such as the skin, lung, and gut. Because TSLP has been shown to activate type 2 immune cells and expand Treg cells, we hypothesized that TSLP could counteract obesity and its associated complications. RESULTS The effect of TSLP on obesity was tested by administering a Tslp-expressing adeno-associated virus serotype 8 (TSLP-AAV) to mice. Compared with mice administered control-AAV, mice given TSLP-AAV displayed selective white adipose tissue (WAT) loss, which protected against diet-induced and genetic models of obesity, insulin resistance, and nonalcoholic steatohepatitis (NASH). Unexpectedly, TSLP-induced WAT loss was not dependent on ILC2s, eosinophils, or Treg cells. Rather, it resulted from direct activation of either CD4+ or CD8+ αβ T cell receptor (TCRαβ) T cells by TSLP in an antigen-independent manner. The adoptive transfer of T cells from the lymph nodes of TSLP-AAV–injected mice also caused WAT loss in TSLP receptor–deficient (Tslpr–/–) mice, suggesting that TSLP-stimulated T cells retain their ability to induce WAT loss. TSLP-induced WAT loss was not associated with decreased food intake, increased fecal caloric excretion, or increased energy metabolism. Instead, the WAT loss was associated with a notable greasy hair appearance. Thin-layer chromatography analysis of extracted hair lipids from TSLP-AAV–injected mice showed that the oleaginous substance was enriched for sebum-specific lipids. Sebum is a calorically dense substance produced by sebocytes in sebaceous glands (SGs) and helps form both the physical and immune-protective skin barrier. Skin histological analysis showed that TSLP promoted sebum secretion and turnover of sebocytes. Sebum hypersecretion was responsible for TSLP-induced WAT loss because TSLP did not induce WAT loss in asebia mice, which harbor hypomorphic SGs. TSLP also induced the migration of T cells to SGs, which was required for the enhanced sebum secretion. Inhibition of T cell migration prevented TSLP-induced sebum hypersecretion and subsequent WAT loss. At homeostasis, TSLP and T cells controlled steady-state sebum secretion. Both Tslpr–/– and T cell–deficient mice exhibited decreased sebum secretion at baseline. Many of the fatty acids within sebum have bactericidal properties, and antimicrobial peptides (AMPs) are also secreted as part of sebum for barrier protection. Accordingly, Tslpr–/– mice expressed lower levels of sebum-associated AMPs in the skin, suggesting that endogenous TSLP plays a role in skin barrier function. This TSLP-sebum axis was also applicable to humans because the expression of TSLP and sebum-associated genes were positively correlated in skin samples from healthy individuals. CONCLUSION Our findings support a model in which TSLP overexpression causes WAT loss by inducing skin T cell migration and increasing sebum hypersecretion. Additionally, TSLP and T cells homeostatically regulate sebum production and skin AMP expression, highlighting an unexpected role for the adaptive immune system in the maintenance of skin barrier function. TSLP activates T cells to promote sebum secretion. Antigen-independent stimulation of T cells through the TSLP receptor (TSLPR) induces their migration to skin SGs and promotes sebum secretion and AMP expression. TSLP overexpression causes marked diversion of circulating lipids toward the skin, which reduces lipid content in the adipose tissue and liver. At baseline, TSLP-stimulated T cells maintain skin barrier function through SGs. VLDL, very-low-density lipoprotein. CREDIT: BIORENDER Emerging studies indicate that the immune system can regulate systemic metabolism. Here, we show that thymic stromal lymphopoietin (TSLP) stimulates T cells to induce selective white adipose loss, which protects against obesity, improves glucose metabolism, and mitigates nonalcoholic steatohepatitis. Unexpectedly, adipose loss was not caused by alterations in food intake, absorption, or energy expenditure. Rather, it was induced by the excessive loss of lipids through the skin as sebum. TSLP and T cells regulated sebum release and sebum-associated antimicrobial peptide expression in the steady state. In human skin, TSLP expression correlated directly with sebum-associated gene expression. Thus, we establish a paradigm in which adipose loss can be achieved by means of sebum hypersecretion and uncover a role for adaptive immunity in skin barrier function through sebum secretion.