Heather L. Caslin

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Background: Weight gain and loss induce adipose CD8+ T cell exhaustion, which persists and may worsen glucose tolerance following weight regain. Because exercise can reduce T cell exhaustion in the blood, we hypothesized that exercise during weight loss would attenuate adipose CD8+ T cell exhaustion and glucose tolerance following weight regain. Methods: Male C57Bl/6J mice were fed low-fat or high-fat diets over 8 to 9-week cycles to generate lean, obese, weight loss, or weight cycled groups. Additional weight loss and weight cycled groups were provided exercise wheels during the weight loss phase. Results: As expected, weight loss increased total and exhausted CD8+ T cells by flow cytometry. Mice that ran the most during weight loss had the lowest proportion of exhausted CD8+ T cells. Notably, exercise reduced the proportion of exhausted CD8+ T cells even after the cessation of exercise and weight regain in all mice. However, exercise did not improve glucose tolerance or macrophage inflammation following weight regain. Moreover, exercise did not affect the induction of innate immune memory in adipose macrophages following weight loss. Conclusion: The addition of exercise to a weight loss intervention remarkably reduced exhausted CD8+ T cells in the adipose tissue even after the cessation of exercise and weight regain. While exercise did not affect macrophage inflammation or glucose tolerance following weight regain, these results illuminate new questions about the persistence and mechanisms by which exercise reduces tissue CD8+T cell exhaustion and the direct role of macrophages in modulating glucose tolerance with weight cycling.

Weight cycling (i.e. cycles of weight gain, loss, and regain) is a growing health concern that has been shown to worsen risk of diabetes beyond that of obesity. Dendritic cells play a causal role in obesity-associated inflammation and metabolic disease. However, whether dendritic cells are impacted by weight cycling is not known. Here, we aimed to test the hypothesis that antigen presentation increases in dendritic cells with weight cycling. C57Bl/6J male mice were put on nutrient-matched low-fat or high-fat diets to elicit lean, weight gain, weight loss, and weight cycled groups. Adipose tissue immune cell populations and proteins related to signal 1, 2, and 3 of antigen presentation were analyzed by single cell-RNA sequencing and flow cytometry. Total adipose tissue dendritic cells, conventional type I and II dendritic cells, and monocyte-derived dendritic cells all increased with weight loss. Regarding major histocompatibility complex (MHC) protein expression, non-classical MHCIb proteins (Qa1, Qa2 and CD1d) were also highest in the weight loss group. Dendritic cells expressing the costimulatory molecule CD86 and the cytokine TNF were highest in the weight loss group. Finally, we assessed effector immune cell populations in the adipose tissue to understand the functional role of antigen presentation. CD8+ T cells, natural killer (NK) T cells, and NK cells also increased consistently with weight loss, but showed evidence of exhaustion. In sum, weight loss, but not weight cycling expands dendritic cells, increases signals for non-classical antigen presentation, and expands CD8+ T cells, NKT cells, and NK cells in the adipose tissue.

Purpose of the Review The innate immune system plays a critical role in mediating many of the physiological consequences of high-fat diet consumption. Dietary lipids, and specifically saturated fatty acids like palmitate and stearate, directly activate innate immune cells and alter the composition of the gut microbiome. Moreover, long-term high-fat diet feeding can induce chronic inflammation, adipose expansion, and the development of obesity. High-fat diet consumption and obesity also worsen the risk for chronic diseases like asthma. Recent Findings It is well known that high-fat diet feeding activates innate immune cells and alters the gut microbiome. However, emerging research provides new insight into the mechanisms by which high-fat diet feeding and obesity affect innate immunity and further disease development. These emerging mechanisms include the induction of lipid-associated macrophages (LAMs), innate immune memory, and innate-adaptive crosstalk leading to T cell exhaustion and granzyme K production. Summary These novel mechanisms help us better understand the effect of high-fat diets on innate immunity, and future studies in these areas may help us better identify new therapeutic strategies for managing obesity and asthma.

Type 2 diabetes (T2D) and Alzheimer's Disease (AD) have seemingly different pathologies and symptoms. However, T2D is a risk factor for AD, and recent evidence suggests there are many mechanistic similarities between the etiologies of each disease including inflammation. Mast cells are tissue resident, sentinel immune cells that reside in the pancreas, adipose tissue, and brain, increase in T2D and AD, and have generally been shown to worsen T2D and AD. However, there are limited studies of local or temporal mast cell deletion, and different phenotypic and polarization states seemingly influence the role of mast cells in the progression of disease. As there are metabolic similarities between T2D and AD including insulin resistance and lipid influx into the brain, we discuss the impact of glucose, insulin, amylin, and different lipid species on the activation and polarization of mast cells, which generally reduce IgE‐mediated degranulation and promote lipid droplet formation and arachidonic acid metabolism. Altogether, this review provides a framework for understanding a shared mechanism of immunometabolic regulation of T2D and AD and provides rationale for future work in this area.

Different stimuli can induce innate immune memory to improve pathogen defense or worsen cardiometabolic disease. However, it is less clear if the same stimuli can induce both the protective and detrimental effects of innate immune memory. Weight loss following high fat diet feeding induces innate immune memory in adipose macrophages that correlates with worsened diabetes risk after weight regain. Here, we investigated the effect of weight gain and loss on adipose macrophage memory and infection outcomes in systemic Staphylococcus aureus infection in C57Bl/6J male mice. Lean controls remained on low-fat diet, weight gain mice started on low-fat diet and were moved to high-fat for 9 weeks, and weight loss mice began on high-fat diet for 9 weeks before moving to low-fat diet for 9 weeks. At 18 weeks, functional analyses were performed on adipose macrophages from each group of mice. Weight loss increased cytokine production and reactive oxygen species compared to lean controls. The remaining mice were infected intravenously with 2.5 × 10⁸ colony forming units S. aureus. There was no effect of weight change on survival; however, weight gain reduced body temperature and increased sepsis scoring, blood neutrophils, and bacterial burden in the kidney. Weight loss increased plasma tumor necrosis factor-alpha and adipose macrophage cytokine production that correlated with reduced bacterial burden in the adipose tissue. Thus, weight loss restores systemic infection defenses that are impaired with weight gain, and weight loss-induced adipose macrophage memory may further reduce local S. aureus growth. Collectively, innate immune memory to weight loss may be protective in local antimicrobial defense.

Cardiovascular and cardiometabolic diseases are leading causes of morbidity and mortality worldwide, driven in part by chronic inflammation. Emerging research suggests that the bone marrow microenvironment, or marrow niche, plays a critical role in both immune system regulation and disease progression. The bone marrow niche is essential for maintaining hematopoietic stem cells (HSCs) and orchestrating hematopoiesis. Under normal conditions, this niche ensures a return to immune homeostasis after acute stress. However, in the setting of inflammatory conditions such as those seen in cardiometabolic diseases, it becomes dysregulated, leading to enhanced myelopoiesis and immune activation. This review explores the reciprocal relationship between the bone marrow niche and cardiometabolic diseases, highlighting how alterations in the niche contribute to disease development and progression. The niche regulates HSCs through complex interactions with stromal cells, endothelial cells, and signaling molecules. However, in the setting of chronic diseases such as hypertension, atherosclerosis, and diabetes, inflammatory signals disrupt the balance between HSC self-renewal and differentiation, promoting the excessive production of proinflammatory myeloid cells that exacerbate the disease. Key mechanisms discussed include the effects of hyperlipidemia, hyperglycemia, and sympathetic nervous system activation on HSC proliferation and differentiation. Furthermore, the review emphasizes the role of epigenetic modifications and metabolic reprogramming in creating trained immunity, a phenomenon whereby HSCs acquire long-term proinflammatory characteristics that sustain disease states. Finally, we explore therapeutic strategies aimed at targeting the bone marrow niche to mitigate chronic inflammation and its sequelae. Novel interventions that modulate hematopoiesis and restore niche homeostasis hold promise for the treatment of cardiometabolic diseases. By interrupting the vicious cycle of inflammation and marrow dysregulation, such therapies may offer new avenues for reducing cardiovascular risk and improving patient outcomes.

Different stimuli can induce innate immune memory to improve pathogen defense or worsen cardiometabolic disease. However, it is less clear if the same stimuli can induce both the protective and detrimental effects of innate immune memory. We previously showed that weight loss induces innate immune memory in adipose macrophages that correlates with worsened diabetes risk after weight regain. In this study, we investigated the effect of weight loss on macrophage cytokine production and overall survival in a mouse model of infection. Male C57Bl/6J mice were put on high-fat or low-fat diets over 18 weeks to induce weight gain or weight loss. Lean mice served as controls. All mice were then infected IV with 2.5×10^6 CFU Staphylococcus aureus . Tissues were collected from 10 mice/group at day 3 and the remaining animals were followed for survival. Weight gain mice had the highest blood neutrophils and the highest bacterial burden in the kidney. However, there was no significant difference in survival. The weight loss group had the highest plasma TNF-α and a significant reduction in bacterial burden in the adipose tissue that correlated with increased adipose macrophage cytokine production. Thus, weight loss-induced adipose macrophage memory may both improve local S.aureus clearance and worsen diabetes risk upon weight regain. Collectively, these findings support the notion that innate immune memory is an evolutionarily protective mechanism that also contributes to the development of cardiometabolic diseases.

IL-33 is an inflammatory cytokine that promotes allergic disease by activating group 2 innate lymphoid cells, Th2 cells, and mast cells. IL-33 is increased in asthmatics, and its blockade suppresses asthma-like inflammation in mouse models. Homeostatic control of IL-33 signaling is poorly understood. Because the IL-33 receptor, ST2, acts via cascades used by the TLR family, similar feedback mechanisms may exist. MicroRNA (miR)-146a is induced by LPS-mediated TLR4 signaling and serves as a feedback inhibitor. Therefore, we explored whether miR-146a has a role in IL-33 signaling. IL-33 induced cellular and exosomal miR-146a expression in mouse bone marrow–derived mast cells (BMMCs). BMMCs transfected with a miR-146a antagonist or derived from miR-146a knockout mice showed enhanced cytokine expression in response to IL-33, suggesting that miR-146a is a negative regulator of IL-33–ST2 signaling. In vivo, miR-146a expression in plasma exosomes was elevated after i.p. injection of IL-33 in wild-type but not mast cell–deficient KitW-sh/W-sh mice. Finally, KitW-sh/W-sh mice acutely reconstituted with miR-146a knockout BMMCs prior to IL-33 challenge had elevated plasma IL-6 levels compared with littermates receiving wild-type BMMCs. These results support the hypothesis that miR-146a is a feedback regulator of IL-33–mediated mast cell functions associated with allergic disease.

All cells of the immune system reside in adipose tissue (AT), and increasing type 2 immune cells may be a therapeutic strategy to improve metabolic health. In our previous study using i.p. IL-5 injections to increase eosinophils, we observed that a standard vehicle control of 0.1% BSA also elicited profound AT eosinophilia. In this study, we aimed to determine whether BSA-induced AT eosinophilia results in metabolic benefits in murine models of diet-induced obesity. I.p. 0.1% BSA injections increased AT eosinophils after 4 wk. Despite elevating eosinophils to >50% of immune cells in the AT, body weight and glucose tolerance were not different between groups. Interestingly, BSA elicited epithelial IL-33 production, as well as gene expression for type 2 cytokines and IgE production that were dependent on IL-33. Moreover, multiple models of OVA sensitization also drove AT eosinophilia. Following transplantation of a donor fat pad with BSA-induced eosinophilia, OVA-sensitized recipient mice had higher numbers of bronchoalveolar lavage eosinophils that were recipient derived. Interestingly, lungs of recipient mice contained eosinophils, macrophages, and CD8 T cells from the donor AT. These trafficked similarly from BSA- and non-BSA–treated AT, suggesting even otherwise healthy AT serves as a reservoir of immune cells capable of migrating to the lungs. In conclusion, our studies suggest that i.p. injections of BSA and OVA induce an allergic response in the AT that elicits eosinophil recruitment, which may be an important consideration for those using OVA in animal models of allergic disease.