Frontline Science: Rapid adipose tissue expansion triggers unique proliferation and lipid accumulation profiles in adipose tissue macrophages

Obesity-related changes in adipose tissue leukocytes, in particular adipose tissue macrophages (ATMs) and dendritic cells (ATDCs), are implicated in metabolic inflammation, insulin resistance, and altered regulation of adipocyte function. We evaluated stromal cell and white adipose tissue (WAT) expansion dynamics with high fat diet (HFD) feeding for 3-56 days, quantifying ATMs, ATDCs, endothelial cells (ECs), and preadipocytes (PAs) in visceral epididymal WAT and subcutaneous inguinal WAT. To better understand mechanisms of the early response to obesity, we evaluated ATM proliferation and lipid accumulation. ATMs, ATDCs, and ECs increased with rapid WAT expansion, with ATMs derived primarily from a CCR2-independent resident population. WAT expansion stimulated proliferation in resident ATMs and ECs, but not CD11c⁺ ATMs or ATDCs. ATM proliferation was unperturbed in Csf2- and Rag1-deficient mice with WAT expansion. Additionally, ATM apoptosis decreased with WAT expansion, and proliferation and apoptosis reverted to baseline with weight loss. Adipocytes reached maximal hypertrophy at 28 days of HFD, coinciding with a plateau in resident ATM accumulation and the appearance of lipid-laden CD11c⁺ ATMs in visceral epididymal WAT. ATM increases were proportional to tissue expansion and adipocyte hypertrophy, supporting adipocyte-mediated regulation of resident ATMs. The appearance of lipid-laden CD11c⁺ ATMs at peak adipocyte size supports a role in responding to ectopic lipid accumulation within adipose tissue. In contrast, ATDCs increase independently of proliferation and may be derived from circulating precursors. These changes precede and establish the setting in which large-scale adipose tissue infiltration of CD11c⁺ ATMs, inflammation, and adipose tissue dysfunction contributes to insulin resistance.

Weight loss independent changes in adipose tissue macrophage and T cell populations after sleeve gastrectomy in mice

OBJECTIVE

In addition to adipocytes, adipose tissue contains large numbers of immune cells. A wide range of evidence links the activity of these cells to regulation of adipocyte and systemic metabolic function. Bariatric surgery improves several aspects of metabolic derangements and at least some of these effects occur in a weight-loss independent manner. We sought to investigate the impact of vertical sleeve gastrectomy (VSG) on adipose immune cell frequencies.

METHODS

We analyzed the frequencies of immune cells within distinct adipose tissue depots in obese mice that had VSG or sham surgery with a portion of the latter group pair-fed such that their body mass was matched to the VSG animals.

RESULTS

We demonstrate that VSG induced a shift in the epididymal adipose tissue leukocyte profile including increased frequencies of CD11c- macrophages, increased frequencies of T cells (CD4+, CD8+, and CD4-/CD8- T cells all increased), but a significantly decreased frequency of adipose tissue dendritic cells (ATDC) that, despite the continued high fat feeding of the VSG group, dropped below control diet levels.

CONCLUSIONS

These results indicate that VSG induces substantial changes in the immune populations residing in the adipose depots independent of weight loss.