Macrophages drive inguinal fat pad and lymph node remodelling in response to peripheral inflammation
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Curated by eLife
eLife Assessment
This study presents potentially important findings linking peripheral inflammation to the remodeling of perinodal adipose tissue and draining lymph nodes, suggesting a mechanism by which local tissue inflammation can reshape LN structure and metabolism. The idea is solid and supported by observations. However, the evidence remains incomplete in parts, as several conclusions rely on correlative weight and cellularity measurements, and macrophage involvement requires further validation.
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Abstract
Adaptive immune responses are intensely energy-dependent and rely on a local source of fuel-producing molecules which have been proposed to be derived from fat pads in which mammalian lymph nodes are embedded. However, the trigger for their release has not been identified. Here we demonstrate that cutaneous inflammation is directly correlated with rapid atrophy of perinodal fat pads and increase in embedded lymph node size. We further demonstrate that the fat pad atrophy is associated with influx of a CCR2-independent, lipid metabolising, macrophage population. Macrophage depletion ameliorates fat pad atrophy, and lymph node expansion, downstream of inflamed sites. Our data therefore identify peripheral inflammation as an antigen-independent trigger of downstream fat pad and lymph node remodelling and contributes to the release of essential nutrients to drive the energetic requirements of the adaptive immune response.
Significance statement
To our knowledge, this striking correlation between peripheral inflammation and reciprocal fat pad and lymph node remodelling has not been reported previously. Our report of this correlation and our mechanistic insight, have clear implications for our understanding of the inflammation-driven rapid release of sources of energy from fat pads to drive the immune response. Our data also potentially shed light on additional aspects of the functionality of pro-inflammatory vaccine adjuvants. We believe that our findings are fully novel and will be of interest to immunologists, infectious disease specialists and researchers interested in adipose tissue derived energetics.
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eLife Assessment
This study presents potentially important findings linking peripheral inflammation to the remodeling of perinodal adipose tissue and draining lymph nodes, suggesting a mechanism by which local tissue inflammation can reshape LN structure and metabolism. The idea is solid and supported by observations. However, the evidence remains incomplete in parts, as several conclusions rely on correlative weight and cellularity measurements, and macrophage involvement requires further validation.
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Reviewer #1 (Public review):
The idea is super interesting, and the subsequent work is potentially significant because it links peripheral inflammation to remodelling of perinodal adipose tissue and draining lymph nodes. This suggests an antigen-independent manner by which local tissue inflammation can communicate with and reshape immune organ structure and tissue metabolism. However, the evidence is suggestive. For instance, many conclusions rely on correlational weight/cellularity relationships, models with confounders (spontaneous wounding; potentially systemic IMQ), and macrophage dependence inferred from a single pharmacologic approach without definitive depletion/lineage or tracer-based causal link.
Major Comments:
(1) "Wounding/fighting" evidence is confounding.
Unless I am mistaken, a large part of the argument for …
Reviewer #1 (Public review):
The idea is super interesting, and the subsequent work is potentially significant because it links peripheral inflammation to remodelling of perinodal adipose tissue and draining lymph nodes. This suggests an antigen-independent manner by which local tissue inflammation can communicate with and reshape immune organ structure and tissue metabolism. However, the evidence is suggestive. For instance, many conclusions rely on correlational weight/cellularity relationships, models with confounders (spontaneous wounding; potentially systemic IMQ), and macrophage dependence inferred from a single pharmacologic approach without definitive depletion/lineage or tracer-based causal link.
Major Comments:
(1) "Wounding/fighting" evidence is confounding.
Unless I am mistaken, a large part of the argument for inflammation-driven perinodal fat pad atrophy and LN expansion relies on spontaneous fighting injuries in co-housed CCR2-/- males, including animals "culled...due to excessive wounding." Because wound severity, duration, infection load, stress, and cage dynamics are uncontrolled, isn't it difficult to assign causality to "cutaneous inflammation"?
(2) The "CCR2-independent macrophage" conclusion.
The manuscript interprets persistence/accumulation of macrophages despite reduced inflammatory monocytes as CCR2-independent recruitment or local proliferation. However, CCR2 deficiency can alter immune baselines and long-term tissue remodelling. Perhaps consider bone marrow chimeras (WT to CCR2-/-, CCR2-/- to WT ????) or an inducible CCR2 deletion approach to separate developmental/systemic effects from acute inflammation-driven mechanisms. If "in situ proliferation" is proposed, include a direct readout (e.g., Ki67 in ATMs in the fat pad).
(3) IMQ and systemic effects.
The work relies on topical Aldara/imiquimod as an "inflammation without antigen" driver of distal LN/fat-pad remodelling. But IMQ is well known (and cited by the authors) to enter circulation and drive systemic responses, which could blur whether effects are truly draining-site specific vs systemic metabolic/inflammatory effects. It would be ideal to provide systemic context: plasma cytokines and/or metabolic readouts (e.g., circulating FFAs) to distinguish local vs systemic drivers.
(4) Macrophage dependence is inferred from CSF1R inhibitor treatment.
However, validation of macrophage depletion and specificity is incomplete. The manuscript uses AZD7507 (CSF1R inhibitor) and observes partial rescue of fat pad/LN phenotype while skin severity (PASI) is unaffected. But, to this reviewer, the data shown do not clearly quantify actual macrophage depletion efficiency in the target fat pad, and LN at endpoint, and CSF1R blockade can affect multiple myeloid populations. Therefore, show absolute macrophage counts (and likely other myeloid populations) in fat pad and LN with/without AZD7507 at the analysed timepoints, not only outcome weights. (The methods describe dosing but not endpoint depletion quantification??)
(5) Fat pad atrophy/LN expansion is a correlation.
The paper emphasises negative correlations between fat pad and LN weights/cellularity at baseline and with inflammation. But correlation does not establish whether fat pad lipolysis drives LN expansion, whether LN changes drive fat remodelling, or whether both reflect systemic mediators. Add tissue-level evidence distinguishing true adipocyte loss vs other contributors to "weight change" (e.g., oedema/fibrosis).
(6) Evidence for "fatty acid donation" from fat pad to LN.
The lipid data are described as "exemplary," and the inference that LN fatty acids originate from the fat pad is based on temporal ordering and relative abundance. This does not rule out plasma spillover, LN-intrinsic metabolism, or altered lymph flow.
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Reviewer #2 (Public review):
The authors aim to demonstrate skin inflammation is associated with fat pad atrophy and lymph node expansion. They further propose that these phenotypes are driven by the recruitment and lipid metabolism of CCR2-independent macrophages.
The authors took advantage of two skin inflammation models, fight-induced and imauimod-induced skin inflammation and analyzed multiple tissues, including skin, fat pads, and lymph nodes. Using a macropahge-depletion method (e.g., CSF-1R inhibitor), the authors further suggest the inverse correlation between fat pads atrophy and lymph node expansion is macropahge-dependent. While the study identifies this intriguing inverse correlation during skin inflammation, the causal pathway linking fat pad atrophy and lymph nodes enlargement has not been clearly established.
To improve …
Reviewer #2 (Public review):
The authors aim to demonstrate skin inflammation is associated with fat pad atrophy and lymph node expansion. They further propose that these phenotypes are driven by the recruitment and lipid metabolism of CCR2-independent macrophages.
The authors took advantage of two skin inflammation models, fight-induced and imauimod-induced skin inflammation and analyzed multiple tissues, including skin, fat pads, and lymph nodes. Using a macropahge-depletion method (e.g., CSF-1R inhibitor), the authors further suggest the inverse correlation between fat pads atrophy and lymph node expansion is macropahge-dependent. While the study identifies this intriguing inverse correlation during skin inflammation, the causal pathway linking fat pad atrophy and lymph nodes enlargement has not been clearly established.
To improve the rigor of the manuscript, the authors address the following concerns;
(1) CCR2-deficient mice showed reduced inflammatory monocytes and monocyte-derived macrophages (PMID:16462739; 16341265). During tissue inflammation, CCR2+ classical monocytes are typically recruited to the injured peripheral tissues, including skin, where they differentiate into monocyte-derived macrophages (PMID:38474365). While inflammatory monocytes were reduced in the skin (Figure 3 d), fat pads (Figure 4a, S2D) of CCR2-deficient mice, macrophage numbers were significantly increased in these mice. It remains unclear whether CCR2-independent macrophages were newly recruited from alternative sources or tissue-resident macrophages underwent local self-proliferation to compensate for the loss of CCR2+ monocyte-derived macrophages.
(2) In line 258, the authors state that there was "a significant reduction in CD11C- CD206+ anti-inflammatory macrophages (Figure 4b i-iii)". However, the quantification data in Figure 4b iii do not appear to show any reduction in anti-inflammatory macrophages in either males or females. Please reconcile this discrepancy between the text and the figure.
(3) Although CD11C and CD206 were historically used as markers of inflammatory and anti-inflammatory markers, respectively. These markers are no longer considered sufficient to define the macrophage polarization state, particularly in adipose tissue, where they are constitutively expressed by resident macrophages (PMID:34210853). Numerous studies have demonstrated substantial macrophage diversity/heterogeneity across iWAT, eWAT, and brown fat tissues. The authors should discuss adipose macrophage diversity beyond the outdated M1/M2 frame.
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