Perinodal Adipose Tissue Participates in Immune Protection through a Lymphatic Vessel-Independent Route

Resident TH2 cells orchestrate adipose tissue remodeling at a site adjacent to infection

Type 2 immunity is associated with adipose tissue (AT) homeostasis and infection with parasitic helminths, but whether AT participates in immunity to these parasites is unknown. We found that the fat content of mesenteric AT (mAT) declined in mice during infection with a gut-restricted helminth. This was associated with the accumulation of metabolically activated, interleukin-33 (IL-33), thymic stromal lymphopoietin (TSLP), and extracellular matrix (ECM)-producing stromal cells. These cells shared transcriptional features, including the expression of Dpp4 and Pi16, with multipotent progenitor cells (MPC) that have been identified in numerous tissues and are reported to be capable of differentiating into fibroblasts and adipocytes. Concomitantly, mAT became infiltrated with resident T helper 2 (T_H2) cells that responded to TSLP and IL-33 by producing stromal cell-stimulating cytokines, including transforming growth factor β1 (TGFβ₁) and amphiregulin. These T_H2 cells expressed genes previously associated with type 2 innate lymphoid cells (ILC2), including Nmur1, Calca, Klrg1, and Arg1, and persisted in mAT for at least 11 months after anthelmintic drug-mediated clearance of infection. We found that MPC and T_H2 cells localized to ECM-rich interstitial spaces that appeared shared between mesenteric lymph node, mAT, and intestine. Stromal cell expression of epidermal growth factor receptor (EGFR), the receptor for amphiregulin, was required for immunity to infection. Our findings point to the importance of MPC and T_H2 cell interactions within the interstitium in orchestrating AT remodeling and immunity to an intestinal infection.

Lymph-derived chemokines direct early neutrophil infiltration in the lymph nodes upon Staphylococcus aureus skin infection

A large number of neutrophils infiltrate the lymph node (LN) within 4 h after Staphylococcus aureus skin infection (4 h postinfection [hpi]) and prevent systemic S. aureus dissemination. It is not clear how infection in the skin can remotely and effectively recruit neutrophils to the LN. Here, we found that lymphatic vessel occlusion substantially reduced neutrophil recruitment to the LN. Lymphatic vessels effectively transported bacteria and proinflammatory chemokines (i.e., Chemokine [C-X-C motif] motif 1 [CXCL1] and CXCL2) to the LN. However, in the absence of lymph flow, S. aureus alone in the LN was insufficient to recruit neutrophils to the LN at 4 hpi. Instead, lymph flow facilitated the earliest neutrophil recruitment to the LN by delivering chemokines (i.e., CXCL1, CXCL2) from the site of infection. Lymphatic dysfunction is often found during inflammation. During oxazolone (OX)-induced skin inflammation, CXCL1/2 in the LN was reduced after infection. The interrupted LN conduits further disrupted the flow of lymph and impeded its communication with high endothelial venules (HEVs), resulting in impaired neutrophil migration. The impaired neutrophil interaction with bacteria contributed to persistent infection in the LN. Our studies showed that both the flow of lymph from lymphatic vessels to the LN and the distribution of lymph in the LN are critical to ensure optimal neutrophil migration and timely innate immune protection in S. aureus infection.

Intracellular infection and immune system cues rewire adipocytes to acquire immune function

Adipose tissue (AT) plays a central role in systemic metabolic homeostasis, but its function during bacterial infection remains unclear. Following subcutaneous bacterial infection, adipocytes surrounding draining lymph nodes initiated a transcriptional response indicative of stimulation with IFN-γ and a shift away from lipid metabolism toward an immunologic function. Natural killer (NK) and invariant NK T (iNKT) cells were identified as sources of infection-induced IFN-γ in perinodal AT (PAT). IFN-γ induced Nos2 expression in adipocytes through a process dependent on nuclear-binding oligomerization domain 1 (NOD1) sensing of live intracellular bacteria. iNOS expression was coupled to metabolic rewiring, inducing increased diversion of extracellular L-arginine through the arginosuccinate shunt and urea cycle to produce nitric oxide (NO), directly mediating bacterial clearance. In vivo, control of infection in adipocytes was dependent on adipocyte-intrinsic sensing of IFN-γ and expression of iNOS. Thus, adipocytes are licensed by innate lymphocytes to acquire anti-bacterial functions during infection.

The role of ectopic adipose tissue: benefit or deleterious overflow?

Ectopic adipose tissues (EAT) are present adjacent to many organs and have predominantly been described in overweight and obesity. They have been suggested to be related to fatty acid overflow and to have harmful effects. The objective of this semi-comprehensive review is to explore whether EAT may play a supportive role rather than interfering with its function, when the adjacent organ is challenged metabolically and functionally. EAT are present adhered to different tissues or organs, including lymph nodes, heart, kidney, ovaries and joints. In this review, we only focused on epicardial, perinodal, and peritumoral fat since these locations have been studied in more detail. Evidence was found that EAT volume significantly increased, associated with chronic metabolic challenges of the corresponding tissue. In vitro evidence revealed transfer of fatty acids from peritumoral and perinodal fat to the adjacent tissue. Cytokine expression in these EAT is upregulated when the adjacent tissue is challenged. In these tissues, glycolysis is enhanced, whereas fatty acid oxidation is increased. Together with more direct evidence, this shows that glucose is oxidized to a lesser degree, but used to support anabolic metabolism of the adjacent tissue. In these situations, browning occurs, resulting from upregulation of anabolic metabolism, stimulated by uncoupling proteins 1 and 2 and possibly 3. In conclusion, the evidence found is fragmented but the available data support the view that accumulation and browning of adipocytes adjacent to the investigated organs or tissues may be a normal physiological response promoting healing and (patho)physiological growth.

Infectious Diseases and the Lymphoid Extracellular Matrix Remodeling: A Focus on Conduit System

The conduit system was described in lymphoid organs as a tubular and reticular set of structures compounded by collagen, laminin, perlecan, and heparin sulfate proteoglycan wrapped by reticular fibroblasts. This tubular system is capable of rapidly transport small molecules such as viruses, antigens, chemokines, cytokines, and immunoglobulins through lymphoid organs. This structure plays an important role in guiding the cells to their particular niches, therefore participating in cell cooperation, antigen presentation, and cellular activation. The remodeling of conduits has been described in chronic inflammation and infectious diseases to improve the transport of antigens to specific T and B cells in lymphoid tissue. However, malnutrition and infectious agents may induce extracellular matrix remodeling directly or indirectly, leading to the microarchitecture disorganization of secondary lymphoid organs and their conduit system. In this process, the fibers and cells that compound the conduit system may also be altered, which affects the development of a specific immune response. This review aims to discuss the extracellular matrix remodeling during infectious diseases with an emphasis on the alterations of molecules from the conduit system, which damages the cellular and molecular transit in secondary lymphoid organs compromising the immune response.