Abstract
Fat-associated lymphoid clusters (FALCs) are atypical lymphoid tissues that were originally identified in mouse and human mesenteries due to that they contain a high number of type 2 innate lymphoid cells/nuocytes/natural helper cells. FALCs are located on adipose tissues in mucosal surfaces such as the mediastinum, pericardium, and gonadal fat. Importantly, these clusters contain B1, B2 and T lymphocytes as well as myeloid and other innate immune cell populations. The developmental cues of FALC formation have started to emerge, showing that these clusters depend on a different set of molecules and cells than secondary lymphoid tissues for their formation. Here, we review the current knowledge on FALC formation, and we compare FALCs and omental milky spots and their responses to inflammation.
Highlights
The interactions between different types of immune cells are essential for both innate and adaptive immune responses to pathogens
These cells are found scattered along the lymphoid clusters and are thought to be different from follicular dendritic cells (FDCs), which require signaling through lymphotoxin β receptor (LTβR) to induce CXCL13 expression [35]
Some reports have shown that gastric, colon, and some liver cancer cells express the chemokine receptors CCR4 and CXCR4, whereas cell populations from omFALCs and mesenteries can express their ligands (CCL22 and CXCL12), which may favor their migration to these structures [51, 52, 55]
Summary
The interactions between different types of immune cells are essential for both innate and adaptive immune responses to pathogens. Such interactions require strategically situated microenvironments to increase the chances that rare antigen-specific lymphocytes become activated. These specialized microenvironments are found in secondary lymphoid organs (SLOs) such as lymph nodes and Peyer’s patches. Paracortical T cell areas are organized by a different type of stromal cells named T zone reticular cells that express CCL21 and CCL19, which attract T cells, and DCs to facilitate their interactions [4]. A novel type of lymphoid tissue called fat-associated lymphoid clusters (FALCs) has been identified in the mesenteries of humans and mice [19]
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