Immunological Functions of the Omentum

Abstract

The omentum is a visceral adipose tissue with unique immune functions. Although it is primarily an adipose tissue, the omentum also contains lymphoid aggregates, called milky spots (MSs), that contribute to peritoneal immunity by collecting antigens, particulates, and pathogens from the peritoneal cavity and, depending on the stimuli, promoting a variety of immune responses, including inflammation, tolerance, or even fibrosis. Reciprocal interactions between cells in the MS and adipocytes regulate their immune and metabolic functions. Importantly, the omentum collects metastasizing tumor cells and supports tumor growth by immunological and metabolic mechanisms. Here we summarize our current knowledge about the development, organization, and function of the omentum in peritoneal immunity.

Figure 1. Structure of Milky Spots in the Omentum

A. Schematic of whole omentum in mice. The omentum is a thin strip of fat (yellow) that is covered by a layer of mesothelial cells (green). Milky spots (blue) are located just beneath the mesothelial layer embedded between adipocytes. The omentum is well vascularized with an extensive network of capillaries (not shown) running between the adipocytes that connect to a large central blood vessel (red). Blood vessels also connect to the milky spots and form a glomerulus-like knot of blood vessels in the lymphoid clusters. A large central lymphatic vessel ((light blue) also runs down the center of the omentum and branches from this vessel lead to milky spots as well as to areas of the omentum that lack milky spots. The human omentum is much larger and resembles an apron hanging in front of the abdomen, but also contains milky spots. B. Structure of milky spot. Milky spots are loose collections of leukocytes embedded between adipocytes (yellow) just beneath the mesothelial layer (green). B cells (blue) form a central cluster, whereas macrophages (brown) and dendritic cells (light green) tend to accumulate around the outside of the milky spot and are also found individually throughout the omentum. T cells (green circles) and ILCs (orange) can be intermixed with the B cells or may cluster around blood vessels (not shown). Cells and antigens are passively collected from the peritoneal cavity through fenestrations in the mesothelial layer by fluid flow or may be actively carried by phagocytic cells like macrophages. Cells can also enter the milky spots from the blood through HEVs (not shown).