Omentum and milky spots the omentum is formed by a double layer of mesothelial cells connect the stomach, pancreas, spleen and colon has immunological and wound-healing properties embedded within the omentum are structures which are clusters of leukocytes, called milky spots

Omentum and milky spots milky spots are mainly composed of macrophages and B1 cells B1cells forma unique subset of B cells can be distinguished from conventional B (B2) cells by expression of distinct cell-surface markers and antigen receptors that can bind common bacterial epitopes have a recognized potential to produce natural antibodies that provide a first protection to bacterial infections B1 cells are localized in distinct locations, such as the peritoneal cavitiy and the spleen MS described to lack dendritic cells as well as follicular dendritic cells

Are milky spots secondary lymphoid organs?

Mouse system want to address the immunological potential of milky spots in the absence of lymph nodes, spleen, and Peyer’s patches used splenectomized lymphotoxin-alpha (LTa)-deficient mice (Lta-/-), which as a result of their deficiency already lacked lymph nodes and Peyer’s patches animals, devoid of secondary lymphoid organs, were reconstituted with wild-type bone marrow (SLP mice) compared to irradiated C57BL/6 mice that were similarly reconstituted with wild-type bone marrow

Antibody responses to peritoneal antigens in the absence of conventional lymphoid organs NP-OVA i.p. TNP-KLH i.p. NP-OVA i.p. WT and SLP mice produce similar titers of NP-specific IgM, IgG1, IgG2a+b and IgG3 after immunization with TNP-KLH SLP mice showed slower generation of IgG titer concentration to see how much secific IgG was produced conventional lymphoid organs are not needed for B cell response

The milky spots of the omentum collect peritoneal cells and antigens HEV + B cell activation of peritoneal cells promotes their migration to the omentum at least some particulates can be passively collected by the omentum in addition to being captured by phagocytic cells

The milky spots of the omentum collect peritoneal cells and antigens peritoneal cells use PTX-sensitive mechanisms to actively migrate to the omentum can also accumulate in the omentum by other mechanisms the segregation of B and T cells and the formation of follicular structures in the MSs are controlled by PTX-sensitive mechanisms

Antigen-specific B cell responses occur in the omentum NP-OVA i.p. SRBC i.p boost d14 GC B cells GC B cells Isotype-switched plasmacells SRBC specific IgG secreting cells specific non-specific d8 d5

Antigen-specific B cell responses occur in the omentum SRBC i.p d14 NP-OVA i.p d14 GC B cells MSs support local germinal center B cell responses to peritoneal antigens

Antigen-specific T cell responses in the omentum T cell activation homing receptor OTII CD4 i.p. 4 hr later immunized T cell responses can also occur in the MSs of the omentum, even in mice that lack spleen, LNs, and Peyer’s patches

Antigen-specific T cell responses in the omentum influenza i.n. 21d p.i. 4-get mice helminth larvae oral 28 d p.i. antigen-experienced CD4+ and CD8+ T cells recirculate through the peritoneal cavity and omentum even when these cells were primed outside of the peritoneal cavity

Milky spot development requires LTα and CXCL13, but not LTi cells inactive genes for CCL19, CCL21 MSs were much smaller or even absent in Lta-/- and Cxcl13-/- mice defects in theMSs of Lta-/- mice seem not be due to an absolute blockade in development

Milky spot development requires LTα and CXCL13, but not LTi cells lymphoid tissue inducer cells even though CXCL13 is very important for the development of the MSs, its expression is not controlled by Lta Lti cells are not needed

CXCL13 is expressed surrounding the B cell areas omentum spleen CXCL13 is expressed around the B cell area absence of conventional FDC networks

CXCL13 is expressed surrounding the B cell areas network of ERTR7+ stromal cells throughout the MSs the cellular architecture and pattern of chemokine expression in the MSs are different than in conventional lymphoid organs

Somatic hypermutation and affinity selection occurs in the absence of conventional secondary lymphoid organs NP-OVA i.p. boost d14 W to L position 33 higher affinity YYYG motif at V-D junction NP-binding sequence somatic hypermutation is still evident in SLP mice but the process of clonal selection is unusual the MSs of the omentum support some aspects of T cell dependent B cell responses

Summary the omentum functions much more broadly as a secondary lymphoid organ it is structurally, developmentally, and functionally unique intersection of recirculating lymphocytes and peritoneal drainage makes the MSs ideal sites for the initiation of local immune responses the structure of the MSs seems inside-out relative to that of conventional lymphoid organs or even ectopic lymphoid follicles the MSs of the omentum are unique secondary lymphoid organs that sample antigens from the peritoneal cavity and promote local, albeit unusual, immune responses