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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

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Omentum and milky spots
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 spots1
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



Mouse system
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
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
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 antigens

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 antigens

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 antigens

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 antigens

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 antigensLTα and CXCL13, but not LTi cells

inactive genes

for CCL19, CCL21

  • MSs were much smaller or even absent in Lta-/- and Cxcl13-/- mice

  • 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 antigensLTα 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 antigens

omentum

spleen

  • CXCL13 is expressed around the B cell area

  • absence of conventional FDC networks


CXCL13 is expressed surrounding the B cell areas antigens

  • 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 antigenshypermutation 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
Summary antigens

  • 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


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