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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
slide8
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
slide9
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

slide10
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
slide11
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
slide12
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
slide13
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
  • 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
slide14
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
slide15
CXCL13 is expressed surrounding the B cell areas

omentum

spleen

  • CXCL13 is expressed around the B cell area
  • absence of conventional FDC networks
slide16
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
slide17
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
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
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