2. 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

3. 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

4. Are milky spots secondary lymphoid organs?

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

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

16. 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

17. 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

18. 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