The document discusses secondary lymphoid organs in teleost fish. It describes how secondary lymphoid organs like the spleen, gut-associated lymphoid tissue (GALT), gill-associated lymphoid tissue (GIALT), mucosa-associated lymphoid tissue (MALT), and skin-associated lymphoid tissue (SALT) allow for antigen contact, lymphocyte proliferation, and generation of effector and regulatory immune cells. It provides details on the structure and immune functions of the spleen, which contains red and white pulp, ellipsoids, and melanomacrophages that trap antigens and initiate immune responses.

1. SECONDARY
LYMPHOID ORGAN IN
TELEOST
H A R A P R I YA B E H E R A

2. • Lymphoid organs are certain organs respectively parts of tissues, in
which lymphocytes can differentiate and proliferate. They are part of the lymphatic
system.
• In primary lymphoid organs, immature lymphocytes differentiate to mature ones into
an antigen sensitive lymphocytes and after maturation, lymphocytes migrate to
secondary lymphoid organs.
• Processes of antigen contact and clonal proliferation of lymphocytes are taking place
in secondary lymphoid organs. In this process effector cells as well as regulator cells
are generated. This differentiation is located in the zone of proliferation of the
secondary lymphoid organ

3. SECONDARY LYMPHOID ORGANS
1. Spleen
2. GALT
3. GIALT
4. MALT
5. SALT

4. SPLEEN
• The spleen is usually a solitary, dark red organ
• It is situated near the greater curvature of the stomach or the flexure of the
intestine
• it has sharply defined edges.
• The splenic capsule is fibrous and devoid of muscle
• The basic elements are :
1. blood vessels,
2. red and white pulps,
3. ellipsoids.
The spleen is covered by a thin, fibrous capsule with little evidence of contractile
ability.
.

5. • Red pulp is an extensive, interconnecting system of splenic cords and sinusoid capillaries
(open capillaries), consisting mainly of erythroid cells and thrombocytes, and usually
comprises the majority of the splenic parenchyma.
• . White pulp, consisting mainly of lymphoid cells, typically surrounds arterial vessels,
• Splenic cords are a mesh of fibroblast-like cells with foci of various blood cells
• The melanomacrophage (MM) is a characteristic immune cell type of teleosts, and is
prevalent in spleen. Ellipsoid in nature and form clusters of parenchyma.
• MM is a phagocyte containing varying amount of pigment, including melanin (black-
brown), hemosiderin, ceroid or lipofuscin (yellow-pink to golden brown) localized in
vacuoles.

6. • erythrocytes and phagocytic cells and are capable of trapping large quantities of
particulate matter from the circulation. Replete macrophages then migrate from
ellipsoids to the melanomacrophage centres.
• The splenic pulp consists of sinusoidal phagocytic tissue in which large numbers of red
blood cells may be held, and haemopoietic tissue, which is supported by argyrophilic
fibres.
• This is mainly lymphopoietic but not exclusively so.
• Melanomacrophage are usually located close to a vessel; they may even be invested
by strands of its externa and usually have a fine reticulin limiting membrane.

7. • The wall of the ellipsoids is composed of densely packed reticulin fibres, in which are
enmeshed macrophages which are active in the uptake of foreign material such as carbon
particles and bacteria.
• Upon antigen stimulation, antibody-producing cells and clusters of pyroninophilic cells
appear in the ellipsoid walls.
• They expand into structures similar to those in the kidney and are thought to develop into
MMC.
• AID expressing cells are also found in spleen of catfish associated with MMC. The reticulin
fibres in the ellipsoids are important in trapping immune complexes which are retained for
prolonged periods of time.
• This process of antigen trapping may be important, as in mammals it is considered to be
involved in the development of immune memory.

8. GALT
• The mucosa of the gut and the lamina propria are well populated by a variety of
leucocytes including macrophages, NCC, T and B cells and plasma cells.
. IgM is present in the gut and may originate from mucosal secretions and the bile
(Abelli et al. 2005).
. Fish do not possess the equivalent of IgA in mammals which is resistant to proteolytic
attack by gut enzymes, but recent evidence suggests that IgT+ B cells are relatively high
at this site (Zhang et al. 2010), and that the majority of gut bacteria are coated in IgT in
trout.

9. IgA has been cloned in fish and shown to associate with gut mucus IgM and IgT, and so
has a potential role in Ig transport. Whilst expression of pIgR can be detected at several
tissue sites
,
• in the gut a population of T-like cells express pIgR in the intestinal epithelium and
lamina propria.
• the mucosal epithelial cells themselves, especially in the second half of the intestine,
can endocytose antigens and transport them to macrophages and lymphocytes
present in the mucosal tissue (Joosten et al. 1997), with some resembling immature M

10. • Ag administration in gut lead to an increase in the number of intraepithelial
leukocytes, induce the production of in the mucosa and bile but not in the serum and
elicit a protective immune responses in the skin mucus.
• oral immunization that induces specific antibodies in serum but not in the mucus also
subsequently leads to immunosuppression.
• Bath immunization of cat fish, Ictalurus punctatus with dinitrophenyl- horse serum and
albumin(DNP-HoSA) enhance secretory immunity but not stimulate systemic
immunity.

11. GIALT
• phagocytosis by reticuloendothelial macrophages lining blood vessels in the secondary
lamellae and the accumulation of lymphocytes on the caudal edge of the
interbranchial septum.
• The gill is considered an important organ of antigen uptake, particularly of particulate
antigens). The gill contains quite high numbers of resident lymphocytes, macrophages
and plasma cells and local antibody production may play an important defence
especially against bacterial diseases and after immersion vaccination

12. MALT
• Oral or immersion vaccination can stimulate antibody responses in mucous
membranes without inducing serum antibody titres.
• oral boosting following immersion vaccination proving to be an effective way to
induce protective immunity.
• Immunisation by the injection route can stimulate antibody producing cells at mucosal
sites although this is not always the case (Xu et al. 2009). Such data give rise to a two-
compartment model of humoral immune responses in fish, systemic and mucosal.

13. SALT
Some lymphocytes, plasma cells and macrophages being
present within the epidermis of fish so the requisite cells for
mounting a local adaptive response are present.
.
.

14. Immunoglobulin is present at low concentration in the skin mucus, with few antibody-
secreting cells (ASC) present in the skin.
skin ASC number can increase some 20- fold post-vaccination (Zhao et al. 2008)
suggesting they are the source of the cutaneous Ig.
the role of IgT+ B cells in skin mucosal immunity will also need to be evaluated

15. REFERENCES
• ATLAS OF FISH HISTOLOGY FRANCK GENTEN EDDY TERWINGHE ANDRÉ DANGUY
Department of Histology and Biopathology of Fish Fauna Laboratory of Functionnal
Morphology Université Libre de Bruxelles (U.L.B)
• Fish Pathology Ronald J. Roberts BVMS, PhD (Glasgow), FRCVS, FRCPath, FSB,
FRSsCommander of the Most Noble Order of the Crown (Thailand)
• The fish immune system Organism, Pathogen, Environment by George Iwama

16. Thank you..