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fish and higher vertebrates, as judged by histology and gene expression patterns employing
mutants and several immune-relevant genes such as RAGs, TCRs, Ikaros (which encodes a
family of haematopoietic-specific zinc finger proteins), lck etc. (reviewed in Langenau and
Zon 2005). Although evidence of intrathymic apoptosis has been obtained in the thymus of
European sea bass and common carp, indicating the occurrence of negative selection of T
cells in fish (Abelli
et al.
1998; Romano
et al.
1999), little is known about the late stages of T
cell development within fish thymus, particularly from double negative (DN, CD4
-
CD8
-
)to
single positive (SP, CD4
+
CD8
-
and CD4
-
CD8
+
) thymocytes. In this context a new hypoth-
esis of positive selection and differentiation of T cells has been proposed in European sea
bass juveniles by Picchietti
et al.
(2008; 2009), providing new evidence about the functional
compartmentalization of the teleost thymus, with a specific role in thymocyte differentiation
pathways. The authors identified CD4
+
and CD8α
+
thymocytes, in each adult thymic lobe,
that largely overlapped and filled the cortex, while in the medulla CD4
+
thymocytes were iso-
lated and CD8α
+
were mainly arranged in cords. These data suggested that intrathymic T cell
development in fish is similar to that in mammals, where αβT cells mature through a CD4/CD8
double-positive stage giving rise to further selection in the medulla to become single positive
thymocytes. More recently, Takizawa and colleagues (2011) found high ratios of CD8α
+
cells
in trout thymus and considerable CD4 expression in CD8α
+
thymocytes, reminiscent of the
CD4
+
/CD8
+
double-positive cortical thymocytes of mammals (Ellmeier
et al.
1999; Laky
et al.
2006). In addition, rainbow trout extrathymic CD8α
+
lymphocytes strongly expressed
CD8α and -β and very low amounts of both CD4-1 and -2, while extrathymic CD8α
−
lympho-
cytes expressed CD4s but not CD8s. Such an expression pattern was in accordance with the
presence of both CD8
+
CD4
−
and CD4
+
CD8
−
T cell subsets (Takizawa
etal.
2011). Similarly,
Toda
et al.
(2011a; 2011b) confirmed the presence of teleost CD4
+
and CD8α
+
T cell subsets
with morphology, tissue distribution and gene expression in clonal ginbuna crucian carp
similar to those of mammalian CD4
+
and CD8
+
T lymphocytes. The authors defined that
more than 15% of the cells in the ginbuna thymus were CD4/CD8 DP T cells, while negligible
numbers of DP cells were present in other tissues, suggesting that the thymus is the site of
T cell development, although the precise differentiation process awaits further investigation.
2.5.2 The head kidney and spleen
The kidney is a paired organ that has been described as having various anatomical and func-
tional compartments. As in all vertebrates, the kidney is located retroperitoneally, exterior to
the dorsal wall or the body cavity. The foremost part of the teleost kidney lacks excretory
tissue and is often referred to as the head kidney or pronephros. In teleosts, the head kidney
is an important haematopoietic organ (Fänge 1986). In fact, studies performed by Kobayashi
et al.
(2006) showed that haematopoietic stem cells, having repopulation and multilineage
potential, reside in the ginbuna kidney, particularly in the trunk kidney. The head kidney is
the site of antibody production (Rombout and Joosten 1998b; Press and Evensen 1999) and
the melanomacrophage accumulations of the parenchyma are able to retain antigens for long
periods of time after administration or vaccination (Lamers and de Haas 1985; Herraez and
Zapata 1986; Tsujii and Seno 1990; Brattgjerd and Evensen 1996), which possibly has a role
in immunological memory (Press
et al.
1996).
It has been suggested that the head kidney is the bone marrow analogue, being the major
source of B cells (Meseguer
et al.
1995; Zapata
et al.
2006; Koppang
et al.
2003; Zwollo
et al.
2005). In rainbow trout this is suggested by a high expression of Ikaros, TdT, RAG 1
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