Immune Defences of Teleost Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 26

and RAG 2 (Hansen and Kaattari 1995; 1996; Hansen et al. 1997; Hansen 1997). In addition,
multiple forms of germline IGI transcripts are expressed in the head kidney of both rainbow
trout and Atlantic cod Gadus morhua (Daggfeldt et al. 1993; Partula et al. 1996). As germline
transcription correlates with accessibility to the V(D)J recombination machinery (Gorman and
Alt 1998), this feature suggests an ongoing development of B cells. Maturation of B cells in
the head kidney appears to be a continuous process during the development of rainbow trout
(Hansen and Zapata 1998) and it has been speculated that developing B cells can mature in
the head kidney and then migrate to the spleen and the mid kidney for activation (Zwollo
et al. 2005). In this context subsequent analysis of the entire rainbow trout kidney, divided
into five contiguous segments K1-K5, reveals a complex pattern of early developing B cells,
late developing B cells, and IgM-secreting cells. Patterns in anterior kidney segment K1 were
most similar to those of mouse bone marrow, while the most posterior part of the kidney, K5,
had many IgM-secreting cells, but lacked early developing B cells. A potential second B lym-
phopoiesis site was uncovered in segment K4 of the kidney (Zwollo et al. 2010). Toda et al.
(2011a; 2011b) found the absence of CD4/CD8 double-positive T cells in the ginbuna kid-
ney and in the other lymphoid organs with the exception of the thymus, implying that T cell
development may not occur in the teleost kidney; then pre-T cells (lymphoid progenitors)
residing particularly in the cyprinid trunk kidney could migrate via the head kidney or directly
to the thymus for differentiation and functional maturation. Flow cytometric analysis of rain-
bow trout cells showed low percentages of CD8α + cells in splenocytes (approximately 2%)
and pronephrocytes (approximately 4%). Accordingly, tissue sections revealed many CD8α +
cells in the thymus, intestine and gill, with only a few scattered CD8α + cells in the spleen
and pronephros (Takizawa et al. 2011). At the same time it has been demonstrated that the
head kidney serves as a secondary lymphoid organ important in the induction and elabora-
tion of immune responses (Kaattari and Irwin 1985) and the spleen, like the head kidney,
is another important source of B cells in adult teleosts. It may also serve as a secondary
lymphoid tissue where mature lymphocytes are activated by antigens and differentiate into
antibody-secreting cells (Kaattari and Irwin 1985; Zapata and Cooper 1990; Bromage et al.
2004; Zwollo et al. 2005). The spleen has a fibrous capsule and small trabeculae extend into
the parenchyma, which can be divided into red and white pulp. The red pulp, which may
occupy the majority of the organ (Grace and Manning 1980; Secombes and Manning 1980),
consists of a reticular cell network supporting blood-filled sinusoids that hold diverse cell pop-
ulations including macrophages and lymphocytes. The white pulp is often poorly developed
but may be divided into two compartments: the melanomacrophage accumulations and the
ellipsoids, which appear to have a specialized function for plasma filtration and the trapping
of blood-borne substances, particularly immune complexes (Secombes and Manning 1980;
Espenes et al. 1995; Solem and Stenvik 2006). In Atlantic salmon scattered CD3ε + cells were
found throughout the spleen with a majority of cells detected in the white pulp surrounding
ellipsoids of the organ (Koppang et al. 2010). This distribution of T cells in the white pulp
was similar to the periarteriolar lymphoid sheet as described in mammals (Ruddle and Akirav
2009), whose white pulp has been described as an organized lymphoid structure (Ruddle and
Akirav 2009) and this seems to apply in salmonids. Recently Barr et al. (2011) defined four
major trout B cell subsets present during terminal B cell differentiation in rainbow trout spleen
using the transcription factor XbpI, concluding that trout B cells follow a highly conserved
activation pathway, albeit slower than that observed in mammalian species. In addition it has
been demonstrated that the teleost spleen possesses abundant IgM + mature B cells (Kaattari