Risposte immunitarie nei vertebrati ectotermi: attività cellulari e molecolari di leucociti del teleosteo Dicentrarchus labrax
Author(s)
Marozzi, Catia
Date Issued
May 31, 2013
Type
Doctoral Thesis
Abstract
Aquaculture, the farming of aquatic organisms, has been the biotechnology activity with
the highest growth rate worldwide in the last four decades. The annual aquaculture
production is at present over 60 million tons, with an approximate value of 85 billion
dollars (FAO, World Review of Fisheries and Aquaculture, 2010). Despite the undeniable
benefits of aquaculture such as the provision of good quality and accessible food for
population and the generation of millions of jobs and billion dollars in budget for the
developing countries, the activity is one of the most criticized worldwide, mainly because
of the environmental impacts that have been and can be caused. Moreover, under intensive
culture conditions, fish are subject to increased stress owing to environmental (water
quality and hypoxia) and health conditions (parasites and infectious diseases). All these
factors have negative impacts on fish well-being and overall performance, with consequent
economic losses (Teles, 2012). European sea bass (Dicentrarchus labrax L.; Moronidae;
Perciformes) is a marine species of great economic importance, especially in
Mediterranean aquaculture. However, numerous pathogenic viruses, bacteria, fungi and
parasites affect the species, causing various infectious diseases. Among those pathologies,
viral encephalopathy and retinopathy (Bovo et al., 1999; Ucko et al., 2004), pasteurellosis
and vibriosis (Afonso et al., 2005) caused by D. labrax encephalitis virus and bacterial
pathogens like Photobacterium damselae subsp. piscicida and Vibrio anguillarum,
respectively, lead to the most heavy losses in aquaculture production of sea bass. In this
respect, knowledge on molecular and genetic mechanisms of resistance to pathogens and
specific features of immune response against various infectious agents should greatly
benefit the development of effective vaccines and proper vaccination strategies in markerassisted
selection (MAS) of fish resistant to a range of infections (Bricknell and Dalmo,
2005; Chinabut and Puttinaovarat, 2005). During the last decades vaccination has become
established as an important method for prevention of infectious diseases in farmed fish.
The majority of aquatic vaccines are delivered by injection, which is by far the most
effective method when compared to oral or immersion deliveries. However it is labour
intensive, costly and not feasible for large numbers of fish. Attempts to develop novel oral
and immersion delivery methods have resulted in varying degrees of success but may have
great potential for the future (Plant and Lapatra, 2011). Recent data have shown that
immune-stimulation and vaccination may modulate transcriptional levels of CD8α and/or
CD4 in some fish species and, together with data showing the antigen uptake occurring in
gills and in the intestine, it appears evident that studies on T cells subtypes and their
distribution and modulation is important to develop strategies for oral and immersion
vaccination. As physical barriers that separate teleost fish from the external environment,
mucosae are also active immunological sites that protect them against exposure to
microbes and stressors. In mammals, the sites where antigens are sampled from mucosal
surfaces and where stimulation of naive T and B lymphocytes occurs are known as
inductive sites and are constituted by mucosa associated lymphoid tissue (MALT).
According to anatomical location, the MALT in teleost fish is subdivided into gutassociated
lymphoid tissue (GALT), skin-associated lymphoid tissue (SALT), and gillassociated
lymphoid tissue (GIALT). All MALT contain a variety of leukocytes including,
but not limited to, T cells, B cells, plasma cells, macrophages and granulocytes. In this
view, the main objective of this thesis was to improve the knowledge about mucosal
immunity in the European sea bass. Special attention has been devoted to study
mechanisms of cellular in vitro proliferation of gills and gut leucocytes after mitogen
stimulation, in order to provide experimental evidence on leukocyte populations involved
and on their mechanisms of action in mucosal immunity. To date, some specific cellular
markers (anti-sea bass T cell marker DLT15/DLT22, anti-sea bass B cells marker DLIg3)
and molecular markers (TCR α-β -γ, CD8α, CD4, TNF-α, IL-1β, MHC-II, IFN-α, IgM,
RAG-1, COX-2, MX, etc.) are available in sea bass. The first step it was to set up a
method for studying cellular proliferation of leucocytes of sea bass, and a number of
experiments has been made in order to develop optimal conditions of cell growth. Cells
obtained from different organs (spleen, intestine, gills, head kidney) and enriched in
leucocytes by Percoll with discontinuous gradient density 1.02 g/cm3 and 1.07 g/cm3,
were stimulated with two different lectins, such as PHA (phytohemagglutinin) and ConA.
(concanavalin A). The cells proliferation was then measured by the fluorescence-based
CFSE assay and monitored by flow cytometry. This type of study is rather innovative,
because in literature there are few information about in vitro mechanisms of leukocyte
proliferation in fish after mitogen stimulation. In the next step, the target cells for the study
were taken from gills and gut tissues. The data obtained clearly show that a lectin-induced
proliferation of gills and gut leucocytes, and that this proliferation was associated with an
increase of DLT15 and DLT22-labelled cells. At last, we have measured the trascription
level of T genes from 12 individuals after 24h and 48h stimulation with PHA and ConA.
Also, part of my work has been focused on T cells population and mechanism of
lymphocyte activation mediated by co-receptors such as CD45. In mammals, the cell
surface-associated CD45 receptor molecule is a highly glycosylated and high molecular
weight enzyme tyrosine phosphatase (Tonks, 1988), also known as leukocyte common
antigen, a type I transmembrane protein present on all hemopoietic cells, except
erythrocytes (Thomas, 1989; Trowbridge, 1991; Trowbridge and Thomas, 1994). The
CD45 is a fundamental regulator of B- and T-cell antigen receptor signalling (Altin and
Sloan, 1997): its long cytoplasmic domain transduces the extracellular signal through a
phosphatase activity that in T cells activate Lck tyrosine kinases, or Lyn/Fyn/Lck kinases
in B cells (McNeill et al., 2007). This co-receptor therefore seems to have a role in the
activation of T lymphocytes and in the immune response. The CD45 receptor gene can be
expressed in mammals leucocytes in five alternatively spliced glycoproteins differing in
the extracellular domain, and regulated in various cell-type specific isoforms that differ in
size, shape and negative charge (Penninger, 2001), with the precise role of these isoforms
still remaining elusive (Falahati and Leitenberg 2007). Naïve T lymphocytes express high
molecular weight CD45 isoforms (around 220 kDa in size), and are usually positive for
CD45RA, whereas after thymic maturation activated and memory T lymphocytes express
CD45RO, a shorter CD45 isoform at 180 kDa (Hathcock et al., 1992). The central
memory CD45RO T helper cells are CCR7+ and CD62L+, whereas effector memory cells
are CCR7- and CD62L- (Sallusto et al., 2004; Krakauer et al., 2006). The biological
significance of the various isoforms of the CD45 gene expressed in leucocytes is still
matter of debate (McNeill et al., 2007, Kozieradzki et al., 1997; Dawes et al., 2006; Earl
and Baum, 2008), but it is assumed that individual isoforms may play important roles in
lymphocyte activation and development. In order to better investigate some features of
immune-biology of CD45 in vertebrates, we employed the European sea bass as a fish
model, and in this species we investigated the transcription of the CD45 gene under in vitro
stimulation of leucocytes. By obtaining an anti-CD45 mAb, called DLT22, we also studied
the distribution of CD45-bearing leucocytes in lymphoid tissues of sea bass, and the
involvement of CD45-bearing cells during lectin-induced proliferation of leucocytes. We
observed a marked difference in the percentages of staining, being very high in thymocytes
(>90%), intestinal leucocytes (>80%), and gill leucocytes (>75%), and very low in head
kidney, spleen, and PBL (around 1-4 % for each tissue). To obtain information on the
molecular size of the DLT22 antigen, western blotting experiments were performed on cell
lysates from thymus and has been observed the presence of multiple immune-reactive
bands. The 130 kDa band was excised from the gel and subjected to sequence analysis,
and when obtained product were compared to a fish peptide database, two peptide
sequences matched with CD45 of Takifugu rubripes. The selected sequence
RYVDILPYDYNRV, matched at position 649-659 of channel catfish CD45,
corresponding in this latter species to enzymatic PTPase domain I. From these data we
strongly supposed that the mAb DLT22 recognized as antigen an epitope present on sea
bass CD45 molecule. In this respect, we have decided to clone partially the CD45 sea bass
gene in order to perform various functional tests described in detail later in this thesis
work.
Additional information
Dottorato di ricerca in Genetica e biologia cellulare