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SHORT COMMUNICATION
SOX2 gene expression in normal human thymus and thymoma
Anca Maria Cimpean • Svetlana Encica •
Marius Raica • Domenico Ribatti
Received: 17 November 2010 / Accepted: 9 December 2010 / Published online: 29 December 2010
� Springer-Verlag 2010
Abstract The SOX gene family encodes a large group of
transcription factors that are strongly involved in the nor-
mal human development and malignancies. Human tumors,
like small cell lung carcinoma, meningioma, gastric, and
pancreatic cancer, have been found to be immunogenic for
SOX2 protein. In this study, we have investigated for the
first time the expression and distribution of SOX2 immu-
noreactive cells in five normal human thymuses (2 fetal and
3 adult) and 10 thymomas bioptic specimens. Results
demonstrated the presence of few positive cells in the fetal
and postnatal normal human thymus with a specific dis-
tribution within thymus parenchyma compartments. On the
contrary, in thymoma immunoreactivity to SOX2 increased
parallel to pathological stage, and a peculiar distribution
was observed in type B3 thymoma with a positive reaction
in both tumor and endothelial cells.
Keywords Human thymus � Immunohistochemistry �SOX2 � Thymoma
Introduction
Transcription factors encoding SOX gene family are
characterized by a highly conserved high-mobility
group (HMG) domain [1–3]. SOX genes are expressed
in a restricted spatial and temporal manner and are
strongly involved in stem cell biology, organogenesis,
and human development. More than 20 Sox genes
have been described in mammals, divided into 6
distinct groups according to their HMG-box homology
[4].
SRY (sex determining region Y)-box 2, also known as
SOX2, is a transcription factor required for pluripotency
during early embryogenesis and for the maintenance of
embryonic stem cell identity. Expression studies suggest
a pivotal role for the SOX2 gene in the developing
central nervous system, in particular in pituitary, fore-
brain, and eye [5]. Scattered data are published until now
concerning SOX2 gene involvement in the normal
development of other organs, like pancreas, liver, or
gastrointestinal tract [6, 7]. Recently, it has been high-
lighted the potential role of SOX2 in human carcino-
genesis in squamous cell carcinoma of gastrointestinal
tract [8] and in basal-like phenotype of breast cancer
[9].
The development of the human thymus is not fully
characterized, and few molecular markers have been
described to be involved in thymus organogenesis and also
in characterization of thymoma [10, 11]. Indirect evidence
in experimental animal models suggested a role for SOX2
in the thymus development [12].
In this study, we have investigated for the first time the
expression and distribution of SOX2 immunoreactive cells
in normal human thymus and in thymoma bioptic
specimens.
A. M. Cimpean � M. Raica
Department of Histology, ‘‘Victor Babes’’ University
of Medicine and Pharmacy, Timisoara, Romania
S. Encica
Department of Pathology, ‘‘Niculae Stancioiu’’ Heart Institute,
Cluj Napoca, Romania
D. Ribatti (&)
Department of Human Anatomy and Histology,
School of Medicine, University of Bari Medical School,
Piazza G. Cesare, 11 Policlinico, 70124 Bari, Italy
e-mail: ribatti@anatomia.uniba.it
123
Clin Exp Med (2011) 11:251–254
DOI 10.1007/s10238-010-0127-0
Materials and methods
Bioptic specimens
Two fetal normal human thymus specimens were obtained
during necropsy of 6–8 month of gestation human fetuses
and three from patients 1 month-5 years aged in the
course of surgical treatment of cardiac malformations.
Surgical specimens from 10 thymomas classified accord-
ingly World Health Organization [13] (type A, n = 2;
type AB, n = 2; type B1, n = 1; type B2, n = 2; type B3,
n = 3) were obtained in the course of surgical treatment
of mediastinal tumor masses. Specimens were fixed in
buffer formalin for 48 h, embedded in paraffin, and 5-lm-
thick sections were stained with hematoxylin-eosin for the
routine diagnosis.
Immunohistochemistry
Sections were incubated with a polyclonal anti-SOX2
antibody (diluted 1:100, Abcam, Cambridge, MA, USA)
for 2 h at room temperature, followed by Advance-HRP
(Dako, Carpinteria, USA) working system protocol. Visu-
alization of the immunoreaction was performed with 3,
30diaminobenzidine and nuclei staining with modified
Lillie’s hematoxylin.
Microscopic evaluation and image acquisition
Microscopic examination was performed with Nikon
Eclipse E 600 microscope (Nikon Corporation, Tokio,
Japan), and captured images were analyzed with Lucia G
software system (Nikon Corporation, Tokio, Japan).
The local research ethic committee approved the pro-
tocol of the study, and informed consent was obtained from
all subjects according to the World Medical Association
(WMA) Declaration of Helsinki.
Results
Two expression patterns of SOX2 were found in human
fetal thymus and thymoma. Nuclear expression was
detectable in both normal thymus and thymomas, whereas
cytoplasmic distribution alone or associated with nuclear
pattern was demonstrated in thymoma.
Expression of Sox2 in human fetal and postnatal
thymus
SOX2-positive epithelial cells were distributed in the thy-
mic cortex, medulla, and cortico–medullary junction of
human fetal thymus, where cells with nuclear positive
staining were grouped in small clusters as well defined
networks. Moreover, in the fetal thymus, few SOX2-posi-
tive cells were tightly associated with cortical small blood
capillaries in the deep cortex and in the medulla (Fig. 1a).
Lymphocytes did not express SOX-2 protein. The same
distribution was found in postnatal normal human thymus,
where epithelial cells of the Hassall corpuscles strongly
expressed SOX2 (Fig. 1b).
Expression of Sox2 in thymoma
An heterogeneous pattern of distribution of SOX2 was
recognizable in thymoma (Fig. 1c–g). In type A and AB
thymoma, neoplastic spindle-like epithelial cells shared
immunoreactivity with both nuclear (predominant) and
cytoplasmic patterns. Few isolated positive cells were
present in the tumor mass. A weak staining for SOX2 was
present in type B1, whereas type B2 thymoma presented
numerous areas with isolated or clustered positive cells.
Moreover, scattered positive cells were found in the peri-
vascular spaces.
In B3 thymoma, immunoreactivity was observed not
only in neoplastic epithelial cells, but also in endothelial
cells of intratumoral blood vessels and an increased
number of positive cells were found in the periphery
of the tumor. Finally, in type B3 thymoma, blood
vessels showed positive cells inside the vascular
endothelium.
Discussion
Human thymus plays a crucial role in the immune system
function, and its peculiar morphological feature is the
presence of stromal epithelial cells, which provide a
special microenvironment for T cell development [14].
Although the morphological and ultrastructural hetero-
geneity of thymic epithelial cells has been extensively
investigated [15], little it is known about the molecular
characterization of normal human thymus and thymomas
[16]. The identity of ‘‘thymus epithelial stem cells’’ is
elusive, and developmental stages of thymic epithelial
cells need to be further elucidated [17]. Moreover, even
if the development of thymic epithelial cells has been
highlighted [18–20], the conclusions of these studies are
not widely accepted. A thymic progenitor epithelial cell
has been described in the murine thymus [21, 22], and it
has been demonstrated that a single epithelial progenitor
cell could give rise to a complete and functional thymic
microenvironment. Nanog, Oct4, and SOX2 have been
described among the markers of these progenitor cells,
and their expression was reduced in thymic epithelial
cells of Aire-deficient mice [23].
252 Clin Exp Med (2011) 11:251–254
123
In this study, we have demonstrated for the first time the
presence of a small population of SOX2-positive epithelial
cells in normal human thymus, suggesting the existence of
a pool of stem cells in the cortico–medullary junction. This
finding agrees with data reported by Jenkinson et al. [24],
demonstrating the presence of epithelial progenitor cells in
the thymus of mice.
Moreover, the oncogenic potential of SOX2 has been
recently highlighted in few human tumors types [25–28],
and in this study we have shown for the first time the
presence of tumor cells immunoreactive to SOX-2 in dif-
ferent types of human thymoma. Our evidence of the
presence of SOX2-positive cells inside the tumor vascular
endothelium in type B3 thymoma specimens suggests the
hypothesis of a presence of an epithelial stem cell
population able to differentiate in both endothelial and
thymic malignant epithelial cells, as it has been indicated
for the p63 positive cells [29, 30].
Overall, in this study, we have described for the first
time the presence of SOX2-positive cells in the normal
thymus and in thymoma bioptic specimens, and we have
suggested a potential oncogenic role of SOX2 in the
development of thymoma.
Acknowledgments This work was supported by grant CNCSIS
IDEI 1147/2009 and grant PN II 41-054 of the Romanian Ministry of
Education and Research. We are grateful to Raluca Ceausu, Dragos
Izvernariu and Diana Tatucu for their technical contribution.
Conflict of interests None to declare.
Fig. 1 Different patterns of SOX2 expression in human normal
thymus and thymomas. Cluster of positive cells at the cortico–
medullary junction in normal fetal thymus (a). Nuclear staining of
epithelial cells of an Hassall corpuscle in adult normal thymus (b).
Nuclear staining in spindle-like areas of a thymoma type AB (c).
Small group of positive cells in a thymoma type B1 (d). Positive
neoplastic epithelial cells around a perivascular space in a thymoma
type B2. Note the nuclear staining also in scattered cells inside
perivascular space (e). Nuclear expression in tumor epithelial cells in
a thymoma type B3. Positive cells are generally located at the
periphery of the tumor (f). Positive tumor endothelial cells in a
thymoma type B3 (g). Original magnifications: a, b, d, g, 9 1,000;
c, 9 200; e, f, 9 400
Clin Exp Med (2011) 11:251–254 253
123
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