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

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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

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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

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