In this study, CD15 expression was observed in WHO
type A and type C thymic epithelial neoplasms but not in
type B. We observed strong positivity in 4 of 5 cases of
type A thymomas. The fifth case that showed +1 staining
appeared morphologically different than the other cases.
This weakly CD15 positive type A case had a prominent
microcystic character and was composed of short spindle
cells. Strongly CD15 positive cases had typical type A
morphology. In all type AB cases the spindle cell component
was positive for CD15. In the light of these findings
it can be suggested that CD15 expression is a characteristic
of medullary differentiation. Previously, CD15
expression was studied in fetal thymus by Engel et al.[
8]
which demonstrated that in normal thymus, Hassals’ corpuscles, medulla and rare cortical epithelial cells are positive.
However, thymomas were to a great extend negative
or showed weak expression with only thymic carcinomas
showing stronger positivity with antibodies against
Lewis-x and sialyl-Lewis-x. The findings in our study and
that of Engel et al. are in accordance. The identification of
Le-x in fetal medulla in Engel et al.’s study also suggests
that this may be a feature of medullary differentiation in
thymomas. While we observed strong CD15 expression
in Type A thymomas they have reported weak expression
in their study. The differences observed between the two
studies may in part be due to composition of thymic epithelial
neoplasms included in the studies and the use of
different monoclonal antibodies to detect CD15.
Differential expression among thymic epithelial neoplasms
with medullary and cortical differentiation has
been shown for two other markers namely CD20 and
CD57[9-11]. CD20, a pan-B-cell marker can be useful
in the differentiation of spindle cell and mixed thymomas
from cortical thymomas[9,10]. CD57 (Leu-7) expression
has been shown in subcapsular cortical epithelium in normal
thymic tissue and in spindle and mixed thymomas,
but not in thymic carcinomas[11].
The results of CEA staining of our study are in agreement
with other reports[12-15]. Troung et al.[14] demonstrated
CEA positivity in 5 of 13 thymic carcinomas.
Papillary carcinoma of thymus was described by Matsuno
et al.[15] and 4 cases of these neoplasms were positive with CEA. Our findings and other reports pointed out that
CEA expression can be seen in thymic carcinomas in a
heterogeneous pattern, but there is no positivist in WHO
type A, AB or B thymomas. This property of thymic carcinomas
is also similar to CD5 immunoreactivity[2,16].
So in addition to the CD5 expression, positivity with CEA
may favor malignancy in thymic epithelial neoplasms.
Our study and previous studies which are related with
immunohistochemical characteristic of thymic epithelial
neoplasms demonstrates that CD5, CD15, CD20, CD57
and CEA are useful markers for subclassification of these
neoplasms. CD5 and CEA are generally positive in thymic
carcinomas, CD57 positivity is seen in medullary differentiation
not in cortical. CD15 and CD20 share similar
staining characteristics in that they are expressed in type
A and C cases.
As a conclusion CD15 positively stains A component
of WHO type A and AB thymomas and WHO type C thymomas.
So it is a useful marker for subclassification of
thymomas to highlight the A component and determination
of malignancy potential. CEA expression can be seen
in type C thymomas but not in any cases of A, AB or B
thymomas.
ACKNOWLEDGEMENT
This work was supported by Hacettepe University
Scientific Research Unit.
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