360
Antigen Presenting Cells (APCs)
Tab. 1
Nonprofessional APCs of the human immune system.
Cell type
Constitutive
MHC class I
Constitutive
MHC class II
Costimulators
CD80/CD86
Thymic cortical
epithelial cells
++
Thymic medullary
epithelial cells
++
Keratinocytes
+−
Myoblasts
+−
T lymphocytes
+−
concerted expression of MHC class II and
costimulatory molecules CD80 and CD86.
Apart from a very few exceptions, for exam-
ple, vascular endothelial cells, only B cells,
macrophages, and DCs are able to express
reasonable amounts of CD80 and CD86
that are essential for activation of naive
T cells. However, a number of other cell
types can express MHC class II molecules
(Table 1).
Thymic cortical epithelial cells (cTECs)
that are critical for positive selection of
CD4
+
thymocytes, and thymic epithelial
cells of the medulla, which are responsible
for deletion of self-reactive CD4
+
thymo-
cytes, express MHC class II molecules
in a constitutive fashion. The impor-
tance of MHC class II expression on
c
T
EC
si
sd
em
o
n
s
t
r
a
t
e
db
yMHCc
l
a
s
s
II–negative mutant mice that lack mature
CD4
+
T cells. Following cytokine stim-
ulation
in vitro
or during inflammatory
states
in vivo
, MHC class II molecules are
induced on a variety of epithelial and en-
dothelial cells. IFN-
γ
induces MHC class
II molecules in keratinocytes of the skin
and even in myoblasts of muscles. Like-
wise, activated T lymphocytes in humans
can become MHC class II positive.
It has been shown that cells becom-
ing MHC class II–positive in the pres-
ence of proinflammatory cytokines, such
as IFN-
γ
, are able to present peptides
to CD4
+
Tc
e
l
l
san
dt
oa
c
t
i
v
a
t
eth
em
,
provided that the T cells do not need cos-
timulation. Neo-MHC class II–expressing
cells that have the potential to present
antigen, are termed
nonprofessional
APCs.
3
Antigens Presented by APCs
3.1
Antigen Types
According to the initial de±nition brought
up by the discoveries of Behring and
Ehrlich, ‘‘antigens’’ are substances that
cause the production of antibodies. How-
ever, this historic de±nition only covers
humoral immune responses. Nowadays,
molecules that are recognized by the anti-
gen receptor of a B lymphocyte (B-cell
receptor) and/or the antigen receptor of a
T lymphocyte (T-cell receptor) are termed
antigens
. Antigens that initiate an im-
mune response are named
immunogens
,
whereas others may induce tolerance and
are named
tolerogens
. Another subset of
antigens that have a low molecular weight
(
<
4000 Da) and are immunogenic only
if they are covalently bound to a carrier
protein, is termed
haptens
.E
x
amp
l
e
so
f
haptens are substances, such as poison
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