Antigen Presenting Cells (APCs)
359
have not been stimulated before and ‘‘see’’
the respective antigen for the Frst time.
APCs that are able to prime naive
T
cells
are
termed
professional
APCs
and include three sets of cells (±ig. 2):
mononuclear phagocytes or macrophages,
dendritic cells (DCs), and B lymphocytes (B
cells). These three classes of APCs can be
distinguished from nonprofessional APCs
in that they express a speciFc combination
of surface proteins that are critical for
priming
naive
T
cells.
These
surface
p
ro
te
insa
re(1
)thegenep
roduc
tso
fthe
major histocompatibility complex (MHC):
MHCc
lassIandc
lassIImo
lecu
lesb
ind
peptides derived from antigenic proteins
and present them to either cytotoxic T
cells, characterized by the coreceptor CD8,
or helper T cells expressing the coreceptor
CD4, respectively; (2) the costimulatory
molecules CD80 (B7.1) and CD86 (B7.2),
which promote growth and differentiation
of T cells upon engaging APCs; and
(3) adhesion molecules such as ICAM-1
or L±A-3 that facilitate the primary contact
between T cells and APCs.
DCs, although being highly diverse in
origin and shape, appear to be the most
potent professional APCs: DCs localize
to
all
putative
antigen
entry
sites
of
the
skin,
mucosa,
and
airways,
they
express high amounts of both classes of
MHC molecules and, as members of the
phagocyte family, they have an enormously
high capacity to pino- and phagocytose
antigenic material. Owing to their superior
impact on the functionality of the acquired
immune system, numerous attempts are
ongoing to exploit DCs in immunotherapy
of cancer and other diseases. Interestingly,
about 100 years ago, the writer George
Bernard Shaw stated in his famous book
The Doctor’s Dilemma: ‘‘There is only
one genuinely scientiFc treatment for
all diseases and that is to stimulate the
phagocytes.’’
2.3
Nonprofessional APCs
One major prerequisite of bone mar-
row–derived professional APCs is their
B lymphocyte
Internalizes and presents:
Soluble antigens
Viral antigens
Toxins
Dendritic cell
Internalizes and presents:
Bacterial antigens
Viral antigens
Soluble antigens
Particulate antigens
Allergens
Macrophage
Internalizes and presents:
Extracellular and
Intracellular pathogens
Particulate antigens
Fig. 2
The three types of antigen presenting cells (APCs). They have overlapping but distinct
repertoires of antigens that they internalize, process and present to T cells.
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