Chemistry Reference
In-Depth Information
expressed on pathogenic microorganisms like
Neisseria meningitidis
,
Haemophilus
infl uenzae
or
Corynebacterium diphtheriae
to name just a few. Apparently sialic
acids are used by these microorganisms for host mimicry. These sialoglycans
expressed on pathogens might also bind to CD22 on B lymphocytes, thereby
modulating the antibody production of B lymphocytes. This is an example for the
ability of pathogens to adapt recognition systems of the host for their own advan-
tage. Having introduced the three predominant groups of lectins known to be
involved in immune reactions, we now give some examples of other lectins taking
part in infl ammatory reactions and host defense against pathogens, in particular
bacteria - host interactions.
27.7
Other Lectins Involved in Antigen Recognition and Infl ammatory Processes
Another important example for lectins that play a role in pathogen recognition is
the dendritic cell - specifi c ICAM - 3 grabbing non - integrin ( DC - SIGN ), a prototype
for type II C-type lectins (see also Chapter 19 and X- ray structure in Figure 16.1 i).
DC-SIGN may function as a pathogen recognition receptor as well as an adhesion
coreceptor for host defense receptors like Toll-like receptors. As an adhesion recep-
tor DC-SIGN is involved in the contact between the professional antigen- present-
ing dendritic cell and T lymphocytes by binding to intercellular adhesion molecule
(ICAM)-3 on T cells. Additionally, and similar to selectins, it mediates rolling of
dendritic cells on vascular endothelium by interaction with ICAM-2. As a pathogen
receptor DC-SIGN recognizes several bacteria, fungi and viruses [11]. DC-SIGN
and its liver homolog L-SIGN have within their N-terminal domain a high affi nity
site for mannose-containing and fucosylated oligosaccharides. In contrast to selec-
tins, DC-SIGN also recognizes the non-sialylated form of Le
x
(CD15) and, for
instance, pseudo - Le
y
of
Schistosoma mansoni.
During uptake of pathogens by dendritic cells, which is a prerequisite for
proper major histocompatibility complex ( MHC ) - mediated presentation of patho-
gen-derived peptides to T cells, DC-SIGN contributes to vesicular antigen uptake.
The C3b receptor CR3 (CD11b, CD18) is a member of the integrin family and
is expressed on neutrophils, NK cells and minor subpopulations of T and B lym-
phocytes (see Chapter 29 for lectin functions of integrins). It functions as an
adhesion molecule and as receptor for ICAM-1. Additionally, it can also bind
β
-glucans of bacteria, thereby bringing the receptor into an activated state, which
permits neutrophil phagocytosis.
It was believed previously that T cells recognize exclusively peptides presented
by the MHC for their activation (see also Chapter 10.4). This dogma has now
changed and it is known that T cells are also able to recognize glycolipids via
their MHC-like CD1 molecules and consequently glycolipids (e.g.,
- galactosyl -
ceramide). These reactions may modify lymphocyte reactivities, and thereby the
outcome of infectious, autoimmune and allergic diseases, and possibly of tumors.
α
