TECHNIQUES
In general, frozen as well as routinely formalin-fixed and paraffin-embedded tissues are suitable for immunohistochemical studies of gastrointestinal mucins. According to their composition of a peptide core and substituting carbohydrate antigens, monoclonal antibodies directed against epitopes formed by these components and also lectins, specifically recognizing carbohydrate binding sites, can be used for the visualization of the various antigens. Up to now, different immunological detection systems have been applied as staining procedures. In the beginning, fluorescein-coupled reagents were mainly employed, whereas at present enzyme-histochemical methods like the peroxidase-anti-peroxidase, biotin-streptavidin-alkaline-phosphatase or biotin-streptavidin-peroxidase technique are preferred.
APPLICATIONS TO THE PATHOLOGY OF THE
DIGESTIVE TRACT
Immunohistochemical methods allow the detection of antigenicity features in certain morphological structures. Therefore, normal as well as pathologically altered tissues can be investigated with respect to alterations of mucin-associated antigens. This point is of special importance regarding inflammatory diseases like gastritis (especially Helicobacter pylori infection), ulcerative colitis, Crohn's disease and others. The present review will be focussed on another aspect, giving a short summary of the various alterations of mucin-associated antigens which accompany the process of malignant transformation including our own results. Potential experimental and clinico-pathological applications are in particular considered.
ALTERATIONS OF THE EXPRESSION OF MUCIN-ASSOCIATED
ANTIGENS DURING CARCINOGENESIS
Generally, mucin-producing cells synthesize specific mucin peptide cores. For
this reason, characteristic patterns of these antigens (MUC1-MUC6) are observed
in the different segments of the gastrointestinal tract. Such typical patterns
of peptide synthesis can be changed during the process of malignant transformation.
For example, the production of MUC2, which is the major mucin of the normal
colon, is reduced in colorectal neoplasms, whereas the immunohistochemical detectability
of MUC1 is increased in comparison to normal tissue. Simultaneously, the activity
of glycosyltransferases catalysing the biosynthesis of oligosaccharide chains
in the different organs or organ segments is altered. Since one or more of such
enzymes may be deleted, reduced or augmented, a complex pattern of modifications
of the carbohydrate chains may be detected by immunohistochemical methods using
carbohydrate-specific monoclonal antibodies (or lectins):
1. Neoexpression of antigens (like ABO antigens in the distal colorectum). ABO blood group antigens incompatible with the patient's blood group may also be expressed.
2. Deletion of antigens (for example compatible ABO blood group antigens).
3. As a result of antigen deletion, cryptic carbohydrate core antigens like TF (Thomsen-Friedenreich) or Tn, which are normally masked by chain elongation or by steric interferences with neighbouring oligosaccharides, may be unmasked.
4. Increased expression of specific backbone structures. For example, the expression of type 2 backbones is raised in colorectal neoplasms. Additionally, type 2 backbone structures can be elongated (forming polylactosaminoglycans) and mono- or polyfucosylated.
5. Increased sialylation of various antigens. By sialylation, core antigens may be altered forming
sialosyl-Tn or sialosyl-TF, respectively. Such a sialylation acts as a stop signal preventing further
addition of carbohydrate residues.
On the other hand, sialosyl-Lewisa and sialosyl-Lewisx
antigens are examples of sialylated antigens of the peripheral region.
CLINICOPATHOLOGICAL RELEVANCE OF SUCH
ANTIGENIC ALTERATIONS
In experimental pathology, the expression of certain antigens is of interest regarding the histogenesis of neoplasms. For example, gastric carcinomas showing a tubular differentiation exhibit a stronger MUC2 expression in comparison to other types of gastric carcinoma. The same antigen is only detected in normal superficial epithelium and areas with intestinal metaplasia. A reappearance of so-called oncofetal patterns can also be observed. In this context, the expression of fucosylated type 2 and ABH antigens in the distal colorectum should be mentioned.
Some carbohydrate structures like mono- and polyfucosylated polylactosaminoglycans, sialylated Lewis x and sialosyl-Tn antigen are associated with an increase of dysplasia during the colorectal adenoma-carcinoma sequence, thus being a marker of malignant transformation.
The expression of certain antigens may have a prognostic impact. Sialosyl-Tn as well as Tn antigen have been repeatedly described to indicate a worse survival of patients with both gastric and colorectal carcinomas. Other antigens have to be evaluated. Hence, immunohistochemistry may be applied to distinguish "low-risk" and "high-risk" groups of patients with colorectal carcinomas.
The above-mentioned phenomenon may be caused by the role of carbohydrate residues in cell-cell-adhesion and, therefore, metastasis. In this context, especially sialosyl-Lewisx and related structures forming the binding counterpart of E-selectin (ELAM-l) and P-selectin (PADGEM) may be of interest as well as TF antigen, which can be bound by an asialo-glycoprotein receptor of hepatocytes and Kupffer cells. In clinico-pathological studies, whether the expression of these antigens displays a correlation to certain patterns of metastasis has to be analysed.
In recent years, oncological immunotherapeutical approaches were tested using distinct tumor-associated carbohydrate antigens as immunogens. If such studies are successful, the determination of tumor antigens couId gain a direct therapeutical impact in the future.