Studies of tyrosine phosphorylation in the human duodenum have indicated that proliferating cells in the middle portion of the duodenal crypt were devoid of this feature, suggesting that tyrosine kinase activation is not a dominant factor in crypt cell
proliferation, and that consequently tyrosine phosphatase activity may be a more critical factor in crypt cell development. We investigated the expression of the leukocyte common
antigen-related receptor (LAR) family of tyrosine phosphatases. A flow cytometry system was used to examine cells from the surface, mid-portion, and lower part of the crypt. Individual cell populations were immunostained with anti-LAR antibodies using
phycoerythrin-conjugated anti-CD3 to discriminate between epithelial cells (CD3−) and intraepithelial lymphocytes (CD3+). Epithelial cells expressed LAR throughout the crypt. Expression of LAR was maximal in the mid-portion of the crypt with lower expression at the top of the
villi. Intraepithelial lymphocytes expressed low levels of LAR at the tips of the villi with stronger expression extending towards the base of the crypt. These findings were confirmed by immunohistochemistry on paraffin-fixed sections. Of note, peripheral blood lymphocytes expressed less LAR than IEL. These observations suggest the possibility that tyrosine phosphatase LAR may be of importance in the regulation of crypt cell
proliferation. Moreover, as the extracellular domain of LAR has homology with adhesion molecules, the finding of this molecule on IEL could suggest a possible functional role in homing of this unique lymphocyte.
