Jeddi, Parvaneh Alipour; (1996) Structure and expression of the beta1, 4-galactosyltransferase gene in relation to IgG glycosylation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Rheumatoid arthritis (RA) is associated with an increase in the level of serum IgG glycoforms lacking terminal galactose residues (i.e., agalactosyl IgG). The agalactosyl IgG shows altered effector functions and there is evidence that it may be pathogenic. Furthermore, levels of agalactosyl IgG have been shown to have a predictive value in RA. There is evidence that the decreased galactosylation of IgG occurs as a pre-secretory event and there are several reports relating this defect to aberrant control of the enzyme β1,4-galactosyltransferase (β1,4-GalTase). This project aimed to examine the structure and expression of the β1,4-GalTase gene in human RA and also in a murine model of arthritis, MRL/Mp-lpr/lpr (MRL lpr/lpr), which shows the same defect in IgG galactosylation. No gross structural alteration of the gene was observed in human RA nor in the MRL lpr/lpr mice, using restriction fragment length polymorphism analysis. An RNase protection assay established that there are similar levels of β1,4-GalTase gene expression in CD19+ cells isolated from peripheral blood of RA patients and normal healthy individuals. IgG-expressing lymphocytes isolated from spleens and lymph nodes of MRL lpr/lpr and CBA/Ca (which exhibit normally galactosylated IgG) mice also showed comparable levels of β1,4-GalTase mRNA. The known pregnancy associated increase in IgG galactosylation was examined in the Balb/c mice. Although the β1,4-GalTase transcription was highly upregulated in the mammary gland in the third trimester of pregnancy and into lactation, no changes in the mRNA and enzyme levels were observed in the lymphocytes isolated from spleens of these mice. The cytokines IL-6 and TNF-? are proposed as glycosylation regulating factors. In addition, IL-6 has been shown to be associated with increased agalactosyl IgG. Therefore, the level of β1,4-GalTase gene expression was measured in IL-6 and TNF-α transgenic mice in relation to the IgG galactosylation level. In these studies, comparable levels of β1,4-GalTase mRNA were observed in the transgenics and their littermates in both cases. Peripheral blood lymphocytes stimulated in vitro with the mitogens PHA, phorbol ester and pokeweed, with the cytokines IL-6 and TNF-α, with the calcium ionophore ionomycin and with the cAMP-inducer forskolin, did not show altered levels of β1,4-GalTase mRNA. However, the addition of prolactin to peripheral blood B cells cultured in the presence of anti-IgM plus IL-2 resulted in a small increase in mRNA levels but with no concomitant increase in IgG galactose. In conclusion, these studies indicate that IgG galactosylation is not regulated at the level of β1,4-GalTase gene expression.

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