Departments of Paediatrics & Anatomy^*, The University of Hong Kong, Hong Kong

Telomerase, a ribonucleoprotein that is capable of synthesizing telomeric repeats, is responsible for telomere length maintenance. When telomeres are shortened to such a critical point that they no longer stabilize chromosome ends, most of the cells then exit from the cell cycle and die. Telomerase is expressed in germline and malignant cells and absent in most normal human somatic cells. Low levels of telomerase activity have been detected in peripheral blood mononuclear cells (PBMC) and hematopoietic cells recently. However, very little is known about their telomerase expression and regulation in cord blood cells. IGF-1 has been shown to block the development of apoptosis in hemopoietic cell lines and rescue cell death as well as expand a cell population. IGF-1 has also been reported to have positive effect on immune function. To address whether telomerase activation is involved in the immune regulation of IGF-1 in cord blood cells, we investigated the effect of IGF-1 on telomerase activities in cord blood mononuclear cells (CBMC) and the relationship with cytokine expression.

Sixteen normal full-term neonates were enrolled in this study. CBMC was cultured in a serum- and hormone-free medium. ³H-methylthymidine (TdR) incorporation was used to assay lymphocyte proliferative responses to PHA with or without IGF-1. Telomerase activities were measured by a PCR-based assay, telomeric repeat amplification protocol (TRAP). Semi- quantitation of interleukin(IL)-2, IL-4 and IL-6 mRNA expression was done by using comparative RT-PCR analysis.

Fresh uncultured and cultured CBMC showed low levels of telomerase activity. When stimulated with PHA, significant up-regulation of telomerase activity and proliferative response of CBMC were detected after 3 days of culture from 16 different infants. IGF-1 alone could not induce CBMC telomerase activity and proliferation, but it could increase the telomerase activities and proliferative responses in PHA-induced CBMC significantly. Comparing the time course of the proliferative response and telomerase activity in PHA-stimulated CBMC from two different infants, we found that the telomerase activity was maintained at a high level, but the proliferative decreased rapidly to a low level on the fourth day. IGF-1 alone could not induce IL-2 and IL-4 mRNA expression in CBMC but could induce IL-6 mRNA expression on its own. IGF-1 could not enhance IL-2 mRNA expression even in the presence of PHA, but it could suppress IL-4 mRNA expression of CBMC stimulated by PHA. IGF-1 could not increase IL-6 mRNA expression further significantly in PHA-stimulated CBMC.

CBMC expressed constitutive telomerase activity and the expression of telomerase in CBMC was activation-induced. The telomerase activity and proliferation of CBMC were not regulated in parallel. IGF-1 could up-regulate PHA-induced telomerase activities. IL-2, IL-4 and IL-6 might not be involved in the regulation of IGF-1 on telomerase activity in CBMC in a simple fashion. Telomerase activation might play an important role in IGF-1 induced antigen-specific T cell clonal survival and expansion, and the protective role of IGF-1 in cells from apoptosis might be mediated by the telomerase activation pathway.