Quantitative and functional expression of somatostatin receptor subtypes in human thymocytes

We recently demonstrated the expression of somatostatin (SS) and SS receptor (SSR) subtype 1 (sst(1)), sst(2A), and sst(3) in normal human thymic tissue and of sst(1) and sst(2A) on isolated thymic epithelial cells (TEC). We also found an inhibitory effect of SS and octreotide on TEC proliferation. In the present study, we further investigated the presence and function of SSR in freshly purified human thymocytes at various stages of development. Thymocytes represent a heterogeneous population of lymphoid cells displaying different levels of maturation and characterized by specific cell surface markers. In this study, we first demonstrated specific high-affinity I-125-Tyr(11)-labeled SS-14 binding on thymocyte membrane homogenates. Subsequently, by RTPCR, sst(2A) and sst(3) mRNA expression was detected in the whole thymocyte population. After separation of thymocytes into subpopulations, we found by quantitative RT-PCR that sst(2A) and sst(3) are differentially expressed in intermediate/mature and immature thymocytes. The expression of sst(3) mRNA was higher in the intermediate/ mature CD3(+) fraction compared with the immature CD2(+)CD3(-) one, whereas sst(2A) mRNA was less abundant in the intermediate/ mature CD3(+) thymocytes. In 7-day-cultured thymocytes, SSR subtype mRNA expression was lost. SS-14 significantly inhibited [H-3] thymidine incorporation in all thymocyte cultures, indicating the presence of functional receptors. Conversely, octreotide significantly inhibited [H-3] thymidine incorporation only in the cultures of immature CD2(+)CD3(-) thymocytes. Subtype sst(3) is expressed mainly on the intermediate/ mature thymocyte fraction, and most of these cells generally die by apoptosis. Because SS-14, but not octreotide, induced a significant increase in the percentage of apoptotic thymocytes, it might be that sst(3) is involved in this process. Moreover, sst(3) has recently been demonstrated on peripheral human T lymphocytes, which derive directly from mature thymocytes, and SS analogs may induce apoptosis in these cells. Interestingly, CD14(+) thymic cells, which are cells belonging to the monocyte-macrophage lineage, selectively expressed sst(2A) mRNA. Finally, SSR expression in human thymocytes seems to follow a developmental pathway. The heterogeneous expression of SSR within the human thymus on specific cell subsets and the endogenous production of SS as well as SS-like peptides emphasize their role in the bidirectional interactions between the main cell components of the thymus involved in intrathymic T cell maturation.