Insulin-like growth factor I messenger RNA and protein are expressed in the human lymph node and distinctly confined to subtypes of macrophages, antigen-presenting cells, lymphocytes and endothelial cells.

Insulin-like growth factor I (IGF-I) is a potent hormone that stimulates growth and differentiation and inhibits apoptosis in numerous tissues. Preliminary evidence suggests that IGF-I exerts differentiating, mitogenic and restoring activities in the immune system but the sites of synthesis of local IGF-I are unknown. Identification of these sites would allow the functional role of local IGF-I to be clarified. The presence of IGF-I in non-immune cells suggests that it acts as a trophic factor, while its occurrence in subtypes of lymphocytes or antigen-presenting cells indicates paracrine/autocrine direct regulatory involvement of IGF-I in the human immune response. The present study investigated the location of IGF-I messenger RNA and protein on archival human lymph node samples by in situ hybridization, immunohistochemistry and double immunofluorescence staining using an IGF-I probe and antisera specific for human IGF-I and CD3 (T lymphocytes), CD20 (B lymphocytes), CD68 (macrophages), CD21 (follicular dendritic cells), S100 (interdigitating dendritic cells) and podoplanin (fibroblastic reticular cells). Numerous cells within the B- and T-cell compartments expressed the IGF-I gene, and the majority of these cells were identified as macrophages. Solitary follicular dendritic cells exhibited IGF-I. A few T lymphocytes, and no B lymphocytes, contained IGF-I immunoreactive material. Furthermore, IGF-I immunoreactive cells outside the follicles that did not react with CD3, CD20, S100 or podoplanin markers were identified as high-endothelial venule (HEV) cells. From this we conclude that the main task of IGF-I in human non-tumoral lymph node may be autocrine and paracrine regulation of the differentiation, stimulation and survival of lymphocytes, antigen-presenting cells and macrophages and the differentiation and maintenance of HEV cells.