Superantigens are presented by and activate thymocytes from infants

A high percentage of human fetal and postnatal thymocytes express MHC class II molecules. This raises the possibility that human thymocytes in early l ife are able to present peptides to other immature T cells and thereby init iate thymic selection of these cells. Here we address this question by expo sing newly harvested infant thymocytes to superantigen (Sag) which binds to the T-cell receptor and to MHC class II chains outside the peptide binding groove. The results show that the thymocytes are able to present Sag and t o be activated to proliferation as well as apoptosis by Sag presented by ot her thymocytes. The absence of responses to Sag with mutations in class II binding sites showed that class II molecules were necessary for the respons es, and very low expression of class II molecules on CD4-8- cells indicates that the demonstrated T-cell/T-cell interactions are confined to T-cell re ceptor-positive CD4+8+, CD4+8-, and CD4-8+ cells. These latter subsets were shown to be able to present Sag to each other. These findings suggest that class II+ thymocytes may participate in the selection of self-restricted T cells during a critical period in the shaping of the human immune system. Copyright (C) 1999 S. Karger AG, Basel.