Death of T cell precursors in the human thymus: a role for CD38

Thymic T cell maturation depends on interactions between thymocytes and cells of epithelial and hematopoietic lineages that control a selective process whereby developing T cells with inappropriate or self-reactive receptors die. Molecules involved in this process are the TCR expressed on thymocytes together with the CD3 complex and MHC-peptide on accessory cells. However, other molecules may favor or prevent death of thymocytes, thus playing a role in selection. CD38 is expressed by the majority of human thymocytes, mainly at the double-positive (DP) stage. In contrast, CD38 is not found on subcapsular double-negative (DN) thymocytes and on a proportion of medullary single-positive (SP) thymocytes. CD38 enhances death of thymocytes when it is cross-linked by goat anti-mouse (GAM) antiserum or by one of its ligands, CD31, expressed by thymic epithelial cells or transfected into murine fibroblasts (L cells). As most thymocytes are at an intermediate (DP) stage of development, it is likely that these cells are most vulnerable to death mediated via MHC-peptide-TCR interactions that is increased by CD38 cross-linking. DN and SP thymocytes are refractory to CD38-induced apoptosis. Accessory molecules, e.g. CD38, are expressed during thymic cell maturation and their presence is relevant for the survival or death of DP T cells in the course of selection. Based on our data, CD38 enhances thymocyte death by interacting with CD31 expressed by accessory cells. In addition, CD28 expression on developing thymocytes also appears to play a role for their selection and it synergizes with CD38 to induce apoptosis of DP thymocytes.     

Assessment of the potential malignancy of Epstein-Barr virus (EBV)-infected B cells in AIDS-related disorders

Spontaneous lymphoblastoid cell lines (LCLs) were established from the peripheral blood of ten human immunodeficiency virus (HIV)-seropositive patients in order to investigate whether or not a progression of the cells toward a malignant state could be traced. The LCLs studied displayed no differences in their surface phenotype, karyotype, or tumorigenicity in nude mice when compared with a wide panel of control LCLs. However, four of the ten LCLs derived from HIV-seropositive patients formed colonies in agar with a cloning efficiency (0.1 to 0.9%) that was much lower than that of a control neoplastic B cell line (50%). Some sublines that were derived form the agar colonies expressed new activation markers (CD10 and Bac-1) but did not produce tumors in nude mice or display chromosomal abnormalities. These sublines might comprise cells that have progressed toward a more transformed state.