CD99, a unique integral membrane protein present on the surface of all human T cells, has previously been shown to regulate cell function and fate. In peripheral T cells, it triggers immediate activation of alpha4b1 integrin and cell arrest on inflamed vascular endothelium, whereas it mediates an apoptotic signal in double-positive thymocytes undergoing the selection process. Two isoforms of CD99 exist, a long form corresponding to the full-length protein and a short form harboring a deletion in the intracytoplasmic segment. Here, we show that while peripheral T cells display exclusive expression of the long form, double-positive thymocytes express both isoforms. Moreover, differential expression of these two CD99 molecules can lead to distinct functional outcomes. Expression of the long form in a CD99-deficient Jurkat T cell line is sufficient to promote CD99-induced cell adhesion, whereas coexpression of the two isoforms is required to trigger T-cell death. When coexpressed, the two proteins form covalent heterodimers, which locate within glycosphingolipidic rafts and induce sphingomyelin degradation. Cholesterol depletion experiments show that this localization is required for the induction of apoptosis. Thus, the surface expression pattern of CD99 isoforms determines T-cell functional outcomes.

The calcium release-activated channel (CRAC) opened in Jurkat cells activated either with CD3 monoclonal antibody or the endoplasmic reticulum Ca2(+)-ATPase blocker, thapsigargin, is blocked by La3+ with an IC50 of 20 nM. Similarly, the entry of Mn2+, used as a surrogate for Ca2+, is also blocked by submicromolar La3+ concentrations. La3+ seems to play its role simply by plugging the CRAC because this ion does not penetrate the cells, as demonstrated by chelation experiments with EGTA. Blocking the Ca2+ influx in activated Jurkat cells results in a lack of expression of CD25, a chain of the interleukin-2 receptor and of CD69, a marker of T-cell activation. By contrast, the very early steps of the T-cell signalling pathway such as the release of Ca2+ from intracellular stores and the subsequent inhibition of phosphatidylserine synthesis are not affected by La3+.

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