IntroductionHuman CD38 is the prototype of a family of proteins that share structural similarities and ectoenzymatic activities involved in the production of calcium-mobilizing compounds. [1][2][3] Aside from its ectoenzymatic activities and, apparently with independent modalities, CD38 may perform as a receptor, ruling adhesion and signaling in T 4 and B lymphocytes, 5 monocytes, 6 and natural killer (NK) cells. 7,8 The receptor functions of CD38 are regulated through interaction with a counterreceptor, identified as CD31. 9 The signaling events initiated by interactions between CD38 and CD31 (and fully mimicked by agonistic anti-CD38 monoclonal antibodies [mAbs]) were initially studied in the dynamic context of circulating CD38 ϩ T lymphocytes adhering to CD31 ϩ endothelial cells. 10 Use of this model allowed definition of some of the events that take place after the interaction and that include calcium (Ca ϩϩ ) mobilization from cytosolic stores, tyrosine phosphorylation of selected substrates, activation of nuclear factors, and secretion of cytokines. 11 It is generally agreed that CD38 controls a specific signaling pathway in T cells, B cells, NK cells, and monocytes. In spite of this evidence, the modalities through which the signal is initiated remain elusive. The molecule has neither the canonical structure of a receptor nor the hallmark domains. Indeed, the cytoplasmic tail is short and lacks docking sites and it is not tyrosine phosphorylated on activation. 12,13 Such negative characteristics are even more evident in CD157, the other member of the protein family, whose signaling features are known, notwithstanding a glycophosphatidylinositol linkage to the cell membrane. 14,15 Some clues can be extrapolated from cocapping experiments, which show that CD38 associates on the cell membrane with professional signaling receptors such as the T-cell receptor (TCR)-CD3 complex in T cells, the B-cell receptor (BCR) in B cells, and CD16 in NK cells. 16 A hypothesis to explain the signaling properties of CD38 is that the molecule exploits the signaling machinery of professional receptors to deliver its own independent signals. This idea was first supported by experiments using CD38 ϩ T-cell lines deficient in components of the signaling apparatus of the TCR-CD3 complex. 17,18 The inability of CD38 to signal in these cells was overcome by reconstituting a complete TCR-CD3 complex, thereby indicating that CD38 signaling depends on the presence of a functional TCR. These observations were recently expanded by studies using T lymphocytes purified from the intestinal The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked ''advertisement'' in accordance with 18 U.S.C. section 1734. For personal use only. on May 11, 2018. by guest www.bloodjournal.org From lamina propria as a model in which the TCR complex is physiologically impaired. 19 A comparative analysis of circulating versus residential T lymphocytes from the s...