For example, affinity of B6H12 and 2D3 mAb for CD47 is higher when monocytes but not RBC, are incubated at 37C instead of 0C. In addition, CD47 displays a different conformation on sickle RBC when compared to normal RBC [18]. 2D3 binds with greater affinity than B6H12 mAb to CD47 on sickle [18] and aged erythrocytes sellectchem [15], which results in adhesion to TSP-1 under ?ow and static conditions. A loss of SIRP-��-Fc but not B6H12 and 2D3 binding, thus acquisition of CD47low status, can be artificially induced either by replacing the transmembrane region of CD47 by CD7, by cholesterol removal, or by a double cysteine mutation that disrupts the S-S disulfide bridge between the transmembrane and extracellular CD47 domains [19], [20]. Nonetheless, the precise molecular mechanism behind the physiologic conformational modification of CD47 remains to be elucidated.

Reduced or enhanced binding to SIRP-��-Fc might result from either CD47 association with other surface molecules, since CD47 was originally identified as an integrin-associated molecule [33], CD47 redistribution at the membrane [28], and/or a modified glycosylation pattern [22]. In the present study, we investigated the functional consequences provoked by the change in CD47 status on CD4 T cells. Upon activation, human CD4 T cells transiently displayed a CD47low status and become sensitive to CD47-mediated cell death by TSP-1. This may represent one mechanism involved in the contraction of the IR, as well as in the resolution of the inflammatory response.

We here showed that the absence of CD47 on murine Ag-specific T cells significantly impaired the contraction of the IR in vivo, demonstrating that the presence of CD47, and more particularly a CD47low status, was necessary for this process to occur. Furthermore, a transient change of CD47low status on CD4 T cells is required to mediate TSP-1-induced cell death in vitro in humans. IL-2 induced a re-expression of CD47high status on human TCR-activated CD4 T cells. T cells themselves represent a source of IL-2 and TSP-1 and CD3 stimulation leads to an increase in the availability of TSP-1 on the cell surface of recently activated T cells [34]. CD47 ligation inhibits early T cell activation, IL-2 production, and CD25 expression [35]. The latter is transiently expressed on activated CD4 T cells in vivo, and CD4+CD25?/? or IL-2?/? effector T cells survive very poorly and generate low numbers of memory T cells in non lymphopenic naive mice [36].

TSP-1 and SIRP-�� bind CD47 IgV loop [37] and TSP-1 can inhibit SIRP-��-Fc binding to CD47 expressing-Jurkat cells [38]. We therefore postulate that the reestablishment of a CD47high phenotype on TCM and re-encounter with SIRP-��+ myeloid cells (macrophages or DCs) might offer an advantage to avoid TSP-1-induced cell death whereas CD47low AV-951 status promotes TSP-1 binding that favors cell death and elimination.