No abstract

We have generated mice deficient in the expression of the lymphocyte cell surface antigen CD48 (Blast-1, BCM1, sgp-60) by gene targeting in embryonic stem cells. Mice homozygous for the CD48 mutation (CD48 ؊͞؊ mice) are severely impaired in CD4 ؉ T cell activation. Proliferative responses to mitogens, anti-CD3 mAb, and alloantigen are all reduced. Experiments in which T cells and antigen-presenting cells from either wild-type or CD48 ؊͞؊ mice were cocultured reveal that CD48 is important on both T cells and antigen-presenting cells. The most dramatic impairment was observed in experiments in which highly purified T cells were stimulated through the T cell receptor in the presence of the phorbol ester, phorbol 12-myristate 13-acetate. The results of these experiments raise the possibility that CD48 plays a role in signaling through the T cell receptor.Besides the T cell receptor, other cell surface molecules participate in the interaction of T cells with antigen-presenting cells (APCs). One of these molecules is CD2, a 55-to 60-kDa protein expressed on the surface of T cells and natural killer cells; in mice, CD2 also is found on B cells (1). There are two known ligands for CD2. In mice (2) and rats (3), CD2 interacts with CD48 (Blast-1, BCM1, sgp-60). CD48 is a glycosylphosphatidylinositol-anchored, 45-to 50-kDa molecule whose expression is tightly regulated and restricted to lymphocytes, macrophages, and dendritic cells (2,4,5). CD48 is expressed in humans; however, human CD48 does not bind CD2 with high affinity. CD58 (LFA-3), a broadly expressed protein of 55-to 70-kDa, is the major ligand for CD2 in humans (6-8). CD58 is structurally and phylogenetically related to CD48 (9). It is currently unknown whether there is a murine homologue of CD58 (2).Although the dual capacity of CD2, to promote T cell-APC adhesion and to transduce T cell activation signals, is well established (10, 11), the physiologic functions of CD2 and of its ligands have remained uncertain. Mice carrying a targeted mutation in the Cd2 gene are phenotypically almost comparable to wild-type mice (12, 13). However, numerous studies using blocking mAbs suggest that CD2 and its ligands participate in immune responses. Antibodies against murine CD2 can be potent suppressants of cell-mediated immunity and can prolong allograft and xenograft survival (14-16). These mAbs also inhibit T cell-dependent B cell activation, indicating that CD2 and its ligands may play a role in humoral immunity (17). Anti-CD48 mAbs also can inhibit immune responses. They can inhibit the proliferation of T cells in vitro (4) and can suppress cell-mediated immunity in vivo, blocking hapten-induced contact sensitivity and the generation of cytotoxic T lymphocytes (18). Anti-CD48 mAbs also can prolong allograft survival (19). To determine the function of CD48 more definitively, we have generated, by gene targeting, mice deficient in CD48 expression (CD48 Ϫ͞Ϫ mice). The results of our experiments demonstrate that CD4 ϩ T cells from CD48 Ϫ͞Ϫ mice are defective in activ...