The YMNM motif that exists in the CD28 cytoplasmic domain is known as a binding site for phosphatidylinositol 3-kinase and Grb-2 and is considered to be important for CD28-mediated costimulation. To address the role of the YMNM motif in CD28 cosignaling in primary T cells, we generated transgenic mice on a CD28 null background that express a CD28 mutant lacking binding ability to phosphatidylinositol 3-kinase and Grb-2. After anti-CD3 and anti-CD28 Ab stimulation in vitro, the initial proliferative response and IL-2 secretion in CD28 Y189F transgenic T cells were severely compromised, while later responses were intact. In contrast to anti-CD3 and anti-CD28 Ab stimulation, PMA and anti-CD28 Ab stimulation failed to induce IL-2 production from CD28 Y189F transgenic T cells at any time point. Using the graft-vs-host reaction system, we assessed the role of the YMNM motif for CD28-mediated costimulation in vivo and found that CD28 Y189F transgenic spleen cells failed to engraft and could not induce acute graft-vs-host reaction. Together, these results suggest that the membrane-proximal tyrosine of CD28 is required for costimulation in vivo. Furthermore, these results indicate that the results from in vitro assays of CD28-mediated costimulation may not always correlate with T cell activation in vivo.

No abstract

The MHC haplotype heterozygosity (H-2d/H-2z) acts as one major predisposing genetic element for autoimmune disease resembling systemic lupus erythematosus (SLE) in the F1 hybrid of NZB (H-2d) and NZW (H-2z) mice. To determine a possible role of mixed-haplotype A molecules, we introduced a transgene A beta z into H-2d/H-2d homozygous (NZB x NZW.H-2d)F1 mice, in which, compared with the original H-2d/H-2z heterozygotes, the incidence and titer of IgG anti-DNA antibodies, serum levels of nephritogenic retroviral gp70/anti-gp70 immune complexes (ICs) and the associated incidence of IC-type lupus nephritis were reduced. Evidence for the formation of mixed-haplotype A alpha d A beta z molecules in the transgenic mice was obtained by A beta z molecule expression on the cell surface of splenic B cells and macrophages, thymic epithelial cells and mature B cells in the bone marrow. A alpha d A beta z-restricted T cell clones showed good proliferative responses to spleen cells from the transgenic mice, to an extent much larger than seen in cells from the original (NZB x NZW)F1 mice, suggesting that the expression of functional A alpha d A beta z molecules on cells in transgenic mice is considerably high. Compared with findings in transgene-negative littermates and the original (NZB x NZW)F1 mice, the A beta z transgene to a greater extent promoted serum levels of IgM anti-double-stranded (ds) DNA antibodies and IgM anti-gp70/gp70 ICs in the transgenic mice. None the less, the production of IgG anti-dsDNA antibodies and IgG anti-gp70/gp70 ICs as well as the development of renal disease was markedly suppressed. Possible mechanisms of such effects of the transgene are discussed.

Conflicting findings regarding proadhesion and antiadhesion in cell-to-cell interactions were previously reported for CD43. We examined possible differences in the role of the 130-kd glycoform and the 115-kd glycoform of CD43 in cellular adhesion in vitro. We generated a monoclonal antibody (MFT3) that discriminates between helper and nonhelper murine T-cell clones. Characterization of MFT3 with use of biochemical analysis and complementary DNA (cDNA) transfection experiments showed that it is specific for the 130-kd glycoform of CD43. T-cell clones that expressed the 130-kd CD43 glycoform showed decreased homocytic aggregation and decreased adhesion to spleen cells, B-lymphoma cell lines, and fibroblastic cell lines compared with T-cell clones negative for the 130-kd glycoform. Expression of core 2 β-1, 6-N-acetylglucosaminyltransferase (C2GnT) cDNA together with CD43 cDNA resulted in expression of both the 130-kd CD43 glycoform and the 115-kd CD43 glycoform in fibroblastic cell lines. Using these cell lines, we showed that the 130-kd glycoform but not the 115-kd glycoform of CD43 has an antiadhesive function in cellular interactions. Our findings suggest that the antiadhesive function of CD43 is primarily carried out by the 130-kd glycoform.