5). In summary, we conclude that both, CD28 and CTLA-4 (at least through its regulation of CD28 at CT99021 the IS), are required for the different efficiencies of CD80 and CD86 costimulation. The increased Ca2+ signals observed after sc CD86/anti-CD33 costimulation compared with sc CD80/anti-CD33 costimulation can, in principle, be a result

of two general mechanisms: increased Ca2+ release or increased net Ca2+ influx. To test this, we separated Ca2+ release and Ca2+ influx. The Ca2+ release was not different when induced by dscFv anti-CD33/anti-CD3 in combination with sc CD86/anti-CD33 compared with dscFv anti-CD33/anti-CD3 in combination with sc CD80/anti-CD33 (Fig. 6a). The figure also shows that Ca2+ release between different donors was extremely homogeneous (Fig. 6b), which was also the case for the influx (data not shown). Both, costimulation with CD80 and CD86 emptied the Ca2+ stores equally well. To analyse Ca2+ influx independently of Ca2+ release, we compared find more Ca2+ influx after the full depletion of Ca2+ stores. The TG was used to fully deplete Ca2+ stores after the initial stimulation with the different bi-specific antibodies. Because Ca2+ release by costimulation does not occur simultaneously in the cells (in Fig. 6 all cells were aligned to the initiation of the Ca2+ release), only a slight but inhomogeneous Ca2+ signal during the release phase

could be observed. In cells with a clear Ca2+ release after costimulation, no further Ca2+ release by TG was detected indicating that TG-sensitive all stores were already fully depleted by the costimulation (Fig. 7). While the Ca2+ release was not influenced by costimulation, the Ca2+ influx was clearly different, as was evident after Ca2+ re-addition. The dscFv anti-CD33/anti-CD3 in combination with sc CD86/anti-CD33 induced a larger Ca2+ entry in comparison with dscFv anti-CD33/anti-CD3 in combination with sc CD80/anti-CD33. This indicates that costimulation increases Ca2+ influx independent of Ca2+ release. Export rates of Ca2+ were not

different for both costimulation methods (data not shown). We conclude that the different amplitudes of Ca2+ signals following dscFv anti-CD33/anti-CD3 in combination with sc CD86/anti-CD33 when compared with dscFv anti-CD33/anti-CD3 in combination with sc CD80/anti-CD33 can only be explained by differences in net Ca2+ entry but are independent of Ca2+ release. Soboloff et al.19 and Parvez et al.21 discovered that STIM2 can inhibit CRAC channel activity. In addition, Parvez et al. showed that STIM2 can also activate a store-independent mode of CRAC/ORAI channels. The store-independent mode of CRAC activation was also observed following the application of low concentrations of 2-aminoethyldiphenyl borate (2-APB) in STIM2/ORAI1 over-expressing HEK-293 cells and in ORAI3 over-expressing HEK-293 cells.