In contrast, administration of belatacept led to higher frequencies of acute rejections. An underlying cause for these acute rejections might be CD8+CD28− T cells that escape inhibition by belatacept. In the present study we investigated the effect of MSC on CD8+CD28− T cells. We identified CD8+CD28− T cells as potentially harmful cells that express granzyme B, TNF-α click here and IFN-γ and are highly proliferative upon allogeneic stimulation. Expression of these cytolytic and proinflammatory molecules by CD8+CD28− T cells has been observed by others
[26-29]. However, data about the ability of CD8+CD28− T cells to proliferate are ambiguous. While some reports confirm our finding [30, 31], other research groups describe that the proliferative response of CD8+CD28− T cells is inhibited [32, 33]. Critical for CD8+CD28− T cell proliferation are the stimulation conditions. Plunkett et al. describe that anti-CD3 stimulation leads only to mild proliferation, while in the presence of irradiated PBMC CD8+CD28− T cells proliferate
strongly . Contrary to these results, we found that CD8+CD28− T cells stimulated with allogeneic PBMC had restrained proliferative abilities. Selleck RXDX-106 CD8+CD28− T cells proliferated as strongly as their counterparts in total PBMC only when CD4+ T cell help was provided. This indicates that certain cytokines or co-stimulatory signals other than CD28 ligands are required for the activation and proliferation of CD8+CD28− T cells. We determined that proliferating CD8+CD28− T cells expressed PD-L1 but lacked CTLA-4. Upon binding to the CD80/86 complex, both molecules transmit inhibitory signals [2, 35-37]. Control of cell proliferation through these inhibiting pathways can therefore be jeopardized by belatacept. However, next to its inhibitory function, PD-L1 has also been described to enhance T cell activation
and thereby might Thalidomide contribute to the proliferative capacities of CD8+CD28− T cells [38, 39]. CD8+CD28− T cells are found predominantly within the (terminally differentiated) effector memory CD8+ T cell subset  and they can have cytotoxic [29, 41-43] or immunosuppressive functions [10, 44-47]. Thus, inhibition of CD8+CD28− T cells by MSC could not only involve suppression of the cytotoxic subset, but also affect the regulatory subset. Our study shows, however, that MSC inhibited CD8+CD28− T cells that express the cytotoxic molecules granzyme B, TNF-α and IFN-γ. In contrast, CTLA-4, which is associated with a regulatory function, was hardly detectable on the CD8+CD28− T cells. Earlier studies by our group demonstrated that terminally differentiated CD8+ T cells contain a large proportion of CD28− cells, and these cells showed no immunosuppressive capacity in vitro .