These findings were in accordance with the previous experiments performed

with LMP2 and LMP7×MECL-1 gene-targeted mice. After adoptive transfer of these T cells, followed by an influenza virus infection of the recipient WT mice, neither LMP2−/− nor LMP7−/−×MECL-1−/− T cells were able to expand to the same extent as C57BL/6 WT cells 7, 10. As a possible explanation, the authors suggest rejection of donor T cells by the host immune response because of either reduced surface MHC expression by LMP7−/− T cells 11 or differences in minor histocompatibility Ag (miHAg). However, it was never thoroughly investigated whether the attenuation of immunoproteasome-deficient T cells in virus infected mice was indeed an artifact of the T-cell transfer experiment based on a host versus graft reaction or whether a so far unknown function of immunoproteasome subunits for T-cell survival or expansion could underlie this phenomenon. An independent hint that immunoproteasome Sirolimus order subunits

may play a so far unappreciated role for T-cell differentiation and/or expansion were the 20–30% reduced number of CD8+ as compared with CD4+ T cells in lymphoid organs of LMP2−/−12 and MECL-1−/−9 mice. Reconstitution experiments of irradiated WT mice with BM from WT and LMP7−/−MECL-1−/− mice showed that the lower CD8+/CD4+ ratio remained among the LMP7/MECL-1 double-deficient T cells although they were selected in the same thymus of recipient CSF-1R inhibitor mice as WT cells with a normal CD8+/CD4+ ratio. This result indicated that the selective reduction of CD8+ T cells lacking LMP7 and MECL-1 was a T-cell intrinsic phenomenon not related to altered Ag presentation in the thymus 13. In this study, we show that a functional requirement for immunoproteasome subunits rather than graft rejection accounts for the loss of LMP2−/−, MECL-1−/− fantofarone and LMP7−/− T cells in virus-infected mice and hence document a novel function of immunoproteasomes which is unrelated to their function in Ag processing. To investigate the proliferative performance of immunoproteasome-deficient T cells elicited

by an LCMV-WE infection in a WT environment, we adoptively transferred MECL-1−/−-, LMP2−/−-, LMP7−/−- or C57BL/6- T cells (all of them carrying the Thy1.2 marker) into LCMV-WE-infected Thy1.1 recipient mice. Eight days post-infection, C57BL/6-derived donor T cells proliferated to an extent of 2.55±0.03% of total lymphocytes, whereas mice that received LMP2−/− T cells comprised only 1.29±0.07% donor T cells. In mice having received MECL-1−/− T cells, we could hardly detect any donor cells on day 8 after infection (0.54±0.17% of total lymphocytes) and a similar loss of the graft was observed for mice which had received LMP7−/− T cells (0.18±0.03%) (Fig. 1A and B). To document the kinetics of donor T-cell expansion, we injected naïve MECL-1−/− or C57BL/6 control T cells into LCMV-WE-infected WT mice and analyzed the presence of donor T cells in blood on several days after transfer (Fig. 1C and D).