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General cellular functions of proteasomes occur through protein degradation, whereas the specific function of immunoproteasomes is the optimization of antigen processing associated with MHC class I. We and others previously reported that deficiency in subunits of immunoproteasomes impaired the activation of antigen-specific CD8+ T cells, resulting in higher susceptibility to tumor and infections. We demonstrated that CD8+ T cells contributed to protection against malaria parasites. In this study, we evaluated the role of immunoproteasomes in the course of infection with rodent malaria parasites. Unexpectedly, Plasmodium yoelii infection of mice deficient in LMP7, a catalytic subunit of immunoproteasomes, showed lower parasite growth in the early phase of infection and lower lethality compared with control mice. The protective characteristics of LMP7-deficient mice were not associated with enhanced immune responses, as the mutant mice showed comparable or diminished activation of innate and acquired immunity. The remarkable difference was observed in erythrocytes instead of immune responses. Parasitized red blood cells (pRBCs) purified from LMP7-deficient mice were more susceptible to phagocytosis by macrophages compared with those from wild-type mice. The susceptibility of pRBC to phagocytosis appeared to correlate with deformity of the membrane structures that were only observed after infection. Our results suggest that RBCs of LMP7-deficient mice were more likely to deform in response to infection with malaria parasites, presumably resulting in higher susceptibility to phagocytosis and in the partial resistance to malaria. [ABSTRACT FROM AUTHOR]

Antitumour immunity against murine melanoma B16 was achieved by genetic immunization with a naked chimeric DNA encoding a fusion protein linking green fluorescent protein (GFP) to the N-terminus of a major CD8+ cytotoxic T lymphocyte (CTL) epitope of tyrosinase-related protein 2 (TRP-2181−188) of murine melanoma, designated as pGFP-TRP-2. Tumour growth was profoundly suppressed in C57BL/6 mice immunized with pGFP-TRP-2, while mice vaccinated with pTRP-2 showed rapid tumour growth and died within 40 days after tumour challenge. Splenocytes of mice immunized with pGFP-TRP-2 showed high CTL activity specific for TRP-2181−188. GFP-TRP-2 expressed in COS-7 cells was rapidly degradated in vitro and the degradation was almost completely prevented by adding a proteasome inhibitor, MG-132, in the culture. Furthermore, the antimelanoma immunity induced by genetic immunization with pGFP-TRP-2 was completely cancelled in mice deficient in proteasome activator PA28α/β. Taken together, GFP-TRP-2 processed by cytosolic proteasome played a central role in breaking peripheral tolerance to a melanoma/melanocyte antigen, TRP-2181−188, by activating CD8+ CTL specific for TRP-2181−188. TRP-2181−188 fused to GFP may be readily cut off from GFP by the ubiquitin-fusion degradation (UFD) pathway and efficiently presented to major histocompatibility complex class I molecules, resulting in effective induction of CD8+ T cells specific for the CTL epitope. Furthermore, CD4+ T cells specific for GFP were shown to play a crucial role in the antimelanoma immunity, probably potentiating activity of TRP-2-specific CTL and/or the ‘ubiquitin-proteasome pathway’. It is noteworthy to document that genetic immunization with pGFP plus pTRP-2181−188 failed to exert the antitumour immunity. [ABSTRACT FROM AUTHOR]

Infection with malaria parasites frequently induces total immune suppression, which makes it difficult for the host to maintain long-lasting immunity. Here we show that depletion of CD4+CD25+ regulatory T cells (Treg) protects mice from death when infected with a lethal strain of Plasmodium yoelii, and that this protection is associated with an increased T-cell responsiveness against parasite-derived antigens. These results suggest that activation of Treg cells contributes to immune suppression during malaria infection, and helps malaria parasites to escape from host immune responses. [ABSTRACT FROM AUTHOR]

Background: Autologous whole tumor cell-based vaccinations would seem to be ideal since such vaccinations, in contrast to vaccination with a single defined antigen, have the potential to elicit a broad type of T-cell immune response to tumor-associated antigens. Objective: We modified formaldehyde (formalin)-fixed mouse melanoma cells and investigated the utility of those cells as sources of tumor antigens for immunotherapy. Methods: C57BL/6 or the proteasome activator PA28α-knockout mice were intradermally inoculated with 1% formalin-fixed B16 cells three times at weekly intervals either before or after tumor challenge. Simultaneously, interleukin-12 gene was transferred into the skin around immunization sites using gene gun technology. The effects were evaluated by tumor growth, antigen-specific interferon-γ production in splenic lymphocytes, and activation of dendritic cells. Results: Fixed cells directly induced production of tumor necrosis factor-α in dendritic cells more effectively than did frozen and thawed cells. More than 60% of the mice immunized with fixed cells and interleukin-12 rejected the challenged B16 tumor. CD4+ T cells from those mice produced a significant amount of interferon-γ in response to melanoma cells. Furthermore, this combined treatment showed antitumor immunity initiated by CD8+ and CD4+ T cells in the therapeutic experiments. PA28α/β appeared not to be required for the development of CD8+ T cells, although it is known to be essential for the development of CD8+ T cells specific for tyrosinase-related protein-2, one of melanocyte-lineage differentiated antigens. Conclusion: These results suggest that formalin-fixed autologous melanoma cells have a potential to function as effective antigen sources for immunotherapy. [Copyright &y& Elsevier]