Proteases of the malaria parasite Plasmodium falciparum have long been investigated as drug targets. The P. falciparum genome encodes ten aspartic proteases called plasmepsins, which are involved in diverse cellular processes. Most have been studied extensively but the functions of plasmepsins IX and X (PMIX and X) were unknown. Here, we show that PMIX is essential for erythrocyte invasion, acting on rhoptry secretory organelle biogenesis. In contrast, PMX is essential for both egress and invasion, controlling maturation of the subtilisin-like serine protease SUB1 in exoneme secretory vesicles. We have identified compounds with potent antimalarial activity targeting PMX, including a compound known to have oral efficacy in a mouse malaria model.

Plasmodium falciparum encodes a single calpain that has a distinct domain composition restricted to alveolates. To evaluate the potential of this protein as a drug target, we assessed its essentiality. Both gene disruption by double cross-over and gene truncation by single cross-over recombination failed. We were also unable to achieve allelic replacement by using a missense mutation at the catalytic cysteine codon, although we could obtain synonymous allelic replacement parasites. These results suggested that the calpain gene and its proteolytic activity are important for optimal parasite growth. To gain further insight into its biological role, we used the FKBP degradation domain system to generate a fusion protein whose stability in transfected parasites could be modulated by a small FKBP ligand, Shield1 (Shld1). We made a calpain-GFP-FKBP fusion through single cross-over integration at the endogenous calpain locus. Calpain levels were knocked down and parasite growth was greatly impaired in the absence of Shld1. Parasites were delayed in their ability to transition out of the ring stage and in their ability to progress to the S phase. Calpain is required for cell cycle progression in Plasmodium parasites and appears to be an attractive drug target. We have shown that regulated knockdowns are possible in P. falciparum and can be useful for evaluating essentiality and function.cell cycle ͉ cysteine protease ͉ essentiality ͉ inducible ͉ malaria

Nonobese diabetic (NOD) mice carrying a segment of chromosome flanking the disrupted IFN-γ receptor gene from original 129 ES cells are resistant to development of diabetes. However, extended backcrossing of this mouse line to the NOD mouse resulted in a segregation of the IFN-γR-deficient genotype from the diabetes-resistant phenotype. These results indicate that the protection of NOD mice from the development of diabetes is not directly linked to the defective IFN-γ receptor gene but, rather, is influenced by the presence of a diabetes-resistant gene(s) closely linked to the IFN-γR loci derived from the 129 mouse strain.

Mice bearing the I-A g7 class II major histocompatibility complex molecules contain a high number of spontaneous autoreactive T cells, as estimated by limitingdilution assays. We found this autoreactivity in various strains that bear the I-A g7 molecule, such as the nonobese diabetic (NOD) mouse strain, which spontaneously develops autoimmune diabetes. However, NOD mice strains that do not express the I-A g7 molecule, but instead express I-A b , do not have a high incidence of autoreactive T cells. About 15% of the autoreactive T cells also recognize the I-A g7 molecule expressed in the T2 line, which is defective in the processing of protein antigens. We interpret this to mean that some of the T cells may interact with class II molecules that are either devoid of peptides or contain a limited peptide content. We also find a high component of autoreactivity among antigenspecific T cell clones. These T cell clones proliferate specifically to protein antigens but also have a high level of reactivity to antigen-presenting cells not pulsed with antigen. Thus, the library of T cell receptors in NOD mice is skewed to autoreactivity, which we speculate is based on the weak peptidebinding properties of I-A g7 molecules.

The murine acquired immunodeficiency syndrome (MAIDS) is induced by a defective murine leukemia virus and has many symptoms similar to those found in patients infected with the human immunodeficiency virus. The presence of both B cells and CD4+ T cells is critical for the development of the disease. Furthermore, a Th2 cytokine response dominates during the progression of the disease. When interleukin-4 (IL-4)-deficient mice that are defective in Th2 cytokine responses were infected, there was no lethality, and the development of the T cell abnormalities associated with MAIDS was delayed. These data suggest that IL-4 or a Th2 response is involved in the development of retrovirus-induced immunodeficiency in mice.