Mice were injected with a monoclonal antibody to interferon-gamma to examine the importance of endogenous production of this lymphokine in resistance against infection with the sporozoan parasite Toxoplasma gondii. Mice with intraperitoneal infections of T. gondii that received no antibody survived and developed chronic T. gondii infection, whereas the infected mice that received the monoclonal antibody died of toxoplasmosis. The activation of macrophages, which kill T. gondii in vivo, was inhibited by administration of the monoclonal antibody, but the production of antibodies to T. gondii was not suppressed. The fact that an antibody to interferon-gamma can eliminate resistance to acute Toxoplasma infection in mice suggests that this lymphokine is an important mediator of host resistance to this parasite.

SummarySince there is a remarkable difference in susceptibility to peroral infection with Toxoplasma gondii among inbred strains of mice, we performed studies to examine the mechanism(s) of this difference in susceptibility. After peroral infection with the ME49 strain of T. gonclii, C57BL/6 (B6) mice all died whereas BALB/c mice all survived. At day 7 of infection (when B6 mice began dying), massive necrosis of the villi and mucosal cells in the ilea were observed in B6 but not in BALB/c mice. To analyze the role ofT cells in resistance against death and development of necrosis in the ilea after infection, studies were performed using athymic nude and euthymic control B6 and BALB/c mice. Athymic B6 mice all died after infection, but surprisingly, they survived significantly longer than control B6 mice, indicating that T cells predispose to early death in these mice. Necrosis in the ilea was observed in control B6 but not in athymic B6 mice; however, significantly less numbers oftachyzoites were observed in the ilea of the former than the latter mice. These results indicate that necrosis in the ilea of the B6 mice was not due to destruction of tissue by tachyzoites but was mediated by T cells. This deleterious effect of T cells appears to contribute to early death in these mice. In contrast, T cells conferred resistance against death in BALB/c mice but did not cause necrosis in their ilea. To analyze the T cell subset(s) that induces necrosis of the ilea in B6 mice, we examined histological changes of the small intestines after infection of mutant mice deficient in different T cell subsets (with the same H-2 b haplotype as B6 mice). Mice deficient in ot/[3 or CD4 + T cells did not develop necrosis in the ilea, whereas wild-type control mice and mice deficient in y/8 or CD8 + T cells did, suggesting that the cells that induce necrosis in the ilea after infection are CD4 + ot/[3 T cells 9 Since interferon (IFN)-~ has been shown to be critical for survival of BALB/c mice after infection with T. gondii, we examined the role of this cytokine in resistance/susceptibility of infected B6 mice 9 Treatment of B6 mice with anti-IFN-'y monoclonal antibody shortly before they developed illness prolonged time to death and prevented necrosis in the ilea in these mice. These results indicate that IFN-y mediates necrosis in the ilea orB6 mice after infection. This CD4 + T cell-dependent, IFN-y-mediated necrosis of the small intestines appears to be a mechanism that underlies the genetic susceptibility of B6 mice to peroral infection with T. gondii, whereas the same cytokine plays a critical role in the resistance of genetically resistant BALB/c mice 9

Toxoplasma gondii is a common human pathogen causing serious, even fatal, disease in the developing fetus and in immunocompromised patients. Despite its ability to reproduce sexually and its broad geographic and host range, Toxoplasma has a clonal population structure comprised principally of three lines. We have analyzed 15 polymorphic loci in the archetypal type I, II, and III strains and found that polymorphism was limited to, at most, two rather than three allelic classes and no polymorphism was detected between alleles in strains of a given type. Multilocus analysis of 10 nonarchetypal isolates likewise clustered the vast majority of alleles into the same two distinct ancestries. These data strongly suggest that the currently predominant genotypes exist as a pandemic outbreak from a genetic mixing of two discrete ancestral lines. To determine if such mixing could lead to the extreme virulence observed for some strains, we examined the F(1) progeny of a cross between a type II and III strain, both of which are relatively avirulent in mice. Among the progeny were recombinants that were at least 3 logs more virulent than either parent. Thus, sexual recombination, by combining polymorphisms in two distinct and competing clonal lines, can be a powerful force driving the natural evolution of virulence in this highly successful pathogen.

Activating transcription factor 3 (ATF3) is induced and functions both as a cellular response to stress and to stimulate proliferation in multiple tissues. However, in the nervous system ATF3 is expressed only in injured neurons. Here we reveal a function of ATF3 in neurons under death stress. Overexpression of ATF3 by adenovirus inhibits the mitogen-activated kinase kinase kinase 1 (MEKK1)-c-Jun N-Terminal Kinase (JNK)-induced apoptosis and induces neurite elongation via Akt activation in PC12 cells and superior nerve ganglion neurons. A DNA microarray study reveals that ATF3 expression and JNK activation induce expression of the heat shock protein 27 (Hsp27). Immunoprecipitation analysis and promoter assay for Hsp27 expression suggest that both ATF3 and c-Jun are necessary for transcriptional activation of Hsp27. Hsp27 expression significantly inhibits JNK-induced apoptosis as well as Akt activation in PC12 cells and superior cervical ganglion neurons. We conclude that the combination of ATF3 and c-Jun induces the anti-apoptotic factor Hsp27, which directly or indirectly activates Akt, and thereby possibly inhibits apoptosis and induces nerve elongation. Our results suggest that ATF3- and c-Jun-induced Hsp27 expression is a novel survival response in neurons under death stress such as nerve injury.