Whether dendritic cells (DC) play a passive or active role in Th2 response induction is poorly understood. In this study, we show that CD8− DC pulsed with Th2-polarizing Ag (soluble egg Ag (SEA)) from Schistosoma mansoni potently stimulate Th2 responses in vivo and in vitro while failing to undergo a conventional maturation process. Thus, in contrast to DC pulsed with the Th1 response inducing Ag Propionebacterium acnes, SEA-exposed DC exhibit a phenotype that is most similar to that of immature DC, failing to up-regulate expression of CD40, CD54, CD80, CD86, or OX40L; producing no detectable IL-4, IL-10, or IL-12; and displaying only a minor increase in MHC class II expression. Importantly, in vitro derived DC exposed to SEA were phenotypically similar to CD8− DC isolated from active S. mansoni infection. By discriminating between different types of pathogen and responding appropriately, CD8− DC play a major role in the decision process to mount either a Th1 or Th2 response.

The role of CD40/CD154 interaction during infection has primarily focused on pathogens that drive inflammatory Th1 responses. In this study, we show that CD40/CD154 interaction is a fundamental requirement for Th2 response development to the parasitic helminth Schistosoma mansoni. Compared with infected wild-type mice, greatly reduced levels of Th2-associated cytokines were measured both in vitro and in vivo, and no IgE or IgG1 was detected in infected CD154−/− mice. In the absence of an overt Th2 response, no exaggerated Th1 response was mounted by CD154−/− mice. Infected CD154−/− mice suffered severe morbidity and mortality, even though parasitemias in wild-type and CD154−/− mice did not differ significantly. These data indicate that CD40/CD154 interaction is required to allow development of a Th2-dominated immune response to S. mansoni and support the view that failure to develop such a response can have fatal consequences.

Coronaviruses are the causative agents of respiratory disease in humans and animals, including severe acute respiratory syndrome. Fusion of coronaviruses is generally thought to occur at neutral pH, although there is also evidence for a role of acidic endosomes during entry of a variety of coronaviruses. Therefore, the molecular basis of coronavirus fusion during entry into host cells remains incompletely defined. Here, we examined coronavirus-cell fusion and entry employing the avian coronavirus infectious bronchitis virus (IBV). Virus entry into cells was inhibited by acidotropic bases and by other inhibitors of pH-dependent endocytosis. We carried out fluorescence-dequenching fusion assays of R18-labeled virions and show that for IBV, coronaviruscell fusion occurs in a low-pH-dependent manner, with a half-maximal rate of fusion occurring at pH 5.5. Fusion was reduced, but still occurred, at lower temperatures (20°C). We observed no effect of inhibitors of endosomal proteases on the fusion event. These data are the first direct measure of virus-cell fusion for any coronavirus and demonstrate that the coronavirus IBV employs a direct, low-pH-dependent virus-cell fusion activation reaction. We further show that IBV was not inactivated, and fusion was unaffected, by prior exposure to pH 5.0 buffer. Virions also showed evidence of reversible conformational changes in their surface proteins, indicating that aspects of the fusion reaction may be reversible in nature.For all enveloped viruses, a critical event during entry into cells is the fusion of the viral envelope with the membrane of the host cell (13). Our current understanding of viral fusion has been driven by fundamental problems first solved with influenza hemagglutinin (HA) (50). Whereas the trigger for HA-mediated fusion is the low pH of the endosome, other viruses (e.g., paramyxoviruses and most retroviruses) undergo a receptor-primed fusion with the plasma membrane at neutral pH (13).Coronaviruses (CoV) have recently received much attention due to the outbreak of severe acute respiratory syndrome (SARS) (22, 28), but there is little consensus as to whether coronavirus entry and fusion occur following endocytosis or at the plasma membrane (6,16,21,43). Coronaviruses are enveloped positive-strand RNA viruses that replicate in the cytoplasm (28). They have a distinctive set of club-shaped spikes on their envelope, and the spike protein (S) is the primary determinant of cell tropism and pathogenesis, being responsible (and apparently sufficient) for receptor binding and fusion (16). However, other envelope proteins are present: the M protein, the E protein, and (in some coronaviruses) an HE protein (28). The coronavirus S protein is categorized as a class I fusion protein, based on the presence of characteristic heptad repeats (3, 9, 26); as such, it shows features of the fusion proteins of influenza virus (HA), retroviruses (Env), and paramyxoviruses (F and HN), for which there is extensive characterization at the structural and biophysical levels (11).Al...

Liver enlargement and hepatocyte proliferation, normal responses in wild-type (WT) mice infected with the parasitic helminth Schistosoma mansoni, were found to be severely impaired in infected IL-4−/− mice. Compared with WT mice, increased levels of O2−, NO, and the more highly reactive ONOO− were detected in the liver and produced by lesional cells isolated from liver granulomas of infected IL-4−/− mice. Concurrently, antioxidant defenses in the liver, specifically catalase levels, diminished dramatically during the course of infection in these animals. This contrasted to the situation in infected WT mice, where catalase levels remained as high as those in normal mice. Actual levels of reactive oxygen and nitrogen intermediates in the livers of infected IL-4−/− animals are thus likely to be considerably higher than those in the livers of infected WT mice. To determine whether these changes contributed to the development of the more severe disease that characterizes infection in the IL-4−/− animals, we treated infected IL-4−/− mice with uric acid, a potent scavenger of ONOO−. This resulted in significantly increased hepatocyte proliferation, decreased morbidity, and prolonged survival. Taken together, these data indicate that IL-4 is playing a protective role during schistosomiasis by controlling the tight regulation of the generation of reactive oxygen and nitrogen intermediates in the liver.