The pathogenesis of Venezuelan equine encephalitis virus (VEE) was examined in the mouse model using V3000, a virus derived from a molecular clone of the Trinidad donkey strain of VEE. These results were compared in parallel experiments with avirulent mutants of VEE derived by site-directed mutagenesis of the clone. Adult mice, inoculated subcutaneously in their left rear footpad with V3000, were followed in a time course study for 6 days in which 15 organs were tested for histopathological changes, for the presence of viral antigen by immunohistochemical staining, for the presence of viral nucleic acid by in situ hybridization analysis, and for content of viable virus. Virus was detected in the footpad inoculation site, but until 12 hr postinoculation (pi), the level of virus did not suggest early viral replication. By 4 hr pi, however, replication of V3000 was evident in the draining popliteal lymph node. At this early time point, no virus could be isolated from any other organ examined. At 12 hr, a significant serum viremia was observed, and virus was detected at a low level in a number of well vascularized organs, including spleen, heart, lung, liver, kidney, and adrenal gland. By 18 hr, high virus titers were present in serum and all the lymphoid organs examined, and these tissues appeared to be the major peripheral sites of V3000 replication. Virus in serum and peripheral organs was cleared by 3-4 days pi. In a second phase of the infection, V3000 invaded the central nervous system (CNS), replicated predominantly in neurons, and persisted in the brain until death by encephalitis. Pathologic findings as well as the results of immunocytochemical and in situ hybridization examination were generally coordinate with virus titration. A site-directed mutant of V3000, V3010, contained a mutation in the gene for the E2 glycoprotein at codon 76 (Glu to Lys) which rendered it avirulent after footpad inoculation. Detection of V3010 replication in the draining lymph node was sporadic and was sometimes delayed to as long as 3 days pi. Infrequent and/or delayed virus spread to other sites also was observed. Analogous experiments were performed with other mutants which were avirulent by the footpad inoculation route: V3014, a mutant differing from V3000 at three loci (E2 Lys 209, E1 Thr 272, and E2 Asn 239), as well as single-site mutants V3032 (E2 Lys 209) and V3034 (E1 Thr 272).(ABSTRACT TRUNCATED AT 400 WORDS)

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To investigate the role of type I interferon (IFN) and its regulatory transacting proteins, interferon regulatory factors (IRF-1 and IRF-2), in early protection against infection with virulent Venezuelan equine encephalitis virus (VEE), we utilized mice with targeted mutations in the IFN-alpha/beta receptor, IRF-1, or IRF-2 genes. IFN-alpha/beta-receptor knockout mice are highly susceptible to peripheral infection with virulent or attenuated VEE, resulting in their death within 24 and 48 h, respectively. Treatment of normal macrophages with anti-IFN-alpha/beta antibody prior to and during infection with molecularly cloned virulent VEE resulted in increased VEE replication. However, treatment with high doses of IFN or IFN-inducing agents failed to alter percentage mortality or average survival times in mice challenged with a low dose of virulent VEE. In IRF-1 and IRF-2 knockout mice (IRF-1(-/-) and IRF-2(-/-)), the 100% protection against virulent VEE that is conferred by attenuated VEE within 24 h in control C57BL/6 mice was completely absent in IRF-2(-/-) mice, whereas 50% of IRF-1(-/-) mice were protected. IRF-2(-/-) mice were deficient in clearing VEE virus from the spleen and the brain compared to the heterozygous IRF-2(+/-) knockout or C57BL/6 (+/+) mice. Furthermore, a distinct pattern of histopathological changes was observed in brains of IRF-2(-/-) mice after VEE exposure. Taken together, these findings imply that the altered immune response in IRF-1 and IRF-2 knockout mice results in altered virus dissemination, altered virus clearance, and altered virus-induced pathology. Thus, type I interferon, as well as IRF-1 and IRF-2, appears to play an important and necessary role in the pathogenesis of, and protection against, VEE infection.

We have studied the induction of neutralizing antibodies by in vivo expression of the human immunodeficiency virus type 1 (HIV-1) envelope by using a Venezuelan equine encephalitis virus (VEE) replicon system with mice and rabbits. The HIV-1 envelope, clone R2, has broad sensitivity to cross-reactive neutralization and was obtained from a donor with broadly cross-reactive, primary virus-neutralizing antibodies (donor of reference serum, HIV-1-neutralizing serum 2 [HNS2]). It was expressed as gp160, as secreted gp140, and as gp160⌬CT with the cytoplasmic tail deleted. gp140 was expressed in vitro at a high level and was predominantly uncleaved oligomer. gp160⌬CT was released by cells in the form of membrane-bound vesicles. gp160⌬CT induced stronger neutralizing responses than the other forms. Use of a helper plasmid for replicon particle packaging, in which the VEE envelope gene comprised a wild-type rather than a host range-adapted sequence, also enhanced immunogenicity. Neutralizing activity fractionated with immunoglobulin G. This activity was cross-reactive among a panel of five nonhomologous primary clade B strains and a Chinese clade C strain and minimally reactive against a Chinese clade E (circulating recombinant form 1) strain. The comparative neutralization of these strains by immune mouse sera was similar to the relative neutralizing effects of HNS2, and responses induced in rabbits were similar to those induced in mice. Together, these results demonstrate that neutralizing antibody responses can be induced in mice within 2 to 3 months that are similar in potency and cross-reactivity to those found in the chronically infected, long-term nonprogressive donor of HNS2. These findings support the expectation that induction of highly cross-reactive HIV-1 primary virusneutralizing activity by vaccination may be realized.

To investigate the roles of type I interferon (IFN-alpha/beta) and other mediators of innate immune responses (e.g., inducible nitric oxide synthase [iNOS]) in early dissemination of Venezuelan equine encephalitis virus (VEE) infection, we used mice with targeted deletions in either their IFN-alpha/beta-receptor (IFNAR-1-/-) or interferon regulatory factor 2 (IRF-2-/-) genes. Following footpad infection, both IFNAR-1-/- and IRF-2-/- mice were more susceptible than control mice to VEE. The IFNAR-1-/- mice also exhibit accelerated VEE dissemination to serum, spleen, and brain, and compared with control mice, they evidenced faster kinetics in the upregulation of proinflammatory genes. In contrast, in IRF-2-/- mice, iNOS gene induction was completely absent following peripheral virulent VEE infection. In evaluating the role of cells involved in iNOS production, primary microglial cell cultures were found to be highly permissive to VEE infection. Moreover, VEE infection increased levels of nitric oxide (NO) in resting microglial cultures but decreased NO production in IFN-gamma-stimulated microglia. Thus, these findings suggest that reactive nitrogen species play an important contributory role in VEE dissemination and survival of the host. Our results further suggest the necessity for a carefully balanced host response that follows a middle course between immunopathology and insufficient inflammatory response to VEE infection.