Differential replicative ability of clinical dengue virus isolates in an immunocompetent C57BL/6 mouse model

Distinct roles for type I and type III interferons in virulent human metapneumovirus pathogenesis

Human metapneumovirus (HMPV) is an important cause of acute lower respiratory infection in children and adults worldwide. There are four genetic subgroups of HMPV and both neutralizing antibodies and T cells contribute to protection. However, little is known about mechanisms of pathogenesis and most published work is based on a few extensively passaged, laboratory-adapted strains of HMPV. In this study, we isolated and characterized a panel of low passage HMPV clinical isolates representing all four genetic subgroups. The clinical isolates exhibited lower levels of in vitro replication compared to a lab-adapted strain. We compared disease phenotypes using a well-established mouse model. Several virulent isolates caused severe weight loss, lung pathology, airway dysfunction, and fatal disease in mice, which was confirmed in three inbred mouse strains. Disease severity did not correlate with lung viral titer, as virulent strains exhibited restricted replication in the lower airway. Virulent HMPV isolates were associated with markedly increased proinflammatory cytokine production and neutrophil influx; however, depletion of neutrophils or genetic ablation of inflammasome components did not reverse disease. Virulent clinical isolates induced markedly increased type I and type III interferon (IFN) secretion in vitro and in vivo. STAT1/2-deficient mice lacking both type I and type III IFN signaling showed reduced disease severity and increased lung viral replication. Inhibition of type I IFN signaling using a blocking antibody or genetic ablation of the type I IFN receptor reduced pathology with minimal effect on viral replication. Conversely, blockade of type III IFN signaling with a neutralizing antibody or genetic ablation of the IFN-lambda receptor had no effect on pathogenesis but restored viral replication. Collectively, these results demonstrate distinct roles for type I and type III IFN in HMPV pathogenesis and immunity.

A murine model of dengue virus infection in suckling C57BL/6 and BALB/c mice

Dengue is a significant public health concern across tropical and subtropical regions worldwide, principally causing disease in children. Very young children are at increased risk of severe manifestations of dengue infection. The mechanism of dengue disease in this population is not fully understood. In this study, we present a murine model of dengue virus primary infection in suckling C57BL/6 and BALB/c mice in order to investigate disease pathogenesis. Three-day-old C57BL/6 mice intraperitoneally infected with DENV-2 NGC were more susceptible to infection than BALB/c mice, showing increased liver enzymes, extended viremia, dissemination to organs and histological alterations in liver and small intestine. Furthermore, the immune response in DENV-infected C57BL/6 mice exhibited a marked Th1 bias compared to BALB/c mice. These findings highlight the possibility of establishing an immunocompetent mouse model of DENV-2 infection in suckling mice that reproduces certain signs of disease observed in humans and that could be used to further study age-related mechanisms of dengue pathogenesis.

CCR2 Plays a Protective Role in Rocio Virus-Induced Encephalitis by Promoting Macrophage Infiltration Into the Brain

Rocio virus (ROCV) is a highly neuropathogenic mosquito-transmitted flavivirus responsible for an unprecedented outbreak of human encephalitis during 1975–1976 in Sao Paulo State, Brazil. Previous studies have shown an increased number of inflammatory macrophages in the central nervous system (CNS) of ROCV-infected mice, implying a role for macrophages in the pathogenesis of ROCV. Here, we show that ROCV infection results in increased expression of CCL2 in the blood and in infiltration of macrophages into the brain. Moreover, we show, using CCR2 knockout mice, that CCR2 expression is essential for macrophage infiltration in the brain during ROCV infection and that the lack of CCR2 results in increased disease severity and mortality. Thus, our findings show the protective role of CCR2-mediated infiltration of macrophages in the brain during ROCV infection.

Differences in the replicative capacities of clinical isolates of dengue virus in C6/36 cells and in urban populations of Aedes aegypti from Colombia, South America

Dengue, the most prevalent arboviral disease worldwide, is caused by any of the four dengue virus (DENV) serotypes that co-circulate constantly in hyperendemic areas such as Medellin (Colombia), and these serotypes are transmitted by mosquitoes of the genus Aedes. In this study, we evaluated the replicative capacity of strains isolated in Medellin between 2003 and 2007 in C6/36 cells and in colonies of Aedes aegypti collected during 2010–2011 from high or low-incidence areas within the same city. The phylogenetic analysis grouped isolates according to the predominant genotypes found in the Americas, and the in vitro characterization showed differences in the morphological changes induced by the isolates of each of the isolated serotypes compared to the reference serotypes. In vitro replicative capacity studies demonstrated that genomic copy number increased at four days post-infection and that cell viability decreased significantly compared to the control for all serotypes. The largest number of genomic copies in C6/36 was produced by DENV-2, followed by DENV-1 and DENV-4; DENV-3 produced the smallest number of genomic copies and had the smallest negative effect on cell viability. Finally, differences in the in vivo replication of intercolonial serotypes between the Rockefeller colony and the field colonies and among the intracolonial serotypes were found. The replication of DENV-2 at 7 and 14 days in both high- and low-incidence colonies was higher than that of the other serotypes, and replication of DENV-3 in the mosquito colonies was the most stable on the days evaluated. Our results support the notion that replication and, possibly, DENV transmission and severity depend on many factors, including serotype and vector characteristics.