Tuesday, October 8, 2013

NIAID-DVI: Pediatric Dengue Cohort Study -- Eva Harris

Neutralizing antibody responses in the Nicaraguan Pediatric Dengue Cohort Study
Eva Harris
Division of Infectious Diseases & Vaccinology
SPH, UC Berkley

So I've always enjoyed talks by Eva Harris...she's always very animated and excited about her work and she almost always has an overwhelming amount of information to show--scientific sensory overload at times...you wish you could just have copies of the slides or write faster to catch everything she says. She has a very active group, active collaborations, so she usually has  a lot to say in a short time. But y'know when you've been involved in work in a country for 23+ years...you're going to have a lot to say. 

I first heard Eva talk at ASTMH about Molly OhAinle's work which came out in 2011 in Science and Translational Medicine...

During two clinical studies of dengue in Managua, Nicaragua, they observed an "abrupt" increase in disease severity across several epidemic seasons of dengue virus serotype 2 (DENV-2) transmission. They found that waning DENV-1 immunity appeared to increase the risk of severe disease in subsequent DENV-2 infections after a period of cross-protection. This increase in severity coincided with replacement of the Asian/American DENV-2 NI-1 clade with a new virus clade, NI-2B. When they examined viral isolates from each of the clades they found a 'fitter' virus which arose during the later season. They also found that the NI-1 clade was more virulent specifically in children who were immune to DENV-1, whereas DENV-3 immunity was associated with more severe disease among NI-2B clade infections (see figure 4 below for clades). 

Fig. 1 Increase in severity of DENV-2 infections increases in two independent studies of pediatric dengue in Nicaragua. (A and B) Proportion of DENV-2 cases, including both primary and secondary infections, classified as DHF/DSS in the early compared to later seasons in the Hospital (P = 0.001, Fisher’s exact test) (A) and Cohort (P = 0.05, Fisher’s exact test) (B) studies. The total number (n) of DENV-2 infections included in the analysis is shown above each graph. (C and D) Proportion of each serotype across four epidemic seasons (2005/6 to 2008/9) in the Nicaraguan Hospital study (C) and five epidemic seasons (2004/5 to 2008/9) in the Nicaraguan Pediatric Dengue Cohort study (D). Pink, DENV-1; green, DENV-2; blue, DENV-3; black, DENV-4. (E) Proportion of all DENV-2 cases classified as primary (light gray bars) or secondary (dark gray bars) DENV infections in the Hospital and Cohort studies, as defined in Materials and Methods

Fig. 4 (edited, see paper for full legend) Association of Nicaraguan DENV-2 clade replacement with increased viral fitness. (A) Maximum likelihood tree of 159 complete coding region nucleotide sequences sampled from the Cohort and Hospital studies, as well as other samples collected in Nicaragua between 1999 and 2008. Twenty-five sequences representing the major genotypes of DENV-2 (American, Asian 1, and Asian/American) were used as outgroup sequences because they were genetically distinct from the Nicaraguan sequences. Only nodes (circles) that are supported at least 60% of the time by bootstrap resampling (1000 replications) of the data are shown. Major Nicaraguan clades are highlighted (red, NI; purple, NI-1; green, NI-2A; blue, NI-2B). (B and C) Changes over time in the proportion of DENV-2 cases caused by infection with major Nicaraguan clades that occurred during the specified epidemic seasons for virus isolates that were either sequenced or genotyped (B) in the Cohort study (2004/5 to 2007/8; n = 97) or (C) in the Hospital study (2005/6 to 2008/9; n = 138). (D and E) Relative fitness (ω) as determined by dual-infection assays of NI-1 isolates in competition with each NI-2B isolate as assayed in (D) mosquito cells (C6/36) and (E) human monocyte–derived iDCs. Box plots show the 25th and 75th percentiles, with a line indicating the median relative fitness value and error bars showing the minimum and maximum fitness values. Five NI-1 viruses were competed against five NI-2B viruses. Viral strain numbers used in each assay are indicated at the bottom of the graph.

In conclusion the data demonstrates 
"...that the complex interaction between viral genetics and population dynamics of serotype-specific immunity contributes to the risk of severe dengue disease." 
The paper is a good read, the work came out to be a really nice piece.

For this year's DVI meeting Eva continues on with Nicaraguan studies, Magelda Montoya's work which came out in PLoS NTD this year: Montoya M, Gresh L, Mercado JC, Williams KL, Vargas MJ, et al. (2013) Symptomatic Versus Inapparent Outcome in Repeat Dengue Virus Infections Is Influenced by the Time Interval between Infections and Study Year. PLoS Negl Trop Dis 7(8): e2357.

In this work:
  • They analyzed over 2,000 DENV infections from 2004-2011 in a prospective pediatric cohort study in Managua, Nicaragua.
  • Symptomatic and healthy subjects were paired and were examined annually to pick out inapparent infections.
  • They used two different serological techniques that measure total anti-DENV antibodies and serotype-specific neutralizing antibodies (inhibition ELISA and reporter viral particle based neutralization assay).
  • They found:
    • Inapparent versus symptomatic outcome did not differ by infection number (whether it was their first, second or so forth infection).
    • There was however, substantial variation in the proportion of symptomatic infections among all DENV infections observed across all study years.
    • Those patients with repeat infections, the time interval between 1st inapparent and 2nd inapparent infection was shorter than the time interval between a 1st inapparent and 2nd symptomatic infection.
    • The difference was not observed in subsequent infections.
    • Age, study year and time interval between consecutive DENV infections influence inapparent versus symptomatic infection outcome.
    • Sex and infection number had no significant effect
  • They conclude:
"...if a second DENV infection happens within a period of ~2 years after the first infection, the second infection is more likely to be inapparent. However, if the time interval between first and second DENV infections is longer, this protection wanes and the infection is likely to be symptomatic."
    • The results suggest that the window for cross-protection induced by 1st infection is approximately 2 years.
    • The results shed light on the factors influencing inapparent versus symptomatic DENV infection outcome.
Eva went on to chat about antigenic cartography and exploring the role of genotype in influencing neutralization titer...but we'll take up those topics in later posts, specifically Leah Katzelnick's work in the next blog to come...