Analysis of host genetic diversity and viral entry as sources of between-host variation in viral load

Abstract

Little is known about the factors that drive the high levels of between-host variation in pathogen burden that are frequently observed in viral infections. Here, two factors thought to impact viral load variability, host genetic diversity and stochastic processes linked with viral entry into the host, were examined. This work was conducted with the aquatic vertebrate virus, Infectious hematopoietic necrosis virus (IHNV), in its natural host, rainbow trout. It was found that in controlled in vivo infections of IHNV, a suggestive trend of reduced between-fish viral load variation was observed in a clonal population of isogenic trout compared to a genetically diverse population of out-bred trout. However, this trend was not statistically significant for any of the four viral genotypes examined, and high levels of fish-to-fish variation persisted even in the isogenic trout population. A decrease in fish-to-fish viral load variation was also observed in virus injection challenges that bypassed the host entry step, compared to fish exposed to the virus through the natural water-borne immersion route of infection. This trend was significant for three of the four virus genotypes examined and suggests host entry may play a role in viral load variability. However, high levels of viral load variation also remained in the injection challenges. Together, these results indicate that although host genetic diversity and viral entry may play some role in between-fish viral load variation, they are not major factors. Other biological and non-biological parameters that may influence viral load variation are discussed.

The Role of Virulence in In Vivo Superinfection Fitness of the Vertebrate RNA Virus Infectious Hematopoietic Necrosis Virus

“…None of the IHNV genotypes used for coinfection studies demonstrated a significant reduction in replication in simultaneous coinfection competition compared to single-infection controls (19,36). This was observed even though fish in coinfection groups were initially challenged with twice the total virus compared to fish in single-infection groups, and total measured viral load was, at times, doubled in coinfected fish compared to that in singly infected fish (19,20).…”

“…However, direct competition within the host seems unlikely in our system when considering results from in vivo coinfection studies previously performed using these same IHNV genotype pairs (17,19,20,36). None of the IHNV genotypes used for coinfection studies demonstrated a significant reduction in replication in simultaneous coinfection competition compared to single-infection controls (19,36).…”

“…For quantification of viral load, cDNA was diluted 1/10 and then subjected to quantitative PCR using genotype-specific assays targeting the virus glycoprotein gene as previously described (19). Briefly, the qPCRs consisted of 5 l diluted cDNA, 200 nM genotypespecific minor grove binder (MGB) probe, and 900 nM each genotypespecific primer in a 12-l total reaction volume as described elsewhere (36). A control reaction mixture containing RNA transcripts of the glycoprotein genes of each genotype were included in each set of cDNA synthesis reactions and then utilized in each qPCR run to generate an 8-step, 10-fold dilution series standard curve for each assay (19).…”

“…Infection with infectious hematopoietic necrosis virus (IHNV) of the natural hosts Oncorhynchus mykiss (rainbow trout) and Oncorhynchus nerka (sockeye salmon) has been used as a tractable model system for studying viral fitness in coinfection in vivo (16,17,19,20,36). IHNV is a significant pathogen of salmonid species which originated in the North American Pacific Northwest and has since spread to Europe and Asia (37).…”

“…Virulence of IHNV variants is defined here as mortality caused to a host as a result of viral infection. In vivo coinfection competition between two virus genotypes of equal relative virulence (genotype B and genotype C) revealed that these viruses were also of equal relative fitness (17,36). Interestingly, in vivo coinfection competition experiments with variants displaying high virulence (genotype HV) and low virulence (genotype LV) demonstrated that observed differences in virulence correlated with significant differences in in-host replication as well as host entry and virus shedding (19,20).…”