Respiratory syncytial virus (RSV) strongly inhibited rhinovirus (RV) when present first, but the reverse order of infection did not offer any inhibition, based on new data presented at a global conference sponsored by the Respiratory Syncytial Virus Foundation.

Coinfection with RSV and RV is common in hospitalized children, but the impact of order of infection on disease severity remains unclear, according to Emily Schultz, MPhil, a PhD candidate in immunology and microbiology at Baylor College of Medicine, Houston, and colleagues.

“RSV-rhinovirus coinfections are the most common coinfection detected in infants hospitalized for bronchiolitis,” Schultz told Medscape Medical News. “Performing our study on RSV-rhinovirus coinfections in an infant-derived ex-vivo model system, the human nasal organoid, was important because clinical cross-sectional studies haven’t been able to tease apart whether the order or coinfection matters in infection outcome, and our model system allows us to evaluate this,” she said.

The researchers used infant human nasal organoids (HNOs) to examine the effect of orders of RSV/RV coinfection on viral activity. They infected five different infant HNOs with alternating orders of RSV/RV coinfection (RSV first, then RV, or RV first, then RSV) to evaluate the effects on viral replication kinetics. They conducted multiple-immunoassays performed for type I and III interferons (IFNs) as well as interleukin (IL)-6, IL-8, TNF-alpha, IP-10, IL-1-alpha and IL-1-beta to investigate epithelial immune responses with the different orders.

The researchers also added recombinant human type III IFN (rhIFN-lambda-1, lambda-2, and/or lambda-3) to RV- or RSV-infected infant HNOs to test their hypothesis that IFN-lambda produced by RSV could inhibit RV replication.

When RSV infection preceded RV in a coinfection, infectious RV decreased by approximately 97%-100% based on area under the curve across all the infant HNO models. By contrast, no such decrease in viral load occurred when RV preceded RSV.

The levels of pro-inflammatory markers increased during coinfection, but no type I IFN was detected and differential induction of type III IFNs (lambda-1, lambda-2, and lambda-3) occurred, the researchers noted. When rhIFN-lambda-1, lambda-2, and/or lambda-3 was added to either RV- or RSV-infected infant HNOs, RV replication was almost entirely inhibited in the presence of each of the type III interferons.

However, RSV replication activity was unaffected by remained unaffected by rhIFN-lambda-1, lambda-2, or lambda-3.

The findings were limited by the small sample size and use of models, but the results provide insights into the mechanisms of coinfection that drive differences in response based on order of infection, the researchers noted.

“We expected the order to coinfection to have an impact on a virus’s ability to replicate but not to the extent of near complete inhibition,” Schultz told Medscape Medical News. “We were surprised to see 99%-100% inhibition of rhinovirus replication when it followed a primary infection of RSV,” she said.

Takeaway: Infection Order Matters

The study findings indicate that the order in which two viruses infect a host can affect the infection outcome, said Schultz. Although the current study utilized RSV and rhinovirus, “the concept and potential for viral interference through host-provoked responses from the respiratory epithelium applies to coinfections between other viruses as well,” she said.

“Since our infant-derived cultures are epithelial only, further studies are needed to evaluate how the cellular immune components, such as Natural Killer cells, macrophages, and T cells impact the responses and interactions occurring during different orders of coinfection,” Schultz added.

Insight and Assessment

Pediatricians often encounter coinfection in hospitalized children with respiratory infections, and clarifying the significance of these coinfections can help assess the expected severity and inpatient course these patients, said Amer Kechli, MD, professor of clinical pediatrics at the Lewis Katz School of Medicine at Temple University, Philadelphia. “Outpatient providers do not routinely test for coinfections or for rhinovirus, as the information has no implications for a childs outpatient course,” said Kechli, who was not involved in the study.

The increase in pro-inflammatory markers in conjunction with coinfection was not surprising as multiple viruses should increase the level of different markers, Kechli noted. However, the massive decrease in RV replication, close to total inhibition, was surprising, he said. “This could indicate the rhinovirus infections occurring after RSV are not significant and perhaps an incidental finding,” he said.

The immediate practical implications of the study findings are minimal as information about the order of infection is rarely available in the emergency department or inpatient settings in which coinfection is often encountered in children with severe respiratory infections, Kechli told Medscape Medical News.

However, the research may be useful in providing insight into whether coinfection correlates with clinical severity, or whether testing can indicate the order of infection and its associated risk for complications, said Kechli. In future studies, “including other mediators, such as chemokines CCL2 and CCL3, may be important for clarifying whether cellular responses and recruitment are affected by coinfection,” he added.

This study was funded through National Institutes of Health grants. Schultz disclosed having no financial conflicts of interest. Kechli disclosed having no financial conflicts of interest.