TLR2 Signaling Decreases Transmission of Streptococcus pneumoniae by Limiting Bacterial Shedding in an Infant Mouse Influenza A Co-infection Model

Abstract

While the importance of transmission of pathogens is widely accepted, there is currently little mechanistic understanding of this process. Nasal carriage of Streptococcus pneumoniae (the pneumococcus) is common in humans, especially in early childhood, and is a prerequisite for the development of disease and transmission among hosts. In this study, we adapted an infant mouse model to elucidate host determinants of transmission of S. pneumoniae from inoculated index mice to uninfected contact mice. In the context of co-infection with influenza A virus, the pneumococcus was transmitted among wildtype littermates, with approximately half of the contact mice acquiring colonization. Mice deficient for TLR2 were colonized to a similar density but transmitted S. pneumoniae more efficiently (100% transmission) than wildtype animals and showed decreased expression of interferon α and higher viral titers. The greater viral burden in tlr2−/− mice correlated with heightened inflammation, and was responsible for an increase in bacterial shedding from the mouse nose. The role of TLR2 signaling was confirmed by intranasal treatment of wildtype mice with the agonist Pam3Cys, which decreased inflammation and reduced bacterial shedding and transmission. Taken together, these results suggest that the innate immune response to influenza virus promotes bacterial shedding, allowing the bacteria to transit from host to host. These findings provide insight into the role of host factors in the increased pneumococcal carriage rates seen during flu season and contribute to our overall understanding of pathogen transmission.

Infant Mouse Model for the Study of Shedding and Transmission during Streptococcus pneumoniae Monoinfection

“…A factor known to enhance pneumococcal shedding is a concurrent or recent viral respiratory infection (6,7). Increased rates of carriage associated with viral respiratory infection are thought to be a consequence of both more-frequent transmission and a higher burden of colonizing bacteria (8)(9)(10).…”

“…Intralitter pneumococcal transmission has also been demonstrated among infant mice, a more tractable model host, but requires influenza coinfection (12,13). Our group used this model to show that increased pneumococcal acquisition by initially uncolonized "contact" pups correlates with increased mucin production and bacterial shedding by colonized "index" pups with influenza A virus (IAV) coinfection (7). These responses among index pups were chiefly driven by the mucosal inflammatory response to IAV, which recapitulates the effects of respiratory viruses in children (6,12).…”

“…Although most pneumococcal transmission in the human population may be seasonal, it is not generally associated with influenza coinfection (14). It is therefore unclear whether the more mild inflammatory response that characterizes pneumococcal monoinfection is sufficient to allow levels of shedding that could promote transmission (7,15). Here, we describe the adaptation of the infant mouse model to enable the study of pneumococcal shedding and transmission without the confounding effects of viral coinfection.…”

Bacterial exploitation of phosphorylcholine mimicry suppresses inflammation to promote airway infection

“…Δpce pneumococci were similarly impaired in an infant mouse model of pneumococcal transmission (5,48). Within litters of mice, we inoculated 1-2 "index" pups with either WT or Δpce bacteria and quantified the acquisition and load of pneumococci among previously uninfected "contact" pups.…”

“…Infant mouse pneumococcal transmission experiments were performed as described previously (5,48). On day 4 of life, we inoculated 1-2 index pups per litter (approximately 1 in 4) with 2,000 CFU of S. pneumoniae strain P1121 in 3 μl PBS.…”