No abstract

This study provides insight into the role of the virus strain, age, immunity, and URT flora on IAV shedding and transmission efficiency. Using the infant mouse model, we found that (i) differences in viral shedding of various IAV strains are dependent on specific hemagglutinin (HA) and/or neuraminidase (NA) proteins, (ii) host age plays a key role in the efficiency of IAV transmission, (iii) levels of IAV-specific immunoglobulins are necessary to limit infectiousness, transmission, and susceptibility to IAV, and (iv) expression of sialidases by colonizing S. pneumoniae antagonizes transmission by limiting the acquisition of IAV in recipient hosts. Our findings highlight the need for strategies that limit IAV shedding and the importance of understanding the function of the URT bacterial composition in IAV transmission. This work reinforces the significance of a tractable animal model to study both viral and host traits affecting IAV contagion and its potential for optimizing vaccines and therapeutics that target disease spread.

Host-to-host transmission is a necessary but poorly understood aspect of microbial pathogenesis. Herein, we screened a genomic library of mutants of the leading respiratory pathogen Streptococcus pneumoniae generated by mariner transposon mutagenesis (Tn-Seq) to identify genes contributing to its exit or shedding from the upper respiratory tract (URT), the limiting step in the organism’s transmission in an infant mouse model. Our analysis focused on genes affecting the bacterial surface that directly impact interactions with the host. Among the multiple factors identified was the dlt locus, which adds d-alanine onto lipoteichoic acids (LTA) and thereby increases Toll-like receptor 2-mediated inflammation and resistance to antimicrobial peptides. The more robust proinflammatory response in the presence of d-alanylation promotes secretions that facilitate pneumococcal shedding and allows for transmission. Expression of the dlt locus is controlled by the CiaRH system, which senses cell wall stress in response to antimicrobial activity, including in response to lysozyme, the most abundant antimicrobial along the URT mucosa. Accordingly, in a lysM−/− host, there was no longer an effect of the dlt locus on pneumococcal shedding. Thus, our findings demonstrate how a pathogen senses the URT milieu and then modifies its surface characteristics to take advantage of the host response for transit to another host. IMPORTANCE Streptococcus pneumoniae (the pneumococcus) is a common cause of respiratory tract and invasive infection. The overall effectiveness of immunization with the organism’s capsular polysaccharide depends on its ability to block colonization of the upper respiratory tract and thereby prevent host-to-host transmission. Because of the limited coverage of current pneumococcal vaccines, we carried out an unbiased in vivo transposon mutagenesis screen to identify pneumococcal factors other than its capsular polysaccharide that affect transmission. One such candidate was expressed by the dlt locus, previously shown to add d-alanine onto the pneumococcal lipoteichoic acid present on the bacterial cell surface. This modification protects against host antimicrobials and augments host inflammatory responses. The latter increases secretions and bacterial shedding from the upper respiratory tract to allow for transmission. Thus, this study provides insight into a mechanism employed by the pneumococcus to successfully transit from one host to another.

1 9 2 0 2 1 2 2 Word count: Abstract (368), Text (4692) 2 3 2 ABSTRACT 2 4 2 5The pandemic potential of influenza A viruses (IAV) depends on the infectivity of 2 6 the host, transmissibility of the virus, and susceptibility of the recipient. While virus traits 2 7 supporting IAV transmission have been studied in detail using ferret and guinea pig 2 8 models, there is limited understanding of host traits determining transmissibility and 2 9 susceptibility because current animal models of transmission are not sufficiently 3 0 tractable. Although mice remain the primary model to study IAV immunity and 3 1 pathogenesis, the efficiency of IAV transmission in adult mice has been inconsistent. 2Here we describe an infant mouse model which support efficient transmission of IAV. 3We demonstrate that transmission in this model requires young age, close contact, 3 4 shedding of virus particles from the upper respiratory tract (URT) of infected pups, the 3 5 use of a transmissible virus strain, and a susceptible recipient. We characterize 3 6shedding as a marker of infectiousness that predicts the efficiency of transmission 3 7 among different influenza virus strains. We also demonstrate that transmissibility and 3 8 susceptibility to IAV can be inhibited by humoral immunity via maternal-infant transfer of 3 9 IAV-specific immunoglobulins, and modifications to the URT milieu, via sialidase activity 4 0 of colonizing Streptococcus pneumoniae (Spn). Due to its simplicity and efficiency, this 4 1 model can be used to dissect the host's contribution to IAV transmission and explore 4 2 new methods to limit contagion.4 3 3 IMPORTANCE 4 4 4 5This study provides insight into the role of the virus strain, age, immunity, and 4 6 URT flora on IAV shedding and transmission efficiency. Using the infant mouse model, 4 7 we found that: (a) differences in viral shedding of various IAV strains is dependent on 4 8 specific hemagglutinin (HA) and/or neuraminidase (NA) proteins; (b) host age plays a 4 9 key role in the efficiency of IAV transmission; (c) levels of IAV-specific immunoglobulins 5 0 are necessary to limit infectiousness, transmission, and susceptibility to IAV; and (d) 5 1 expression of sialidases by colonizing Spn antagonize transmission by limiting the 5 2 acquisition of IAV in recipient hosts. Our findings highlight the need for strategies that 5 3 limit IAV shedding, and the importance of understanding the function of the URT 5 4 bacterial composition in IAV transmission. This work reinforces the significance of a 5 5tractable animal model to study both viral and host traits affecting IAV contagion, and its 5 6 potential for optimizing vaccines and therapeutics that target disease spread. 5 7 5 8 5 9

Binding of Streptococcus pneumoniae (Spn) to nasal mucus leads to entrapment and clearance via mucociliary activity during colonization. To identify Spn factors allowing for evasion of mucus binding, we used a solid-phase adherence assay with immobilized mucus of human and murine origin. Spn bound large mucus particles through interactions with carbohydrate moieties. Mutants lacking neuraminidase A (nanA) or neuraminidase B (nanB) showed increased mucus binding that correlated with diminished removal of terminal sialic acid residues on bound mucus. The non-additive activity of the two enzymes raised the question why Spn expresses two neuraminidases and suggested they function in the same pathway. Transcriptional analysis demonstrated expression of nanA depends on the enzymatic function of NanB. As transcription of nanA is increased in the presence of sialic acid, our findings suggest that sialic acid liberated from host glycoconjugates by the secreted enzyme NanB induces the expression of the cell-associated enzyme NanA. The absence of detectable mucus desialylation in the nanA mutant, in which NanB is still expressed, suggests that NanA is responsible for the bulk of the modification of host glycoconjugates. Thus, our studies describe a functional role for NanB in sialic acid sensing in the host. The contribution of the neuraminidases in vivo was then assessed in a murine model of colonization. Although mucus-binding mutants showed an early advantage, this was only observed in a competitive infection, suggesting a complex role of neuraminidases. Histologic examination of the upper respiratory tract demonstrated that Spn stimulates mucus production in a neuraminidase-dependent manner. Thus, an increase production of mucus containing secretions appears to be balanced, in vivo, by decreased mucus binding. We postulate that through the combined activity of its neuraminidases, Spn evades mucus binding and mucociliary clearance, which is needed to counter neuraminidase-mediated stimulation of mucus secretions.