Hierarchical effects of pro-inflammatory cytokines on the post-influenza susceptibility to pneumococcal coinfection

Abstract

In the course of influenza A virus (IAV) infections, a secondary bacterial infection frequently leads to serious respiratory conditions provoking high hospitalization and death tolls. Although abundant pro-inflammatory responses have been reported as key contributing factors for these severe dual infections, the relative contributions of cytokines remain largely unclear. In the current study, mathematical modelling based on murine experimental data dissects IFN-γ as a cytokine candidate responsible for impaired bacterial clearance, thereby promoting bacterial growth and systemic dissemination during acute IAV infection. We also found a time-dependent detrimental role of IL-6 in curtailing bacterial outgrowth which was not as distinct as for IFN-γ. Our numerical simulations suggested a detrimental effect of IFN-γ alone and in synergism with IL-6 but no conclusive pathogenic effect of IL-6 and TNF-α alone. This work provides a rationale to understand the potential impact of how to manipulate temporal immune components, facilitating the formulation of hypotheses about potential therapeutic strategies to treat coinfections.

Figures

Figure 1. Experimental scheme.

(a) C57BL/6 J wildtype mice were intranasally infected with a sub-lethal dose of IAV (A/PR8/34) followed by bacterial infection with the S. pneumoniae strain T4 on day 7. Bronchoalveolar lavage (BAL), post-lavage lung and blood were collected at the indicated time points post secondary bacterial infection (hpi). (b) The infection groups were single viral infection (IAV), single bacterial infection (T4) and coinfection (IAV+T4). (c) The bacterial burden, viral titers and cytokine concentrations were determined as the experimental readouts.

Figure 2. Organ-wide bacterial burden in the single and coinfected animals.

Bacterial titers in single T4 infected and IAV+T4 coinfected mice were determined in (a) post-lavage lung, (b) BAL and (c) blood. All experiments were performed in groups of 4–7 WT C57BL/6 J mice, raw data can be found in the Supplementary Fig. S1. Statistical analysis was performed using the Mann-Whitney test. Asterisks indicate significant differences between single and coinfected mice: *p < 0.05; **p < 0.01.

Figure 3. Absolute numbers of alveolar macrophages (AM) in post-lavage lungs of single and coinfected animals.

Statistical analysis was performed using the Mann-Whitney test. Asterisks indicate significant differences between single and coinfected mice: **p 

Figure 4. Pro-inflammatory cytokine profiles of the BAL of coinfected and single T4 infected mice.

Protein concentrations of (a) IFN-γ, (b) TNF-α, (c) IL-6 and (d) MCP-1 were determined in the BAL fluid at the indicated time points after secondary T4 infection on day 7 post IAV or single T4 infection on day 7 post PBS treatment. Raw data can be found in the Supplementary Fig. S2. Statistical analysis was performed using the Mann-Whitney test. Asterisks indicate significant differences between single and coinfected mice: *p < 0.05; **p < 0.01.

Figure 5. Simulations for the coinfection model M6.

In silico neutralizations of the different pro-inflammatory responses in model M6 are presented in the panel (a). The time-dependent contributions of pro-inflammatory cytokines to the impairment of bacterial clearance by the function fx are depicted in the panel (b). When the mathematical function fx is 1 means that there is no impairment to the bacterial clearance.