Pneumolysin Attenuates Lethal Streptococcus pneumoniae Superinfection Following SARS-CoV-2 Infection

Streptococcus pneumoniae (Spn) is a common bacterial co-infection in patients with respiratory viral infections, including influenza and SARS-CoV-2, and is associated with increased morbidity and mortality. However, the mechanisms underlying the severe disease caused by SARS-CoV-2 and Spn co-infection remain poorly understood. Here, we show that Spn infection 3 days after infection with mouse-adapted SARS-CoV-2 MA10 markedly increased morbidity and mortality in wild-type mice. In contrast, SARS-CoV-2 infection 3 days after Spn infection did not increase mortality, demonstrating that disease severity depends on the sequence of infection. Increased susceptibility to Spn following SARS-CoV-2 infection was associated with enhanced viral burden, exacerbated lung pathology, pronounced neutrophil accumulation, and increased expression of IFN-β and the neutrophil-recruiting chemokine CXCL2. To determine whether pneumolysin (PLY), a major Spn virulence factor, contributes to superinfection-induced pathology, we infected mice with a PLY-deficient Spn strain. Unexpectedly, PLY deficiency further increased mortality and lung pathology without altering bacterial or viral titers. Immunofluorescence analysis revealed increased accumulation of neutrophils and macrophages in the lungs of mice co-infected with PLY-deficient Spn. These results indicate that Spn superinfection exacerbates SARS-CoV-2-induced lung disease, whereas PLY unexpectedly attenuates disease severity. We propose that PLY limits immunopathology during SARS-CoV-2–Spn superinfection by restraining macrophage- and neutrophil-mediated inflammation.