An ahemolytic pneumolysin of Streptococcus pneumoniae manipulates human innate and CD4⁺ T-cell responses and reduces resistance to colonization in mice in a serotype-independent manner

Abstract

Background: Some Streptococcus pneumoniae serotypes express an ahemolytic pneumolysin (PLYa). Serotypes that commonly express PLYa, including serotype 8 (ST8) and ST1, are often associated with a low prevalence during colonization but a higher propensity to cause invasive disease. We sought to study the host response to ST8 PLYa in a homologous and heterologous capsular background.

Methods: We genetically exchanged the PLYa of ST8 strain 6308 with the hemolytic PLY (PLYh) of ST3 A66.1 and vice versa and determined the impact of the exchange on nasopharyngeal colonization in mice. Then, to compare the response of human cells to PLYa-expressing and PLYh-expressing strains, we infected human peripheral blood mononuclear cells (PBMCs) with PLY-switched strains and assessed dendritic cell and CD4(+) T-cell responses by intracellular cytokine staining.

Result: Mice colonized with PLYa-expressing strains had significantly higher colonization densities than those colonized with PLYh-expressing strains, irrespective of capsular background. Compared with infection of PBMCs with PLYh-expressing strains, infection with PLYa-expressing strains induced diminished innate (dendritic cell cytokines, costimulatory receptor, and apoptotic) and adaptive (CD4(+) T-cell proliferative and memory interleukin 17A) responses.

Conclusion: Our findings demonstrate that PLYa has the potential to manipulate host immunity irrespective of capsule type. PLY exchange between STs expressing PLYa and PLYh could lead to unexpected colonization or invasion phenotypes.

Keywords: Streptococcus pneumoniae; T cell; apoptosis; colonization; dendritic cell; invasion; mice; pneumolysin; serotype.