Human neutrophils recognize group B streptococci via formylated peptide receptors and toll-like receptor 8

Neutrophils, the most abundant leukocyte population, play a central role in host defense against group B Streptococcus (GBS), an encapsulated bacterium that frequently causes severe infections in humans, including sepsis and meningitis. However, the molecular pathways by which human neutrophils detect and respond to GBS remain incompletely defined.

Neutrophils were isolated from the peripheral blood of healthy donors and stimulated with live and killed GBS or their nucleic acids, in the presence of specific inhibitors of immune recognition receptors. Cytokine responses were assessed by enzyme-linked immunosorbent and quantitative polymerase chain reaction assays, while reactive oxygen species were detected using fluorescence flow cytometry.

We initially found that neutrophils produce similar levels of several pro-inflammatory cytokines when stimulated with either live or heat-killed GBS. However, live GBS induce significantly higher levels of interleukin-8 (IL-8) and reactive oxygen species (ROS) compared to killed bacteria by a mechanism that requires activation of formyl peptide receptors 1 or 2 (FPR1 or FPR2). We identified two formylated signal peptides (f-pep8 and f-pep10) which selectively activate FPR1 and FPR2, respectively, promoting neutrophil chemotaxis and ROS generation. We also found that Toll-like receptor 8 (TLR8), but not TLR2, is required for recognition of both live and killed GBS through its ability to recognize bacterial RNA.

These findings reveal a tripartite sensing mechanism involving TLR8, FPR1 and FPR2, which together amplify neutrophil responses to live GBS. While FPR1 and FPR2 signaling are critical for the augmented production of IL-8 and ROS, they are dispensable for the induction of most other pro-inflammatory cytokines. These data may be useful for better understanding the mechanisms by which neutrophils sense the presence of GBS and for guiding the development of therapeutic strategies targeting innate immune pathways in bacterial infections.