Meningitis caused by Streptococcus suis serotype 2 (SS2) in humans and pigs is an acute nervous disorder associated with serious sequelae. Bacterial meningitis is tightly associated with immune cell responses and the local immune microenvironment. However, the dynamic changes of the immune system during the disease progression in the brain remains unclear. Here, single-cell mass cytometry analyses are used to comprehensively profile the composition and phenotypes of female mouse brain immune cells at different stages of SS2 meningitis. Ten major immune cell lineages are identified among which T cells and dendritic cells significantly increased during meningitis, with B cells increasing in the late stage. Specifically, SS2⁺PD-L1⁺ neutrophils with strong phagocytosis, bactericidal and apoptotic effects accumulate in the acute phase of SS2 infection. Microglia sequentially display the features of homeostasis, proliferation, and activation (enhanced MHCII and TLR2 signals and TNF-α secretion) during the process of meningitis. Both border-associated and monocyte-derived macrophages contribute to the process of SS2-induced meningitis, exhibiting upregulation of CD38 and MHCII. Interestingly, CD11c⁺CD8⁺T cells are the main contributor of IFN-γ and specifically appeared during SS2 infection. In addition, the appearance of other lymphocytes such as CCR6^+/lo B cells, CX3CR1⁺ NK and MHCII⁺ ILC3 are related to the progression of meningitis. Moreover, correlation analysis between the composition of immune cell clusters and the SS2 infection process yield a dynamic immune landscape in which key immune clusters, including some previously unidentified, mark different stages of infection. Together, these data reveal the unique infection-stage immune microenvironment during the progression of meningitis caused by SS2 and provide resources for the analysis of immunological pathogenesis, potential diagnostic markers and therapeutic targets for bacterial meningitis. (Figure presented.)

Due to the increase in bacterial resistance, improving the anti-infectious immunity of the host is rapidly becoming a new strategy for the prevention and treatment of bacterial pneumonia. However, the specific lung immune responses and key immune cell subsets involved in bacterial infection are obscure. Actinobacillus pleuropneumoniae (APP) can cause porcine pleuropneumonia, a highly contagious respiratory disease that has caused severe economic losses in the swine industry. Here, using high-dimensional mass cytometry, the major immune cell repertoire in the lungs of mice with APP infection was profiled. Various phenotypically distinct neutrophil subsets and Ly-6C+ inflammatory monocytes/macrophages accumulated post-infection. Moreover, a linear differentiation trajectory from inactivated to activated to apoptotic neutrophils corresponded with the stages of uninfected, onset, and recovery of APP infection. CD14+ neutrophils, which mainly increased in number during the recovery stage of infection, were revealed to have a stronger ability to produce cytokines, especially IL-10 and IL-21, than their CD14- counterparts. Importantly, MHC-II+ neutrophils with antigen-presenting cell features were identified, and their numbers increased in the lung after APP infection. Similar results were further confirmed in the lungs of piglets infected with APP and Klebsiella pneumoniae infection by using a single-cell RNA-seq technique. Additionally, a correlation analysis between cluster composition and the infection process yielded a dynamic and temporally associated immune landscape where key immune clusters, including previously unrecognized ones, marked various stages of infection. Thus, these results reveal the characteristics of key neutrophil clusters and provide a detailed understanding of the immune response to bacterial pneumonia.