Introduction and Objectives Pseudomonas aeruginosa (PA) is an important respiratory pathogen resulting in damaging neutrophilic responses. The cytokine IL-17 is important in orchestrating such inflammation. Cells producing IL-17, such as Th17 cells, have been shown to be important in host defense in chronic pulmonary PA infection. We set out to determine the origin and role of IL-17 in a model of chronic pulmonary PA infection.
Methods Experimental chronic pulmonary infection in mice was produced by intra-tracheal instillation of mucoid PA strains embedded in agar-beads; sterile beads were utilised as controls. Thoracic lymph nodes (LNs), splenocytes and peritoneal B1a cells were restimulated with PA followed by cytokine assay and immunostaining to define responding cell subsets. PA-specific immunoglobulins were measured in sera and culture supernatants. Intrapulmonary B cells were identified via immunohistochemistry. Mice genetically engineered to lack B cells (µMT strain) were utilised to examine the effect on pathogenesis in the absence of B cell responses.
Results Chronic PA infection developed in 43% (SD: 25%) of infected animals at 14-days.
Following infection, the pulmonary LN B cell compartment expanded, with a large B1 population (B220+ CD19+ CD43+ IgM hi IgD lo and predominantly CD5+ ) that expressed intracellular IL-17A. Infected animals also developed peribronchial B220+ cellular foci.
In mediastinal LNs following infection, PA-specific responses were dominated by B220+ CD19+ CD43+ CD23-B5+ cells expressing and producing IL-17A and IL-22 as well as PA-specific IgM but not IgG. This PA-specific B1 response was not seen in the thoracic lymph nodes of sterile-bead treated animals. In splenocytes, there was a pre-existing B cell response to PA with identical features. Peritoneal B1a cells isolated from untreated controls also produced IL-17A, IL-22 and anti-PA IgM following infection, confirming the existence of pre-existing B1 cells that can respond to PA. In µMT animals, chronic colonisation rates, bacterial burden and neutrophilic inflammation did not differ from WT littermates. However, classical PA-specific Th17 responses dominated following infection in µMT animals, suggesting alternative compensatory IL-17 sources acting in the absence of B cells.
Conclusions In chronic pulmonary PA infection, innate-like B1 cells migrate to the site of infection and are a novel source of pro-inflammatory IL-17 cytokines.