Stepwise changes in the murine salivary gland immune response during virally-induced ectopic lymphoid structure formation.

Objectives: Sjögren's syndrome (SS) is a chronic autoimmune disease characterised by lymphocytic infiltration into the salivary glands (SG) and, in a subset of patients, formation of ectopic lymphoid structures (ELS) in the glands. However, the mechanisms of how ELS form ectopically are not fully elucidated. Here we used a viral inducible murine model of ELS formation in the SG to elucidate the key immunological steps regulating the formation of ELS in the SG.

Methods: We have utilised an inducible murine model of sialadenitis whereby retrograde cannulation of the submandibular SG with a replication-deficient adenovirus 5 leads to the formation of ELS. Flow cytometry, immunofluorescence and gene expression was performed on the SGs at regular time points after cannulation to follow the organisation of ELS.

Results: Innate immune cells (neutrophils, eosinophils and monocytes) rapidly infiltrated the SG by 3 days post cannulation (dpc) whereby monocytes started to differentiate into resident macrophages. Myeloid dendritic cells accumulated inside leukocytic aggregates whereas macrophages were excluded from the developing ELS. Meanwhile, CD11b+ cells upregulated Il18, Cxcl13, Ltb, April and other lymphoid genes stimulating the influx of T cells by 12 days and B cells shortly after. Infiltration of T-follicular helper (Tfh) cells correlated with an increase in GL7+ germinal centre B cells, which peaked at 19 dpc.

Conclusions: Immune cell infiltration in virally-infected murine SG follows a highly reproducible step-wise process whereby early innate immune cells reshape the SG myeloid compartment leading to upregulation of genes involved in the ectopic lymphoid neogenesis process. This in turns leads to T and B cell recruitment, differentiation and activation, culminating in the organization of ELS and localised germinal centres responses.