Follicular helper T (Tfh) cells play a critical role in germinal center formation and B cell activation, both of which are hallmarks of primary Sjögren's syndrome (SS). CCR9-expressing T helper cells have "Tfh-like" characteristics and their numbers are increased at mucosa-associated sites in several inflammatory conditions. Because the characteristics of these cells are unique and evaluation has been limited, this study was undertaken to investigate the local and systemic CCL25/CCR9 axis in patients with primary SS.
Levels of CCL25 protein and messenger RNA (mRNA) and CCR9+ T helper cells were evaluated in the labial salivary glands (LSGs) of patients with primary SS and patients with sicca syndrome without a diagnosis of primary SS (non-SS sicca controls). CCL25 levels were assessed for correlation with parameters of inflammation and clinical features. Circulating CCR9+ and CXCR5+ T helper cells were compared on the basis of phenotypic and functional properties.
CCL25 protein and mRNA levels were elevated in the LSGs of patients with primary SS as compared to non-SS sicca controls. Increased levels of CCL25 were associated with B cell hyperactivity, autoimmunity, and levels of interleukin-21 (IL-21) and soluble IL-7 receptor α-chain (IL-7Rα). Furthermore, the frequency of CCR9-expressing cells in the LSGs was increased and levels of circulating CCR9+ T helper cells expressing programmed death 1 and inducible T cell costimulator were elevated in patients with primary SS. CCR9+ T helper cells displayed higher expression of IL-7Rα and secreted higher levels of interferon-γ, IL-17, IL-4, and IL-21 as compared to CXCR5+ T helper cells, ex vivo and upon triggering with antigen or IL-7. Both CCR9+ and CXCR5+ T helper cells induced IgG production by B cells more potently than that induced in the cultures with CCR9-CXCR5- T helper cells.
Enhanced expression of CCL25 in LSGs of patients with primary SS can facilitate attraction of CCR9+ T helper cells, and these cells secrete high levels of proinflammatory cytokines when triggered with antigen or IL-7. The observed associations with B cell hyperactivity, autoimmunity, and markers of lymphoid neogenesis indicate that the CCL25/CCR9 axis plays a significant role in the immunopathology of primary SS, suggesting that this axis could represent a novel therapeutic target for the disease.