To identify the cells that produce BAFF in the salivary glands of patients with primary Sjögren's syndrome (SS), and to analyze BAFF receptor expression by local T and B lymphocytes.
We used 3 methods to identify the source of BAFF: in situ hybridization of the transcripts for BAFF combined with staining of membrane markers, regular and real-time reverse transcription-polymerase chain reaction (RT-PCR) of cultured epithelial cells, and RT-PCR of sorted single-cell T and B lymphocytes eluted from salivary glands. Cells expressing TACI, BCMA, and B lymphocyte stimulator receptor 3 (BR-3) were disclosed by combining each specific staining of the receptors with each specific staining of the cells. The function of BAFF generated by epithelial cells on B lymphocytes was determined in short-term cocultures.
Transcripts for BAFF were seen in epithelial cells and infiltrating T lymphocytes and, for the first time, were detected in local B cells. It is interesting that BR-3 was present on these B cells but not on T cells. In contrast, TACI and, to a lesser degree, BCMA were observed on transitional B lymphocytes, whereas T lymphocytes were devoid of receptors for BAFF. Furthermore, this cytokine was shown to be functional, in that epithelial cell-bound BAFF extended the survival of normal B cells, but cell-free BAFF released in the supernatants did not.
These experiments establish that in primary SS, BAFF is produced not only by epithelial cells and T cells but also by B cells. The expression of receptors for BAFF would thus allow these receptors to participate in an autocrine pattern of self-stimulation.