CD6 is one of a pair of related genes encoding CD5-associated receptors on all T cells and a subset of B cells. The current availability of "T1h", a humanized anti-CD6 monoclonal antibody for B cell-mediated autoimmune disorders revives analysis of the B-cell subset expression of CD6, particularly in primary Sjögren's syndrome (SS). Refined phenotype of B-lymphocytes peripheral blood (PB), bone marrow and tonsils revealed that the overlap between the expression of CD6 is less close to that of CD5 than currently acknowledged. In contrast to CD5, CD6 is absent on transitional B cells, while present on mature and memory B cells. Interestingly, the PB proportion of CD6(+) B cells is decreased in patients with primary SS, as opposed to those with rheumatoid arthritis. The reduction in primary SS does not result from the shedding of CD6 from the membrane of B cells, but from the lowering of memory B lymphocytes. It may result from the ability of CD6 to make transmigration of CD27(+) memory B cells into the salivary glands (SGs) easier. Consistent with this view is our finding that CD166 (one of the ligands for CD6) is highly expressed on epithelial cells of patients' SGs. This study is relevant in that the humanized T1h anti-CD6 becomes an alternative to anti-CD20 for treatment of primary SS.
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