A positive feedback loop between CXCL13 and LTα1β2 on B cells
In NPs with eLTs, B cells were found to be the major producer of CXCL13 (Fig 1, E), suggesting an involvement of B cells at a later stage of eLT formation in NPs. B cells are also known as the main source of the membrane-bound form of LT (LTα1β2) in SLOs.34, 35 We found here that the mRNA expression levels of LTα were upregulated in both types of NPs, whereas the levels of LTβ were only upregulated in NPs with eLTs compared to those in control tissues (Fig 4, A), indicating an upregulation of membrane LTα1β2 in NPs with eLTs rather than in NPs without eLTs. Consistent with the ability of B cells to express LT, our immunofluorescence staining demonstrated expression of LTα and LTβ on B cells in eLTs in NPs (Fig 4, B). Furthermore, we found that after B cell depletion, LTβ mRNA expression was markedly decreased in patients with NPs with eLTs (mean, 62% reduction), but not in patients with NPs without eLTs (mean, 32% reduction). In contrast, a mild and comparable reduction of LTα mRNA expression was noted in both NPs with and without eLTs after depleting B cells (mean, 27% and 38% reduction, respectively) (Fig 4, C). These results suggest that B cells are the main producer of LTα1β2, but not LTα3, in NPs with eLTs.
Of a particular interest, it has been shown that activation of CXCR5 induced murine splenic B cells to produce membrane LTα1β2 which is required for GC formation in SLOs.36 Here, the expression of CXCR5 on B cells in eLTs in NPs was verified by immunofluorescence (Fig E4, A). We further discovered that CXCL13 induced the upregulation of LTβ mRNA expression and membrane LTα expression in purified polyp B cells (Fig 4, D and E). However, there was no increase of secreted LTα3 in the supernatant of B cell culture (Fig E4, B). Collectively, these findings indicate an upregulation of membrane type, but not secretory type of LT, in polyp B cells under CXCL13 stimulation.
In SLOs, LTβR pathway is critical for the induction of CXCL13.37, 38 In this study, we found that B cell were the major source of CXCL13 in NPs with eLTs (Fig 1, E). Thereby we tested whether the production of CXCL13 in B cells can be induced via LTβR pathway. We found that LTβR was expressed by B cells in eLTs in NPs by immunofluorescence staining (Fig E4, C). Moreover, we found that LTα1β2 treatment promoted mRNA (Fig 4, F) and protein production of CXCL13 in polyp B cells as reflected by CXCL13+ B cell frequencies (Fig E4, D) and CXCL13 levels in culture supernatants (Fig 4, G).