Role of Mast cell expressed-BAFF in B cell biology:promotion of B cell survival in normal and pathological context

Mast cells (MCs) are best known for their critical role in allergic response but a growing body of evidence shows that they are important players of a variety of IgE-independent biological processes. It has been demonstrated that MCs are able to interact with B cells and to modulate both their effector and regulatory functions. Since several outcomes of the crosstalk between these two cell types resemble the effects of B-cell activating factor (BAFF), in this work we investigated the possibility that MCs could produce BAFF and we focused on its role in the crosstalk between MCs and B cells. The association between elevated BAFF levels and human autoimmunity and diseases makes the study of the role of BAFF in the B/MC interaction a very intriguing task. Through experiments of real time PCR we show that both murine and human MCs constitutively express BAFF mRNA. BAFF mRNA expression is associated with protein production both on the cell membrane and in intracellular stores, as demonstrated by flow cytometry and western blot experiments. Conversely, soluble BAFF was not detected, through ELISA, in culture supernatants of both murine and human MCs. Once demonstrated the production of BAFF by MCs, we were interested in elucidating its role in the B/MC crosstalk. Our data show that B cell survival and proliferation are reduced when using BAFF-deficient MCs to perform the MC/B cell co-cultures. On the contrary, BAFF produced by MCs seemed not to be fundamental neither in the induction of activation-induced cytidine deaminase (AID) expression nor in the expansion of regulatory B cells, two processes in which MCs were shown to play an important role. Moreover, we investigated the modulation of the expression of BAFF receptors on B cell surface and we have evidences that BAFF-R and TACI are decreased in the presence of MCs. Finally, we demonstrate that the expression of BAFF by MCs is particularly relevant in the context of multiple myeloma (MM). MCs are known to Abstract infiltrate MM and facilitate new vessel formation. Interestingly, we show that BAFF is absent in the rare MCs present in bone marrow biopsies from healthy subjects while its expression, increases in patients with monoclonal gammopathy of undetermined significance (MGUS) and with MM, following the worsening of the disease. In vitro co-culture experiments with the myeloma cell line RPMI 8226 and the human MC line HMC-1.2 suggest that the malignant plasma cells are responsible of the up-regulation of BAFF on MC-surface. In conclusion, our data demonstrate that MCs produce BAFF and uncover an interesting relation between BAFF, B cells and MCs which might result particularly relevant in the context of clonal B cell neoplasias such as MM.