Pierre Frandji

This paper describes a new role for mast cells as being able to present Ag to immune T cells. A mouse bone marrow-derived mast cell population obtained after 3 wk of culture in a conditioned medium has been shown to express a variety of membrane-associated Ag, including MHC class II and class I Ag, CD23, CD32, high affinity receptor for IgE, and CD4. Expression of MHC class II molecules was up-regulated upon stimulation with LPS but not with IFN-gamma and was down-regulated after exposure of mast cells to IL-3 treatment. We have demonstrated that mast cells were able to present native Ag as well as immunogenic peptides to MHC class II-restricted T cell hybridoma. The inhibition of Ag presentation after mast cells have been treated with ammonia suggests that Ag catabolism in intracytoplasmic compartment as a key step in Ag handling takes place in these cells. The MHC class II molecule is the restricting element for the presentation of OVA and the lambda repressor from bacteriophage lambda to a panel of specific T cell hybridomas, as demonstrated by the blocking effect of anti-MHC class II mAb on the Ag-presenting function. A characteristic feature of mast cells is the generation of a narrower immunogenic peptide repertoire as compared with A20 and LBB 3.4.16, a B lymphoma cell line, and a B cell hybridoma, respectively. This novel function of mast cells brings to a much closer connection inflammatory and immunologic processes and sheds new light on the biology of mast cells and particularly on the specific allergic responses.

In the present work, we explored the cytokine-dependent regulation of bone marrow-derived mast cell (BMMC) antigen-presenting cell (APC) function, and co-stimulation requirements, and analyzed the nature of antigens presented to T cells. We observed an up-regulation of the APC function of mast cells induced by granulocyte/macrophage-colony-stimulating factor (GM-CSF) and a complete abrogation by interferon (IFN)-gamma. Expression of co-stimulatory molecules CD80 and CD86 was suggested by the ability of mast cells to activate purified lymph node-derived T cells. Indeed, addition of the fusion protein mCTLA4-Ig strongly inhibited antigen presentation by mast cells to normal T cells and to the T cell hybridoma 3DO-54.8. The regulatory mechanisms of APC function by GM-CSF and IFN-gamma were investigated by measuring CD80 and CD86 transcripts in mast cells. GM-CSF-treated must cells showed a strong increase in the expression of both CD80 and CD86 transcripts, whereas in IFN-gamma-treated mast cells, this expression was completely abrogated. Thus, up- and down-regulation of CD80 and CD86 expression by GM-CSF and IFN-gamma is directly correlated to the APC function. In addition, we analyzed antigen presentation by mast cells of endogenous self-antigens. Mast cells failed to activate anti-I-A or anti-I-E-specific T cell hybridomas and alloreactive T cells in primary mixed lymphocyte reactions (MLR). Furthermore, mast cells did not present the mouse beta 2-microglobulin (m beta 2-m) peptide 25-40, constitutively expressed on B cells. However, mast cells, especially those treated with GM-CSF, activated an anti-m beta 2-m-specific T cell hybridoma in the presence of exogenous peptide. The minor lymphocyte-stimulating antigen-1 Mls-1a is a viral superantigen (vSAG) encoded by the the mouse mammary tumor provirus-7 (MMTV-7). Mast cells, despite a reasonable amount of major histocompatibility complex class II on the cell surface and the presence of MMTV transcripts predicted to encode the vSAG, cannot stimulate in vivo or in vitro V beta 6+ T cells specific for Mls-1a. In contrast, mast cells could present the exogenous bacterial SAG, staphylococcal enterotoxin B (SEB), to specific V beta 8+ T cells. The selective ability of mast cells to present exogenous antigens may have physiological relevance in that mast cells could participate in immune response regulatory mechanisms by discriminating self from nonself.

The present report describes a novel function of mast cells that consists of a B cell growth activity. The B cell response occurred without any stimulation or preactivation of mast cells. A small number of mast cells was required, since mast cell/B cell ratios as low as 1/100 to 1/10,000 lead to effective B cell activation. Mast cell-dependent B cell activation resulted, within 48 h of incubation, in blast formation, proliferation, and IgM production. Both low and high density B cells were responsive to mast cells. Supernatants from unstimulated mast cells could also activate B cells, suggesting that a B cell-stimulating activity (MC-BSA) is mediated by a soluble factor(s). The addition of anti-IL-4 or anti-IL-6 mAbs or even proteases to the mast cell-derived supernatants did not alter B cell activation. However, treatment of mast cells with mitomycin C or actinomycin D, or paraformaldehyde fixation totally abrogated MC-BSA. Fractionation of mast cell supernatant by gel filtration chromatography resulted in four peaks, ranging from > 200 to 15 kDa, all of which were biologically active on B cells. Because mast cells are known to continuously release proteoglycans, MC-BSA was subjected to chondroitinase and heparinase treatment, but no significant inhibition of B cell activation was obtained. This direct T cell-independent stimulatory effect of mast cells on B cells could account for a mechanism by which plasma cells are continuously produced in lymphoid organs and particularly in bone marrow.

We have previously shown that mouse bone marrow-derived mast cells (BMMC) can process and present immunogenic peptides to CD4 T cells. Here, we report on a T cell-dependent MHC class II-mediated mast cell activation resulting in IL-4 transcription and protein release. Presentation of optimal doses of ovalbumin peptide 323-339 resulted in IL-2 production by a specific T cell hybridoma and increase in IL-4 mRNA transcription in mast cells. IL-4 mRNA transcription increased by 200-fold in mast cells treated in IL-3/IL-4/granulocyte-macrophage colony-stimulating factor (high presenters) whereas only a tenfold increase or no increase were obtained with IL-3/IL-4/IFN-gamma- or IL-3-treated mast cells (low presenters), respectively. Induction of IL-4 mRNA transcription in purified mast cells by direct ligation of MHC class II molecules, using anti-I-A and anti-I-E-coated beads, indicates that MHC class II molecules are critical in this signaling pathway. However, when compared to T cells, anti-MHC class II-coated beads were less efficient, indicating a potential role of accessory molecules in this mast cell activation process. IgE-independent IL-4 production by mast cells as a result of cognate interaction with CD4 T cells could be critical for the development of type 2 responses. This novel mechanism may contribute to the induction and/or amplification of specific IgE-mediated allergic responses.