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.

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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.

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.

SUMMARYThe discovery that bone marrow-derived mast cells can express major histocompatibility complex class II molecules and act as antigen-presenting cells prompted us to evaluate this function when antigen is internalized through fluid-phase endocytosis or via specific uptake by using IgG and IgE antibodies. This study was performed using a specific T-cell hybridoma developed against Lol p 1, the major allergen of grass pollen Lolium perenne. Expression of FccR and FceRI by mast cells led us to investigate the influence of IgG-and IgE-targeted antigen on the antigenpresenting function of mast cells. Internalization of Lol p 1 through different specific IgG monoclonal antibodies (mAb) resulted in the activation of Lol p 1-specific T-cell hybridoma at concentrations about 100-fold less than that required for T-cell stimulation by uncomplexed antigen. IgE-complexed Lol p 1, which facilitates trapping of antigen by mast cells, induced an accelerated and more efficient antigen-presenting capacity of mast cells than that obtained with uncomplexed antigen. However, aggregation of anti-dinitrophenyl (DNP) IgE mAb by the irrelevant antigen DNP-human serum albumin did not substantially increase the capacity of mast cells to present Lol p 1 to T cells. This suggests that the mere aggregation of FceRI is not sufficient for enhanced antigen presentation mediated by IgE. Tissue distribution and strategic location of mast cells at the mucosal barriers and their capacity to process the antigen through efficient fluidphase pinocytosis as well as IgG-and IgE-dependent targeting of antigens provide mast cells with a prominent role in immune surveillance.