The cellular basis of immunological memory remains a controversial issue. Here we show that basophils bound large amounts of intact antigens on their surface and were the main source of interleukins 6 and 4 in the spleen and bone marrow after restimulation with a soluble antigen. Depletion of basophils resulted in a much lower humoral memory response and greater susceptibility of immunized mice to sepsis induced by Streptococcus pneumoniae. Adoptive transfer of antigen-reactive basophils significantly increased specific antibody production, and activated basophils, together with CD4(+) T cells, profoundly enhanced B cell proliferation and immunoglobulin production. These basophil-dependent effects on B cells required interleukins 6 and 4 and increased the capacity of CD4(+) T cells to provide B cell help. Thus, basophils are important contributors to humoral memory immune responses.
SummaryThe formation of gametes is a key step in the life cycle of any sexually reproducing organism. In flowering plants, gametes develop in haploid structures termed gametophytes that comprise a few cells. The female gametophyte forms gametic cells and flanking accessory cells. During a screen for regulators of egg-cell fate, we isolated three mutants, lachesis (lis), clotho (clo) and atropos (ato), that show deregulated expression of an egg-cell marker. We have previously shown that, in lis mutants, which are defective for the splicing factor PRP4, accessory cells can differentiate gametic cell fate. Here, we show that CLOTHO/GAMETOPHYTIC FACTOR 1 (CLO/GFA1) is necessary for the restricted expression of egg-and central-cell fate and hence reproductive success. Surprisingly, infertile gametophytes can be expelled from the maternal ovule tissue, thereby preventing the needless allocation of maternal resources to sterile tissue. CLO/GFA1 encodes the Arabidopsis homologue of Snu114, a protein that is considered to be an essential component of the spliceosome. In agreement with their proposed role in pre-mRNA splicing, CLO/GFA1 and LIS co-localize to nuclear speckles. Our data also suggest that CLO/GFA1 is necessary for the tissue-specific expression of LIS. Furthermore, we demonstrate that ATO encodes the Arabidopsis homologue of SF3a60, a protein that has been implicated in pre-spliceosome formation. Our results thus establish that the restriction of gametic cell fate is specifically coupled to the function of various core spliceosomal components.
Binding of intact Ag is a hallmark of Ag-specific B cells. Apart from B cells, a small number of non-B cells can bind Ag with comparable efficacy as B cells and are found in the peripheral blood, spleen, and bone marrow of mice. This population has been observed for a long time and recently named “Ag-capturing cells.” Their identity remained enigmatic. In this study, we show that these cells are basophilic granulocytes. Their ability to capture Ags is dependent on surface IgE receptors and on Ag-specific plasma IgE molecules appearing after immunization. Several surface markers including surface bound IgE, IL-3R, CD45, CD16/32, and the chemokine receptor CCR2 were used to clearly identify these cells. Cross-linkage of surface Igs results in the release of large amounts of IL-4 and IL-6. The data identify basophils as Ag-capturing cells and support the concept of basophils as important regulators of humoral immune responses.
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