SMALL SPLENIC B-CELLS THAT BIND TO ANTIGEN-SPECIFIC T-HELPER (TH) CELLS AND FACE THE SITE OF CYTOKINE PRODUCTION IN THE TH CELLS SELECTIVELY PROLIFERATE - IMMUNOFLUORESCENCE MICROSCOPIC STUDIES OF TH-B ANTIGEN-PRESENTING CELL-INTERACTIONS

Antigen (Ag)-specific T helper (Th) cells regulate the proliferation a nd differentiation of Ag-specific B cells by secreting cytokines and b y expressing activating receptors like gp39. In vitro, the cytokines a nd the activating receptors function in an Ag-nonspecific manner. It i s unclear, therefore, how Ag specificity is imposed on B cell response s in physiological Th-B cell interactions. Here we studied, at the sin gle cell level, the interactions between cloned Th cells and small spl enic B cells, which served as Ag-specific antigen-presenting cells (AP Cs) to the Th cells. Digital confocal immunofluorescence microscopy of Th-B cell conjugates revealed significant variability in the molecula r and cellular properties of these interactions, in spite of the fact that all the interactions in this system were expected to be Ag specif ic. After 30 h of incubation B cells began to divide, and this process was entirely dependent on the presence of both Th cells and Ag. Immun ofluorescence microscopic studies showed that essentially all the mito tic B cells were bound to Th cells and faced the microtubule organizin g center (MTOC) in the Th cells where interleukin 4 was highly concent rated. Other B cells that were bound to the same Th cells but were not close to the Th-MTOC remained in interphase. These results provide th e first direct structural and functional evidence that the site of int eraction of B cells with Th cells affects their immune response. We pr opose that, during Ag-induced Th-B cell interactions, B cells that are bound facing the Th-MTOC proliferate preferentially because they are the recipients of locally secreted cytokines. In addition, these B cel ls may interact with newly expressed receptors, which may also be loca lly inserted into the Th membrane. The polarized delivery of activatin g molecules towards the Th-bound APCs may impose functional specificit y on effector molecules that otherwise are not Ag specific.