We have produced and investigated cell couples formed between cloned Th cells or T hybridoma cells, and either Ag-presenting B hybridoma or B lymphoma cells. The specific direct interaction between a Th and B-APC is here demonstrated by two rearrangements occurring inside the bound Th cell; the MTOC (and presumably the GA) is oriented to face the cell contact region with the B cell, and a membrane-associated cytoskeletal protein, talin, becomes concentrated under the contacting Th membrane. In the absence of the specific Ag or the correct Ia determinant, nonspecific T-B cell couples form that are morphologically indistinguishable from specific cell couples in the light microscope, but neither the MTOC nor the talin rearrangement occurs inside the bound T cell of such nonspecific couples. Furthermore, Ag processing by the B cell is required to produce the MTOC and talin rearrangements within the T cell in specific T-B couples. In the case of allogeneic Th-B cell couples, similar specific MTOC and talin rearrangements are observed inside the Th. Extracellular Ca2+ is required for the MTOC orientation to occur inside the specifically bound Th cell, but not for the talin rearrangement. It is proposed that the MTOC (and GA) reorientation and the talin rearrangement are involved in the directed secretion of GA-derived lymphokines from the Th cell to the bound B cell.

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