Cellular interactions involved in Th cell memory.

The cellular interactions involved in maintaining CD4+ T cell memory have hitherto not been identified. In this report, we have investigated the roles played by B cells and dendritic cells (DCs) in this process. We show that long-lasting Th cell memory depends on the presence of B cells, but that direct Ag presentation by B cells is not required. Instead, Ag presentation by DCs is critical for the survival of memory Th cells. DCs presenting specific Ag can be detected in animals long after immunization. These findings support a model in which B cells provide an environment in which Ags may be trapped and retained. This Ag is periodically presented to memory CD4+ T cells by DCs, providing an essential survival signal.