T‐T cell interactions during cytotoxic T cell responses IV. Murine lymphoid dendritic cells are powerful stimulators for helper T lymphocytes

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Abstract
Enriched populations of Ia+ Fc receptor‐negative dendritic cells were compared to other cell types for their stimulatory activity in primary mixed lymphocyte reactions to alloantigens and 2,4,6‐trinitrophenylated syngeneic cells. Dendritic cells were 20‐100 times more effective than unfrationated splenocytes. A second cell type exhibiting strong stimulatory activity was an Ia+ Fc receptor‐positive transiently adherent cell. Both types of stimulatory cells were only effective when able to produce the monokine interleukin 1. Thus glutaraldehyde‐fixed cells were not stimulatory unless extraneous interleukin 1 was added. Stimulation of helper cells by either dendritic cells or Ia+ Fc receptor‐positive cells resulted in the production of interleukin 2. The data are discussed in view of the recently formulated interleukin model as a minimal scheme to explain T‐T cell interactions during the in vitro induction of cytotoxic T lymphocytes.