Low sialic acid‐bearing mouse thymocytes do not express helper T cell properties in vitro

Immature, presumably cortical, mouse thymocytes were isolated by removing mature thymocytes by agglutination with the sialic acid‐specific lectin, lobster agglutinin 1 (LAg1). These immature cells do not respond to the mitogenic effects of concanavalin A (Con A), even in the presence of interleukin 2. Moreover, they do not exhibit two properties of helper T cells; they do not secrete interleukin 2 when stimulated with Con A, nor do they provide T help for an in vitro immune response by spleen B cells to the T‐dependent antigen, sheep erythrocytes. LAg1‐negative thymocytes fail to provide T help even though Con A is added to the cultures, regardless of the number of LAg1‐negative thymocytes added per culture, and even in the presence of exogenous macrophages. Unseparated thymocytes, LAg1‐positive thymocytes and cortisone‐resistant thymocytes all provide T cell help under these conditions. These experiments indicate that immature, presumably cortical mouse thymocytes, isolated by virtue of their low levels of surface sialic acid, are inherently unable to provide T cell help in vitro.