Lipopolysaccharide Blasts Reactive to T

B lymphocytes can be induced to divide and form Ig-secreting clones either by Tcell help or by mitogens (1, 2). About one in three splenic lymphocytes respond to Bcell mitogens, and such activation is polyclonal: 1 clone in 1,000 produces antibodies against sheep erythrocytes (SRC), 1 1 in 500 against horse erythrocytes (HRC), and 1 in 50 against trinitrophenyl (TNP) (3-5). The presence of mitogen is required for each consecutive cell cycle (6; W. Lernhardt and F. Melchers. Manuscr ipt in preparation.), but the various B-cell mitogens are largely interchangeable (6). Both the frequencies of rnitogen-reactive cells and their multireactivity suggests that these cells are also the targets of T-cell help. T-cell help is generated by the interaction of antigen-specific helper T-cells and adherent cells (2, 7, 8). By the use of recently developed methods it could be shown that such antigen-activated T-cell help was as effective in inducing ant i -SRC and an t i -HRC clones as was lipopolysaccharide (LPS) or lipoprotein (LP) (9). By combining the improved method of generating antigen-activated T-cell help (9, 10) 2 with the techniques developed for testing responses of B blasts at the single cell level (3), we could investigate three questions: (a) Is T-cell help directed toward the same set of B cells as are inducible by B-cell mitogens? (b) Is this help limited in any way, either by the affinity of antigen for cellular receptors or by some other kind of restriction (11, 12)? (c) Is antigen-activated T-cell help as specific as is its induction?