A novel lymphocyte subpopulation, designated "B' cell" because of its characteristic dual expression of Ig and Lyt-1 antigen, is described in relation to its ability to augment the in vitro secondary antibody response. The cells are found in the spleens of normal unprimed mice as well as those of athymic nude mice and represent a small of normal unprimed mice as well as those of athymic nude mice and represent a small number (2-3%) of immunoglobulin-positive cells. No other distinguishing surface markers of conventional T and B cells, such as Thy-1, Lyt-2, Ia, and ThB antigens, are detected on the B' cell. In the in vitro anti-hapten secondary antibody response, the addition of a small number of B' cells from unprimed mice to the mixture of T and B cells greatly augmented the anti-hapten antibody formation when the number of carrier-specific helper T cells was limited. This augmentation was observed only when B and B' cells shared the same set of IgVH genes. The identity of the immunoglobulin gene between T cells and B or B' cells was not necessary for optimum antibody production. The results indicate that the presence of B' cells is necessary for the induction of an optimum antibody response when helper T cells are limited. It is suggested that B' cells deliver an additional signal to the B cell network to magnify the antibody response.

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