Resting and sensitized T lymphocytes exhibit distinct stimulatory (antigen-presenting cell) requirements for growth and lymphokine release.

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Abstract
Unprimed or resting T lymphocytes will grow and release lymphokines when stimulated by dendritic cells (DC). The stimulatory requirements for antigen-primed or blast-transformed T cells were examined. The latter were derived from dendritic/T cell clusters that developed during the primary mixed leukocyte reaction (MLR). The specificity of the blasts were established by a binding assay in which most T cells aggregated small B lymphocytes of the appropriate haplotype within 2 h at 4.degree. or 37.degree. C. Since unprimed T cells did not aggregate allogeneic B cells, it is suggested that DC induce T lymphocytes to express additional functioning receptors for antigen. Lyt-2- T blasts did not grow or release interleukin 2 or B cell helper factors unless rechallenged with specific alloantigen, whereupon growth (generation time of 14-18 h) and lymphokine release rapidly resumed. The blasts could be stimulated by allogeneic macrophages, B cells and B lymphoblasts, whereas the primary MLR was initiated primarily by DC. Responsiveness appeared restricted to the I region of the major histocompatibility complex, and varied directly with the level of Ia antigens on the stimulator cells. The interaction of B and T blasts was bidirectional. The T blasts would grow and form B cell helper factors, while the B cells grew and secreted antibody. However, the efficacy of T cell-mediated antibody formation was enhanced some 10-fold by the addition of specific antigen. Responses of resting helper T cells, then, are initiated by antigen plus DC. Once sensitized, T blasts interact independently with antigen presented by other leukocytes.

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