The cellular basis of accelerated human lymphocyte activation following preliminary culture: The role of T lymphoblasts

When human peripheral blood mononuclear cells are precultured, they demonstrate greatly accelerated responses to mitogens and alloantigens. This increased responsiveness, termed cultural augmentation, was investigated to help elucidate the process of lymphocyte activation. The cellular basis of accelerated reactivity was studied by determining the capacity of purified lymphocytes to undergo cultural augmentation after depletion of monocytes. Lymphocyte preparations with less than 0.2% monocyte contamination demonstrated augmentation after preliminary culture comparable to that of mononuclear cells containing 15–25% monocytes. After preculture, preparations of purified lymphocytes contained 5–15% very large cells (three times the diameter of uncultured lymphocytes). When fractions enriched or depleted with respect to these large cells were obtained by velocity sedimentation at unit gravity, the degree of cultural augmentation was found to correlate with the percentage of large cells present. Morphologically the large cells appear to be lymphoblasts. They lack esterase or peroxidase staining granules, do not ingest latex particles, and do not have IgM or IgD on their surface. A majority of the large cells form spontaneous rosettes with sheep red blood cells indicating they are T lymphoblasts. The cells responsible for cultural augmentation had recently synthesized DNA, further suggesting a crucial role of the lymphoblasts; BUDR treatment 2 days before the end of preliminary culture, followed by light prior to culture with mitogen, ablated most of the augmented response.