Stimulation of subpopulations of human lymphocytes by a vaccine strain of Francisella tularensis.

When purified T lymphocytes from individuals vaccinated with a viable, attenuated strain of Francisella tularensis were incubated in vitro in the presence of heat-killed bacteria or a membrane preparation of the vaccine strain, they were stimulated to form blast cells and to synthesize deoxyribonucleic acid. The blast cells had the characteristics of T cells, being devoid of surface immunoglobulin and able to form rosettes with sheep erythrocytes. The stimulation occurred only when monocytes were present. A lymphocyte preparation enriched in B lymphocytes did not respond to the heat-killed bacteria or to the membrane preparation. In a stimulated mononuclear leukocyte preparation, about 70% of the blast cells formed rosettes with sheep erythrocytes, and 10 to 20% of them had surface immunoglobulin. The results show that there is an enlarged population of specifically committed T lymphocytes after tularemia vaccination. It is suggested that the lymphocyte stimulation test measures mainly T-lymphocyte reactivity when membranes or whole bacteria of F. tularensis LVS are used as antigen, and that the stimulation of human T lymphocytes by whole bacteria or bacterial membranes is completely monocyte or macrophage dependent. The present experimental procedure may provide a model for study of antigen-induced stimulation of human lymphocytes under controlled conditions. The technique used gave a reproducible, extremely purified preparation of T lymphocytes and a preparation of monocytes especially suitable for microcultures.