Substantial generation of human target-specific cytotoxic lymphocytes (CTL) was exhibited in allogeneic in vitro sensitization (IVS) cultures employing subimmunogenic numbers of allogeneic stimulator lymphocytes when supplemented with partially purified and lectin-freed T-cell growth factor (PP-TCGF). These results indicate that very few allogeneic stimulator cells are able to provide the optimal specificity signal required for allogeneic IVS. Augmentation was unique to PP-TCGF since neither PHA nor the unpurified TCGF exhibited this property. PP-TCGF augmented the generation of cytotoxic lymphocytes when added at the initiation of the 7-day culture (Day 0) or on Day 1, 2, 3, or 4. The level of cytotoxic activity generated was found to be proportional to the quantity of PP-TCGF used and was not due to immune interferon contained in the PP-TCGF preparations. PP-TCGF was also found to augment alloantigen stimulated proliferation as measured by [3H]tdr uptake. Finally, in two model IVS systems in which allogeneic stimulator cells had been rendered nonimmunogenic by uv irradiation or by heat killing, PP-TCGF supplementation was found to totally restore the generation of target specific CTL by providing a proliferative stimulus. These results show that human PP-TCGF contains a soluble factor, functional in controlling the proliferation and development of human CTL in vitro. The use of PP-TCGF in IVS cultures may represent an useful technique for augmenting the in vitro immune response to weak immunogens.
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