| Abstract: | Stable cell lines lacking cytotoxic activity against specific target cells were derived from highly active murine CTL clones by the omission of antigen from the culture for several weeks. Several independent CTL clones cultured in the absence of antigen showed a gradual decline in cytotoxic activity, resulting in complete loss by 5 to 10 wk. Such noncytotoxic (NC) cells lacked the ability to form stable conjugates with specific target cells, but were able to kill all target cells tested in the presence of Con A. It was shown by subcloning at limiting dilution that all cells in the starting population were cytolytically active, and that all cells in the NC population derived from such a clone were cytolytically inactive against target cells bearing an appropriate antigen under normal assay conditions. By using the monoclonal antibody F23.1, which reacted with the antigen receptors of two of the CTL clones, it was shown that the NC cells derived from these clones continued to express the receptor at normal levels. Levels of expression of Thy-1.2, Lyt-2.2, and LFA-1 were also similar in all cytotoxic cell lines and their noncytotoxic derivatives. The F23.1 antibody induced an increase in cytoplasmic free Ca2+ in both CTL and NC cells, and NC cells lysed F23.1 hybridoma cells in the absence of Con A. When cells expressing appropriate target cell antigen were added back to cultures of NC cells, cytotoxic activity of appropriate specificity was fully recovered in 2 wk. These results indicate that expression of an apparently functional antigen receptor alone is insufficient for stable binding of CTL to specific target cells, and that other factors dependent upon antigen stimulation may be involved in the recognition process. A difference in affinity for antigen between CTL and NC cells is suggested as a possible explanation for these observations. |
