A T Cell Line Growing in Culture Is Subjected to a Chemical

Multiple Choice

A T cell line growing in culture is subjected to a chemical mutagen, and individual mutant lines are derived from this population. The individual mutant cell lines are each screened for their ability to proliferate in response to stimulation with antibodies to the T-cell receptor plus CD28 (anti-CD3 + anti-CD28) . In addition, the cells are treated with varying doses of added IL-2, and three days later, T cell proliferation is measured by ³H-thymidine incorporation (cpm) . The data for one mutant line and the wild-type control are shown in Figure Q16) .
Figure Q16) The gene that is defective in this mutant T cell line most likely encodes:

A) The CD3 epsilon subunit of the T-cell receptor complex
B) The co-stimulatory molecule CD28
C) CD25, also known as the IL-2 receptor $\alpha$ chain
D) The IL-2 receptor $\beta$ chain
E) The cytokine IL-2