L-arginine deprivation and its effect on CD3ζ expression. (Upper panel) Expression of CD3ζ in Jurkat T-cells (JKT) cultured for 48 hours alone (control) or co-cultured in trans-wells with CL-2, CL-19, or Renca cell. Differences in CD3ζ expression were measured by mean fluorescence intensity. (Lower panel) L-arginine levels in supernatants after 48 hours of co-culture of Jurkat cells with the cell lines. Levels of L-arginine were significantly lower (* P < 0.001) in CL-19 co-cultures than in Jurkat control or in co-cultures with CL-2 and Renca cell lines. Data shown is representative of a single experiment at 48 hours in culture.