Effects of ochratoxin A upon early and late events in human T-cell proliferation.

We have investigated the effect of ochratoxin A (OA) on both early and late events during activation of human T lymphocytes. As early activation parameters we chose to measure both changes in intracellular Ca2+ levels and the activity of protein kinase C following the triggering process. Our results demonstrate that concentrations of OA that inhibit overall activation had no effect neither on the absolute levels nor on the duration of the Ca2+ response. Furthermore, PKC activity as measured by phosphorylation of two specific cytosolic substrate substrate proteins, was also unaffected. However, when the cells were prestimulated for 48 h to measure effects on late events in the activation cascade, addition of graded concentrations of ochratoxin A down to 6.4 microns completely inhibited the DNA synthesis. This shows that ochratoxin A is able to block DNA synthesis efficiently even if the activation process has been running for 48 h in advance. To investigate whether the inhibitory effect on DNA synthesis could be ascribed to inhibition of protein synthesis, we carried out experiments to measure protein synthesis both in resting and activated T-cells. In resting T-cells protein synthesis was nearly abrogated by 12.5 microM ochratoxin A, but only minor effects were observed in stimulated cells. The low impact of ochratoxin A on protein synthesis in activated cells may indicate that other mechanisms than just a general inhibition of protein synthesis are operating.