pp60^c-src expression is induced by activation of normal human T lymphocytes

We have re-examined whether pp60^c-src, the normal cellular homologue of the transforming protein of Rous sarcoma virus, is present in human T cells. By in vitro immune-complex kinase assay or Western blotting with the anti-pp60^c-src mAbs 327 or GD11, pp60^c-src was found to be present in lysates of T cell lines, including the Jurkat T cell line. The 327 and GD11 mAbs have been reported to be specific for pp60^c-src and not to cross-react with other src family members or other kinases. Furthermore, the size of the pp60^c-src bands present on Western blotting and in vitro kinase assay were clearly different from those of p56^lck or p59^fyn. In addition, pp60^c-src is detected in the HTLV-I-derived T cell lines S1T and C8, which lack expression of p56^lck and p59^fyn. RNase protection assays confirmed that pp60^c-src mRNA is present in Jurkat T cells. We also found pp60^c-src protein to be constitutively present in freshly isolated thymocytes. In contrast, pp60^c-src was absent, or present at extremely low levels, in normal, resting peripheral blood T lymphocytes, which is in agreement with previous findings. However, after stimulation of resting T cells with the mitogenic lectin PHA or with Ab to the TCR complex, pp60^c-src expression is induced in both CD4⁺ and CD8⁺ T cell subsets, with peak expression detectable 12 to 24 h after T cell activation. The levels of pp60^c-src are low in all T cells except Jurkat, where levels of pp60^c-src are comparable to levels found in a glioblastoma cell line (T98G). Nevertheless, significant levels of pp60^c-src kinase activity are readily detectable in thymocytes and activated normal T cells as well as in T cell lines. The finding that pp60^c-src is inducible following activation through the TCR suggests that pp60^c-src may play a specific role in the normal T cell activation pathway.