Abstract
T-lineage acute lymphoblastic leukemia (T-ALL) cells have abundant cytoplasmic CD3/Ti but express low amounts on the cell surface and are deficient in CD3/Ti-mediated signal transduction. Nevertheless, plating T-ALL cells on dishes containing immobilized anti-CD3 monoclonal antibodies with a source of growth factors induced the expression of CD25 (interleukin-2 receptor alpha chain) and stimulated the formation of blast colonies in 12 of 14 cases studied. The proliferative response to CD3 ligation was modulated by the presence of antibodies to the CD2, CD4, or CD8 accessory T-cell receptors. The effect of these accessory receptors on signal transduction mediated by CD3/Ti was next investigated by monitoring cytoplasmic calcium concentration [( Ca2+]i) and by measuring tyrosine phosphorylation after stimulation. Crosslinking CD3, CD2, CD4, or CD8 alone did not induce cytoplasmic calcium mobilization in T-ALLs, but crosslinking the accessory receptors with CD3/Ti induced calcium responses in three of the T-ALLs and enhanced calcium responses in three of the T-ALL cell lines, including HPB-ALL, MOLT-4, and CEM. Crosslinking CD4 but not CD2 with CD3/Ti greatly enhanced tyrosine phosphorylation of multiple substrates in comparison with crosslinking either CD4 or CD3/Ti separately on both normal mature T cells and the CEM T-ALL cell line. Thus, CD4 regulates CD3/Ti signal transduction in T-ALL cells through the tyrosine phosphorylation of substrates whereas CD2 may regulate [Ca2+]i signal transduction through a separate mechanism.
