T-lymphocyte killing by T101-ricin A-chain immunotoxin: pH-dependent potentiation with lysosomotropic amines.

To maximize T-lymphocyte killing with anti-pan-T-lymphocyte immunotoxin (IT), prepared by linking ricin A-chain to monoclonal antibody (MoAb) T101 (T101-RTA IT), we have established the nature and the extent of parameters that influence the sensitivity of T lymphocytes to the IT. We showed that peripheral blood T lymphocytes, which are much less susceptible than malignant T cells to the T101-RTA IT, could become highly sensitive to the IT when used in conjunction with NH4Cl. However, enhancement of the IT by NH4Cl only occurred when the pH rose above neutrality. This pH-sensitive process of IT activation by NH4Cl, which led to an all-or-nothing effect within an extremely narrow pH window of 0.7 pH unit width, is due to the fact that NH3 is the effective enhancing component of NH4Cl. We also showed that F(ab')2 or Fab containing IT were much more effective than those produced using the whole IgG counterpart. From these data, we defined a procedure for an optimal and specific elimination of T lymphocytes in vitro by treating them with (Fab)T101-RTA at 10(-8) mol/L at pH 7.8 in the presence of NH4Cl for two hours. This peripheral blood cell processing elicited an abrogation of three logs of functional T-cell response. Under the same conditions, there was no reduction in the number of marrow hematopoietic precursor granulocyte-macrophage colony-forming units (CFU-GM).