| Abstract: |
The cellular processing of three anti T cell ricin A chain (RTA) immunotoxins (IT) by two human target T cell populations was evaluated. Immunofluorescence studies with 4A2-RTA (anti-CD7), H65-RTA (anti-CD5) and M236-RTA (anti-CD8) on peripheral blood mononuclear cells (PBMC) demonstrated significant modulation of the CD5 and CD7 antigens (Ags) and poor modulation of the CD8 Ag. Antigen reexpression with time was greater for the CD5 Ag than for the CD7 Ag. The internalization of the ITs by resting (E + PBL) and activated T cells was studied by immunogold electron microscopy. The percentage of IT internalized was greater for 4A2-RTA and H65-RTA than with M236-RTA. While the rate of internalization was greater with 4A2-RTA compared to H65-RTA with E + PBL, the rate was comparable on activated T cells. With 4A2-RTA, gold particles were found more consistently in non coated vesicles, tubulovesicular structures, and in the region of the Golgi apparatus as compared to the other two ITs. Although endocytosis occurred more rapidly with activated T cells than with E + PBL, the rate of intracellular routing to the lysosomal compartment and therefore the rate of degradation, was also more rapid. These data are significant in that they demonstrate that the activity of an immunotoxin could be influenced not only by the Ag modulation and reexpression, but also by the internalization and intracellular routing of the conjugate. Furthermore, each of these factors appear to be target cell dependent, which is of potential significance for the design of clinical trials. |
