Cell-type-specific cytotoxicity of anti-CD4 and anti-CD8 ricin immunotoxins against human alloreactive T-cell clones

Potent T-cell subset-directed immunotoxins (ITs) were generated by conjugating the anti-CD4 monoclonal antibody (MoAb) G17-2 and the anti-CD8 MoAb G10.1 to the ribosome-inhibitory protein, ricin. The cell-type-specific cytotoxicities of the generated ITs were evaluated at the clonal level using human alloreactive T-cell clones. The kinetics of anti-CD4 ricin-induced inactivation of protein synthesis in target CD4⁺ cloned T-cells was first order with no detectable lag period and a maximum rate of 0.07 logs per hour (t₁₀ = 13.6 hours; first-order rate constant/K = 0.17 hr^-1). The alloantigen specific lytic function of the CD4⁺ cytolytic T-cell clone JMAC28 was acutely sensitive to anti-CD4 ricin, and no residual lytic activity against allogeneic targets was detectable 24 hours after treatment with as little as 0.5 nmol/L anti-CD4 ricin. Notably, both anti-CD4 ricin and anti-CD8 ricin elicited a selective and dose-dependent inhibition of clonal proliferation of target T-cell clones with a maximum kill of >3 logs at 5 nmol/L. No significant 'bystander effects' were observed for non-target cells. Bone marrow progenitor cells CFU-GM, BFU-E, and CFU-GEMM were only minimally affected by either IT. We conclude that these ITs show considerable potential for effective depletion of T-cell subpopulations from allogeneic donor marrow grafts for clinical graft-versus-host disease (GVHD) prophylaxis.