In vivo administration of anti-cd3 monoclonal antibodies or immunotoxins in murine recipients of allogeneic T cell-depleted marrow for the promotion of engraftment

The role of host anti-donor cells in rejection of fully allogeneic donor T cell-depleted marrow was investigated by using mAb or immunotoxins directed against T cell or NK cell determinants. Immunotoxins consisting of mAb conjugated to a low oligosaccharide-containing fraction of purified ricin toxin A chain (RTA) facilitated in vivo-depletion of target cell populations. BALB/c and DBA/1 donors were selected based upon their expression (BALB/c) or lack of (DBA/1) hemopoietic histocompatibility (Hh1) Ag, which may serve as targets for donor rejection in C57BL/6 hosts. When studies directed toward eliminating CD3⁺ cells were performed in both systems, injections of intact anti-CD3 mAb or anti-CD3-RTA reproducibly produced the highest engraftment values. The fact that engraftment values obtained with anti-CD3 or anti-CD3-RTA therapy in allogeneic systems were substantially higher than in syngeneic controls suggested that engraftment stimulatory proteins were released upon TCR engagement. Elevated levels of cytokines and a high mortality rate in allogeneic recipients confirmed that this was the case. Nonstimulatory preparations of anti-CD3F(ab′)₂ fragments and anti-CD3F(ab′)₂-RTA promoted engraftment of both types of allogeneic marrow, as measured by short term ¹²⁵I-IUdR assays, suggesting that stimulation was not a prerequisite for engraftment. Recipients of anti-CD3F(ab′)₂ or anti-CD3F(ab′)₂-RTA showed a marked reduction of host CD3⁺ cells as measured by immunofluorescence and flow cytometry. In long term chimerism studies, recipients of Hh1-disparate marrow and anti-CD3F(ab′)₂ had a dramatic increase in donor cell engraftment as compared to controls, indicating that positive effects on engraftment were long lived. Studies further showed that BALB/c donor cells exhibiting an Hh1 disparity were rejected by host cells expressing NK1.1 or Ly-1 (NK cells and T cells). In contrast, DBA/1 donor cells that were not Hh1-disparate were rejected by cells expressing Ly-1, but not NK1.1 (T cells only). These studies provide definitive data that CD3⁺ cells participate in the rejection of either Hh1⁺ or Hh1^null T cell-depleted allografts and offer new strategies for alloengraftment using regimens containing nonmitogenic anti-CD3.