Background: The interaction between intercellular adhesion molecule-1 (ICAM-1) and its ligand, leukocyte function associated antigen-1 (LFA-1), is especially relevant in allograft rejection. We have previously shown that the simultaneous blockade of ICAM-1 and LFA-1 by monoclonal antibodies (mAbs) results in specific immunologic tolerance to cardiac allograft in a mouse model.
Methods: We evaluated the roles of these adhesion molecules in xenograft rejection by using a rat-to-mouse concordant xenograft model to identify critical molecules for immunosuppression.
Results: Lewis rat hearts transplanted into C3H/He mice were rejected within 5 to 7 days without treatment. A significant prolongation of xenograft survival (mean survival time, 11.6 days) was observed after treatment with anti-rat ICAM-1 and anti-mouse LFA-1 mAbs, when compared with nontreated mice or mice treated with different combinations of mAbs. Graft survival was prolonged in mice treated with FK506 (1 mg/kg/day), anti-rat ICAM-1, and anti-mouse LFA-1 mAbs (mean survival time, 22.2 days), whereas the same dose of FK506 alone was not effective. The mixed lymphocyte reaction showed that a combination of mAbs against mouse LFA-1-rat ICAM-1 and rat LFA-1-mouse ICAM-1 significantly inhibited the proliferation of mouse responders to rat stimulators and rat responders to mouse stimulators, respectively. Infiltration of mouse CD4 positive, mouse CD8 positive, and mouse LFA-1 positive cells, as well as dense deposition of mouse immunoglobulin G (IgG), IgM, and up-regulation of rat ICAM-1, on the graft endothelial cells were demonstrated by immunopathologic analysis of the rejected hearts. Flow cytometric analysis with rat spleen cells demonstrated the presence of xenoreactive antibodies (mouse IgG and IgM) in the recipient's serum. This xenoreactive antibody production was delayed but not inhibited by treatment of the recipients with anti-rat ICAM-1 and anti-mouse LFA-1.
Conclusions: Blockade of the donor side ICAM-1 and the recipient side LFA-1 is critical for immunosuppression with anti-ICAM-1-LFA-1 treatment. Humoral factors may be responsible for xenograft rejection that occurs even after inhibition of the cell-mediated immune response by anti-ICAM-1 and anti-LFA-1 mAbs.