M-T7 is a myxoma virus-encoded protein that has been found to bind and disrupt human chemokine gradients. This study examined whether purified M-T7 could prevent chronic rejection in a rat renal allograft model. Fisher F344 renal allografts were transplanted into Lewis rats. Recipients were randomly grouped into two groups: control animals treated with cyclosporine alone and animals treated with cyclosporine combined with low-, medium- and high-dose M-T7 viral protein. The survival rate was not significantly different between allograft groups. Renal allografts treated with high-dose M-T7 demonstrated a significant reduction in tubular atrophy, glomerular atrophy, vascular hyalinization, cortical scarring, and lymphocyte infiltration. Morphometric analyses demonstrated that the high-dose M-T7 group also showed a significantly decreased amount of glomerulosclerosis and transplant arteriosclerosis. These data demonstrate for the first time that the immunoregulatory viral protein M-T7 can effectively attenuate chronic rejection in rat renal allografts.
Vascular leak syndrome (VLS) is the major dose-limiting toxicity of immunotoxin and interleukin-2 therapy. It has been evidenced that VLS-inducing molecules share a three-amino acid consensus motif, (x)D(y), which may be responsible for initiating VLS. Here we have constructed a recombinant immunotoxin (SMFv-PE38KDEL) by genetically fusing PE38KDEL to a single-chain antibody derived from SM5-1 monoclonal antibody, which has a high specificity for melanoma, hepatocellular carcinoma and breast cancer. In order to eliminate VLS induced by this PE38KDEL-based immunotoxin, a panel of mutants were generated by changing amino acid residues adjacent to its three (x)D(y) motifs in the three-dimensional structure. One of the SMFv-PE38KDEL mutants, denoted as mut1, displayed a similar protein synthesis inhibitory in a reticulocyte lysate translation assay compared to the wild-type SMFv-PE38KDEL (wt). The in vitro cytotoxicity assay indicated that mut1 specifically killed SM5-1 binding protein-positive tumor cells, although its cytotoxicity was slightly less than wt. In contrast, mut1 was shown to be much weaker in inducing VLS in mice than wt. The LD(50) values of wt and mut1 in mice were investigated with the result that the LD(50) of mut1 was about tenfold higher than that of wt. The in vivo antitumor activity of wt and mut1 were also compared in tumor-bearing nude mice. Both wt and mut1 were effective in inhibiting the tumor growth but mut1 showed improved therapeutic efficacy. These studies suggest mut1 may be a novel PE-based immunotoxin with much less toxicity for clinical use.
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