Importance of natural killer cells in the rejection of hamster skin xenografts.

Background: In the hamster to rat xenogeneic combination, antibodies, T cells, and natural killer (NK) cells have all been implicated in the process of rejection. 3.2.3 is a mouse IgG1kappa monoclonal antibody (mAb) directed against NKR-P1A on rat NK cells. The purpose of this study was to evaluate the effect of this mAb independently and in combination with other immunosuppressive agents in a hamster to rat skin graft model in order to elucidate the mechanisms involved in xenograft rejection.

Methods: Lewis rats were recipients of hamster skin grafts. Various groups received antilymphocyte serum (ALS) (days -1, 0, and +2), rapamycin (3 mg/kg; alternate days from day +1 through day +13), and 3.2.3 mAb (days 0, +1, and +2). Anti-hamster antibody production was determined serially with a complement-dependent cytotoxicity assay. Lewis anti-hamster mixed lymphocyte reaction and cell-mediated lympholysis assays were performed within 7 days after rejection of the skin graft. NK cell function was tested using a cytotoxicity assay versus YAC-1 target cells on day 14 or day 15 after skin grafting.

Results: Median graft survival in untreated animals was 7 days. There was only modest prolongation in rats treated with rapamycin alone (median survival time [MST]=9 days) or ALS alone (MST=10 days). The use of 3.2.3 mAb in untreated rats (3.2.3 alone MST=7 days) and in ALS-treated rats (ALS+3.2.3 MST=9.5 days) did not improve graft survival. The combination of ALS+rapamycin substantially improved graft survival (MST=13 days), and even greater prolongation was seen with the addition of 3.2.3 mAb (ALS+rapamycin+3.2.3 MST=18.5 days). Cytotoxic antibodies, secondary mixed lymphocyte reaction responses, cytotoxic T cells, and normal NK activity were seen at the time of rejection in untreated rats as well as those treated with 3.2.3 mAb alone, ALS alone, ALS+3.2.3 mAb, and rapamycin alone. ALS+rapamycin completely blocked the formation of anti-hamster antibodies and cytotoxic T cells but did not suppress NK activity. The use of 3.2.3 mAb produced a marked but transient suppression of NK activity in all groups.

Conclusion: Hamster skin xenografts can be rejected by Lewis rats in the absence of cytotoxic antibodies and cytotoxic T cells. ALS, rapamycin, and ALS+rapamycin do not suppress NK activity in Lewis rats, although their use produces a modest prolongation of hamster skin graft survival. The administration of 3.2.3 mAb to Lewis rats results in a marked but transient suppression of NK cell function, which substantially prolongs hamster skin graft survival only when antibody and cytotoxic T-cell production have also been suppressed.