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 IgG(1 kappa) monoclonal antibo
dy (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 com
bination 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 w
as determined serially with a complement-dependent cytotoxicity assay,
Lewis anti-hamster mixed lymphocyte reaction and cell-mediated lympho
lysis assays were performed within 7 days after rejection of the skin
graft, NK cell function was tested using a cytotoxicity assay versus Y
AC-1 target cells on day 14 or day 15 after skin grafting, Results, Me
dian graft survival in untreated animals was 7 days. There was only mo
dest prolongation in rats treated with rapamycin alone (median surviva
l 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 (AL
S+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 m
ixed lymphocyte reaction responses, cytotoxic T cells, and normal NK a
ctivity were seen at the time of rejection in untreated rats as well a
s those treated with 3.2.3 mAb alone, ALS alone, ALS+3.2.3 mAb, and ra
pamycin alone. ALS+rapamycin completely blocked the formation of anti-
hamster antibodies and cytotoxic T cells but did not suppress NK activ
ity. 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 c
ytotoxic T cells. ALS, rapamycin, and ALS+rapamycin do not suppress NK
activity in Lewis rats, although their use produces a modest prolonga
tion of hamster skin graft survival. The administration of 3.2.3 mAb t
o Lewis rats results in a marked but transient suppression of NK cell
function, which substantially prolongs hamster skin graft survival onl
y when antibody and cytotoxic T-cell production have also been suppres
sed.