Suppression of allograft responses by combining donor alloantigen-specific intravenous presensitization with suboptimal doses of FK506.

C57BL/6 (B6) mice were injected i.v. with class I H-2-disparate B10.QBR spleen cells (10(7)/mouse). This regimen, termed "donor alloantigen-specific i.v. presensitization" (DSP), induced almost complete elimination of anti-B10.QBR mixed lymphocyte reaction/IL-2 production but did not affect the generation of CTL responses. Repeated (5 or 11 times) administration in vivo (over 7 or 18 days) of FK506 at suboptimal doses (0.75-1.0 mg/kg/day) failed to eliminate the capacities to exhibit MLR/IL-2 production and to generate CTL responses. Prolongation of skin graft survival was not induced by either of a single DSP or FK treatment (0.75-1.0 mg/kg/day, 11 times during 18 days) alone, but by the combination of these. Such combined treatment also resulted in almost complete reduction of CTL responses before (5 rounds of FK injection) or after (11 rounds of FK injection) recipients were engrafted with B10.QBR skin grafts. Under conditions in which lymphoid cells from mice receiving both treatments failed to generate CTL responses, the addition of recombinant IL-2 to cultures restored the CTL generation, suggesting that CTL precursors themselves are not attenuated by the combined treatment. Both prolongation of graft survival and suppression of CTL responses were obtained when the administration of FK506 was started before but not after DSP. Prolongation of graft survival could also be obtained in class I and II MHC-disparate combinations when the combined treatment was performed in a particular protocol. These results indicate that (1) DSP alone fails to prolong graft survival in class I and class I and II MHC-disparate combinations; (2) such failure is ascribed to the induction of CTL responses by CTL precursors and CTL helpers, both of which are DSP-resistant; (3) the administration of suboptimal doses of FK506 is not sufficient for the suppression of CTL responses, but is effective selectively for suppressing DSP-resistant CTL helpers; and (4) the combination of DSP with FK506 treatment in an appropriate protocol can thus prolong graft survival through the suppression of CTL-involved as well as CTL-independent graft rejection pathways.