Donor alloreactive NK cells trigger recipient DCs to synthesize proteins that accelerate post-BMT immune reconstitution. (A) Recipient murine chimeras were constructed in which hematopoietic lineage cells and nonhematopoietic tissues differed in their MHC class I, thus rendering tissues either potentially susceptible (H-2^b) or resistant (H-2^d or H-2^d/b) to donor (H-2^d) NK cell alloreactivity. Chimeras were conditioned and received NK cells and BMT from H2^d mice. Illustrations show potential target cell susceptibility (→) or resistance (→|) to NK cell alloreactivity for each individual chimera. Post-BMT cell counts showed that, unlike NK-susceptible nonhematopoietic tissue (chimera 1), NK-susceptible hematopoietic cells were necessary for accelerated immune reconstitution (chimera 2). In particular, NK-susceptible DCs were the only recipient hematopoietic lineage cell that was necessary for accelerated immune rebuilding; accelerated post-BMT immune reconstitution did not occur in chimeras that had NK-resistant DCs (chimera 3), whereas accelerated post-BMT immune reconstitution did occur in chimeras with NK-susceptible DCs (chimera 4). (B) H-2^d NK cells (NK^allo) were cocultured with allogeneic H-2^b DCs to generate alloreactive NK/DC coculture supernatants. In control experiments, in order to obtain nonalloreactive NK/DC coculture supernatants, H-2^d NK cells (NK^syn) were cocultured with syngeneic H-2^d DCs. The supernatants were infused into lethally irradiated mice before BMT. Infusion of alloreactive NK/DC coculture supernatants accelerated post-BMT immune recovery. Infusion of supernatants from alloreactive NK/DC cocultures in which DNA transcription was blocked in DCs by actinomycin D (actD), but not in NK cells, failed to accelerate immune reconstitution (cell counts at day +20 are shown). (C) Human alloreactive NK/DC coculture supernatants (NK^allo), obtained by coculturing alloreactive NK cell clones (from HLA class I C1/C2 group heterozygous individuals) and NK-susceptible DCs (from HLA class I C group homozygous individuals), were added to thymocyte/TEC cocultures. Unlike supernatants obtained from human nonalloreactive (autologous) NK/DC combinations (NK^auto), human alloreactive NK/DC coculture supernatants increased human thymocyte counts in vitro. DNA transcription blockade in DCs, but not in NK cells, prevented the increase in human thymocyte counts (cell counts at day 10 of culture are shown). ∗P < .05, ∗∗P < .01.