IDENTIFICATION OF DONOR-DERIVED DENDRITIC CELL PROGENITORS IN BONE MARROW OF SPONTANEOUSLY TOLERANT LIVER ALLOGRAFT RECIPIENTS1,2

Multilineage donor-derived hematopoietic cell chimerism is a persistent feature of spontaneously tolerant mouse liver allograft recipients. We have shown previously that normal liver-derived precursors of "chimeric" dendritic cells (DC) propagated in vitro migrate in vivo to T-dependent areas of allogeneic lymphoid tissue, where they or their progeny appear to persist indefinitely. In this study, granulocyte-macrophage colony-stimulating factor (GM-CSF)+interleukin-4 (IL-4) were used to propagate DC progenitors from freshly isolated mouse bone marrow. The progenitor cells gave rise in 7-10 days to potent antigen-presenting cells (APC) that stimulated naive allogeneic T cells in primary mixed leukocyte cultures (MLC). The culture method, together with the reverse transcriptase-polymerase chain reaction (RT-PCR) for the detection of donor and recipient strain major histocompatibility complex (MHC) class II mRNA was used to test whether donor-derived DC could be propagated from the bone marrow of unmodified, orthotopic liver allograft recipients. Freshly isolated bone marrow from these transplanted animals contained small numbers of donor cells and responded to GM-CSF+IL-4 stimulation. In addition to cells expressing recipient (B10) phenotype (H-2Kb+; Iab+), a minor population of donor (B10.BR)-derived cells (H-2Kk+; Iak) were also propagated from liver graft recipients euthanized two weeks posttransplant. DC sorted from these cultures exhibited stimulatory activity for recipient strain T cells consistent with a low level (< 1%) of donor DC propagation. The immunologic role of donor-derived DC progenitors in liver allograft recipients and its relation to the induction and maintenance of donor-specific unresponsiveness remains to be determined.