The Fate of Human Langerhans Cells in Hematopoietic Stem Cell Transplantation.

The fate of Langerhans cells (LC) and other antigen-presenting cells (APC) in haematopoietic stem cell transplantation (HSCT), their depletion by conditioning regimens, reconstitution and chimerism are important factors in understanding graft versus host disease and the outcome of transplantation. Hitherto untested predictions in humans state that depletion of recipient LC may prevent acute graft versus host disease (GVHD), the acquisition of donor chimerism in LC may drive the evolution of clinical GVHD from acute to chronic and the persistence of recipient LC may explain late acute GVHD phenomena and donor lymphocyte infusion toxicity.

Between April 2003 and June 2005 we obtained 184 skin biopsies from 76 patients at 4 UK centres. Using confocal microscopy of intact epidermal sheets to image up to 2000 LC at a time, we find that full intensity transplant (FIT) conditioning with TBI-based regimens or busulfan and cyclophosphamide depletes 55% of LC at day 0 (n 14; p 0.001). In contrast, fludarabine-based reduced intensity transplant (RIT) depletes only 9% of LC (n 23; p 0.061). A minimum LC density is reached at 14-21 days in both regimes with little difference at the nadir, suggesting that the effect on density at day 0 is primarily kinetic. Rapid recovery occurs by 40 days in the absence of acute cutaneous GVHD but is delayed beyond 100 days in the presence of GVHD (n 50; p 0.006)

We assessed LC chimerism in sex-mismatched transplants by allowing single cells to migrate from small epidermal sheets in vitro and applying a two-step Giemsa/FISH assay. At 40 days median LC donor chimerism is 97% after FIT and 36.5% after RIT (n 11; p 0.004). At 100 days median donor chimerism is 100% after FIT and 97.5% after RIT (n 28; p 0.133.). The majority of transplants - 16/28 (57%) - achieve 100% LC donor chimerism at day 100 and all are at least 90%. Attainment of full donor chimerism at 100 days shows only a trend with FIT (p 0.133), and with complete myeloid chimerism in the blood (p 0.080) but is strongly associated with prior acute cutaneous GVHD (p 0.002). At one year post transplant it is possible to detect rare recipient cells in a minority of transplants (12/1746 interphase nuclei - less than 1%), confirming that LC may regenerate in the skin under quiescent conditions.

In conclusion we find that: 1) recipient LC survive conditioning, especially after RIT, suggesting that targeted APC depletion might have potential as a novel means of preventing acute GVHD; 2) LC turnover is accelerated by full conditioning, complete myeloid chimerism and acute GVHD; 3) the majority of LC are donor-derived in all transplants at 100 days, consistent with the hypothesis that donor engraftment of APC drives the transition of acute to chronic GVHD. 4) there is little evidence using these conditioning regimens that acute GVHD or DLI-toxicity occurring after 100 days is related to the persistence of recipient APC.