111In-oxine and 99mTc-HMPAO labelling of antigen-loaded dendritic cells:: in vivo imaging and influence on motility and actin content

In cancer vaccination trials, antigen-loaded dendritic cells (DCs) are usually injected intradermally and are expected to rapidly move to a regional lymph node where antigen presentation should occur. In this study we investigated the influence of indium-111 oxine (In-111) and technetium-99m hexamethylpropylene amine oxime (Tc-99m-HMPAO) labelling on the motility and actin content of antigen-loaded DCs in parallel with in vivo migration in humans. Human autologous monocyte-derived DCs loaded with a tumour antigen were labelled with In-111 (0.11, 0.37 or 0.74 MBq/10(7) DCs) or Tc-99m-HMPAO (18.5 or 185 MBq/10(7) DCs). In-111 labelling was much more stable than 99mTc-HMPAO labelling. Quantitative videomicroscopy showed that the mean distance of displacement of DCs increased in accordance with the In-111 activity used for labelling. Monomeric (G) and filamentous (F) actin content of DCs evaluated by quantitative immunofluorescence demonstrated that the ratio of filamentous to globular actin content in labelled DCs increased significantly in accordance with the activity used for labelling with both tracers. Twelve patients enrolled in a phase I/II vaccination trial received injections of 10(7) antigen-loaded DCs labelled with either 0.74 MBq of In-111 (group A, n=6/12) or 18.5 MBq of Tc-99m-HMPAO (group B, n=6/12) in the proximal part of the legs, one intradermally on one side, one subcutaneously on the opposite side. In three of the six patients of each group, antigen-loaded DCs were incubated with monophosphoryl lipid A (MPL) just before the labelling, in order to initiate the maturation process (subgroup MPL+). Only one MPL+ patient of group A exhibited faint focal uptake in the inguinal region on the late images. Group B presented a more complex pattern of radioactivity distribution (early bladder activity without brain uptake) indicating that Tc-99m-HMPAO is not a suitable radiopharmaceutical for labelling of loaded DCs. The activity cleared from DCs as a labelled molecule different from the lipophilic Tc-99m-HMPAO. Only one of the six patients had nodular inguinal uptake on the intradermally injected side (DCs not incubated with MPL). In conclusion, the present study did not demonstrate migration of loaded labelled DCs from intradermal or subcutaneous sites of injection to regional lymph nodes. This provides an indication that a large proportion of antigen-loaded DCs, as used in current human trials for cancer therapy, may not reach regional lymph nodes.