Development of 111In-labeled tumor-associated antigen peptides for monitoring dendritic-cell-based vaccination

Dendritic cells (DC) are professional antigen-presenting cells capable of inducing potent immune responses. In our ongoing clinical trials, human leukocyte antigen (HLA)-A2.1+ melanoma patients are vaccinated with mature DC, presenting tumor-derived peptides in major histocompatibility complexes (MHC) to naive T cells. Previously, we have shown that both intradermally and intranodally injected 111In-labeled mature DC migrate to draining lymph nodes. However, little is known about the fate of the MHC–peptide complex after injection of these peptide-loaded DC.The aim of the present study was to develop radiolabeled, tumor-derived peptides to monitor their binding to MHC Class I.MethodsThe HLA-A2.1 binding peptide gp100:154–162mod (gp100:154m) was conjugated with diethylenetriamine pentaacetic acid (DTPA) either at the N-terminus (α-DTPA-gp100:154m) or at the epsilon amino group of the Lys154 residue (ε-DTPA-gp100:154m) and labeled with 111In.ResultsThe maximum specific activity for both peptides was 13 GBq/μmol. The IC50 of the α-[111In]DTPA-gp100:154m peptide was >75 μM. The IC50 of the 111In-labeled ε-DTPA-gp100:154m was 3 μM, similar to the unconjugated peptide. MHC binding studies showed specific binding of the ε-[111In]DTPA-gp100:154m peptide to the JY cells at 4°C. Interestingly, no specific binding was observed for the α-[111In]DTPA-gp100:154m peptide. In contrast to the α-[111In]DTPA-gp100:154m peptide, the ε-[111In]DTPA-gp100:154m peptide was recognized by cytotoxic T cells.ConclusionWhen DTPA was conjugated to the epsilon NH2 group of the Lys154 residue, MHC binding of the peptide was preserved and could still be recognized by cytotoxic T cells. These studies allow the noninvasive determination of the behavior of MHC–peptide complexes on DC in vivo.