Characterization of Cellular Immune Responses to a Recombinant Idiotype Vaccine by ELISPOT and Identification of MHC Class I-Restricted T Cell Epitopes by Peptide Mapping.

Induction of tumor-specific immune responses by idiotype vaccination is a promising strategy for biological therapy of indolent B cell lymphomas. In a previous report, we have described immune responses in a subset of patients participating in a phase I clinical trial primarily designed to demonstrate safety and efficacy of a recombinant idiotype vaccine (Veelken et al., ASH abstract #3342, 2003). In this trial, B-NHL patients who had relapsed after standard chemotherapy received repetitive intradermal vaccinations with recombinant idiotype Fab fragment derived from their tumor mixed with lipid-based adjuvant and concurrent subcutaneous GM-CSF at the same site. We now present the final analysis of cellular immune responses in this cohort. Peripheral blood lymphocytes (PBL) were obtained prior to and on various time points during and after vaccinations. Cryopreserved PBL were stimulated twice by autologous dendritic cells (DC) exposed to the autologous Fab protein for cross-presentation as MHC class I-bound peptides. INFγ-secreting cells were subsequently quantified by ELISPOT with Fab-presenting DC. Alternatively, freshly thawed PBL were directly assayed with recombinant Fab by ELISPOT without prestimulation. An increase in the frequency of Fab-responding PBL was detected in 7 of 15 evaluable patients with the prestimulation assay and in 4 of 10 patients by direct quantitation, resulting in a combined cellular response rate of 53% (9 of 17). A cellular immune response showed a trend for correlation with extended progression-free survival (p=0.08). T cell responses were predominantly idiotype-specific since lesser or no increases in IFNγ-secreting cells were detected against light chain- and VH family-matched control Fabs. Interestingly, a much higher base-line reactivity was observed against the control Fabs in comparison to the patient’s lymphoma Fab in four patients, pointing to the possibility of tumor-specific anergy in lymphoma patients that can at least be partially corrected by active immunization. In an effort to identify the MHC class I-presented idiotype-derived peptides, potential binding motifs were defined by reverse immunology with the SYFPEITHI algorithm (www.syfpeithi.de). Ten candidate peptides from the variable and constant region of an immune responder’s idiotype heavy chain were synthesized and evaluated with post-vaccination PBL by ELISPOT without prestimulation. A peptide derived from the CDR2 region showed a significantly higher response compared to an unrelated peptide control (p=0.0013). Additional peptides derived from the FWR1, CDR1, and CDR2 also showed a significant stimulation, but only in comparison to a no peptide control. ELISPOT offers a valuable tool to monitor cellular immune reponses and demonstrates successful induction of tumor immunity in pretreated, tumor bearing and immunosuppressed B cell lymphoma patients. Supported by Deutsche Krebshilfe