It is likely that the low responder numbers at the lowest dose was a function of dose rather than MHC class II allele distribution. Alexander et al. described a de novo designed non-natural pan-DR epitope peptide (PADRE) that binds promiscuously to common HLA-DR alleles [2]. The PADRE peptide has been tested in a number of clinical trials. BCR-ABL peptides linked to PADRE and co-administered with GM-CSF to patients with chronic myeloid leukemia elicited a PADRE-specific recall response in 14 of 14 subjects tested [31]. Gefitinib price PADRE peptide admixed with MAGE3 peptide in incomplete Freunds adjuvant administered

to melanoma patients elicited detectable but low levels of PADRE-reactive effector cells in 7 of 9 subjects [32]. PADRE peptide and WT-1, Muc-1, and proteinase-3 CTL epitopes admixed with CpG oligonucleotides in montanide and administered to patients with acute myeloid leukemia

and multiple myeloma induced an increase in PADRE-reactive effector T cells in all subjects, although these T cells showed an apparent defect in IL-2 secretion [33]. In contrast, a DNA vaccine encoding 21 HIV-specific CTL epitopes and PADRE was tested in 42 healthy volunteers and elicited only one positive recall response to PADRE as measured by ELISpot [34]. Finally, autologous dendritc cells pulsed with the PADRE elicited an ex vivo recall response to PADRE in 10 of 18 subjects in one study [35] and low level Akt inhibitor responses in another study [36]. Not surprisingly, the efficacy and universality of the PADRE peptide may be dependent Thiamine-diphosphate kinase upon the context in which the peptide is administered, such as dose, regimen, route, adjuvant, and form (free peptide, linked peptide, DNA-encoded, or pulsed DCs). One of the potential advantages of using a universal T cell helper peptide based on TT and DT is that pre-existing CD4 T cell memory to TpD from prior immunization with DT and TT may confer an advantage for a TpD-containing nanoparticle vaccine by generating a larger pool of antigen-specific T cells that

can provide faster and more efficient help to B cells in a secondary challenge [37], [38] and [39]. In addition CD4 memory T cells have several functional characteristics that facilitate a more robust response to antigen. For example, CD4 memory T cells have a lower threshold for activation by antigen than naïve cells and show polarized differentiation to specific T cell subsets (e.g. Th1, Th2, Th17, and T follicular helper (Tfh) subsets), and multi-cytokine expression (e.g., TNF-α, IL-2 and IFN-γ) [40]. In particular, CXCR5 expressing memory CD4 cells have been found to provide accelerated help to B cells, perhaps due to their ability to localize to B cell follicles [41]. Overall the data suggests that the existence of CD4 memory T cells will be beneficial in producing a more rapid and robust induction of antibody production. As a result there may be an advantage in targeting memory T cell activation to enhance a response in vaccines.