CTL play a pivotal role in anti-viral and anti-tumor selleck chemical immunity. Vaccination to date has been unsuccessful for treatment of cancer patients with established disease. It is accepted that
the generation of high-frequency T-cell responses is not necessarily an indication of the induction of a competent immune response. The presence of Ag-specific T cells rarely correlates with positive clinical responses in patients, whereas T-cell avidity may be a better indicator of clinical response 1–4. In both viral infection and tumor models, only high-avidity and not low-avidity CTL mediate viral clearance and tumor eradication 1, 3, 5. Avidity is defined by the amount of peptide required for activation of effector function 3, 6, 7 and is therefore a measure of the overall strength PS-341 of the interaction between a CTL and a target cell 3, 8, 9. Although avidity has been shown to be important, the mechanisms by which high CTL are generated in vivo remains unclear. Several factors have however been implicated in
the regulation of functional avidity, e.g. the cytokines IL-12 and IL-15 10, 11, CD8αβ expression 7, 12, TCR affinity, the level of co-stimulatory molecules expressed by APC 10, 13 and the maturation state of DC. The challenge is therefore to find a vaccine approach that mimics these conditions. Several groups have used Ab to stimulate immune responses 14. They showed that it was possible to genetically replace CDR-H3 with helper and B-cell epitopes and stimulate immune responses 15, 16. Zaghouani et al. also attempted to of replace CDRH3 with class I restricted
CTL epitopes. Although they showed that transfectomas expressing recombinant Ig were capable of inducing CTL responses, the purified Ig was unable to do so 17, 18. Recent studies with this mouse IgG2b expressing a nucleoprotein CTL epitope (NP-Ig) have shown that it is possible to stimulate CTL responses if co-administered with the TLR agonist dsRNA, which upregulates Fer receptor IV (FerγIV) receptor IV (FcγRIV) and downregulates FcγRIIb 19. This group did not assess T-cell avidity. We have shown that a human monoclonal IgG1 anti-idiotypic Ab, which expressed a T-cell mimotope of CD55 Ag within its CDR, can stimulate helper and cytotoxic T-cell responses in over 300 cancer patients with no associated toxicity 20–22. Two of the osteosarcoma patients were cured of their disease and survived for at least 10 years post treatment. When the Fc region of this Ab was removed it displayed 1000-fold less efficiency at stimulating T cells 23. Immature circulating DC in the blood express only low levels of FcγRI to avoid binding serum Ig, but this is transiently upregulated by IFN-γ on extravasation into inflamed tissue 24. It can then bind, internalize and process any IgG whether free or forming small immune complexes within the inflamed tissue. Large immune complexes can be cross-presented by FcγRIIa (FcγRIV in mice) but only if the inhibitory FcγRIIb is blocked or downregulated 25.
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