New strategies for second generation vaccines based on cellular immunity. CD4⁺ helper T cells mature and activate APCs through recognition of epitopes presented by class II MHC molecules (MHC II) and interaction of CD40 and CD40 ligand (CD40L). The CD40-CD40L interaction causes the APC to upregulate expression of costimulatory molecules such as CD80 and CD86 and to secrete cytokines IL-12 and IL-15. The costimulatory molecules interact with CD28 on the CD8⁺ CTL to provide a second CTL activation signal in addition to T cell receptor (TCR) recognition of an antigenic peptide presented by a class I MHC molecule (signal 1). IL-12 also contributes to activating the CTL and polarizing the T helper cell to produce Th1 cytokines, such as IFN-γ. IL-15 contributes to induction and maintenance of CTL memory and longevity. Regulatory T cells, including NK T cells and CD25⁺CD4⁺ T cells, can dampen or inhibit the CTL response in order to prevent autoimmunity, but also reduce the immune response to the vaccine. Various strategies may be employed to improve the natural T cell response. Epitope enhancement of class I or class II MHC–binding peptides can increase their affinity for the respective MHC molecules and their immunogenicity. Incorporation of (a) cytokines such as IL-15 to recruit more memory CTLs; (b) IL-12 to steer the T helper cell population towards a Th1 response; (c) GM-CSF to recruit dendritic cells; or (d) costimulatory ligands such as CD40L to activate and induce maturation of the dendritic cells recruited by the GM-CSF can synergistically amplify the immune response. In addition, CpG-containing oligonucleotides can act through toll-like receptor 9 (TLR 9) to activate dendritic cells. Increased levels of costimulatory molecules can selectively induce higher-avidity CTLs that are more effective at clearing virus infections. Agents that block factors secreted or induced by regulatory T cells, such as IL-13 and TGF-β, can synergize with other strategies to allow the CTL response to reach its full potential. Similarly, blockade of the inhibitory receptor, CTLA-4, on the T cell, can increase the T cell response. See multiple references in the text for the other strategies.