DCs 1 are antigen-presenting cells that regulate several components of the immune system. The mature or terminal stage of DC development induces specific T-cell immunity and resistance to experimental tumors in vivo. However, DC maturation is induced by inflammatory and microbial stimuli, so it is unlikely that mature DCs normally present antigens from tumor cells in cancer patients. Accordingly, clinical studies have begun in which DCs are generated ex vivo, charged with tumor antigens, exposed to maturation stimuli and reinfused to immunize patients. This approach has the potential to control responses to cancer antigens in a specific and nontoxic manner, in both vaccination and therapeutic settings.
DCs can mobilize several immune resistance mechanisms. These include CD8 ฯฉ CTLs, CD4 ฯฉ helper T cells, NK and NKT cells. Each of these lymphocytes recognizes targets through a distinct mechanism and has the capacity to kill tumor cells and release valuable protective cytokines like IFN-โฅ. CD4 ฯฉ T cells also provide essential help for the expansion and maintenance of CD8 ฯฉ cytolytic cells, while NK and NKT cells can eliminate targets that dampen presentation on MHC class I to escape CTL recognition. The stimulation and concerted action of these classes of lymphocytes can now be studied directly in patients, using ex vivo-derived DCs.
Several findings have emerged from studies of healthy volunteers who have been immunized with DCs charged with model antigens, KLH protein and influenza virus matrix peptide. Mature DCs elicit a polarized Th1 type of CD4 T-cell response, only a single injection being required. Vaccination with antigen-bearing DCs also markedly improves the functional affinity of CD8 ฯฉ T cells. These findings are important in the context of immunotherapy because Th1 cells are more efficient helper cells in experimental models of viral infection and tumors, while high-affinity CTLs should improve recognition of tumor-derived peptides. An important caution also has surfaced when DCs are not adequately differentiated. Immature DCs can silence adaptive T-cell responses, e.g., by inducing IL-10 -producing, T-reg cells.
DC-based active immunization does not result in major short-