CTL induction by Apcs. Antigens are taken up and degraded into peptide fragments by antigen presenting cells (APC), such as immature DC. At some point on their path to the cell surface, newly synthesized MHC class II or I molecules bind the peptide antigen fragments and transport the peptides to the cell surface. CD8+ T cells recognizing the antigen expressed by weakly costimulatory cells become activated only in the presence of CD4+ T cells bound to the same APC. This happens via CD4+ T cells recognizing antigens presented by APCs and being triggered to induce increased levels of costimulatory activity by the antigen-presenting cell. The CD4+ T cells also produce increased amounts of IL-2, which drives CD8+ T cell proliferation. CD8+ T cells then become cytotoxic T lymphocytes (CTL).
