Fig. 3.

Cell extrinsic recognition of infected cells. A virally-infected non-antigen presenting cells recognizes pathogen-associated molecular patterns (PAMP) (blue lines indicate viral nucleic acids) within the cytosol via the RIG-I-like receptors (RLRs), leading to secretion of type I interferons (IFNs) and other factors that activate dendritic cells. Infected dead cells are taken up by non-infected dendritic cells and viral PAMPs (blue) are recognized through endosomal Toll like receptors (TLRs). Viral antigens (red) are processed and presented on major histocompatability (MHC) class II (via the conventional endosomal pathway) or MHC class I (via cross-presentation). TLR signaling leads to the induction of costimulatory molecules and cytokines necessary for activation and differentiation of T lymphocytes.