HPV innate immunity.

HPV infections of the epidermis and anogenital tract occur frequently in healthy individuals, and 'high risk' HPV types are a major risk factor for cervical cancer. The first line of defense against HPV is the innate immune system, which provides non specific protection against a variety of pathogens and also enhances the adaptive immune response. However, HPV-infected cells often evade innate immune recognition and elimination. HPV gene expression and release of virus occur in superficial squamous cells where virus antigens are not readily detected, and keratinocytes are not lysed during HPV infection so there is no inflammatory response. In addition, HPV early proteins inhibit specific components of the innate immune system. E6 and E7 inhibit signaling by type I interferons and decrease expression of multiple interferon-inducible genes. E5 and E7 inhibit expression of major histocompatibility complex class I proteins on the cell surface. HPV-infected cells are resistant to lysis by natural killer (NK) cells, but are sensitive to cytokine-activated NK cells. Activated macrophages also kill HPV-infected cells and control malignant development. Thus, innate immunity is important for prevention of HPV infections, but HPV often persists due to evasion or inactivation of innate defenses.