Immunology of Infection

Viral infections are controlled by a combination of non-antigen-specific and antigen-specific immune responses. Most viruses induce these immune responses by causing lytic cell death which, in turn, causes inflammation and stimulates the production of cytokines. PV infection, in contrast, is non-lytic and, consequently little or no local inflammation is induced. This situation probably reflects the reduced ability of PVs to invoke effective immune responses that are capable of eliminating established lesions (Frazer, 1996). Nonetheless, there is evidence of involvement of the immune system in the control of PV infections.

Humoural (antibody) immune responses directed against almost all PV proteins have been detected in infected individuals. However, only antibodies directed against LI or L2 have been found to be neutralizing and to protect against infection. Antibodies against the E6 and E7 proteins of high risk HPVs, although not effective at the prophylactic level, are commonly associated with carcinoma of the uterine cervix. Antibodies against the E2 and E4 proteins have also been associated with carcinoma of the cervix, but not universally (Frazer, 1996; Shah and Howley, 1996).

The persistence of PV-induced lesions suggests that the development of an effective cellular immune response against PVs following infection is neither immediate nor universal. Nonetheless, several observations suggest that the host's cell-mediated immune response is responsible for limiting the growth and promoting the regression of PV-induced lesions. First, there is a high prevalence of PV-induced lesions and malignant tumours in hosts with suppressed cellular immunity. Second, the regression of anogenital and skin warts in humans is associated with a pronounced local infiltration of mononuclear cells (activated T lymphocytes, macrophages and, to lesser extents, NK cells and B lymphocytes) invading the epidermis and destroying the neoplastic tissue. On this basis, the cellular immune response in spontaneously regressing warts appears to be consistent with a delayed type hypersensi-tivity (DTH) reaction to foreign antigen. Nonetheless, the presence of specific cytotoxic CD8 + T lymphocytes (CTLs), which are normally involved in the resolution of viral infections, has been notoriously difficult to demonstrate in HPV infections. Indeed, only a handful of studies (i.e. Tarpey et al., 1994; Nakagawa et al., 1997) have shown E6- or E7- specific CTLs in humans. Finally, vaccination with PV antigens has been found to induce a specific T cell proliferative or CTL responses against L1, L2, E2, E6 and E7 in animals and humans (Frazer, 1996; Shah and Howley, 1996).

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