Studies with influenza A and paramyxovirus indicated that immunoprophylaxis requires the generation of an immune response to viral proteins that will protect individuals on exposure to the infecting virus. The proteins inducing such a response are termed 'protective antigens' and, in the main, are represented by surface-located glycoproteins in enveloped viruses and capsid proteins in non-enveloped viruses (Epstein et al. 1993; Tao et al. 2000). Some non-structural proteins have also been found to be protective, such as the NS1 protein of dengue, T antigen of SV40 and Tat and Nef of HIV-1. Currently (2006), 16 viruses are covered by licensed vaccines in the United States. These protect against adenovirus, hepatitis A and B, influenza A and B, Japanese encephalitis virus, measles, mumps, papillomavirus, poliovirus, rabies, rotavirus, rubella, smallpox, varicella and yellow fever.
In general, although MHC Class I-restricted CD8+ cytotoxic T-lymphocytes (CTLs) have a major role in detecting and destroying virus-infected cells, the principal mediator of resistance to infection is by antibody, with the assistance of MHC Class II-restricted CD4+ helper T-cells (TH). Most of the vaccines listed above work by inducing circulating low-titre antibody against blood-borne or blood-distributed viruses such as hepatitis A and B (HAV/HBV), measles, polio, rubella, smallpox and varicella (VZV). Live vaccines induce both humoral and CTL responses, whereas peptide, inactivated or subunit vaccines are generally poor inducers of cell-mediated responses and, in the case of subunits, may require the addition of adjuvants to induce the required humoral response. At present, only two vaccines, one against hepatitis B and the other against human papillomavirus, are manufactured using recombinant DNA. The remainder of this section summarizes the current efforts being made to produce new recombinant vaccines.
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