Evasion of Immune Mechanisms by Viruses

1-Viruses can alter their antigens by point mutations or by reassortment of RNA genomes in RNA viruses:

• The antigens affected may be targets of antibodies or T cells, and large numbers of antigenically different strains of many viruses are known to exist.
•  Because of antigenic variation, a virus may become resistant to immunity generated in the population by previous infections.
• E.g.: influenza virus, Rhinovirus, HIV

·         The influenza pandemics that occurred in 1918, 1957 and 1968 were due to different strains of the virus, and subtler variants arise more frequently

2- Some viruses inhibit class I MHC-associated presentation of cytosolic protein antigens:

·         Several mechanisms that inhibit antigen presentation have been described with different viruses.

·         Inhibition of antigen processing and presentation blocks the assembly and expression of stable class I MHC molecules and the display of viral peptides.

·         As a result, cells infected by such viruses cannot be recognized or killed by CD8⁺ CTLs.

·         NK cells may be a host adaptation to kill these infected cells because NK cells are activated by the absence of class I MHC molecules.

·         Some viruses blockade TAP transporter

·         E.g.: Herpesvirus, Cytomegalovirus

3-Some viruses produce cytokines homologues molecules that inhibit innate and adaptive immunity:

·         Poxviruses encode molecules that bind to several cytokines, including IFN-γ , TNF, IL-1, IL-18 and chemokines, and these molecules are secreted by infected cells.

·         The secreted cytokine-binding proteins may function as competitive antagonists of the cytokines.

4-Viruses may infect and either kill or inactivate immunocompetent cells:

·         The obvious example is HIV, which survives by infecting and eliminating CD4⁺ T cells, the key regulators of immune responses to protein antigens.

5-Production of immunosuppressive cytokines:

·         Epstein-Bar virus secrete IL -10