Antigenic shift and drift

Antigenic shift and drift

1. Antigenic Shift & Antigenic Drift Presented By ABDULLAH Roll no 03 M.phil Microbiology Batch 03 Faculty of Life sciences Department of Microbiology Abdul Wali Khan University, Mardan Pakistan

2. Antigenic Shift A sudden shift in the antigenicity of a virus resulting from the recombination of the genomes of two viral strains.  The shift occurs when different strains of same virus or strains of different viruses combine and form a new subtype. The resulting new subtype having a mixture of surface antigens of the two or more original strain. Only in influenza virus the antigenic shift is seen. Specially influenza A virus.

3. Continue…….. Antigenic shift confers a phenotypic change by the reassortment of viral genomes. New viral pathogens follows the pathway of antigenic shift to get entry into the new environment thus antigenic shift is important for the emergence of new viral pathogens . Influenza virus is the best known example of antigenic shift as it mostly specifically applied to it, but the process of antigenic shift may also occur in other viruses such as Visna virus that cause encephalitis and chronic pneumonitis in sheep.

4. Structure of Influenza Virus

5. A ntigenic Shift in Influenza virus By the types of hemagglutinin and neuraminidase surface proteins flu viruses are named. For example, H3N2 is named that it have type-3 hemagglutinin and type-2 neuraminidase surface proteins. When human cell is infected by the flu virus such as H3N2 and H5N1 at the same time, the virus uncoated and release its viral genome to the host cell and transcribed to mRNA which is then translated to viral proteins and a new virion is formed by assemblage.

6. Continue…… Now in this case the host cell is infected by two different strains of influenza virus H3N2 and H5N1 so the possibility is present that it form a new viral strain H5N2 . The H5N2 strain is new to the host which cause difficulties in recognizing and illness. Transmission from person to person can occurs and lead to epidemic or pandemic.

7. Pig model and Antigenic Shift In case of pigs which is infected by swine influenza and susceptible to avian influenza cause infection in ducks and human influenza virus. Infected pigs with these three types of different viruses shows similar symptoms such as cough, fever, and runny nose etc . During replication and recombination of viruses it is possible that a new strain is formed by the different three viral genome combinations.

8. Continue…….. For example, a pig was infected by the human and avian influenza simultaneously. In this case antigenic shift could occur, which lead to produce a new virus which probably had most genes from human influenza virus and hemagglutinin and neuramindase surface proteins . The resulting virus is new and cause infection in humans as well as transmissible from person to person. The population will not be immune to this new strain as it new and thus no immune protection a pandemic may occur.

9. Antigenic Drift The mechanism of mutation and variation in viruses within the genes that code for antibody-binding sites is known as antigenic drift. The result of this mechanism or phenomena is a novel strain of virus particles which cannot be inhibited by antibodies effectively as they were originally inhibited in previous strains of the same virus, by this mechanism the virus is able to spread throughout the body and invade or may escape the immune system. Antigenic drift occurs in both strains of influenza A & B virus.

10. Antigenic drift in Influenza virus Hemagglutinin (HA ) and neuraminidase (NA) are the two relevant surface proteins antigens present in the influenza virus. The responsibility of hemagglutinin in the influenza virus is binding and entry into host epithelial cells while on the other hand neuraminidase is responsible in the process of budding out of new virions budding from the host cells. Sites present on the hemagglutinin and neuraminidase surface proteins are recognized by host immune cells are under constant selective pressure. Due to the selective pressure antigenic drift allows the influenza virus for evasion of these host immune cells by creating small mutations in the genes coding for hemagglutinin and neuraminidase that make the protein unrecognizable to pre-existing host immunity.

11. Continue….. So antigenic drift is thus described as continued process of genetic and antigenic changes among the flu virus strains. Individuals that are immune or vaccinated in human populations, exert selective pressure for single point mutations in the gene coding for hemagglutinin protein that increase receptor binding avidity, while those individuals who are not immune or vaccinated exerting selective pressure for single point mutations that decrease receptor binding avidity. The observed rapid evolution in the gene coding for hemagglutinin proteins in influenza virus are due to above dynamic selection pressures that facilitate it.

12. Continue….. Specifically, in the HA1 domain of the hemagglutinin gene 18 specific codons have been identified as undergoing positive selection to change their encoded amino acid. Thus vaccines that are able of broad protection against heterovariant strains are needed against seasonal, epidemic and pandemic influenza to cope or overcome the challenge of antigenic drift.

13. Antigenic Drift in HIV Antigenic drift has also been reported in Retrovirus (HIV) and rhinovirus which cause common cold. In case of HIV infection body immune system produce high titer of broadly neutralizing antibodies ( bNAbs ) against HIV. But however antigenic drift occurs in gp120 surface glycoprotein of the human immune deficiency virus due some selection pressure by which it escape from the immune system. The bNAbs not invading the modified gp120 strain of HIV.

14. Hong Kong Flu (1968–1969) The  Hong Kong Flu  was a category 2 flu pandemic caused by a strain of H3N2 descended from H2N2 by  antigeni shift, in which genes from multiple subtypes reassorted to form a new virus . K illed an estimated one million people worldwide. I nfected an estimated 500,000 Hong Kong residents In the United States, about 33,800 people died .

15. Antigenic Shift vs Antigenic Drift