I feel that Julius wishes to communicate that the current generation of virologists should be aware that a virus is not only a threat for their i-phon but can give e.g. vaccine developers important Information if they combine to classical virology with in-silico virology. In contrast to (RNA) viruses  with small genomes where a try and error approach may be helpful, large DNA viruses like herpesvires, asfarviruses, poxviruses are to complex to not make use of mutants attenuated by passiging in cell culture, eggs, non-target animals. For me the best example are herpesvirus vaccines against Aujeszky's disease of pigs or against IBR/IPV of cattle whose development made use of the knowledge about deletions introduced in the viral genome by in vitro passages.

I have experience with the generation of defective interference particles of tospoviruses and potyviruses. They are RNA viruses. Are you interested in more information? 

Interesting thoughts Július. 

Attenuation can occur after passage in a different host, the influenza strain PR8 being one such example.  The viral interplay with the host cell may depend on culture conditions, like those used to generate DI particles, but may also depend on the host itself.

Perhaps it can explain the incomplete viral forms observed in the brain of fetuses of schizophrenic mothers with strong families antecedents of schizophrenia. It could be as a long passage history viruses.

Thank you all for your remarks. Indeed, my ideas concern mainly DNA viruses, especially the Herpesvirus family. In the case of the famous Sabin polio vaccine, essential mutations concerned the non-coding vRNA sequence, which correct folding is important for the interaction ribosomes. Thank you and expact further remarks from anybody interested.

Great thought! Maybe in some way, cell culture is also not the best place for this kind of virus parasitism, and a selective pressure is always there. 

The defetive interfering particles represent a different issue. They appear in culture mainly if the cells are repeatedly infected by high MOI in the course of passaging.  

Numerous passages regularly carried on cell culture or animal. Sometimes, the carrier be change. It can cause an adaptation of the virus to the host.

This adaptation whose mechanisms are in part genetic, result in the loss or acquisition of new powers. Ativities such as reduced pathogenicity (attenuation) , a short incubation period and these techniques have been used for the development of vaccines.

Two different genes code a viral ability of attaching to the host receptors, and its ability of causing disease. Life vaccine may be produced from tissue culture, that provide attachment receptors but it does not produces diseases. Continue propagating of virus in tissue culture increase the frequency of attachment gene expression, but not the expression of pathogenic gene. After many generation of propagation in tissue culture, viral attachment ability increased (enhance vaccine attachment efficiency), whereas vaccine does not produce diseases in tissue culture, the pathogenic gene gradually become dormant or even lost (Mutation) the ability of causing disease during many generation of propagation in tissue culture.

Very interestin discussion.

The reason for attenuation of a given virus after several passages in cell culture is quite simple; the virus become under pressure in a different environment and must change or adapt. This adaptation process is viewed in several studied to be related to nucleotide sequence, block of sequences insertion, deletion or substitution. Additionaly, adaptation can occur through recombination or reassortments. The resultant virus variant is able to propagate quickly (time to first cytopathic effect) and to a hgher titre. The methodology to study the genetic bases for attentuation is the complete or partial genome sequencing and genetic analysis. If you know the virulence genes of your virus go for partial, if not then the best is the complete genome sequencing that involve the wild type original virus and the high passage virus. If you want more data such as at what passe the attenuation occur the use low, middle and high passae viruses.

I feel that Julius wishes to communicate that the current generation of virologists should be aware that a virus is not only a threat for their i-phon but can give e.g. vaccine developers important Information if they combine to classical virology with in-silico virology. In contrast to (RNA) viruses  with small genomes where a try and error approach may be helpful, large DNA viruses like herpesvires, asfarviruses, poxviruses are to complex to not make use of mutants attenuated by passiging in cell culture, eggs, non-target animals. For me the best example are herpesvirus vaccines against Aujeszky's disease of pigs or against IBR/IPV of cattle whose development made use of the knowledge about deletions introduced in the viral genome by in vitro passages.

The last remark by Guenther seems very well reflect what I had in mind. I was happy to participate in a team at our Institute, which discovered the Murine herpesvirus (MuHV 4), a new Gammaherpesvirus, which casues the appearance of atypical mononuclears in WBC smears of mice. Its genome is closer the KHSV than to EBV,but  it has over 10 genes common with both human Gammaherpesviruses and at least 16 MHV-specific genes. For further details, if being interested, please consult the correponsing papers of mine, Mistriková and/or Kudelová.  The point being that different isolates undergo deletions when passaged in vitro at the 5´-end. This causes the loss of an important gene encoding an immune evasion protein. The corresponding strain  in a paradoxic way may become relatively more virulent, since it can replicate to higher titers at the inoculation site than does the counterpart which can express the chemokine binding polypeptide. This thing is even more interesting, because the increased replication on other hand, may not be accompanied with an usual inflammatory response, since as a rule, the latter is  interleujkin dependent...In any case this is a very interesting model confirming my idea (for some colleagues still controversial) namely that virology, pathology and immunology should not be regarded for so widely independent disciplines than some specialits believe... 

Yes, we can see that effect in some of the fish viruses, especially in herpesviruses. In most of the RNA viruses such an effect is only there if the viruse is adapetd to a heterologous cell lines within serial passages at different temperatures. This might be a special issue for fish viruses because I could not see that when I passaged Marek's disease Virus (MDV) in fish cells at temperatures between 15 and 20°C. In other DNA viruses this is also not really happen like ranaviruses. Passage one and passage 100 is inducing a severe disease in the original hosts.

perhaps  because  you do not dilute  enough  the inoculum it needs to dilute 1/1000 or less  exemle  measles  VSV  influenzae

HIV-1, despite the small RNA genome,  gets attenuated after few in vitro passages. The sequence of several proteins changes to adapt to the cell culture system at the price of a reduced in vivo proliferation.