PFU does NOT = infectious particles

[DANDERSON at PRL.PULMONARY.UBC.CA]

ryan at mbcf.stjude.org wrote:

>     A PFU titre (or any measure of infectivity) has meaning only for
>     the virus/host cell combination in which it was determined.  

I agree, that was my one of my points, but I was taking it further 
and by adding the question:  Where does the infectious process 
"breakdown" in a moderately permissive cell compared to a permissive 
cell? 

>     The PFU titer _is_ the 
>     number of infectious particles per unit volume, usually defined in the 
>     most permissive system available.  Snip....

     >2) this could mean that a single cell must get hit 50 times by 
     >viable virus to accumulate enough virus information to get a productive 
     >infection started. If this were the case I think the conclusion would be 
     >"this virus doesn't form plaques".

This is the only point where we differ.  Is not a PFU a Plaque Forming 
Unit, or the unit volume of virus solution you need to develop a 
plaque?  I understand what you are saying about limiting dilution, 
but prove to me that in one PFU you only have only one infectious 
particle.  Why can there not be 50 infectious particles.  When you 
dilute "something" you dilute it to the point where it is not 
detectable.  If it takes 50 infectious particle to infect a cell and 
you dilute it to 5 infectious particles you lose the plaque.  You 
dilute it beyond what you are detecting.  

This assay does not address the issue of how many infectious 
particles are present in that volume, but a PFU simply reflects the 
number of particles necessary to infect a cell on average!

    >What a PFU
   >  titre tells me is that at a certain inoculum dilution I can expect to see
    > a certain number of plaques. It doesn't tell me anything about the virus
   >  particles that don't form plaques: they might have no DNA/RNA in them,
   >  they might be only partially defective due to mutation, they might be 
   >  perfectly viable & healthy but failed to find a cell receptor due to rare 
   >  bad luck. I can only score as infectious the ones that do productively
   >  infect; a PFU assay says nothing about the others...      

Exactly, as I indicated above.

The study of virus tropism has classically been in the context of 
virus mutants (antibody, cold and etc).  What if you turn the 
situation around?  What host factors are important when comparing 
moderately susceptible and susceptible cell lines.  Replication of 
virus has often been deemed strictly depend upon a recepter (the door 
to a permisive infection) , but what about the efficiency of 
replication.  Is this host cell protein dependent?  Probably, as 
there are papers in recent print which support the role of host cell 
proteins in regulating virus replication!  Finally in our model I 
described, this occurs with the same number of infectious particles, 
yet the efficiency of replication (at some level) is decreased.  What 
is the difference between these cells?  They can both bind and 
endocytose virus.  As well, I can, and have, adapted the virus to 
each cell line.  Now what is the difference?  Is this mutation (towards 
susceptibility) always at the level of the recepter binding site, or is it 
affecting the interactions of the ribosomes with the viral genome, or 
affecting other protein genome interactions?  Why would virus 
replication such be independent of regulation of protein and genome 
interactions?  I'm getting off on a tangent.  I will stop!

Dan