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
> 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!