LIVE POLIO IRUS VACCINES

Discussion
47
ble, by introduction into children, of producing
viremia which is not usually observed with original
strains.
We therefore took monolayer cultures and
conducted blood tests on children at various
periods after the introduction of such strains to
test the possible appearance of viremia and the
increase of neurovirulence.
The results are now in progress, but we have
preliminary data with Type 3 strain which show
that this more neurovirulent strain does not cause
viremia in children. I believe that these results
are more important for the evaluation of safety
of these viruses than are the various laboratory
markers, which are so interesting in theoretical
investigations of different variants. Viremia,
after all, is the first stage which can be a menace
to the generalization of such viruses into the
central nervous system and its evaluation, among
other markers, seems to be an especially important
task.
DR. DULBECCO: The question of relationship
between various markers and neurovirulence is
actually a part of the general problem of correlation
in variation of markers of any kind that
occurs in poliovirus.
In the work carried out in our laboratory by
Dr. McBride, Dr. Vogt, and myself, we have
obtained the information which is summarized
in Table 1. We have markers which show extreme
cystine requirement, the inhibition by
cystine, the temperature resistance for inactivation,
the d character, the 30 and 40 centigrade
growth, the MS marker, the resistance to inhibitors,
and neurovirulence.
Now, there are many pluses and many minuses.
The plus indicates, in a qualitative way, that
the coexistence of the markers in the same virus
is possible and frequent. The minus indicates
that very frequently we have exclusion between
these markers.
Now there are exceptions as we very well
know. You will note that there are quite a
number of minuses, meaning that if mutation
occurs, let us say, from the d plus to the d, then
it is very likely that at the same time the temperature
resistance marker will also change
simultaneously.
A close look at the minuses shown in Table 1
will reveal that these concern essentially a group
of markers which include the temperature resistance,
the 30 and 40 degree growth, the MS
character, the d character, cystine requirement,
and neurovirulence.
The bovine and equine inhibitor markers are
completely out of the group. This means that
whenever a marker in this group changes, the
bovine or equine inhibitor markers are not affected,
and vice versa. Therefore, all the markers
constitute two groups. In one there is an
extensive degree of covariation, meaning that a
variation of one marker may involve a variation
of another one, or variation of one marker may
cause exclusion of another marker.
I believe that, in general, the use of markers
serves a dual purpose in the study of vaccines.
One of these would be, as emphasized very
frequently at this session, to have markers that
can be used as in vitro markers for neurovirulence.
The other would be to have a marker
which can be used to trace a certain strain
introduced in the population, in order to determine,
for instance, whether the strain recovered
is identical to the strain which was introduced.
Markers of the first group may be useful for
the first purpose. For the second purpose, it is
obvious that none of the markers in this group is
useful because if a mutation arises, it may involve
the marker employed. Only the bovine
and equine markers are suitable for the second
purpose. Perhaps the marker of antigenicity
referred to earlier could be considered.
As for the covariation of markers of the first
group, many exceptions have already been
pointed out. None of these markers truly reflects
the behavior of neurovirulence.
This also raises further questions: the direction
that research will have to take in this field
and the significance of all these findings.
I should like to call attention to a theoretical
point, namely, the fact that we have so many
different markers for poliovirus does not imply
that we have many genes in the poliovirus.
The hypothesis of many genes, which may
seem to be the simplest explanation, does not
take into account two facts: one, the construction
of the virus; the other, the extent of covariation.
If there were different genes, one
would not expect this extensive covariation.
The structure of the virus is against the hypothesis
of many genes because we know that the
RNA of poliovirus, the genetic material, is very
small, of a size equivalent to that considered as