LIVE POLIO IRUS VACCINES

Application of Genetic Markers to Development and Control of Vaccine 63
TABLE 14.
EFFECT OF D20 IN MEDIUM ON PLATING EFFICIENCY OF CHAT VIRus ON MS CELLS
EXP. MEDIUM NO. OF PLAQUES PRODUCED ON CELL MONOLAYERS
MS
MK
1 H2 O0 14
2
D 2 0* 14 N. T.
2 H 2 0 1 33
D 2 0* 20 N. T.
N.T. = Not tested. * 40-50% D20 in medium.
D 2 0 medium is markedly increased and similar
to that observed in fresh monkey-kidney cells
grown in H20 medium. Thus, the substitution
of H 2 ions by deuterium in the tissue culture
system seems to increase the plating efficiency
of MS cells to between 50 and 100 per cent.
As mentioned before, the cold CHAT strain
cannot be grown at 400 C. However, it seemed
to be of interest to verify whether infectious
nucleic acid is produced in cultures kept at these
temperatures shortly after infection with the
CHAT strain. In this experiment monkey-kidney
tissue-culture monolayers were infected respectively
with CHAT and Mahoney virus and incubated
for 6-8 hours at 37 ° and 40 ° C. Following
this period of incubation cultures were harvested
and RNA extracted. The infectivity of
the extracted RNA is shown in Table 14a. It
may be observed that although the yield of infectious
RNA from the virulent Mahoney virus
was similar regardless of the temperature at
which cultures were incubated, the yield of
CHAT RNA from cultures kept at 40 ° C. was
very small compared to that obtained from cultures
kept at 370 C. These results seem to indicate
that a cold variant, when kept at high
temperatures, appears to lose the capacity to
produce even an infectious nucleic acid.
As shown in Table 15, incorporation of D20
into growth medium and agar overlay of monkeykidney
monolayers infected with the CHAT virus
and kept at 40 ° C. resulted in formation of
plaques in numbers representing 10 to 63 per
cent of those observed at 370 C. Progeny of the
TABLE 14a. INFECTIVITY OF RNA EXTRACTED FROM "HOT" MAHONEY AND "COLD" CHAT VIRus
C,ROWN FOnR 8 HOURS AT Two DIFFERENT TEMPERATURES IN MONKEY-TIssíUE CUILTIRE
VIRUS TBiWPERATURE NO. OF PLAQUES PRODUCED BY THE cXTRACTED RNA
MAHONEY 370 C 39
40 0° 39
CHAT 37 ° ' C 43
40 0 C 4