71’ LETTEHS TO THE EDITOltS

2. Virus solutions corresponding to the infectivity of t,he RNA preparations t 10-? t.o 10e3 !,Ti) were birefringent when tested in the Rao Flowhirefringence Apparatus. rlt, 300 see-’ the birefringence was 8-10” (Senarmount compensator). No isoclines were visible when the RNA preparations were observed up to 2000 see-1.

3. Ribonuclease (0.35 pg/ml) completely destroyed the infectivity of RXA preparations, whereas it did not affect the infectivity of PVS prepared in dis- tilled water. The infectivity of PVS treated wit,h citrate was diminished brit never completely eliminated (Table 1, column 3).

WC have also tried to prepare infectious RSA from I’\‘S by means of the Irhcrrol techniqrte and can essentially,- confirm the unsatisfactory results of Bawden and Klrczkowski (4).

RFFE’REYCES 1J /

1. REIC’HX4SS. >I. b:.. (‘un. J. (‘hewI. 36, 160331611 (1958). i?. ~tEIC.Hhl.4NS, ;c1. F:., CCL% ./. (:hC?Vl. 37, -I-lo (1959). 8. NORTHROP, T. G., and SIXSHEIMER, R. L., J. (‘he,u. Ph!/s. 22, 703-707 (1%X).

4. B.IWI)EN, F. C., and KIJWZKOWSKI, 9., I’irology 7, 375-384 (1959).

(‘nnutlrc .lgriculfure, Research Sfafion,

w. The I’nil’ersily of British t’olunrbiu.

T~crnco~rre~~ 8, B.C., Canada

Received September 15, 1959

nI. I+:. REICHMANS

1~. STAC~~LIIT~~

Concerning: “Mutual Exclusion of Strains of Tobacco Mosaic Virus”

In a recent paper, Siegel (I) interpreted his results to indicate “that an esrlrl-

sion mechanism may be operative such that when an infection is initiated wit,h a particle of one strain of virus, a particle of a second strain cannot participate in the same infect,ion.” Siegel observed that when two strains of tobacco mosair virus (TMV), Ul and U2, were inoculated together onto leaves of Nicofinnn s&

wsfri.s Spegaz. and Comes, the number of local lesions tended to be fewer than when U2 was inoculated alone, the concentration of U2 being the same in both cases. The decrease in lesion number depended on the relative and absolute COII- crntrations of the tw-o strains. When the inoculation of the two strains was sep:i- rated hy 1 hour, however, there was no inhibitory effect. on t,he number of lesions formed by U2. Siegel concluded from the lack of inhibition when the two strains were inoculated separately that the reduced number of lesions observed when the t,wo strains were inoculated together resulted from “a phenomenon concerned with the initiation of infection” and not, from a phenomenon concerned with symp-

tom expression. In my opinion, this conclusion does not follow because the two experimental treatments are not comparable. When the two strains are inoculated sepamtely, the chance for t,he occurrence of a double infection is very small, and

this is so for technical reasons which do not apply when the two strains are inoru lated together. Among t,hese technical reasons the following may be cited:

LETTERS TO THE EDITORS 713

(A) So matter how much inoculum is used, a large proportion of the ~11s of the leaf surface would not seem to he infected, as shown by the maximum number of local lesions being much less than t,he number of cells. a second inoculation would presumably infect other cells than the first inoculat,ion. This has been t,he interpretation of Kunkel’s classic experiments (2) which showed that as the time between the inoculations of two strains increased, the number of loral lesions formed by the superinfecting strain decreased. This “protection” from the second strain is believed to derive from the progressively more thorough infection of the leaf by the first strain.

CR) It. has been shown that rubbing an inoculated leaf redllces the ninnber of lesions (3); this indicates that the additional rubbing has altered the infected sites sufficiently to prevent lesion formation. Thus, even if both viruses shollld reach the same cell, the cell probably would be so altered by the additional wound- ing necessary to introdllce t,he second st,rain as not t,o support virus multiplic:ition at all.

In Siegel’s discussion, the argument for exclusion depends upon a coml~arison of experimental results with calculations derived from various models. If it is accepted that, Siegel may have dealt with symptom espression and not lvith the initiation of infection, then the various factors which may influence symptom formation should be evaluated, particularly in the model which supposes mixed infection. Among these fact.ors may be cited the possible differences between the two strains, when present, together, in the rate at which they attach to multi- plicative sites, the rate at which they multiply, the interval from the initiation of infection to the appearance of the first particle, t,he total number of particles that are formed, and the average time required in one cell before infection of the neighboring cell. The disagreement bet,ween the data and model C of Siegel may well be the result of the excessively simple assumptions used in the model concern- ing the process of mixed infection.

If I may inject, my own interpretat,ion, I would consider the case of N. s~~/z~~stris to be more readily interpreted as showing no exclusion mechanism for the ordinary and yellow aucuba strains of TMV (these two strains correspond in their behavior to Ul and U2, respectively). The best, evidence for cellular immunizat,ion has been the difficulty of showing multiplication of a superinfecting, second strain when it is inoculated onto a leaf already thoroughly infected. The data, of Sadas- ivan (4) and of Siegel show that this difficulty may be purely technical and that multiplication may indeed occur, but, that it would not be detected in any test because of the preponderance of the first strain.

Furthermore, the presence of white spots on systemically infected leaves (5) shows that the ordinary strain, in moving from the inoculated leaf, actually facilitates t~he movement of t,he yellow aucuba strain and prevents the necrotic response to this strain, at least temporarily. It is this altered response of the in- fected tissue to which I would attribute the reduced number of lesions so evident in protection.

REFERENCES

1. SIEGEL; A., Virology 8, 470-477 (1959). 2. KITNKEL, L. O., Phytopathologu 24, 437-466 (1934)

711 LETTERS TO THE EDITORS

3. BAWDEN, F. C., and HARRISOX, B. I>., J. Gen. Microbio!. 13, 491-508 (1955). 4. S.~I)ASIV.~~, T. S., Ann. :lppl. Biol. 27, 3X-367 (1940,. 5. i3ESD.4, C:. T. -4., Virology, 3, ml-602 (1957).

c;. T. A. 13ltsn.4

Pulmonary Consolidation Caused by Mumps Virus in Mice

When sllfficient amounts of mumps virus were inoculated into young mice intra- nasally. consolidation occ~wretl without an!’ demonstrable multiplication of the virus in the lungs. The experiment is as follows: Eight-day-old chick embryos were inoruluted with the Enders strain into the amnionic and allantoic cavities and incrrhated at 36” for 4 days. .4 concentrated virus suspension with a hemagglutinin (HA) titer of 4096 per milliliter was obtained from the amnionic and allantoic

Fro. 1. Lung of a mouse sacrificed 48 hours after inoculation of mumps virlls (klnders strain) intranasally. Hematosylin-eosin s&in. Magnification: approsi- mate]? X 100.