EVOLUTIONARY CHANGES IN MYXOMA VIRUS IN BRITAIN. AN EXAMINATION OF 222 NATURALLY OCCURRING STRAINS OBTAINED FROM 80 COUNTIES DURING THE PERIOD OCTOBER-NOVEMBER 1962

Samples of lesions containing viable myxoma virus were collected from 222 infected wild rabbits captured in 80 counties of Britain during October–November 1962. They were dispatched by air to Australia and passaged once in domestic rabbits before being tested for their virulence by the intradermal inoculation of groups of six laboratory rabbits with small doses of virus.

The results showed that a wide range of viruses of differing properties now coexist in Britain. Their virulence ranges from very high (99 % lethal) to low (about 50 %), but the majority of strains fall into the grade III virulence group, with estimated case mortality rates varying between 70 and 95%. The viruses also produce disease of varying symptomatology, skin lesions being very prominent, raised or flat. There was no obvious association between lesion type and virulence.

Tests made on the wild rabbits from which the samples were obtained, and on Inoculated domestic rabbits, showed that virulence could not be predicted by gel-diffusion tests.

INFECTIVITY OF HISTONE-POLIOVIRUS RIBONUCLEIC ACID PREPARATIONS

Ludwig, Ernest H. (The Pennsylvania State University, University Park) and Christine E. Smull. Infectivity of histone-poliovirus ribonucleic acid preparations. J. Bacteriol. 85:1334-1338. 1963.-Some properties of histone-poliovirus ribonucleic acid (RNA) preparations, as relate to infectivity for HeLa cell monolayers, were investigated. The histone-RNA preparations were found to lose their infectivity rapidly at room temperature. They were considerably more stable at ice-bath temperature. Dilution of a histone-RNA preparation in a diluent containing an appropriate amount of histone yielded a plaque count which was proportional to the dilution factor, and which regressed linearly through the point of origin. Dilution of a histone-RNA preparation in a diluent containing no histone resulted in a rapidly decreasing plaque count which was not proportional to the dilution factor. The ability of histone to enhance the infectivity of poliovirus RNA was found to be dependent upon the presence of a low concentration of a monovalent salt in the histone-RNA preparations. Histone-RNA preparations containing approximately isotonic levels of NaCl exhibited a high degree of infectivity. When concentrations of NaCl outside this range were used, a great loss in infectivity of the histone-RNA preparations occurred. The NaCl could be replaced by KCl but not by CaCl(2), MgCl(2), or MgSO(4).

LEUKOCYTES AND INTERFERON IN THE HOST RESPONSE TO VIRAL INFECTIONS. I. MOUSE LEUKOCYTES AND LEUKOCYTE-PRODUCED INTERFERON IN VACCINIA VIRUS INFECTION IN VITRO

1. Investigation of the role of leukocytes in vaccinia virus infection is reported in an in vitro model, in the absence of an immune response. 2. Mouse leukocytes were shown to be capable of inhibiting the progression of vaccinia virus infection in primary mouse embryo fibroblast cultures. The degree of protection varied from slowing of spread of infection to complete control of the infection with eventual elimination of detectable virus and recovery of the culture. 3. Interferon production by leukocytes is thought to be an important factor in the observed protective effect.

THE SUSCEPTIBILITY OF CHICK EMBRYO SKIN ORGAN CULTURES TO INFLUENZA VIRUS FOLLOWING EXCESS VITAMIN A

The conversion of chick embryonic epidermis to mucous epithelium by excess vitamin A in organ culture as reported by Fell and Mellanby (5) was shown to be accompanied by a corresponding change of susceptibility to influenza and vaccinia viruses. Untreated epidermis of 10- to 12-day chick embryos supported the growth of influenza (PR8) virus in organ cultures and a maximum infectivity (EID(50)) titer was reached 2 to 3 days after infection. At the same time) the epidermis showed squamous keratinization, beginning about the 4th day of cultivation. Addition of excess vitamin A (40 microg per ml) to the skin organ culture induced the following changes: (a) mucous metaplasia of the epidermis which was usually first evident after 4 to 5 days in the vitamin A medium, (b) increase in the daily and maximum yield of influenza virus, if the epidermis had been grown for 4 or more days in the vitamin A medium before infection took place, and (c) decrease in the production of vaccinia virus under similar conditions. The maximum yield of both viruses remained unchanged, however, if excess vitamin A was introduced to the organ culture at the time of virus inoculation. The magnitude of increase in the yield of influenza virus in this organ culture system was found to be proportionally related to the concentration of vitamin A added 4 or more days before inoculation of this virus. Increasing doses of vitamin A however, had no effect on the short-term growth of influenza virus in tissue cultures of chorio-allantoic membrane. Observation on the early period (2 to 12 hours) of influenza virus growth initiated in the 4-day organ cultures of chick embryonic skin showed no significant difference in virus production between the normal and the vitamin A medium groups. The change of virus specificity apparently is not due to the presence of excess vitamin A per se, but appears to be related to the change of differentiation produced in the organ culture system.

EFFECTS OF HEAT ON THE INFECTING, ANTIBODY-ABSORBING, AND INTERFERING POWERS OF VACCINIA VIRUS

Galasso, G. J. (University of North Carolina School of Medicine, Chapel Hill), and D. G. Sharp. Effects of heat on the infecting, antibody-absorbing, and interfering powers of vaccinia virus. J. Bacteriol. 89:611-616. 1965.-At 56 C the infectivity of vaccinia virus particles is destroyed rapidly, but even when it is reduced by a factor of 10(6) the particles are capable of producing strong interference in L cells. The rate constant K for thermal inactivation of plaque-forming power is greater than that for interfering power by the factor e(3.8) or about 45 times. At 37 C both properties of the virus decline more slowly and at equal rates. The temperature coefficient of K is discontinuous in the region of 40 C, indicating quite different activation energies for the reactions above and below this critical point. The degradation of interfering power exhibits a similar discontinuity, although less in magnitude, but none has been found in the antibody-reactive power of the virus, which is much more heat resistant.

STUDIES ON PROTEIN AND NUCLEIC ACID METABOLISM IN VIRUS-INFECTED MAMMALIAN CELLS. THE FORMATION OF A VIRUS-SPECIFIC ANTIGEN IN KREBS II ASCITES-TUMOUR CELLS INFECTED WITH ENCEPHALOMYOCARDITIS VIRUS

1. Krebs II mouse ascites-tumour cells infected with encephalomyocarditis virus were found to contain, in addition to mature virus, a virus-specific protein antigen. An assay, based on the ability of this antigen to block the neutralization of purified virus by its specific antiserum, was developed. 2. This antigen was present both in the culture fluid 17 hr. after the infection of cells with virus and intracellularly, where its titre increased at a time when viral capsid protein was being synthesized. Within the cell, it was mostly localized in the soluble cell sap. 3. In contrast with virus, the antigen did not agglutinate sheep erythrocytes, and its immunological properties were destroyed by digestion with trypsin. Ribonucleic acid was not detected in concentrated preparations of the antigen, nor was the titre of antigen affected by ribonuclease. 4. The antigen had a sedimentation coefficient (20 degrees ) of approx. 14s, and its diffusion coefficient, determined by the method of Allison & Humphrey (1960), was 3.2x10(-7) cm.(2)sec.(-1). The particle weight of the antigen was hence 420000+/-40000. 5. The capsid protein from purified encephalomyocarditis virus could be degraded by treatment with ethanolamine into a protein of sedimentation coefficient (20 degrees ) of approx. 4s. The 14s antigen, when similarly treated, yielded a protein of similar size. However, no such smaller antigen was detected in virus-infected cells. 6. It is concluded that the non-haemagglutinating antigen represents a polymeric form of the basic viral capsid-protein molecule and that it is synthesized in the cytoplasm of infected cells. It may be either an intermediate or a by-product in the process of viral capsid-protein synthesis.

STUDIES ON THE PATHOGENESIS OF RINDERPEST IN EXPERIMENTAL CATTLE. II. PROLIFERATION OF THE VIRUS IN DIFFERENT TISSUES FOLLOWING INTRANASAL INFECTION

The pathogenesis of rinderpest virus was studied in twenty-nine grade cattle, which were infected by the intranasal route with a virulent strain of virus recently isolated in East Africa (RGK/1). These animals were killed or died at intervals of 1–16 days after infection and a number of tissues from each of them (usually 21) was titrated for virus infectivity in monolayer cultures of primary bovine kidney cells.

Temperature reactions were first detected on the 3rd to 5th days (mean 4·1), mouth lesions on the 6th to 9th and diarrhoea on the 8th or 9th days, following infection. The course of the disease was divided into four phases, viz. incubation (days 1–4), prodromal (days 5–7), mucosal (days 8–12) and early convalescence (days 13–16). Virus proliferation in different tissues was related to these clinical phases, detailed results being presented in tabular and graphical form.

No primary multiplication was detected in the nasal mucosa but virus was demonstrable within 48 hr. in its associated lymph nodes. Low-level viraemia began on the 2nd or 3rd days after infection and generalization had occurred by the end of the incubation period. The virus had established itself at this time throughout the alimentary tract and, sometimes, in the lungs.

The prodromal phase was characterized by plateaux of high virus titres in the lymphopoietic and lympho-epithelial tissues; there was a descending gradient from the cephalic nodes to the superficial lymph and haemolymph nodes of the body, to the visceral lymph nodes and spleen. There was a similar gradient in the titre of virus in the mucosae of the gastro-intestinal tract—from caecum to colon, to ileum and pylorus. Virus first appeared in the turbinate mucosa on the 5th day, post-infection and lung titres were high towards the end of this period. Some virus proliferation may have occurred in the liver, but none was certainly demonstrable in the kidney, myocardium or brain.

The mucosal phase began with continuing high titres of virus in all the major sites of proliferation, but a decline set in from the 9th day onwards. This at first involved the cephalic lymph nodes but soon extended to the spleen and other lymphopoietic tissues. It was most delayed in lympho-epithelial structures such as the tonsil, lung and gastro-intestinal mucosae. It was suggested that the decline in virus titres was due to the destruction of susceptible cells, accompanied by the local production and later circulation of antibody. Neutralizing antibody was present in the serum on and after the 9th day.

During the early convalescent period virus had disappeared from four animals, with the exception of one recovery from the lung tissue. Antibody titres were high during this time.

These results were discussed with reference to previously existing information on the distribution of rinderpest virus in infected cattle. An attempt was made to correlate them with published information on the pathogenesis of human measles and canine distemper. The data were also used to explain some previously reported observations on the excretion of rinderpest virus by experimentally infected cattle.

This study would not have been possible without the enthusiastic co-operation of Messrs C. S. Rampton, A.I.M.L.T., R. F. Staple, A.I.M.L.T., and R. Pillinger, F.I.M.L.T., in organizing and carrying out a very considerable number of laborious titrations. Dr B. Liess helped with some of the animals killed at earlier stages of the infection. Our junior African staff gave invaluable assistance with their careful and efficient handling of many thousands of tissue cultures; amongst them we should particularly like to mention Mr Francis Ngugi and Mr Harrison Agili.

This paper is published with the permission of the Director, E.A.V.R.O., Mr H. R. Binns, C.M.G., O.B.E.

A LABILE INTRANUCLEAR RNA ASSOCIATED WITH THE DEVELOPMENT OF ADENOVIRUSES

The nucleic acids produced intracellularly during the replication cycles of both DNA and RNA viruses can now be identified rapidly using a sensitized procedure based on staining with the fluorochrome acridine orange. Cellular DNA, viral DNA (both single and double stranded forms), cellular RNA, and RNA arising as a result of viral stimulus can be differentiated. The intracellular development of virus specific DNA, RNA, and protein has been studied in monkey kidney cells infected with adenoviruses types 3 and 7. It has been possible to detect a labile RNA in the nucleus from 16 to 20 hours after inoculation. When the cultures are treated with puromycin at this time, this RNA can be accumulated under certain conditions in the nucleus and demonstrated cytochemically. At the same time the production of specific viral protein as determined by staining with fluorescein-labeled antibodies is markedly inhibited. However, intranuclear double stranded DNA continues to be formed for a time. When puromycin is added to the system early in the eclipse period virus-specific DNA and labile RNA cannot be detected.

THE ENTRY AND DISTRIBUTION OF HERPES VIRUS AND COLLOIDAL GOLD IN HELA CELLS AFTER CONTACT IN SUSPENSION

The way in which herpes virus of a well adapted strain penetrates susceptible HeLa cells has been investigated using thin sectioning techniques for electron microscopy. Mature virus particles and cells were mixed together in suspension cultures for 15, 30, 60, or 120 minutes so that the stages in virus uptake could be followed in sequence. The ingestion of particles of colloidal gold by HeLa cells under similar conditions was studied for comparison in parallel experiments. After 15 minutes' contact, the mature virus was found adsorbed on the surface of the cells but separated from them by a narrow gap in which phosphotungstic acid staining was sometimes able to reveal an extraneous coat which appeared as an amorphous layer on the outer aspect of the plasma membrane. When mixing continued for longer the particles were present in deep invaginations or actual cytoplasmic vacuoles, with their outer layers in various stages of stripping and digestion. The stripped, naked, central portion of the virus was occasionally found in these vacuoles but was more commonly free in the cytoplasmic matrix; the mode of transition between these sites could not be determined. Where contact continued for 2 hours these phenomena were much less frequently observed. The larger particles of colloidal gold were ingested in the same way as the virus, but smaller ones were taken up in micropinocytosis vesicles. The gold passed through membrane-bounded cytoplasmic spaces to accumulate in vacuoles from which, in contrast to herpes particles, it did not escape. These findings are discussed, and considered with particular reference to their bearing on the initiation of infection, the uptake and disposal of particles by cells, and the influence on the latter of virus morphology.

CONDITIONAL-LETHAL MUTANTS OF AN ANIMAL VIRUS: IDENTIFICATION OF TWO CISTRONS

Two different temperature-sensitive conditional-lethal mutants of Sindbis virus, an animal virus that contains RNA, have been isolated. When cultured in chick fibroblast monolayers at 42 degrees C, these mutants yield less than 0.05 percent as much virus as does the wild type, whereas at 27 degrees C they grow normally. One mutant appears to be altered in the synthesis of a protein that is produced early in the infection and is required for viral RNA synthesis. The other mutant produces as much infectious RNA as the wild type at 42 degrees C and appears to be altered in the synthesis of a protein produced late in the infection.