Thermal inactivation of Newcastle disease virus. I. Coupled inactivation rates of hemagglutinating and neuraminidase activities

Thermostability study of virulent Newcastle disease viruses isolated in Southern Angola

Newcastle disease (ND) is endemic in Angola. Several outbreaks of ND occurred in small backyard flocks and village chickens with high mortality in the southern provinces of the country, Cunene, Namibe and Huíla, in 2016 and 2018. In those years, 15 virulent ND virus (NDV) strains were isolated and grouped within subgenotype 2 of genotype VII (subgenotype VII.2). We now present a study on the thermostability of the isolates, aiming at the selection of the most thermostable strains that, after being genetically modified to reduce their virulence, can be adapted to the production of vaccines less dependent on cold chain and more adequate to protect native chickens against ND. Heat-inactivation kinetics of haemagglutinin (Ha) activity and infectivity (I) of the isolates were determined by incubating aliquots of virus at 56 °C for different time intervals. The two isolates from Namibe province showed a decrease in infectivity of 2 log10 in ≤ 10 min, therefore belonging to the I-phenotype, but while the NB1 isolate from 2016 maintained the Ha activity up to 30 min and was classified as thermostable virus (I-Ha+), the Ha activity of the 2018 NB2 isolate decreased by 2 log2 in 30 min, being classified as a thermolabile virus (I-Ha-). Of the 13 NDV isolates from Huíla province, 10 isolates were classified as thermostable, eight with phenotype I+Ha+ and 2 with phenotype I-Ha+. The other three isolates from this province were classified as thermolabile viruses (I-Ha-).Contribution: This study will contribute to the control and/or eradication of Newcastle disease virus in Angola. The thermostable viral strains isolated from chickens in the country can be genetically manipulated by reverse genetic technology in order to reduce their virulence and use them as a vaccine in the remote areas of Angola.

Thermostability profile of Newcastle disease virus (strain I-2) following serial passages without heat selection

I-2 is an avirulent strain of Newcastle disease virus. During establishment of the I-2 strain master vaccine seed, a series of selection procedures was carried out at 56 degrees C in order to enhance heat resistance. This master seed is used to produce a working seed, which is then employed to produce the vaccine. These two passages are done without further heat selection; however, it is not known how rapidly and to what extent thermostable variants would be lost during further passage. The study was therefore conducted to determine the effect of passage on thermostability of strain I-2. The virus was serially passaged and at various passage levels samples were subjected to heat treatment at 56 degrees C for 120 min. The inactivation rates for infectivity and haemagglutinin (HA) titres were assayed by use of chicken embryonated eggs and HA test, respectively. Thermostability of HA and infectivity of I-2 virus were reduced after 10 and 5 passages, respectively, without heat selection at 56 degrees C. These results suggest that 5 more passages could be carried out between the working seed and vaccine levels without excessive loss of thermostability. This would result in increased vaccine production from a single batch of a working seed.

A differential scanning calorimetric study of Newcastle disease virus: identification of proteins involved in thermal transitions

The irreversible thermal denaturation of Newcastle disease virus was investigated using different techniques including high-sensitivity differential scanning calorimetry, thermal gel analysis intrinsic fluorescence, and neuraminidase activity assays. Application of a successive annealing procedure to the scanning calorimetric endotherm of Newcastle disease virus furnished four elementary thermal transitions below the overall endotherm; these were further identified as coming from the denaturation of each viral protein. The shape of these transitions, as well as their scanrate dependence, was explained by assuming that thermal denaturation takes place according to the kinetic scheme N-->(k)D, where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state; and D is the denatured state. On the basis of this model, activation energy values were calculated. The data obtained with the other methods used in this work support the proposed two-state kinetic model.

Thermostabilities of virion activities of Newcastle disease virus: evidence that the temperature-sensitive mutants in complementation groups B, BC, and C have altered HN proteins

Four virion activities of Newcastle disease virus (hemagglutinating, neuraminidase, hemolytic, and infectious activities) were examined before and after heat stress in low-salt buffer and physiological salt buffer (phosphate-buffered saline). The hemagglutinating and neuraminidase activities of the Australia-Victoria wild-type (AV-WT) strain were thermostable at both salt concentrations tested, whereas the thermostabilities of the hemolytic and infectious activities were salt dependent (thermostable in phosphate-buffered saline but not in low-salt buffer). Virions of RNA(+) temperature-sensitive (ts) mutants of AV-WT were tested for the stabilities of the four activities. Some mutants in groups B, BC, and C were as stable as AV-WT in all functions, but others were much less stable in all functions. The unstable mutants in groups B, BC, and C affirmed the assignment of the ts lesions of these mutants to the hemagglutinin/neuraminidase (HN) protein gene because HN function(s) are required for all four activities. The instability of these ts mutants was not related to their decreased virion HN protein content and was not due to physical loss of the HN protein from the virions. Three of four ts(+) plaque-forming revertants of the least stable mutant, BC2, coreverted for stability, confirming that the unstable phenotype is indeed the result of the mutation responsible for the ts phenotype. Group D mutants were approximately as stable as AV-WT in hemagglutinating, neuraminidase, and hemolytic activities; this is consistent with this group representing a lesion in a gene other than the HN protein gene. However, the infectivities of two of the three group D mutants were less stable than the infectivity of AV-WT in low-salt buffer.

Selective inactivation of hemagglutinin and neuraminidase on mumps virus

The thermal stability and the effect of guanidine on the hemagglutinin and neuraminidase of three strains of mumps virus were compared. The heat inactivation of hemagglutinin resulted in the concomitant loss of neuraminidase. The effect of guanidine at various molarities showed that the neuraminidase was more sensitive than the hemagglutinin and a selective inactivation was obtained after exposure to 1.5 M guanidine. However differences in sensitivity of both activities (hemagglutinin and neuraminidase) to heat and guanidine inactivations were observed among strains and correlated with differential susceptibility to non-specific inhibitors of the strains.

Thermostability of Newcastle disease virus strains of different virulence

The thermal inactivation rate constant for infectivity of a total of 24 Newcastle disease virus (NDV) strains was determined at 50 degrees and 56 degrees C. The greater part of the examined NDV strains were found, irrespective of virulence properties, to be thermostable, since the loss of infectivity titre did not exceed 2 logarithmic orders after exposure at 50 degrees C for 60 minutes. Thermostable (I+) and thermolabile (I minus) strains with respect to infectivity were uniformly encountered among the 12 avirulent (lentogenic) and 10 fully virulent (velogenic) strains studied. Strains with both heat stable (Ha+) and heat labile (Ha minus) haemagglutinin were found in the lentogenic group, whereas haemagglutinins of all examined velogenic strains were heat stable. On the basis of I/Ha character, i.e. combination of thermosensitivity of infectivity and haemagglutinin, all examined strains could be classified into one of three categories: I minus Ha minus (8 lentogenic strains), I+Ha+ (4 lentogenic and 7 velogenic strains) and I minus Ha+ (2 mesogenic and 3 velogenic strains). The possible fourth combination, I+Ha+ minus, was not encountered among the strains studied. Determination of the I/Ha character may be helpful in clarifying the origin of a lentogenic NDV strain.