Quantitative studies on the dependence of NDV penetration and elution on the attachment multiplicity

Dependence of virus adsorption to the cell surface on the input multiplicity of infection

A state of "saturation" of the cell surface ensues when only 5-10% of the cell surface is "covered" by the adsorbed virions. Hence, this state cannot be explained by an "overcrowding effect" caused by the virions attached to the cell surface. A hypothesis of heterogeneity of the cell surface with regard to the virus absorbtion ("the domain hypothesis") is suitable for explanation.

Interaction of Newcastle disease virus strains differing in virulence with chicken red blood cell receptors

Nine NDV strains belonging to lentogenic, mesogenic and velogenic groups were studied. Virus adsorption to chicken red blood cell (RBC) surface was performed at 4 degrees C, and after a temperature shift from 4 degrees to 37 degrees C elution of pre-adsorbed virus and accumulation of free N-acetyl-neuraminic acid (NANA) split from RBC receptors as a result of neuraminidase (Nase) activity was detected. In the case of high multiplicity of adsorption the elution was very fast (complete elution within 5 minutes) for all the strains irrespective of their virulence. Although physical saturation of RBC surface with the adsorbed virus was not achieved, a certain minimal (strain-specific) amount of the pre-adsorbed virus which splits a maximally possible (for a given strain) quantity of the NANA was found (a state of "enzymatic saturation"). Below a certain low multiplicity of adsorption elution was delayed for about 20-30 minutes while the accumulation of the split NANA began immediately after the temperature shift. This phenomenon was interpreted as a result of "crawling" of the adsorbed virions upon the RBC surface followed by "browsing" of RBC receptors and liberation of NANA. Thus, the Nase activity of the attached virus ("in situ Nase activity") is a factor providing both elution and "crawling" of the virus (depending on the multiplicity of adsorption). The in situ Nase activity of all the strains used was determined quantitatively by (1) parameters of enzymatic kinetics (Vmax, Km and Km/Vmax) and (2) parameters of enzymatic efficiency related to a certain quantity of the adsorbed virus, namely, per amount of: a) "crawling" virus, b) that providing "enzymatic saturation", and c) that equal to Km. Computation of these parameters revealed inverse correlation between the in situ Nase activity and the strain virulence. Thus, these indications can be in vitro markers of the in vivo virulence.

Cell receptors for paramyxoviruses

Treatment of chick embryo fibroblasts, calf kidney and BHK cells for 30 minutes with the enzyme neuraminidase from Vibrio cholerae causes an enhancement of the per cent of attached NDV virions. This enhancement does not depend on the multiplicity of infection. The quantity of spontaneously eluted and cellbound virus is two times greater than the quantity of the same virus derived from control cells. N-acetyl-neuramin lactose inhibits the effect of Vibrio cholerae neuraminidase. After prolonged action of this enzyme, the quantity of adsorbed NDV diminishes. Treatment of the same cells with neuraminidase from influenza virus decreases the per cent of adsorbed NDV with respect to controls. The other paramyxovirus--bovine parainfluenza 3 virus adsorbs also more intensively on cells treated with Vibrio cholerae neuraminidase. It is suggested that partial hydrolysis of NANA molecules causes a rearrangement of the cell surface charged groups and thus allows a more effective contact between paramyxoviruses and the cell.