Agglutination-separation reactions of red blood cells sensitized with Newcastle disease virus: quantities, agglutination characteristics, and serology of altered virus and HN spikes released following neuraminidase reactivation.

Red blood cells (RBC) become sensitized following the elution of strain 575 Newcastle disease virus (NDV). The neuraminidase (NA) in the haemagglutinin (HA)-sialic acid configuration is inactive. The HA on sensitized RBC agglutinates normal RBC. The sialic acid on normal RBC initiated reactivation of the NA-a newly described function. Then normal-sensitized RBC agglomerates separated at 37 degrees C in the irreversible agglutination-separation (AS) reactions. With separation the AS products. HN spikes (150-200 kDa) and altered NDV, which contain fewer HN spikes than intact allantoic NDV, were removed from the sensitized RBC and the NDV membrane. Extraction of HN spikes from the membrane required more sialic acid than the removal of AS products from RBC. Thus 2 reactions were delineated for the orderly removal. Amounts of each released AS product suggest the source of the HN spikes. AS reactions and ether treatment of NDV increased the HA titres up to 19.2 fold. HA-sialic acid configurations were estimated by the amounts of normal RBC agglutinated by sensitized RBC and also by agglutination with fetuin. Elution of B1 vaccine, HN spikes from ether-treated NDV as well as AS products separated on sephadex resulted in incompletely sensitized RBC; fewer configurations were titrated with normal RBC; all failed to respond to anti-NA antibody. In contrast, sensitized RBC or suspensions of 575 NDV, but not the B1 vaccine strain, responded to both anti-NA and anti-HA antibody. Sensitization, slow elution, and responding to anti-NA antibody, which was accompanied by fluorescent foci on sensitized RBC, required intact NDV and the infrequent HA-sialic acid configuration. The NA was inactive for the HA-sialic acid configuration but cleaved fetuin, indicating substrate specificity. An inactive NA would allow time for fusion and NDV penetration rather than elution by an active NA early in NDV infection.