An inhibition radioimmunoassay and a two-site immunoradiometric (four-layer) assay for the quantification of measles virus nucleocapsid antigen were established and their sensitivities compared. Both assays exhibited threshold sensitivities of from 1 to 10 ng nucleocapsid antigen per ml. Concentrations of reagents used in the assays were shown to be an important factor determining the sensitivities of the assays. Purified mumps or parainfluenza 2 virus nucleocapsids did not compete with measles virus nucleocapsids in the inhibition assay but some degree of cross-reactivity with canine distemper virus nucleocapsid was observed. Pretreatment of virus-infected cells with detergent had a significant effect on the amount of antigen detectable by the assays. SDS greatly decreased the reactivity of measles virus nucleocapsid but Triton X-100 and, to a larger degree, sodium deoxycholate released antigen from the cells in quantities greater than that detected when no detergent pretreatment was employed. Complexing of antibodies to measles virus nucleocapsids decreased dramatically the threshold sensitivity of the nucleocapsid assays. The antigen-antibody complexes could be disrupted and the components separated but the conditions employed destroyed 90% of the antigenic reactivity of the nucleocapsid and also had a deleterious effect on the antibody. Nucleocapsid antigen was readily detected in brains from hamsters with acute measles encephalitis. The assays, however, were not sensitive enough for routine detection of antigen in brains from hamsters with chronic measles encephalitis.


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