An IgM monoclonal antibody to Japanese encephalitis virus recognizing a cross-reactive epitope on nuclear histones

An IgM class of monoclonal antibody (MAb) raised against 'envelope' (E) glycoprotein of Japanese encephalitis (JE) virus, cross reacted with nuclear histones, in addition to recognizing the viral antigen present in the cytoplasm of infected cells by indirect fluorescent antibody (FA) technique. The experiments on histone depletion by the acid treatment of uninfected PS (porcine kidney) cells, revealed the loss of nuclear immunofluorescence (IF) which was regained after the reconstitution of acid treated cells with histones, prior-to reacting with MAb NHA-2. The IgM MAb recognized specifically the viral antigens expressed on the surface of JE virus infected PS cells by a modified indirect FA. The adsorption of MAb NHA-2 with calf thymus histones (type II-AS) showed a comparative higher drop in the reactivity to JE virus (54.2% reduction) as compared to that against uncomplexed histones (33.3%) by ELISA, thus indicating a higher MAb affinity to the former. In contrast, the adsorption of MAb with chicken RBC nuclei resulted in comparatively more reduction in the reactivity to the uncomplexed histones (52.4% reduction) as against JE virus (37.5%), suggesting that DNA plays some role in modifying and presenting these epitopes. The cross-linkage of epitopes by glutaraldehyde treatment of JE virus antigen and histones showed a 2-fold and higher rise in the MAb reactivity as against those with unfixed or methanol fixed antigens (no cross-linkage), suggesting that the epitope is conformation dependent. Thus, histones seem to share a partial conformational homology with 'E' glycoprotein of JE virus and immune reaction with histones might lead to an autoimmune disorder.