Serologic cross-reactions between nucleocapsid proteins of human respiratory syncytial virus and human metapneumovirus.

Human respiratory syncytial virus (hRSV) and human metapneumovirus (hMPV) share virologic and epidemiologic features and cause clinically similar respiratory illness predominantly in young children. In a previous study of acute febrile respiratory illness in Bangladesh, we tested paired serum specimens from 852 children presenting fever and cough for diagnostic increases in titers of antibody to hRSV and hMPV by enzyme immunoassay (EIA). Unexpectedly, of 93 serum pairs that showed a ≥ 4-fold increase in titers of antibody to hRSV, 24 (25.8%) showed a concurrent increase in titers of antibody to hMPV; of 91 pairs showing an increase to hMPV, 13 (14.3%) showed a concurrent increase to hRSV. We speculated that common antigens shared by these viruses explain this finding. Since the nucleocapsid (N) proteins of these viruses show the greatest sequence homology, we tested hyperimmune antisera prepared for each virus against baculovirus-expressed recombinant N (recN) proteins for potential cross-reactivity. The antisera were reciprocally reactive with both proteins. To localize common antigenic regions, we first expressed the carboxy domain of the hMPV N protein that was the most highly conserved region within the hRSV N protein. Although reciprocally reactive with antisera by Western blotting, this truncated protein did not react with hMPV IgG-positive human sera by EIA. Using 5 synthetic peptides that spanned the amino-terminal portion of the hMPV N protein, we identified a single peptide that was cross-reactive with human sera positive for either virus. Antiserum prepared for this peptide was reactive with recN proteins of both viruses, indicating that a common immunoreactive site exists in this region.