Detection of antibodies against human metapneumovirus by Western blot using recombinant nucleocapsid and matrix proteins

Detection of antibodies against individual proteins of human metapneumovirus (hMPV) is important in the analysis of immune responses to hMPV. Specific antibodies against nucleocapsid (N) and matrix (M) proteins in 97 serum samples were tested by Western blot using recombinant N and M proteins of hMPV expressed in Escherichia coli. The results were compared with those of immunofluorescence assays (IFAs) based on hMPV-infected LLC-MK2 cells, which expressed the whole hMPV proteins. Thirty (61.2%) and 31 (63.3%) of 49 serum samples with titers of > or = 1:160 by IFA reacted with N and M proteins, respectively. Only 2 (4.2%) of 11 serum samples with titers of 1:80 by IFA reacted with N and M proteins. Antibodies against N and M proteins were not detected in 37 serum samples with titers of < 1:40 by IFA. These results indicate that the antibodies against N and M proteins are highly specific (100%) but less sensitive (42.1%, N protein; 40.8%, M protein) than those against whole proteins of hMPV detected by IFA. The reactivity of sera with the recombinant N protein and that with the recombinant M protein correlated well (correlation coefficient of 0.79), and the concordance of reactivities was 91% (kappa = 0.79). In summary, both recombinant N and M proteins of hMPV were antigenic, and the responses to N and M protein varied among patients. Therefore, Western blot using N and M proteins provide a useful tool for analysis of immune responses to hMPV.