Immune reactivity of human sera to the glycoprotein B of human herpesvirus 7

Abstract

The glycoprotein B (gB) is highly conserved among distinct human herpesvirus 7 (HHV-7) strains. Similarly to other herpesvirus glycoproteins, gB has been assumed to induce a specific human immune response. However, it did not appear as an immunodominant protein in conventional immunoblot assays. Recombinant gB, obtained from either Escherichia coli or baculovirus expression systems, did react specifically with HHV-7-seropositive sera, and the main corresponding epitopes were located in its N-terminal part. A 24-amino-acid peptide, corresponding to a predicted hydrophilicity peak and presenting no extensive homology with other betaherpesvirus glycoproteins, was selected in this region at positions 129 to 152 of the gB sequence. When tested by enzyme-linked immunosorbent assay (ELISA), this peptide specifically reacted with HHV-7-seropositive sera. This reactivity was significantly inhibited by the preincubation of sera with the peptide itself, lysates of gB-expressing cells, or lysates of HHV-7-infected cells. The reactivity was not significantly modified when sera were preincubated with lysates of either human cytomegalovirus (HCMV)- or HHV-6-infected cells. In cross-sectional studies including both children and adults, 49 out of 61 serum samples (80%) were found to be positive by HHV-7 ELISA, independent of their reactivity to HCMV. A longitudinal serological study of 17 children during the first 4 years of life showed that the level of ELISA-detected antibodies significantly decreased within a few weeks after birth and then increased in the following months, likely reflecting, respectively, the loss of maternal antibodies and the occurrence of seroconversion. These results demonstrate that gB peptide ELISA might be a useful tool for the serological study of HHV-7 infection.

FIG. 1.

Immune reactivity of human sera to the recombinant gB expressed in bacteria. The recombinant protein gB was expressed in bacteria as a fusion protein with a polyhistidine tract at its N terminus. The proteins were separated by denaturing SDS-PAGE, transferred to a nitrocellulose sheet, and then allowed to react with distinct human sera (lanes C to K) or to the mouse antipolyhistidine antibody (lane L). In lanes A and B, P1 serum was tested against lysates of bacteria transformed with the plasmid pTrc-HisC containing no insert and the plasmid pTr-gB-rec in the absence of IPTG induction respectively, as negative controls. Numbers on the left of gel are in kilodaltons. The star to the right of gel indicates the position of recombinant gB. NA, not applicable.

FIG. 2.

Immune reactivity of human sera to the recombinant gB expressed in Sf21 insect cells. The recombinant proteins were expressed in Sf21 insect cells following infection with the recombinant baculoviruses Bac-gB-rec and Bac-gB.N-rec. The proteins, either the entire gB (gB) or its N-terminal part (gB_N), were separated by denaturing SDS-PAGE, transferred to nitrocellulose, and then allowed to react with distinct human sera (lane pairs C to K) or the mouse antipolyhistidine antibody (lane pair L). In the lanes A and B, serum P1 was tested with either lysates of uninfected Sf21 or Sf21 cells infected with wild-type baculovirus, respectively, as negative controls. Numbers to the left and right of gel are in kilodaltons. NA, not applicable.