Humoral responses to Cytomegalovirus glycoprotein B vaccine with MF59 adjuvant

Solid organ transplant (SOT) patients are at risk of end-organ diseases such as pneumonitis, hepatitis or enteritis caused by HCMV. HCMV infection can occur via primary infection of a seronegative recipient or upon reinfection or reactivation in a seropositive transplant recipient. Seronegative recipients have the greatest risk of viraemia and disease, showing that pre-existing natural immunity provides substantial protection. This, in turn, underpins vaccination as a viable strategy to control HCMV in the transplant setting. To test this, a clinical trial with a vaccine based on HCMV glycoprotein B (gB) antigen plus MF59 adjuvant was performed in SOT awaiting transplantation. The study showed that antibody titres against the gB antigen were significantly increased in both seropositive and seronegative recipients of the vaccine in comparison to the patients who received placebo, and importantly, higher titres correlated directly with reduced viraemia post-transplant. The aim of my thesis was to identify the component of the specific humoral response responsible for this effect. In comparison with placebo recipients, I could find no evidence for the protection being due to induction of antibodies that mediate neutralisation, antibody dependent cellular cytotoxicity, or prevent cell to cell spread of virus in culture. In contrast, analysis of antigenic domains of gB bound by the antibodies revealed that vaccination of seropositive individuals enhanced antibody responses against antigenic domain 2 and that these correlated with reduced viraemia post-transplant. Antibodies against three other antigenic domains were induced by the vaccine, but did not correlate with protection. These results suggest that antigenic domain 2 should be an important component of future HCMV vaccines to boost pre-existing immune responses that protect from HCMV infection. The protection afforded to seronegatives remains unidentified, but could be explained if another antigenic domain on gB remains to be discovered

Original antigenic sin shapes the immunological repertoire evoked by HCMV gB-MF59 vaccine in seropositive recipients

A cytomegalovirus (CMV) vaccine is urgently needed to protect against primary infection and enhance existing immunity in CMV infected individuals (CMV+). Using sera from CMV+ gB/MF59 vaccine recipients prior to transplant we investigated the composition of the immune response. Vaccination boosted pre-existing humoral responses in our CMV+ cohort but did not promote de-novo responses against novel linear epitopes. This suggests that prior natural infection has a profound effect on shaping the antibody repertoire and subsequent response to vaccination (‘original antigenic sin’). Thus vaccination of CMV+ may require strategies of epitope presentation distinct from those intended to prevent primary infection

Epitope-specific humoral responses to human cytomegalovirus glycoprotein-B vaccine with MF59: Anti-AD2 levels correlate with protection from viremia

The human cytomegalovirus (HCMV) virion envelope protein glycoprotein B (gB) is essential for viral entry and represents a major target for humoral responses following infection. Previously, a phase-2 placebo-controlled clinical trial conducted in solid organ transplant candidates demonstrated that vaccination with gB plus MF59 adjuvant significantly increased gB ELISA antibody levels whose titer correlated directly with protection against post-transplant viremia. The aim of the current study was to investigate in more detail this protective humoral response in vaccinated seropositive transplant recipients. We focussed on four key antigenic domains (AD) of gB; AD1, AD2, AD4 and AD5 measuring antibody levels in patient sera and correlating these with post-transplant HCMV viremia. Vaccination of seropositive patients significantly boosted pre-existing antibody levels against the immunodominant region AD1 as well as against AD2, AD4 and AD5. A decreased incidence of viremia correlated with higher antibody titers against AD2 but not with antibody titers against the other three ADs. Overall, these data support the hypothesis that antibodies against AD2 are a major component of the immune protection of seropositives seen following vaccination with gB/MF59 vaccine and identify a correlate of protective immunity in allograft patients

A temperature dependent virus binding assay reveals the presence of neutralising antibodies in human cytomegalovirus gB vaccine recipients’ sera

Human cytomegalovirus (HCMV) remains an important cause of mortality in immune compromised transplant patients and following congenital infection. Such is the burden an effective vaccine strategy is considered highest priority. The most successful vaccines to date have focused on generating immune responses against glycoprotein B (gB) – a protein essential for HCMV fusion and entry. We have previously reported that an important component of the humoral immune response elicited by gB/MF59 vaccination of patients awaiting transplant is the induction of non-neutralising antibodies that target cell associated virus which is concomitant with little evidence of concomitant classical neutralizing antibodies. Here we report that a modified neutralization assay which promotes prolonged binding of HCMV to the cell surface reveals the presence of neutralizing antibodies in sera taken from gB vaccinated patients which cannot be detected using standard assays. We go on to show that this is not a general feature of gB neutralizing antibodies suggesting specific antibody responses induced by vaccination could be important. Although we can find no evidence that these neutralizing antibody responses are a correlate of protection in vivo in transplant recipients their identification demonstrate the utility of the approach in identifying these responses. We hypothesise that further characterization has the potential to aid the identification of functions within gB important during the entry process and could potentially improve future vaccine strategies directed against gB if they prove to be effective against HCMV at higher concentrations

Protection from cytomegalovirus viremia following glycoprotein B vaccination is not dependent on neutralizing antibodies

Human cytomegalovirus (HCMV) is an important pathogen in transplant patients and in congenital infection. Previously, we demonstrated that vaccination with a recombinant viral glycoprotein B (gB)/MF59 adjuvant formulation before solid organ transplant reduced viral load parameters post transplant. Reduced posttransplant viremia was directly correlated with antibody titers against gB consistent with a humoral response against gB being important. Here we show that sera from the vaccinated seronegative patients displayed little evidence of a neutralizing antibody response against cell-free HCMV in vitro. Additionally, sera from seronegative vaccine recipients had minimal effect on the replication of a strain of HCMV engineered to be cell-associated in a viral spread assay. Furthermore, although natural infection can induce antibody-dependent cellular cytotoxicity (ADCC) responses, serological analysis of seronegative vaccinees again presented no evidence of a substantial ADCC-promoting antibody response being generated de novo. Finally, analyses for responses against major antigenic domains of gB following vaccination were variable, and their pattern was distinct compared with natural infection. Taken together, these data argue that the protective effect elicited by the gB vaccine is via a mechanism of action in seronegative vaccines that cannot be explained by neutralization or the induction of ADCC. More generally, these data, which are derived from a human challenge model that demonstrated that the gB vaccine is protective, highlight the need for more sophisticated analyses of new HCMV vaccines over and above the quantification of an ability to induce potent neutralizing antibody responses in vitro