Promiscuous Coxiella burnetii CD4 Epitope Clusters Associated With Human Recall Responses Are Candidates for a Novel T-Cell Targeted Multi-Epitope Q Fever Vaccine

Abstract

Coxiella burnetii , the causative agent of Q fever, is a Gram-negative intracellular bacterium transmitted via aerosol. Regulatory approval of the Australian whole-cell vaccine Q-VAX® in the US and Europe is hindered by reactogenicity in previously exposed individuals. The aim of this study was to identify and rationally select C. burnetii epitopes for design of a safe, effective, and less reactogenic T-cell targeted human Q fever vaccine. Immunoinformatic methods were used to predict 65 HLA class I epitopes and 50 promiscuous HLA class II C. burnetii epitope clusters, which are conserved across strains of C. burnetii . HLA binding assays confirmed 89% of class I and 75% of class II predictions, and 11 HLA class II epitopes elicited IFNγ responses following heterologous DNA/DNA/peptide/peptide prime-boost immunizations of HLA-DR3 transgenic mice. Human immune responses to the predicted epitopes were characterized in individuals naturally exposed to C. burnetii during the 2007–2010 Dutch Q fever outbreak. Subjects were divided into three groups: controls with no immunological evidence of previous infection and individuals with responses to heat-killed C. burnetii in a whole blood IFNγ release assay (IGRA) who remained asymptomatic or who experienced clinical Q fever during the outbreak. Recall responses to C. burnetii epitopes were assessed by cultured IFNγ ELISpot. While HLA class I epitope responses were sparse in this cohort, we identified 21 HLA class II epitopes that recalled T-cell IFNγ responses in 10–28% of IGRA+ subjects. IGRA+ individuals with past asymptomatic and symptomatic C. burnetii infection showed a comparable response pattern and cumulative peptide response which correlated with IGRA responses. None of the peptides elicited reactogenicity in a C. burnetii exposure-primed guinea pig model. These data demonstrate that a substantial proportion of immunoinformatically identified HLA class II epitopes show long-lived immunoreactivity in naturally infected individuals, making them desirable candidates for a novel human multi-epitope Q fever vaccine.

Coxiella burnetii epitope-specific T-cell responses in chronic Q fever patients

“…Using immunoinformatically predicted T-cell epitopes derived from C. burnetti sero-reactive and type IV secretion systems (TSS4) substrate proteins, we previously analyzed antigenicity in naturally infected subjects with past symptomatic or asymptomatic C. burnetii infection; these are here-after referred to as ‘convalescents’ since infection was cleared. In these naturally exposed subjects, we demonstrated long-lived immunoreactivity to promiscuous CD4 T-cell epitopes, while HLA class I epitope responses were sparse in this cohort (7). One possible explanation for the latter was that class I responses might have contracted faster than class II responses, as previously observed following smallpox infection or vaccination and tuberculosis treatment (8–11).…”

“…In this study we compared the repertoire of HLA class II T-cell epitopes recognized by subjects treated for persistent C. burnetii infection (chronic Q fever) with those recognized by convalescent individuals (i.e. those with resolved past acute or asymptomatic infection) in our previous study (7). We find that individuals treated for chronic Q fever have effectively generated a central memory C. burnetii -specific T-cell response, as measured by cultured ELISpot, that closely resembles that of convalescent patients.…”

“…The principle difference may be that unlike Q fever, herpes simplex infection is always considered a persistent infection but can remain asymptomatic nonetheless. Instead, the same set of promiscuous HLA class II epitopes identified previously in the convalescent cohort (7) could in principle be used to further boost already primed T-cell responses in individuals with persistent infection. Whether this would speed up resolving of infection in this patient group, however, is unclear.…”

Better Epitope Discovery, Precision Immune Engineering, and Accelerated Vaccine Design Using Immunoinformatics Tools

“…Figure 3B shows recent HLA binding studies for HLA class II-restricted epitopes, performed at EpiVax (published and unpublished). Additional T cell epitope and HLA binding validation studies have been published in the course of grant-funded research collaborations, describing T cell immune responses to predicted epitopes in vitro using human lymphocytes [see (4,(24)(25)(26)(27)(28)(29)(30)(31)(32)] and also in prospective, in vivo immunogenicity and vaccine efficacy studies in murine models [see (10,18,(33)(34)(35)(36)(37)(38)].…”

“…Building on our experience with the "rapid fire" development of a vaccine for Lassa Fever (60), EpiMatrix and JanusMatrix were used to identify 50 promiscuous class II epitopes from Coxiella burnetii antigens that were tested in HLA binding assays and screened for immunogenicity in HLA-DR3 transgenic mice by colleagues at the Vaccines and Immunotherapy Center (VIC, Harvard) and Innatoss, Oss, the Netherlands (28,29). Significant epitope-specific IFNγ responses were found for 11/50 peptides, all of which are predicted HLA-DRB1 * 0301 epitopes (Fisher's exact p-value: 0.023); all but one bound to DRB1 * 0301 in vitro.…”