A Systems Biology Approach Identifies B Cell Maturation Antigen (BCMA) as a Biomarker Reflecting Oral Vaccine Induced IgA Antibody Responses in Humans

Abstract

Vaccines against enteric diseases could improve global health. Despite this, only a few oral vaccines are currently available for human use. One way to facilitate such vaccine development could be to identify a practical and relatively low cost biomarker assay to assess oral vaccine induced primary and memory IgA immune responses in humans. Such an IgA biomarker assay could complement antigen-specific immune response measurements, enabling more oral vaccine candidates to be tested, whilst also reducing the work and costs associated with early oral vaccine development. With this in mind, we take a holistic systems biology approach to compare the transcriptional signatures of peripheral blood mononuclear cells isolated from volunteers, who following two oral priming doses with the oral cholera vaccine Dukoral®, had either strong or no vaccine specific IgA responses. Using this bioinformatical method, we identify TNFRSF17, a gene encoding the B cell maturation antigen (BCMA), as a candidate biomarker of oral vaccine induced IgA immune responses. We then assess the ability of BCMA to reflect oral vaccine induced primary and memory IgA responses using an ELISA BCMA assay on a larger number of samples collected in clinical trials with Dukoral® and the oral enterotoxigenic Escherichia coli vaccine candidate ETVAX. We find significant correlations between levels of BCMA and vaccine antigen-specific IgA in antibodies in lymphocyte secretion (ALS) specimens, as well as with proportions of circulating plasmablasts detected by flow cytometry. Importantly, our results suggest that levels of BCMA detected early after primary mucosal vaccination may be a biomarker for induction of long-lived vaccine specific memory B cell responses, which are otherwise difficult to measure in clinical vaccine trials. In addition, we find that ALS-BCMA responses in individuals vaccinated with ETVAX plus the adjuvant double mutant heat-labile toxin (dmLT) are significantly higher than in subjects given ETVAX only. We therefore propose that as ALS-BCMA responses may reflect the total vaccine induced IgA responses to oral vaccination, this BCMA ELISA assay could also be used to estimate the total adjuvant effect on vaccine induced-antibody responses, independently of antigen specificity, further supporting the usefulness of the assay.

Keywords: BCMA (TNFRSF17); IgA responses; biomarkers; enteric diseases; global health; oral vaccines and futures.

Figure 1

Transcriptomics analysis of PBMCs from Dukoral® vaccinated volunteers. (A) RNA-Seq analysis was performed on PBMCs from six Swedish volunteers who following two oral priming doses had either strong (n = 4, closed squares) or no (n = 2, open squares) IgA responses to the CTB and OSP antigens in the oral cholera vaccine. Volunteers with two-fold increases in IgA antibody responses are considered as vaccine responders (dashed line). Bars represent geometric means. (B) Heatmap of DEGs (padj < 0.05) of high vs. non-IgA vaccine responders that were enriched in Reactome pathways at day 19 post-vaccination. Unique DEGs to strong IgA vaccine responders are highlighted in red. (C) Heatmap of unique PBMC DEGs (padj < 0.05) that were enriched in Reactome pathway analysis at day 19 post-vaccination. Unique DEGs that were also identified in the strong vs. non-IgA vaccine analysis at day 19 post-vaccination are highlighted in orange. Color scales on Heatmaps; Red = up, White = no change, Blue = down regulated transcripts.

Figure 2

BCMA responses in ALS specimens correlate with Dukoral® ALS vaccine responses in Swedish volunteers. (A) Concentrations of BCMA (pg/mL) in ALS specimens were quantified by ELISA in 29 individuals before (pre) and after (post) primary and booster Dukoral® vaccination. Horizonal bars represent geometric mean values. (B) Correlation between the magnitudes of ALS CTB IgA responses and ALS BCMA fold rises following primary and booster Dukoral® vaccinations. (C) Correlation between the magnitudes of ALS OSP IgA and ALS BCMA responses following primary and booster Dukoral® vaccinations. Each symbol represents one individual.

Figure 3

ALS BCMA responses is higher in orally vaccinated subjects than in placebo recipients. Magnitudes of ALS BCMA responses from volunteers were primed with 2 oral doses 2 weeks apart (day 0 and day 14), of either vaccine buffer (placebo group, n = 21), or the oral ETVAX vaccine, administered with or without 10 μg dmLT (n = 46). Each symbol represents one individual. Filled diamonds or triangles represent individuals who responded to at least one ETVAX vaccine antigen in ALS IgA. Open diamonds and triangles represent subjects that did not respond to any of the vaccine antigens in ALS. ALS BCMA responses higher than the arbitrarily chosen level of 1.4 is indicated by a dashed line. Bars represent geometric means with 95% CI.

Figure 4

ALS BCMA responses correlate with ALS IgA responses to ETVAX vaccine antigens. Correlations between the vaccine specific ALS IgA responses against each individual ETVAX vaccine antigen (A–E) and magnitudes of ALS BCMA responses following primary (n = 46) and booster (n = 35) ETVAX vaccination. (A) LTB IgA, (B) CFA/I IgA, (C) CS3 IgA, (D) CS5 IgA, (E) CS6 IgA. (F) Correlations between the combined ALS IgA responses against the five major ETVAX vaccine antigens (response index) and the magnitudes of ALS BCMA responses following primary (n = 46) and booster (n = 35) ETVAX vaccination. Each symbol represents one individual. Filled squares represent vaccine responders with an ALS IgA combined response index >150. Open squares represent vaccine responders with an ALS IgA combined response index <135.

Figure 5

ALS BCMA response magnitudes correlate with ETVAX induced plasmablast responses. (A) Correlations of the magnitudes of total plamablast responses (fold rises in frequencies of CD19⁺CD27⁺CD38^high cells among cells detected by flow cytometric analysis in post compared to pre-vaccination samples) induced by primary and booster ETVAX vaccination with ALS BCMA responses (n = 17). (B) Correlations of the magnitudes of IgA⁺ plamablast responses induced by primary and booster ETVAX vaccination with ALS BCMA responses (n = 16). Each symbol represents one individual. Filled squares represent vaccine responders with an ALS IgA combined response index >150. Open squares represent vaccine responders with an ALS IgA combined response index <135.

Figure 6

The usefulness of ALS BCMA as an oral vaccination biomarker. (A) Correlation between magnitudes of ALS BCMA responses following primary ETVAX vaccination and the combined booster ALS IgA responses to all five major ETVAX vaccine antigens (n = 29). (B) Magnitudes of ALS BCMA responses in volunteers who received ETVAX with 10 μg dmLT (n = 24) compared to ETVAX only (n = 21). Bars represent geometric means with 95% CI. Filled squares represent vaccine responders with an ALS IgA combined response index >150. Open squares represent vaccine responders with an ALS IgA combined response index <135.