Elevated IgG Responses in Infants Are Associated With Reduced Prevalence of Mycobacterium tuberculosis Infection

Abstract

Background: It is unclear whether antibodies can prevent Mycobacterium tuberculosis (Mtb) infection. In this study, we examined the relationship between total plasma IgG levels, IgG elicited by childhood vaccines and soil-transmitted helminths, and Mtb infection prevalence, defined by positive QuantiFERON (QFT) test.

Methods: We studied 100 Mtb uninfected infants, aged 4-6 months. Ten infants (10%) converted to positive QFT test (QFT+) within 2 years of follow-up for Mtb infection. Antibody responses in plasma samples acquired at baseline and tuberculosis investigation were analyzed by enzyme-linked immunosorbent assay and ImmunoCAP^® assay.

Results: QFT- infants displayed a significant increase in total IgG titers when re-tested, compared to IgG titers at baseline, which was not observed in QFT+ infants. Bacille Calmette-Guérin (BCG) vaccine-specific IgG2 and live-attenuated measles vaccine-specific IgG were raised in QFT- infants, and infants who acquired an Mtb infection did not appear to launch a BCG-specific IgG2 response. IgG titers against the endemic helminth Ascaris lumbricoides increased from baseline to QFT re-testing in all infants.

Conclusion: These data show raised IgG associates with a QFT-status. Importantly, this effect was also associated with a trend showing raised IgG titers to BCG and measles vaccine. Our data suggest a possible protective association between raised antibody titers and acquisition of Mtb infection, potentially mediated by exposure to antigens both related and unrelated to Mtb.

Keywords: Bacille Calmette-Guérin; Mycobacterium tuberculosis infection; antibody; enzyme-linked immunosorbent assay; helminth; vaccine.

Figure 1

Infants recruited to the tuberculosis (TB)/helminth study. Infants enrolled in the MVA85A trial were randomly assigned to the placebo or MVA85A vaccination arms (A). 112 infants were recruited to the TB/helminth study; incomplete clinical data and sample sets were available for n = 12 infants, leaving n = 100 available for further analysis. Time-points of blood collection for QFT (QuantiFERON) (B) were at baseline, study day 336, at last scheduled visit and/or upon TB investigation. Per participant, serum was analyzed at baseline and upon TB investigation. The age range of infants at baseline was 4–6 months, and the age range at TB investigation was 8–39 months.

Figure 2

Comparisons of tuberculosis (TB) investigation total IgG responses to baseline total IgG, age and QFT outcome. Total serum IgG in participants at baseline and upon TB investigation visit (n = 100) as measured by enzyme-linked immunosorbent assay (A). Total IgG titers vs. age in months at TB investigation (n = 97), with samples from QFT positive participants indicated in purple (B); overlaid are the line-of-best-fit and 95% confidence bands (dashed lines). Association between age at TB investigation and QFT result (C). Antibody titers are presented as log-transformed arbitrary values. The Wilcoxon matched-pairs signed rank test was used to assess significance of the comparison in (A), and the Mann–Whitney test was used to assess significance in (C). The Spearman correlation was used to assess the strength of the correlation in (B).

Figure 3

Total IgG titers stratified by QFT outcome and MVA vaccination status. Relationship between QFT outcome and total IgG titers from baseline and tuberculosis (TB) investigation participant samples (A). A before/after comparison of samples subdivided as in (A) is presented in (B). Relationship between QFT outcome, MVA85A vaccination status, and total IgG titers from baseline and TB investigation participant samples (C). Column sample numbers left-right (C): 45, 4, 45, 6, 45, 4, 45, 6. Antibody titers are presented as arbitrary values. Statistical analysis was performed with the Kruskal–Wallis test and Dunn’s post hoc test to assess significance of the comparisons in (A,C). The Wilcoxon matched-pairs signed rank test was used to assess significance of the associations in (B).

Figure 4

Antibody responses at tuberculosis investigation to childhood vaccines. Anti-Bacille Calmette-Guérin (BCG), measles, and tetanus IgG titers (A), as well as anti-BCG IgG1, IgG2, and IgG3 titers (B) as measured by enzyme-linked immunosorbent assay and stratified by QFT result. One sample was excluded in the anti-measles IgG analysis (A) (participants had not received measles vaccination). Two fewer samples reported for BCG IgG1 (B) due to a lack of sample availability. Antibody titers are presented as arbitrary values. Comparisons were assessed for significance by the Mann–Whitney test.

Figure 5

Soil-transmitted helminth-specific IgG titers and QFT outcome at baseline and upon tuberculosis investigation. Anti-Ascaris lumbricoides IgE (n = 50), IgG, and IgG4 (n = 100) titers (A). Anti-A. lumbricoides IgG and IgG4 titers stratified by QFT outcome (B). A before/after comparison of anti-A. lumbricoides IgG titers as subdivided in (B) is presented in (C). Antibody titers are presented as arbitrary values. The Mann–Whitney test was used to assess two-group comparisons for significance in (A), the Kruskal–Wallis test with Dunn’s post hoc test was used for the multi-group comparisons in (B), and the Wilcoxon matched-pairs signed rank test was used to assess for significance in (C).