Fig. 6: Association between protective IgG profiles and in vitro inhibition of function and promotion of opsonophagocytosis.

a, Correlation coefficients (Spearman’s method) between binding IgG titers and functional immunoassays in n = 114 serum samples randomly selected across a broad range of binding IgG levels. Highlighted squares indicate the specific relationship between binding titers and the immunoassay that directly measures the function of the corresponding antigen. Blue square represents SLO, orange represents SpyCEP, yellow represents SpyAD and green represents GAC. b–d, Relationship between binding IgG levels and functional activity in serum samples (n = 114): IgG binding levels to SLO and inhibition of SLO-mediated hemolysis (n =114) (b), IgG binding levels to SpyCEP and inhibition of SpyCEP-mediated IL-8 cleavage (n = 114) (c) and IgG binding levels to SpyAD and promotion of phagocytosis of SpyAD-coated beads by THP-1 cells (n = 114) (d). Box plots show the IQR (box) and 1.5× IQR whiskers. Points beyond whiskers represent outliers. IC₅₀ values between those above and those below 50% protective thresholds were compared with two-sided Wilcoxon tests. e, Proportion of participants with titers above and below 50% protective thresholds with any detectable opsonophagocytosis of M1 bacteria. Proportions between groups were compared with a two-sided Fisherʼs exact test. f, Relationship between IgG binding titers to SLO, SpyAD, SpyCEP and opsonophagocytosis of M1 bacteria by THP-1 cells. Binding IgG level above (purple) and below (red) the 50% protective threshold is demonstrated.