Specificity of subcapsular antibody responses in Ethiopian patients following disease caused by serogroup A meningococci

Abstract

Dissecting the specificities of human antibody responses following disease caused by serogroup A meningococci may be important for the development of improved vaccines. We performed a study of Ethiopian patients during outbreaks in 2002 and 2003. Sera were obtained from 71 patients with meningitis caused by bacteria of sequence type 7, as confirmed by PCR or culture, and from 113 Ethiopian controls. Antibody specificities were analyzed by immunoblotting (IB) against outer membrane antigen extracts of a reference strain and of the patients' own isolates and by enzyme-linked immunosorbent assay for immunoglobulin G (IgG) levels against lipooligosaccharide (LOS) L11 and the proteins NadA and NspA. IB revealed that the main antigens targeted were the proteins PorA, PorB, RmpM, and Opa/OpcA, as well as LOS. MenA disease induced significant increases in IgG against LOS L11 and NadA. The IgG levels against LOS remained elevated following disease, whereas the IgG anti-NadA levels returned to acute-phase levels in the late convalescent phase. Among adults, the anti-LOS IgG levels were similar in acute-phase patient sera as in control sera, whereas anti-NadA IgG levels were significantly higher in acute-phase sera than in controls. The IgG antibody levels against LOS and NadA correlated moderately but significantly with serum bactericidal activity against MenA strains. Future studies on immune response during MenA disease should take into account the high levels of anti-MenA polysaccharide IgG commonly found in the population and seek to clarify the role of antibodies against subcapsular antigens in protection against MenA disease.

FIG. 1.

Examples of immunoblotting results from selected Ethiopian MenA patients. Patients in group I (patients 3018, 5001, 5007, 5009, 9090, and 9117) were observed to have no detectable rSBA titer in their acute-phase serum, whereas those in group II (patients 8006, 8009, and 9115) showed high rSBA titers already in their acute-phase serum. Letters indicate the positions of individual antigen bands, as determined by the guidestrips incubated with MAbs (PorA, PorB, RmpM, OpcA, NspA, and L11) or polyclonal sera (Omp85 and FetA) as follows: A, NadA; B, Omp85; C, FetA; D, PorA; E, PorB; F, RmpM; G, OpcA; H, NspA; I, LOS L11. Pat. No., patient number; D.A.O., days after reported onset of disease; rSBA, SBA against MenA strain F8238 with rabbit complement; hSBA, SBA against MenA strain Mk 686/02 with human complement; IgG APS, approximate IgG concentration (μg/ml) against MenA polysaccharide as measured by ELISA (40); nd, not done due to limited amount of serum available.

FIG. 2.

Development of IgG against LOS L11 in sera from MenA disease patients as measured by ELISA. (a) Response in paired sera from acute versus early convalescent phase. The 45° line indicates the location of values that show no difference in IgG binding between acute- and early-convalescent-phase sera. (b) Age-related differences in anti-LOS IgG levels among Ethiopian control sera, expressed as percentages of sera above or equal to thresholds (the thresholds used are either the GMCs of these control sera or 50% of the GMC).

FIG. 3.

Development of IgG against NadA in sera from MenA disease patients as measured by ELISA indicated by the response in acute-phase versus early-convalescent-phase paired sera. The 45° line indicates the location of values that show no difference in IgG binding between acute-and early-convalescent-phase sera.

FIG. 4.

(a) Scatter plot showing the correlation between results in the hSBA assay and in the anti-L11 LOS IgG ELISA in all sera (acute, early convalescent, and late convalescent phases) from Ethiopian MenA disease patients infected with L11-positive strains (n = 52). (b) Similarly, the correlation between log-transformed results in the rSBA assay and in the anti-NadA IgG ELISA in all sera from patients infected with NadA-positive strains (n = 18) is shown. The Pearson correlation coefficient r and the least-squares linear regression line equation are given.