Relative importance of complement-mediated bactericidal and opsonic activity for protection against meningococcal disease

Abstract

Killing of Neisseria meningitidis can result from complement-mediated serum bactericidal activity (SBA) or opsonophagocytosis (OPA), or a combination of the two mechanisms. While SBA titers > or =1:4 confer protection, recent evidence suggests that this threshold titer may not be required. For example, the incidence of meningococcal disease declines between ages 1 and 4 years without evidence of acquisition of SBA titers > or =1:4. Meningococcal polysaccharide vaccination also elicited OPA and lowered the risk of disease in patients with late complement component deficiencies whose sera did not support SBA. Sera from healthy adults immunized with an outer membrane vesicle vaccine showed OPA killing of N. meningitidis with C6-depleted complement, and whole blood from complement-sufficient non-immunized adults with SBA titers <1:4 also frequently had killing activity. Collectively the data indicate that SBA titers <1:4 and/or vaccine-induced OPA can confer protection against meningococcal disease.

Figure 1

Age-incidence (black triangles) of meningococcal disease in relation to percent of population with SBA titers ≥1:4 (open circles [group B] or squares [group C]). Panel A, adapted from a figure published by Goldschneider et al. from a study in the U.S. in the 1960s [14] with permission from the publisher. Panel B, adapted from a figure published by Trotter et al. from a study in the U.K. in 2000-2004 [20] with permission from the publisher. Note that the respective X and Y axes of the figures in the two panels have different scales.

Figure 2

Effect of addition of human fH on down-regulation of rabbit C3 and human serum vaccinee bactericidal titers measured with rabbit complement against group C N. meningitidis strain 4243. Panel A. Live bacteria were incubated with 20% infant rabbit serum alone (open histograms shown as solid lines), or rabbit serum plus human fH, 25 μg/ml (shaded histograms). The control, no serum, is shown by the dotted lines. Rabbit C3 deposition was measured by flow cytometric detection. Panel B. Bactericidal titers of sera of 19 children, aged 4- to 5- years, obtained 1 month after meningococcal polysaccharide vaccination. Titers were measured with 20% rabbit complement alone (none), or with 25 μg/ml of human fH or, as a negative control, 25 μg/ml of human C1-esterase inhibitor. The differences between the respective geometric mean titers measured in human sera using rabbit complement in the absence or presence of human fH were significant (P<0.001). Adapted from published data [44] with permission of the publisher.

Figure 3

Survival of N. meningitidis group B bacteria in whole blood of two adults (Donor 3 (left) and Donor 4 (right)) with SBA titers <1:4. Data are for group B strains 8047, NZ98/254 and BZ232. Adapted from published data of the author [26] with permission of the publisher. Data on Y axes represent CFU/ml on a log₁₀ axis. Error bars represent range in duplicate samples for each time point. Dashed lines represent data for blood containing 50 μg/ml of purified group B polysaccharide as an inhibitor.

Figure 4

Analysis of time to onset of meningococcal disease in patients with late complement component deficiencies in relation to meningococcal polysaccharide (A, C, Y and W-135) vaccination. The observed differences between the two groups were significant (P<0.01, see text). Adapted from a figure previously published [12] with permission of the publisher.

Figure 5

Killing of strain NZ98/254 by antibody, PMNs and C6-depleted human serum (C6D) as a complement source. Panel A. Anti-PorA P1.4 mAb activity in the presence of 20% C6D and 40 human PMNs per CFU of bacteria (black triangles with solid line); or in the absence of PMNs (no PMNs, open circles with dashed line); or with PMNs, plus antibody without C6-depleted complement (X, showing data point at one anti-P1.4 mAb concentration). Panel B. An example of killing of meningococci in an opsonophagocytic assay by a 1:5 dilution of heat-inactivated post-immunization serum from an adult immunized with an OMV vaccine plus recombinant GNA 2132. Hatched bars; sera were assayed with human PMNs, and 20% C6D as the complement source. There was no bacterial killing by the sera when tested with C6-sufficient complement in the absence of added PMNs (white bars). Data represent results of four replicate experiments for each test serum. Adapted from previously published data of the author [27] with permission of the publisher.

Figure 5

Killing of strain NZ98/254 by antibody, PMNs and C6-depleted human serum (C6D) as a complement source. Panel A. Anti-PorA P1.4 mAb activity in the presence of 20% C6D and 40 human PMNs per CFU of bacteria (black triangles with solid line); or in the absence of PMNs (no PMNs, open circles with dashed line); or with PMNs, plus antibody without C6-depleted complement (X, showing data point at one anti-P1.4 mAb concentration). Panel B. An example of killing of meningococci in an opsonophagocytic assay by a 1:5 dilution of heat-inactivated post-immunization serum from an adult immunized with an OMV vaccine plus recombinant GNA 2132. Hatched bars; sera were assayed with human PMNs, and 20% C6D as the complement source. There was no bacterial killing by the sera when tested with C6-sufficient complement in the absence of added PMNs (white bars). Data represent results of four replicate experiments for each test serum. Adapted from previously published data of the author [27] with permission of the publisher.

Figure 6

Serum bactericidal (C6-sufficient complement) and opsonophagocytic killing (C6-deficient complement) of sera from vaccinated adults measured against N. meningitidis group B strain H44/76, which was used to prepare the OMV vaccine. OMV vaccine group (N=17, white bars); combination OMV vaccine + recombinant GNA 2132 vaccine group (N=15, gray bars). Bars with hatches represent pre-immunization sera and bars without hatches represent sera obtained 1 month after a third dose of vaccine. Panel A. Complement-mediated bactericidal activity with C6-sufficient complement. Panel B. Opsonophagocytic killing of meningococci measured with C6-depleted complement and human PMNs. Comparisons of the respective percentages of subjects in the two groups with opsonophagocytic killing activity after vaccination, *P=0.02. Similar respective trends were observed for two other heterologous test strains but the differences were not statistically significant. Adapted from published data of the author [27] with permission of the publisher.