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We studied the concentration of circulating human immunoglobulin G (IgG) antibody to the native capsular polysaccharide of group B streptococcus (GBS) type Ib necessary to protect mice against lethal challenge by laboratory and clinical GBS Ib strains. Antibody was measured by an enzyme-linked immunosorbent assay in which native polysaccharide antigen coupled to human serum albumin was used. The assay was standardized by a quantitative precipitation test, using native antigen and specific human IgG antibody purified by affinity chromatography. IgG anti-GBS Ib antibody level-protection curves for 90% lethal dose challenge of mice were sigmoidal. The curves of whole serum and affinity-chromatographed IgG anti-GBS Ib were superimposable. The serum concentrations of human antibody required for complete protection of mice varied with the infecting strain and ranged from 0.038 to 0.175 microgram/ml. Protective levels of human IgG anti-GBS Ib were lower than those we found previously for homologous protection against GBS Ia challenge (range, 0.25 to 1.0 microgram/ml).

We evaluated the chemotactic and bactericidal capacities of human cord blood monocytes, and the ability of cord lymphocytes and sera to generate chemotactic (LDCF) and opsonic factors for monocytes. Our results suggest that the intrinsic locomotive capacity, and the receptor system for LDCF, are adequately developed in the newborn monocyte. Moreover, newborn lymphocytes appear to produce adequate amounts of LDCF, capable of attracting adult monocytes. Poor chemotaxis was observed only when cord monocytes were exposed to supernatants of cord lymphocytes, which suggests that both contribute to this abnormal response. An inhibitory factor for which only cord monosensitivity of monocytes and strength of chemotactic factor between adults and newborns would explain these results. Bactericidal capactiy of cord monocytes against Escherichia coli K-12 opsonized by either cord or adult serum was comparable to that of adult monocytes.

Although the current use of 51Cr for the evaluation of monocyte chemotaxis has yielded encouraging results, certain properties of this radionuclide leave room for improvement. Technetium-99m was evaluated as an alternative label for this purpose. A cell loss of 50% was found, but the recovered cells showed excellent viability and function. Chemotaxis was measured using a modified Boyden's chamber and a lymphocyte-derived chemotactic factor. The chemotactic properties of 99mTc-labeled human monocytes were preserved, and an excellent correlation between radioactive measurement and microscopic counting of migrating cells was observed.

The phagocytosis-induced metabolic burst of human newborn monocytes, as evaluated by their capacity to reduce NBT, was comparable to that of adult monocytes. The NBT reduction assay constitutes a simple method of ascertaining the functional capacity of human monocytes.

We determined the serum concentration of human IgG antibody to the native capsular polysaccharide of group B Streptococcus (GBS) type III needed to passively protect mice against lethal homologous challenge. Antibody was measured by an ELISA, standardized by two methods, and corrected for nonprecipitating antibody. A concentration of 1.3 micrograms of IgG antibody to GBS type III/ml protected 126 (97%) of 130 mice from an 80%-96% lethal dose bacterial challenge. Concentrations of IgG antibody to GBS type III in sera from 42 infected infants were less than or equal to 0.3 micrograms/ml. Concentrations of antibody ranged from less than 0.02 to 21.7 micrograms/ml in sera from 102 unselected pregnant women (median, 0.05 microgram/ml); 13% had concentrations greater than or equal to 1.3 microgram/ml. Levels in 25 women colonized with GBS type III who gave birth to normal infants were significantly higher and ranged from 0.1 to 10.7 microgram/ml (median, 0.78 micrograms/ml). In a study of transplacental passage of antibody, protective levels were found in a number of infants with gestational ages between 28 and 36 weeks.

Human IgG antibody to the type-specific polysaccharide antigen of group B Streptococcus type Ia in the sera of mice was measured by enzyme-linked immunosorbent assay, previously standardized by quantitative precipitation, 24 hr after passive immunization with human serum or affinity-chromatographed antibody. The concentrations of antibody needed to protect mice against 90% lethal dose challenge varied with the bacterial inoculum and ranged from 0.25 to 1 microgram/ml using five strains of group B Streptococcus type Ia. Affinity-chromatographed antibody gave results comparable to serum, indicating the specificity of the antibody and the absence of other humoral factors in protection with this serum. Sera from 11 infected infants and their mothers had concentrations of antibody of less than or equal to 0.17 micrograms/ml, below the protective level delineated in the experimental model. Twelve percent of 50 adult women and 36% of 25 women colonized with group B Streptococcus type Ia had antibody levels of greater than or equal to 1 microgram/ml.

Strain differences have been postulated to explain the observation that group B Streptococcus type III (GBS III) late-onset disease occurs in only a fraction of colonized infants. To determine the distribution of type-specific polysaccharide antigen (Ag) in GBS III, Ag was measured by rocket immunoelectrophoresis in both supernatant fluids and EDTA extracts and by radial immunodiffusion in multiple HCl extracts of the pellet from cultures of 10 strains of GBS III. Capsular Ag was defined as the sum of Ag in EDTA extracts + Ag in multiple HCl extracts. Both Ag in EDTA extracts and Ag in supernatant fluids correlated with capsular Ag (r = 0.94). GBS III strains were obtained from the blood of 19 infants with late-onset sepsis, from the cerebrospinal fluid or blood of 22 infants with late-onset meningitis, and from mucosal surfaces of both 18 infants and 12 mothers of infants with low levels of type-specific antibody and asymptomatic colonization. Mean values of Ag in supernatant fluids in strains from infants with late-onset sepsis (1.50 +/- 0.08 micrograms/ml) and late-onset meningitis (1.67 +/- 0.09 micrograms/ml) were significantly greater than those in asymptomatic colonization strains (1.14 +/- 0.05 micrograms/ml; P less than 0.001). The number of organisms required for a 50% lethal dose in the chick embryo, determined in 29 strains, was inversely related to Ag in supernatant fluids (r = -0.60). The demonstration that the quantity of capsular Ag produced by GBS III strains is related to their virulence in chick embryos and to their invasiveness in susceptible infants supports the hypothesis that Ag is a virulence factor in humans.

An ELISA was developed to measure human IgG antibody to the native polysaccharide antigen of GBS serotype Ia. Because the polysaccharide binds poorly to polyvinyl chloride, its adherence was enhanced by activation with cyanogen bromide and coupling to HSA in a molar ratio of polysaccharide to albumin of 1:4.5. There was minimal loss of sialic acid during coupling, and the coupled antigen showed identity with uncoupled native antigen by Ouchterlony analysis. OD values obtained by ELISA showed a log-linear relation to concentration of specific antibody in whole and affinity-chromatographed human sera measured by quantitative precipitation over a range of 0.25 to 3.5 microgram/ml. In replicate ELISA experiments using serially diluted human serum, dilutions with antibody content as low as 0.016 microgram/ml could be reliably differentiated from PBS or agammaglobulinemic serum. The concentration of antibody in 98 selected human sera measured by ELISA correlated well (r = 0.89, p less than 0.001) with results obtained by indirect IF assay. This quantitative ELISA for GBS antibody is rapid, convenient, economical, and suitable for routine use.