Human Immunity to Group B Streptococci Measured by Indirect Immunofluorescence: Correlation with Protection in Chick Embryos

Abstract

An indirect immunofluorescence (IF) assay has been developed as a useful semiquantitative method for determination of type-specific IgG antibody in human sera to the five serotypes of group B Streptococcus. Antibody titers measured by IF correlated with passive protection in chick embryos, and antibody titers associated with chick embryo protection were delineated. Except for types Ia and Ic, IF antibody to each of the streptococcal types was completely absorbed by homologous strains, and antibody titers were unchanged by incubation with heterologous bacteria. For types Ia and Ic, IF antibody was absorbed by either the Ia or the Ic strain and by native Ia carbohydrate antigen. Antibody titers measured by IF and chick embryo protection against types Ia and Ic were similar, but were divergent for Ib and Ic, a finding suggesting that antibody is predominantly directed to the major carbohydrate determinants. In addition, 29 of 31 sera that had been tested in chick embryos yielded comparable results in mice against challenge with type Ia group B Streptococcus, a finding further validating the chick embryo assay. Sera from all of 43 mothers of infants infected with group B streptococci had antibody titers by IF that were less than titers associated with protection in chick embryos.

Antibody-independent Classical Pathway-mediated Opsonophagocytosis of Type Ia, Group B Streptococcus

“…In three of these studies, however, the presence of type Ia antibody in a serum was measured exclusively by functional'assays (opsonophagocytosis [7,30] or opsonization related to chemiluminescence [8]) that employed the mousepassed strain 090 as the only test organism. Our finding that phagocytosis and killing of this strain is dependent upon a critical amount of specific antibody (quantitated in a radioimmunoassay) is consistent with these observations (7)(8)(9)30). However, the notable disparity between the opsonophagocytic requirements of human sera for this laboratory-adapted, highly encapsulated strain and those for type Ia strains freshly isolated from patients described in the present study suggest that the precise role of type-specific antibody and complement in human infection due to type Ia, group B Streptococcus should be reexamined.…”

“…Employing prototype strains, passed in mice to enhance virulence, and type-specific rabbit antisera prepared to these strains, they were able to show passive protection after lethal challenge with homologous, but not heterologous, strains representing group B streptococcal serotypes Ia, Ib, Ic, and'II. Although type III strains were not lethal in this standard animal model, type III-specific protective immunity has now been established in models for lethal infection in mice (23)(24)(25), chick embryos (9,26), and suckling rats (27 (1,3,8,9,16). This presumably "protective" antibody has specificity for the native III polysaccharide (2,5), is an IgG immunoglobulin (1,8,9), and is placentally transferred in concentrations that, beyond 35-36 wk of gestation, approach that in maternal serum (1,9).…”

“…Although type III strains were not lethal in this standard animal model, type III-specific protective immunity has now been established in models for lethal infection in mice (23)(24)(25), chick embryos (9,26), and suckling rats (27 (1,3,8,9,16). This presumably "protective" antibody has specificity for the native III polysaccharide (2,5), is an IgG immunoglobulin (1,8,9), and is placentally transferred in concentrations that, beyond 35-36 wk of gestation, approach that in maternal serum (1,9). In addition, it promotes opsonophagocytosis and killing of type III strains by polymorphonuclear leukocytes in the presence of complement (4,18), is protective in animal models when sufficient concentrations are administered before or concomitant with challenge (9,25,26), and can modify the terminal sialic acid moieties of the type III capsule in a manner that permits activation of the alternative pathway (4).…”

“…This presumably "protective" antibody has specificity for the native III polysaccharide (2,5), is an IgG immunoglobulin (1,8,9), and is placentally transferred in concentrations that, beyond 35-36 wk of gestation, approach that in maternal serum (1,9). In addition, it promotes opsonophagocytosis and killing of type III strains by polymorphonuclear leukocytes in the presence of complement (4,18), is protective in animal models when sufficient concentrations are administered before or concomitant with challenge (9,25,26), and can modify the terminal sialic acid moieties of the type III capsule in a manner that permits activation of the alternative pathway (4). Previous reports have suggested that human immunity to type Ia, group B Streptococcus is analogous to that of serotype III (7)(8)(9)30).…”

“…In addition, it promotes opsonophagocytosis and killing of type III strains by polymorphonuclear leukocytes in the presence of complement (4,18), is protective in animal models when sufficient concentrations are administered before or concomitant with challenge (9,25,26), and can modify the terminal sialic acid moieties of the type III capsule in a manner that permits activation of the alternative pathway (4). Previous reports have suggested that human immunity to type Ia, group B Streptococcus is analogous to that of serotype III (7)(8)(9)30). In three of these studies, however, the presence of type Ia antibody in a serum was measured exclusively by functional'assays (opsonophagocytosis [7,30] or opsonization related to chemiluminescence [8]) that employed the mousepassed strain 090 as the only test organism.…”