Antibody to group B Streptococcus type III in human sera measured by a mouse protection test.

Antibody to group B Streptococcus type III (GBS-III) was measured in human plasma and sera by using a test which measures the ability to protect outbred albino mice from an intraperitoneal challenge of GBS-III calculated to be lethal to 90% of the mice (LD90 dose). Of three samples from three different lots of commercial human immune serum globulin, none were protective despite the presence of antibody to the native type III polysaccharide. Nine specimens were tested from recipients of multivalent pneumococcal vaccine, and none were protective. Five specimens were tested from recipients of GBS-III polysaccharide vaccines who had responded with greater than 50 micrograms of specific antibody per ml in the blood. All of these were protective and could be diluted to titers of 1:10 to 1:40. Of two prevaccination sera with low levels of specific antibody, neither was protective. An unexpected finding which may limit the sensitivity of the mouse protection assay was that the human immune serum globulin and human serum with a very low level of antibody appeared to increase the lethality of the GBS-III test strain, resulting in LD50 values reduced to 0.01 of the usual LD50.