In vitro lymphocyte proliferation in response to type III group B streptococci

The in vitro cell-mediated responses to group B streptococci (GBS) and the relationship of cell-mediated immunity to specific humoral immunity to type III GBS were investigated. Blood was obtained from 20 adult volunteers, and lymphocytes were isolated and cultured in microtiter plates. Each well contained 2 x 10(5) lymphocytes, 15% autologous serum, and either GBS (cell-to-organism ratio of 1:10, 1:1, or 1:0.1), phytohemagglutinin, streptokinase-streptodornase, or RPMI 1640. Cells were harvested at 5, 6, or 7 days, and DNA synthesis was quantitated. Serum antibody titers were determined with an enzyme-linked immunosorbent assay. Maximal lymphocyte responses occurred at 6 days of culture and at a cell-to-organism ratio of 1:1. Individuals with significant antibody titers to type III GBS, as well as those with undetectable antibody, responded to GBS (stimulation index greater than 10). There was a significant difference (P less than 0.001) between mean antibody concentrations in responders (stimulation index greater than 10) and nonresponders (stimulation index less than 10). Thus, the in vitro responses to GBS may be both to a specific antigen and to a nonspecific mitogen and may be important in host immunity to GBS.