Immunoregulatory role of the spleen in antibody responses to pneumococcal polysaccharide antigens.

Antibody responses to two structurally different pneumococcal polysaccharides, type 3 (SIII) and type 14 (SXIV), were examined in intact and splenectomized (Sx) A/J mice to determine whether the role of the spleen in immune responses to these antigens varies with respect to the dosage, the antigenic structure, or the interval between immunization and assay. Antibody responses to SIII and SXIV, measured over a 4-week period by radioimmunoassay, differed in antigenic load requirements, kinetics, and extent of dependence upon the spleen. Intact mice given 50 or 100 ng of SIII produced peak antibody responses on day 5, which tapered off by days 14 and 21. Intact mice given SXIV required doses 100 times greater than those of SIII to stimulate high levels of antibody response; antibody responses increased on day 5 and remained elevated through day 28. In Sx mice given 50 or 100 ng of SIII, the peak antibody response on day 5 was obliterated, but extrasplenic sources produced low levels of antibody which peaked by day 14. In Sx mice given SXIV, all anti-SXIV responses were abrogated regardless of the dose or day of assay. Differences between the anti-SIII and anti-SXIV responses in dependence upon the spleen were probably due to structural differences between the two antigens and to the localization of each to different sites in the reticuloendothelial system. These results attest to the importance of the spleen in antibacterial resistance. They show that, even in the presence of extrasplenic antibody synthesis, the spleen is required for early antibody production, the timing of which is critical for the effective clearance of bacteria.