Immune responses of systemic and mucosal lymphoid organs to Pnu-Imune vaccine as a function of age and the efficacy of monophosphoryl lipid A as an adjuvant

A murine model system was established to study immune responses to the Pnu-Imune vaccine, which is made up of 23 different pneumococcal capsular polysaccharides. In this animal model, antibody-forming cell responses to 21 of 23 individual polysaccharides in the vaccine were detected. The Pnu-Imune vaccine elicited good antibody responses from the spleens and mesenteric lymph nodes (MLN) of young mice, whereas a variety of other peripheral lymph nodes were unresponsive. The immunoglobulin M plaque-forming cell (PFC) response in the spleen to the Pnu-Imune vaccine (given intraperitoneally or subcutaneously) decreased dramatically with increasing age. However, the spleen and MLN differed in their susceptibility to an age-associated decline in immune function. While the PFC responses in the spleen declined with age, the PFC response in the mucosa-associated MLN did not decline with age but instead remained constant over the entire age span of 4 to 28 months studied. These studies showed that the spleen, peripheral lymph nodes, and MLN did not demonstrate parallel age-associated defects in antibody responses to pneumococcal polysaccharides when the antigen was administered systemically. Also, the deficient splenic antibody response to Pnu-Imune vaccine in aged mice could be enhanced by injecting a combination of Pnu-Imune vaccine and the nontoxic adjuvant monophosphoryl lipid A. Moreover, an immunoglobulin G response was induced when the immunogen was a mixture of vaccine and adjuvant.