Both plasma and memory B cells are stimulated following exposure to PPS. In contrast to T-independent immune responses, priming by either PCV, previous encounter with S. pneumoniae or a cross-reacting antigen prior to 23vPPS vaccination, could stimulate immunological memory by presentation of polysaccharide-protein

conjugate antigens to the immune system (T-dependent) [34]. Given the T-independent nature of PPS antigens, 23vPPS may stimulate the existing pool of memory B cells to differentiate into plasma cells and secrete antibody without replenishment Pomalidomide price of the memory B cell pool. This has been proposed as one mechanism

for the hyporesponsiveness observed following polysaccharide vaccine administration [35]. Upon subsequent booster with 23vPPS or a natural infection, immune hyporesponsiveness could be induced Rho kinase assay as a result of a decreased memory B cell population and result in the reduced antibody concentrations observed in this study. In addition, the development of immune hyporesponsiveness may also be the result of immune regulation via the establishment of pneumococcal-specific tolerogenic immune responses. Increased expression of the immunosuppressive cytokine interleukin 10 [19] and [36] and suppressor T cell activity may suppress the response to PPS [37]. Recent evidence also suggests a role for CD4+ T-lymphocytes in the immune response to pneumococcal

antigens [38]. Studies have demonstrated the importance of co-stimulatory signals (CD40-CD40L) for a robust immune response to pneumococcal antigens and that CD4+ T-lymphocytes can protect mice against pneumococcal colonization independent of specific antibody. These findings strongly suggest a role for cellular immunity in protection against pneumococcal infection [39], [40], [41], [42] and [43]. Furthermore, it is possible that regulatory Sitaxentan T-lymphocytes (Treg) may suppress antibody production and other immune responses in the context of chronic antigen exposure. Hyporesponsiveness induced by Treg has been described during bacterial, viral and parasitic infections with up-regulation of CD4+CD25+ Treg and IL-10 and TGF-β secretion [37] and [44]. Limited data is available on the role of Treg in the attenuation immune response to pneumococcal antigens. However, a high level of exposure to pneumococci, particularly in early life, could induce Treg activity that suppresses serotype-specific IgG, thereby increasing IPD risk following 23vPPS immunization. The clinical relevance of this immunological finding in this study is not known.