Qualitatively, the splenocytes population from mice receiving the standard booster dose is composed by one half of IL-2 positive cells (51%), the remaining half being divided into IL-5 (28%) and IFN- (21%) positive cells (Figure?5E). of serotype-specific antibodies, with analog avidity and opsonophagocytic properties. On the other hand, when CMI was evaluated, the presence of CRM197-specific IL-5 and IL-2 producing cells was evident in splenocytes from mice immunized with the full dose, while in those immunized with the fractional booster dose, IFN- producing cells responsive to both protein and polysaccharide antigens were significantly increased, whereas the number of IL-5 and IL-2 positive cells remained unaffected. Overall the present findings show that PCV13 humoral response in mice is associated to a Th2 predominant response at the full booster dose, while the fractional one favors a mixed Th1/Th2 response, suggesting an important role of CMI besides measurement of functional protective antibodies, as an additional and important key information in vaccine development. conjugated to an immunogenic protein carrier, the cross-reactive material 197 (CRM197), a nontoxic form of diphtheria toxoid, and aluminium phosphate adjuvant (AlPO4) . Conjugation to the carrier is essential to convert the free-polysaccharide T-cellCindependent immune response into a T-cellCdependent immune response, enabling PCV13 to elicit long-lasting immunity in children, and contributing to vaccine-induced herd immunity in the adult population [7, FTI-277 HCl 8]. Since 2007, PCVs have been worldwide included in childhood immunization programmes although with differences in age of administration, timing and number of priming doses, but also presence and timing of a booster vaccination . Similar efficacy in reducing pneumonia symptoms in children with pneumococcal pneumonia and emphysema was reported for different schedules, supported by WHO recommendations, regarding the number of priming and booster doses and timing of immunization showing no differences in the production of functional antibodies [10, 11]. More recently, it has also been shown that even reducting the number of primary doses (followed by a booster) does not compromise the long-term immunogenicity in terms of serotype-specific IgG concentration and opsonophagocytic activity (OPA) , thus confirming that immunogenicity of the PCV booster dose FTI-277 HCl is relatively independent from the number of priming doses. Other than SLRR4A the prime-boost regimen, and the number and intervals among doses, also the amount of antigen contained in FTI-277 HCl the priming versus boosting doses is an important determinant of immune response. Some reports have demonstrated that not always higher antigen doses, for both priming or boosting, are the most effective, both in human clinical trials and animal models [13, 14]. A greater antibody protection and affinity maturation has been shown to improve by a delayed fractional booster dose following administration of a malaria vaccine . To our knowledge, preclinical studies in mice to evaluate the immunogenic properties of PCVs, have focused on the influence of mice strain, vaccine composition, routes of immunization and intervals on FTI-277 HCl antibody levels and functionality [16, 17, 18, 19, 20], but no reports have investigated the effect of the booster dose, in both humoral and cell-mediated immune response. In the present study, the immunogenic responses produced by PCV13 in a murine model were investigated using two different boosting dose regimens, i.e. the full booster dose was compared to a fractional dose, both following two priming administrations with the full dose. Immune responses were evaluated in terms of antibody titer, avidity, and opsonophagocytic activity, and compared to cell-mediated immune responses by determining the frequency of cytokine (IFN-, IL-2, IL-4, IL-5)-producing splenocytes after recall with both protein or polysaccharide antigens or polyclonal activation. Results show that the use of a fractional booster dose of PCV13 vaccine, although not modifying both quantitatively and qualitatively.