Abstract

CD4(+) T-cell-dependent acquired immunity confers antibody-independent protection against pneumococcal colonization. Since this mechanism is poorly understood for extracellular bacteria, we assessed the antigen specificity of the induction and recall of this immune response by using BALB/c DO11.10Rag(-/-) mice, which lack mature B and T cells except for CD4(+) T cells specific for the OVA(323-339) peptide derived from ovalbumin. Serotype 6B Streptococcus pneumoniae strain 603S and unencapsulated strain Rx1Delta lytA were modified to express OVA(323-339) as a fusion protein with surface protein A (PspA) (strains 603OVA(1) and Rx1Delta lytAOVA(1)) or with PspA, neuraminidase A, and pneumolysin (Rx1Delta lytAOVA(3)). Whole-cell vaccines (WCV) were made of ethanol-killed cells of Rx1Delta lytA plus cholera toxin (CT) adjuvant, of Rx1Delta lytAOVA(1) + CT (WCV-OVA(1)), and of Rx1Delta lytAOVA(3) + CT (WCV-OVA(3)). Mice intranasally immunized with WCV-OVA(1), but not with WCV or CT alone, were protected against intranasal challenge with 603OVA(1). There was no protection against strain 603S in mice immunized with WCV-OVA(1). These results indicate antigen specificity of both immune induction and the recall response. Effector action was not restricted to antigen-bearing bacteria since colonization by 603S was reduced in animals immunized with vaccines made of OVA-expressing strains when ovalbumin or killed Rx1Delta lytAOVA(3) antigen was administered around the time of challenge. CD4(+) T-cell-mediated protection against pneumococcal colonization can be induced in an antigen-specific fashion and requires specific antigen for effective bacterial clearance, but this activity may extend beyond antigen-expressing bacteria. These results are consistent with the recruitment and/or activation of phagocytic or other nonspecific effectors by antigen-specific CD4(+) T cells.