Cord blood Streptococcus pneumoniae-specific cellular immune responses predict early pneumococcal carriage in high-risk infants in Papua New Guinea

Pneumococcal conjugate vaccine primes mucosal immune responses to pneumococcal polysaccharide vaccine booster in Papua New Guinean children

Introduction

Invasive pneumococcal disease remains a major cause of hospitalization and death in Papua New Guinean (PNG) children. We assessed mucosal IgA and IgG responses in PNG infants vaccinated with pneumococcal conjugate vaccine (PCV) followed by a pneumococcal polysaccharide vaccine (PPV) booster.

Methods

Infants received 7-valent PCV (7vPCV) in a 0–1–2 (neonatal) or 1–2-3-month (infant) schedule, or no 7vPCV (control). At age 9 months all children received 23-valent PPV (23vPPV). IgA and IgG to 7vPCV and non-7vPCV (1, 5, 7F, 19A) serotypes were measured in saliva collected at ages 1, 2, 3, 4, 9, 10 and 18 months (131 children, 917 samples). Correlations were studied between salivary and serum IgG at 4, 10 and 18 months.

Results

Salivary IgA and IgG responses overall declined in the first 9 months. Compared to non-7vPCV recipients, salivary IgA remained higher in 7vPCV recipients for serotypes 4 at 3 months, 6B at 3 months (neonatal), and 14 at 3 (neonatal), 4 and 9 months (infant); and for salivary IgG for serotypes 4 at 3, 4 and 9 months, 6B at 9 months, 14 at 4 (neonatal) and 9 months, 18C at 3, 4, and 9 (infant) months, and 23F at 4 months. Following 23vPPV, salivary 7vPCV-specific IgA and IgG increased in 7vPCV-vaccinated children but not in controls; and salivary IgA against non-PCV serotypes 5 and 7F increased in 7vPCV recipients and non-recipients. Salivary and serum IgG against 7vPCV-serotypes correlated in 7vPCV-vaccinated children at 4 and 10 months of age.

Conclusions

PCV may protect high-risk children against pneumococcal colonization and mucosal disease by inducing mucosal antibody responses and priming for mucosal immune memory that results in mucosal immune responses after booster PPV. Saliva can be a convenient alternative sample to serum to study PCV-induced systemic IgG responses.

Prediction and Validation of Immunogenic Domains of Pneumococcal Proteins Recognized by Human CD4+ T Cells

CD4⁺ T-cell mechanisms are implied in protection against pneumococcal colonization; however, their target antigens and function are not well defined. In contrast to high-throughput protein arrays for serology, basic antigen tools for CD4⁺ T-cell studies are lacking.

CD4⁺ T-cell mechanisms are implied in protection against pneumococcal colonization; however, their target antigens and function are not well defined. In contrast to high-throughput protein arrays for serology, basic antigen tools for CD4⁺ T-cell studies are lacking. Here, we evaluate the potential of a bioinformatics tool for in silico prediction of immunogenicity as a method to reveal domains of pneumococcal proteins targeted by human CD4⁺ T cells. For 100 pneumococcal proteins, CD4⁺ T-cell immunogenicity was predicted based on HLA-DRB1 binding motifs. For 20 potentially CD4⁺ T-cell immunogenic proteins, epitope regions were verified by testing synthetic peptides in T-cell assays using peripheral blood mononuclear cells from healthy adults. Peptide pools of 19 out of 20 proteins evoked T-cell responses. The most frequent responses (detectable in ≥20% of donors tested) were found to SP_0117 (PspA), SP_0468 (putative sortase), SP_0546 (BlpZ), SP_1650 (PsaA), SP_1923 (Ply), SP_2048 (conserved hypothetical protein), SP_2216 (PscB), and SPR_0907 (PhtD). Responding donors had diverging recognition patterns and profiles of signature cytokines (gamma interferon [IFN-γ], tumor necrosis factor alpha [TNF-α], interleukin-13 [IL-13], and/or IL-17A) against single-epitope regions. Natural HLA-DR-restricted presentation and recognition of a predicted SP_1923-derived epitope were validated through the isolation of a CD4⁺ T-cell clone producing IFN-γ, TNF-α, and IL-17A in response to the synthetic peptide, whole protein, and heat-inactivated pneumococcus. This proof of principle for a bioinformatics tool to identify pneumococcal protein epitopes targeted by human CD4⁺ T cells provides a peptide-based strategy to study cell-mediated immune mechanisms for the pneumococcal proteome, advancing the development of immunomonitoring assays and targeted vaccine approaches.