CpG-motifs enhance initial and sustained primary tetanus-specific antibody secreting cell responses in spleen and bone marrow, but are more effective in adult than in neonatal mice

Unmethylated CpG oligonucleotides (CpG-ODN) increase adult and neonatal primary antibody responses to T-dependent antigens, at yet unidentified stages of antigen-specific B cell differentiation. In adult mice, a single dose of CpG-ODN adjuvanted tetanus toxoid (TT) vaccine markedly enhanced and prolonged splenic TT-specific antibody-secreting-cell (ASC) responses and significantly increased the size of the bone marrow (BM) ASC pool. Surprisingly, this was not associated with changes of germinal center (GC) numbers, size, apoptosis or function. In 1-week-old mice, CpG-ODN also enhanced TT-specific splenic ASC responses, but failed to correct limitations of the GC reaction and of the development of the BM ASC pool

Reduced ability of neonatal and early-life bone marrow stromal cells to support plasmablast survival

In human infants (<1 year), circulating IgG Abs elicited in response to most T-dependent Ags rapidly decline and return to baseline within a few months after immunization for yet-unknown reasons. In mice immunized between 1 and 4 wk of age, a limited establishment of the bone marrow (BM) pool of long-lived plasma cells is observed. In this study, we show that tetanus toxoid (TT)-specific plasmablasts generated in the spleen are efficiently attracted in vitro and in vivo toward early-life BM stromal cells, which express adult levels of CXCL12. Similarly, adoptively transferred TT plasmablasts efficiently reach the BM compartment of 2-wk-old and adult mice. In contrast, TT plasmablasts fail to persist in the early-life BM compartment, as indicated by the persistence of a significantly lower number of TT plasmablasts in the early-life compartment than in the adult BM compartment 48 h after transfer. This limited persistence is associated with an increased rate of in vivo apoptosis of TT-specific plasmablasts that have reached the early-life BM and with a significantly lower survival rate of TT-specific plasmablasts cocultured on early-life BM stromal cells compared with adult BM stromal cells. Thus, early-life BM stromal cells fail to provide the molecular signals that support plasmablast survival and differentiation into surviving plasma cells

Unresponsiveness to lymphoid-mediated signals at the neonatal follicular dendritic cell precursor level contributes to delayed germinal center induction and limitations of neonatal antibody responses to T-dependent antigens

The factors limiting neonatal and infant IgG Ab responses to T-dependent Ags are only partly known. In this study, we assess how these B cell responses are influenced by the postnatal development of the spleen and lymph node microarchitecture. When BALB/c mice were immunized with alum-adsorbed tetanus toxoid at various stages of their immune development, a major functional maturation step for induction of serum IgG, Ab-secreting cells, and germinal center (GC) responses was identified between the second and the third week of life. This correlated with the development of the follicular dendritic cell (FDC) network, as mature FDC clusters only appeared at 2 wk of age. Adoptive transfer of neonatal splenocytes into adult SCID mice rapidly induced B cell follicles and FDC precursor differentiation into mature FDC, indicating effective recruitment and signaling capacity of neonatal B cells. In contrast, adoptive transfer of adult splenocytes into neonatal SCID mice induced primary B cell follicles without any differentiation of mature FDC and failed to correct limitations of tetanus toxoid-induced GC. Thus, unresponsiveness to lymphoid-mediated signals at the level of neonatal FDC precursors delays FDC maturation and GC induction, thus limiting primary Ab-secreting cell responses to T-dependent Ags in early postnatal life

Influence of complement C3 amount on IgG responses in early life: immunization with C3b-conjugated antigen increases murine neonatal antibody responses

Complement component C3, which plays an important role in both the innate and adaptative immune response, is present at low level in human infants. We show here that: (i) serum C3 amount is weak also in infant mice, (ii) these young animals fail to upregulate C3 to adult levels following tetanus toxoid immunization, (iii) neonatal macrophages have a limited capacity to synthesize C3 upon LPS exposure, (iv) conjugation of antigen to C3b significantly enhances antibody response elicited in 1-week-old mice--although it does not increase primary IgG response in adult mice. Altogether, this identifies C3 as one of the factors limiting early life antibody response and emphasizes the potential interest of immunization strategies overcoming this limitation

CpG oligodeoxynucleotides can circumvent the Th2 polarization of neonatal responses to vaccines but may fail to fully redirect Th2 responses established by neonatal priming

Neonatal murine responses to a panel of conventional vaccines differ qualitatively from adult responses by a particular polarization toward a Th2 pattern and a frequent limitation of the Th1 and CTL responses required for protection against intracellular microorganisms. In contrast, DNA vaccines induce adult-like Th1/CTL neonatal responses against the same vaccine Ags. In this report, we show that this can be related to their content in unmethylated CpG motifs. Oligodeoxynucleotides (ODN) containing CpG motifs activate neonatal APCs to produce IL-12 in vitro and induce adult-like Th1 responses to tetanus toxoid and measles Ags in vivo, with production of IgG2a-specific Abs and adult-like secretion of IFN-gamma and IL-5 by Ag-specific T cells. However, in spite of their capacity to trigger neonatal B cell proliferation in vitro, CpG-ODN only partially enhanced early life Ab responses. Finally, using Th1-driving CpG-ODN with the boosting dose of a protein vaccine was sufficient to redirect adult but not neonatally primed Th2 responses. These observations could be important for the development of novel vaccines that will have to be effective early in life

Co-administration of CpG oligonucleotides enhances the late affinity maturation process of human anti-hepatitis B vaccine response

We assessed the avidity maturation process elicited by human immunization with alum-adsorbed HBsAg alone or with a novel adjuvant containing CpG motifs (CpG 7909). Mean avidity indexes and distribution of low- and high-avidity anti-HBs indicated that avidity maturation essentially takes place late after priming. CpG 7909 markedly enhanced this affinity maturation process, increasing the pool of high-avidity antibodies. The influence of CpG 7909 was antigen-specific, isotype-specific and distinct from the influence on anti-HBs production, as avidity did not correlate with anti-HBs IgG titers. This is the first demonstration that a novel human adjuvant may induce antibodies with higher antigen-binding affinity