Fitness of B-Cell Responses to SARS-CoV-2 WT and Variants Up to One Year After Mild COVID-19 - A Comprehensive Analysis.

Objective: To comprehensively evaluate SARS-CoV-2 specific B-cell and antibody responses up to one year after mild COVID-19.

Methods: In 31 mildly symptomatic COVID-19 participants SARS-CoV-2-specific plasmablasts and antigen-specific memory B cells were measured by ELISpot. Binding antibodies directed against the proteins spike (S), domain S1, and nucleocapsid (N) were estimated using rIFA, ELISA, and commercially available assays, and avidity measured using thiocyanate washout.

Functional limitations of plasmacytoid dendritic cells limit type I interferon, T cell responses and virus control in early life.

Infant mortality from viral infection remains a major global health concern: viruses causing acute infections in immunologically mature hosts often follow a more severe course in early life, with prolonged or persistent viral replication. Similarly, the WE strain of lymphocytic choriomeningitis virus (LCMV-WE) causes acute self-limiting infection in adult mice but follows a protracted course in infant animals, in which LCMV-specific CD8⁺ T cells fail to expand and control infection. By disrupting type I IFNs signaling in adult mice or providing IFN-α supplementation to infant mice, we show here that the impaired early life T cell responses and viral control result from limited early type I IFN responses.

Environmental and T cell-intrinsic factors limit the expansion of neonatal follicular T helper cells but may be circumvented by specific adjuvants.

Follicular Th (T(FH)) cells have emerged as a new Th subset providing help to B cells and supporting their differentiation into long-lived plasma cells or memory B cells. Their differentiation had not yet been investigated following neonatal immunization, which elicits delayed and limited germinal center (GC) responses. We demonstrate that neonatal immunization induces CXCR5(high)PD-1(high) CD4(+) T(FH) cells that exhibit T(FH) features (including Batf, Bcl6, c-Maf, ICOS, and IL-21 expression) and are able to migrate into the GCs.

Homing and adhesion patterns determine the cellular composition of the bone marrow plasma cell niche.

According to commonly held concepts, plasma cell (PC) longevity in bone marrow (BM) depends upon their access to survival niches. These are thought to exist in nursery cell types, which support PCs by secreting PC survival factors. To better define PC survival niches and their functioning, we adoptively transferred traceable Blimp-1-(GFP) PCs into recipient mice lacking a proliferation-inducing ligand (APRIL), IL-6, or macrophage migration inhibitory factor.

APRIL is critical for plasmablast survival in the bone marrow and poorly expressed by early-life bone marrow stromal cells.

The persistence of serum IgG antibodies elicited in human infants is much shorter than when such responses are elicited later in life. The reasons for this rapid waning of antigen-specific antibodies elicited in infancy are yet unknown. We have recently shown that adoptively transferred tetanus toxoid (TT)-specific plasmablasts (PBs) efficiently reach the bone marrow (BM) of infant mice.

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.

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.

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-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.

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.

Generation of adult-like antibody avidity profiles after early-life immunization with protein vaccines.

The capacity to induce high-avidity antibodies following early-life immunization has long been questioned, and the possibility of inducing such antibodies soon after birth is a recognized goal for a number of vaccination strategies. Therefore, we assessed the capacity to develop high-avidity antibodies to peptidic vaccines in 1-week-old BALB/c mice. The dynamics of the generation of antibody molecules of increasing avidity were analyzed on circulating serum antibodies and, where feasible, at the single-cell level on spleen and bone marrow antibody-secreting cells.