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

Lung ILC2s are activated in BALB/c mice born to immunized mothers despite complete protection against respiratory syncytial virus

Activated lung ILC2s produce large quantities of IL-5 and IL-13 that contribute to eosinophilic inflammation and mucus production following respiratory syncytial virus infection (RSV). The current understanding of ILC2 activation during RSV infection, is that ILC2s are activated by alarmins, including IL-33, released from airway epithelial cells in response to viral-mediated damage. Thus, high levels of RSV neutralizing maternal antibody generated from maternal immunization would be expected to reduce IL-33 production and mitigate ILC2 activation. Here we report that lung ILC2s from mice born to RSV-immunized dams become activated despite undetectable RSV replication. We also report, for the first time, expression of activating and inhibitory Fcgamma receptors on ILC2s that are differentially expressed in offspring born to immunized versus unimmunized dams. Alternatively, ex vivo IL-33-mediated activation of ILC2s was mitigated following the addition of antibody: antigen immune complexes. Further studies are needed to confirm the role of Fcgamma receptor ligation by immune complexes as an alternative mechanism of ILC2 regulation in RSV-associated eosinophilic lung inflammation.

Developmental adaptations of γδ T cells and B cells in blood and intestinal mucosa from birth until weaning in Holstein bull calves

This study aimed to characterize the development of systemic and colon tissue resident B and γδ T cells in newborn calves from birth until weaning. At birth, calves have limited capacity to initiate immune responses, and the immune system gradually matures over time. Gamma delta (γδ) T cells are an important lymphocyte subset in neonatal calves that confer protection and promote immune tolerance. A total of 36 newborn calves were enrolled in a longitudinal study to characterize how systemic and colon tissue resident B and γδ T cells develop from birth until weaning. Blood and colon biopsy samples were collected on d 2, 28, and 42 to determine the proportions of various B and γδ T cell subsets by flow cytometry. We classified γδ T cells into different functional subsets according to the level of expression intensity of the coreceptors WC1.1 (effector function) and WC1.2 (regulatory function). Furthermore, naive B cells were classified based on the expression IgM receptor, and activation state was determined based on expression of CD21 and CD32, 2 receptors with opposing signals involved in B cell activation in early life. Additional colon biopsy samples were used for 16S sequencing, and microbial diversity data are reported. At birth, γδ T cells were the most abundant lymphocyte population in blood, accounting for 58.5% of the lymphocyte pool, after which the proportions of these cells declined to 38.2% after weaning. The proportion of γδ T cells expressing WC1.1 decreased by 50% from d 2 to d 28, whereas no change was observed in the expression of WC1.2. In the colon, there was a 50% increase of γδ T cells after weaning and the proportion of WC1.2+ γδ T cells doubled from d 28 to 42. The proportion of IgM+ B lymphocytes in blood increased from 23.6% at birth to 30% after weaning, were the proportion of B cells expressing CD21 increased by 25%, while the proportion of B cells expressing CD32 decreased by 30%. While no changes were observed for the overall proportion of IgM+ B lymphocytes in the colon, there was a 6-fold increase in the proportion of CD21+ B cells from pre- (d 28) to postweaning (d 42). Microbial diversity increased from d 2 of life to 28 and declined abruptly after weaning. The reduction in microbial diversity during weaning was negatively correlated with the increase in all γδ T cell subsets and CD21+ B cells. These data suggest that developmental adaptations after birth coordinate expansion of γδ T cells to provide early systemic protection, as well as to steer immune tolerance, while B cells mature over time. Additionally, the increase of colonic γδ T cells on d 42 suggests a protective role of these cells during weaning.

The Neonatal Immune System and Respiratory Pathogens

Neonates are more susceptible to some pathogens, particularly those that cause infection in the respiratory tract. This is often attributed to an incompletely developed immune system, but recent work demonstrates effective neonatal immune responses to some infection. The emerging view is that neonates have a distinctly different immune response that is well-adapted to deal with unique immunological challenges of the transition from a relatively sterile uterus to a microbe-rich world, tending to suppress potentially dangerous inflammatory responses. Problematically, few animal models allow a mechanistic examination of the roles and effects of various immune functions in this critical transition period. This limits our understanding of neonatal immunity, and therefore our ability to rationally design and develop vaccines and therapeutics to best protect newborns. This review summarizes what is known of the neonatal immune system, focusing on protection against respiratory pathogens and describes challenges of various animal models. Highlighting recent advances in the mouse model, we identify knowledge gaps to be addressed.

Early life vaccination: Generation of adult-quality memory CD8+ T cells in infant mice using non-replicating adenoviral vectors

Intracellular pathogens represent a serious threat during early life. Importantly, even though the immune system of newborns may be characterized as developmentally immature, with a propensity to develop Th2 immunity, significant CD8+ T-cell responses may still be elicited in the context of optimal priming. Replication deficient adenoviral vectors have been demonstrated to induce potent CD8+ T-cell response in mice, primates and humans. The aim of the present study was therefore to assess whether replication-deficient adenovectors could overcome the risk of overwhelming antigen stimulation during the first period of life and provide a pertinent alternative in infant vaccinology. To address this, infant mice were vaccinated with three different adenoviral vectors and the CD8+ T-cell response after early life vaccination was explored. We assessed the frequency, polyfunctionality and in vivo cytotoxicity of the elicited memory CD8+ T cells, as well as the potential of these cells to respond to secondary infections and confer protection. We further tested the impact of maternal immunity against our replication-deficient adenoviral vector during early life vaccination. Overall, our results indicate that memory CD8+ T cells induced by adenoviral vectors in infant mice are of good quality and match those elicited in the adult host.

Ability of Antibiotic-Resistant Nonvaccine-Type Pneumococcal Clones to Cause Otitis Media in an Infant Mouse Model of Pneumococcal-Influenza Virus Coinfection

The introduction of the 7-valent pneumococcal conjugate vaccine in Portugal resulted in reduced carriage in children by vaccine-type strains and an increased carriage of three major antibiotic-resistant clones, ST2191, ST276, and ST63 expressing capsules 6A, 19A, and 15A, respectively. Pneumococcal otitis media (OM), a frequent infection among preschool age children, is often associated with viral coinfection. To evaluate the ability of these three antibiotic-resistant clones to cause disease, we used an infant mouse model of influenza virus pneumococcal coinfection. The 6A and 19A clonal types induced OM, while 15A induced pneumococcal pneumonia and bloodstream infection, suggesting potential for invasive disease.

The adjuvant LT-K63 can restore delayed maturation of follicular dendritic cells and poor persistence of both protein- and polysaccharide-specific antibody-secreting cells in neonatal mice

Ab responses in early life are low and short-lived; therefore, induction of protective immunity requires repeated vaccinations. One of the major limitations in early-life immunity is delayed maturation of follicular dendritic cells (FDCs), which play a central role in mediating the germinal center (GC) reaction leading to production of Ab-secreting cells (AbSCs). We assessed whether a nontoxic mutant of Escherichia coli heat-labile enterotoxin (LT-K63) and CpG1826 as model adjuvants could accelerate FDC maturation and immune response in neonatal mice, using a pneumococcal polysaccharide of serotype 1 conjugated to tetanus toxoid (Pnc1-TT) as a model vaccine. In neonatal NMRI mice, a single dose of Pnc1-TT coadministered with LT-K63 enhanced Pnc1-TT-induced GC reaction. In contrast, CpG1826 had no effect. Accordingly, LT-K63, but not CpG1826, accelerated the maturation of FDC networks, detected by FDC-M2(+) staining, characteristic for adult-like FDCs. This coincided with migration of MOMA-1(+) macrophages into the GCs that can enhance GC reaction and B cell activation. The FDC-M2(+) FDC networks colocalized with enhanced expression of TNF-α, which is critical for the maintenance of mature FDCs and is poorly expressed in neonates. The accelerated maturation of FDC networks correlated with increased frequency and prolonged persistence of polysaccharide- and protein-specific IgG(+) AbSCs in spleen and bone marrow. Our data show for the first time, to our knowledge, that an adjuvant (LT-K63) can overcome delayed maturation of FDCs in neonates, enhance the GC reaction, and prolong the persistence of vaccine-specific AbSCs in the BM. These properties are attractive for parenteral vaccination in early life.

Immunohistochemical investigation of cells expressing CD21, membrane IgM, CD32 and a follicular dendritic cell marker in the lymphoid tissues of neonatal calves

Activation of B lymphocytes in the presence of passive maternal antibodies depends on expression of CD21, membrane IgM and CD32. On colligation with IgM, CD32 inhibits activation whereas CD21 enhances it. Recently, we assessed expression of CD21 and CD32 on IgM(+) cells from lymphoid tissues of newborn calves by flow cytometry, but this approach does not provide information about spatial distribution within lymphoid compartments. Therefore, histologic sections of lymphoid tissues from newborn and 7-month-old calves were examined using an immunoperoxidase technique. In all calves, CD21 and IgM stained cells were collocated in the cortex and paracortex of the retropharyngeal lymph node, in the marginal zone of the spleen and in lymphoid aggregates of palatine tonsils. Most CD32(+) cells were in the mantle zone of lymphoid follicles in 7-month-old calves, whereas only weak staining was observed in newborns. A few CD32(+) cells were also observed in the paracortex at both ages. Absence of CD32(+) cells in the center of follicles suggests that IgM(+)CD32(-) cells observed previously by flow cytometry were from germinal centers. Overall, there were few organized lymphoid aggregates within lymphoid tissues of newborn calves, and follicular dendritic cells were virtually undetectable. Their absence may be an important limitation for neonatal immunization.

Expression of complement receptor 2 (CD21), membrane IgM and the inhibitory receptor CD32 (FcgammaRIIb) in the lymphoid tissues of neonatal calves

Limited active antibody responses in neonates following vaccination have been attributed to immaturity of the immune system and to the suppressive effects of maternal antibodies. The activating receptor CD21 (CR2), when co-ligated with membrane IgM (mIgM) by complement-bound antigen lowers the threshold for activation of B lymphocytes. The inhibitory receptor CD32 (FcgammaRII) when co-ligated with mIgM by antigen-antibody complexes raises the threshold for activation. Expression of these receptors, which potentially play roles in regulation of B cell responses in the presence of maternal antibodies in neonates, has been recently characterized in blood lymphocytes in neonatal calves. Little is known however about expression of these receptors in the lymphoid tissues, where immune responses are initiated. In this study, expression of CD21, mIgM and CD32 receptors by B lymphocytes was studied in a range of lymphoid tissues including spleen, lymph nodes and bone marrow from newborn and 7-week-old calves using flow cytometry. The proportion of naïve B lymphocytes in the lymphocyte gate was significantly lower in blood and spleen of newborn calves compared to 7-week-old calves. Over 90% of B lymphocytes expressed CD21 in the lymphoid tissues. In the lymph nodes and spleen, a lower proportion of mIgM(+) B lymphocytes expressed CD32 compared to blood. In addition, intensity of expression of CD32 on B cells in lymph nodes was significantly lower compared to that in blood, suggesting a lower potential for inhibitory signalling in B cells in the lymphoid microenvironment. Investigation of the CD5(+) B cell population (as an indicator of B1 B cells) suggested an increase in the proportion of IgM(+)CD5(+) cells with age in calves, in both blood and lymphoid tissue, in contrast to the situation in humans and mice. Overall, the majority of naïve B lymphocytes in lymphoid tissues in neonatal calves expressed both activating (CD21, mIgM) and inhibitory (CD32) receptors. These receptors may provide targets for novel adjuvants, to lower the threshold for activation of B cells in neonates, and enhance antibody responses.

Variation in expression of membrane IgM, CD21 (CR2) and CD32 (Fcgamma RIIB) on bovine lymphocytes with age: a longitudinal study

Typically, neonatal calves have poor active antibody responses to vaccination, attributed to immaturity of the neonatal immune system and suppressive effects of maternal (colostral) antibodies. Responses of naïve B cells are regulated by ligation of opposing activating (CD21, membrane IgM [mIgM]) and inhibitory (CD32) receptors. Expression of these receptors on blood lymphocytes of 15 calves, from birth to 6 months of age, was investigated by three-colour flow cytometry. Although the absolute number of mIgM(+) B lymphocytes was low in calves under 6 weeks, the intensity of mIgM expression per cell was significantly higher than for adults and >90% expressed both CD21 and CD32. The intensity of CD21 expression in calves did not differ significantly from adults, whereas CD32 expression was lower. Paradoxically, these findings suggest that responses of neonates should bias toward activation at the B cell level, warranting further investigation to reveal strategies for development of vaccines that are efficacious at an early age.

A novel humanized neonatal autoimmune blistering skin disease model induced by maternally transferred antibodies

All mammal neonates receive maternal Abs for protection against pathogenic organisms in the postnatal environment. However, neonates can experience serious adverse reactions if the Abs transferred from the mother recognize self-molecules as autoAgs. In this study, we describe a novel model for autoimmune disease induced by transferred maternal Abs in genetically transformed Ag-humanized mice progeny. Bullous pemphigoid is the most common life-threatening autoimmune blistering skin disease that affects the elderly, in which circulating IgG autoAbs are directed against epidermal type XVII collagen (COL17). We have established a genetically manipulated experimental mouse model in which maternal Abs against human COL17 are transferred to pups whose skin expresses only human and not mouse COL17, resulting in blistering similar to that seen in patients with bullous pemphigoid. Maternal transfer of pathogenic Abs to humanized neonatal mice is a unique and potential experimental system to establish a novel autoimmune disease model.

B-cell responses to vaccination at the extremes of age

Infants and the elderly share a high vulnerability to infections and therefore have specific immunization requirements. Inducing potent and sustained B-cell responses is as challenging in infants as it is in older subjects. Several mechanisms to explain the decreased B-cell responses at the extremes of age apply to both infants and the elderly. These include intrinsic B-cell limitations as well as numerous microenvironmental factors in lymphoid organs and the bone marrow. This Review describes the mechanisms that shape B-cell responses at the extremes of age and how they could be taken into account to design more effective immunization strategies for these high-risk age groups.

The role of complement in the success of vaccination with conjugated vs. unconjugated polysaccharide antigen

The complement system, a well-characterised arm of the innate immune system, significantly influences the adaptive immune response via direct cell-cell interaction and maintenance of lymphoid organ architecture. Development of vaccines is a major advance in modern health care. In this review, we highlight the importance of the marginal zone in response to both, polysaccharide and conjugated vaccines, and discuss the relevance of complement herein, based on findings obtained from animal models with specific deletions of certain complement components and from vaccination reports of complement-deficient individuals. We conclude that both, intactness of the complement system and maturity of expression of its components, are relatively more important to aid in the immune response to polysaccharide vaccine than to conjugated vaccines.

The relative importance of local and systemic complement production in ischaemia, transplantation and other pathologies

Besides a critical role in innate host defence, complement activation contributes to inflammatory and immunological responses in a number of pathological conditions. Many tissues outside the liver (the primary source of complement) synthesise a variety of complement proteins, either constitutively or response to noxious stimuli. The significance of this local synthesis of complement has become clearer as a result of functional studies. It revealed that local production not only contributes to the systemic pool of complement but also influences local tissue injury and provides a link with the antigen-specific immune response. Extravascular production of complement seems particularly important at locations with poor access to circulating components and at sites of tissue stress responses, notably portals of entry of invasive microbes, such as interstitial spaces and renal tubular epithelial surfaces. Understanding the relative importance of local and systemic complement production at such locations could help to explain the differential involvement of complement in organ-specific pathology and inform the design of complement-based therapy. Here, we will describe the lessons we have learned over the last decade about the local synthesis of complement and its association with inflammatory and immunological diseases, placing emphasis on the role of local synthesis of complement in organ transplantation.

Innate immunity of the newborn: basic mechanisms and clinical correlates

The fetus and newborn face a complex set of immunological demands, including protection against infection, avoidance of harmful inflammatory immune responses that can lead to pre-term delivery, and balancing the transition from a sterile intra-uterine environment to a world that is rich in foreign antigens. These demands shape a distinct neonatal innate immune system that is biased against the production of pro-inflammatory cytokines. This bias renders newborns at risk of infection and impairs responses to many vaccines. This Review describes innate immunity in newborns and discusses how this knowledge might be used to prevent and treat infection in this vulnerable population.