Highlights d CD27 dull and CD27 bright MBCs share their VH repertoire but have different functions d CD27 dull MBCs are the long-lived substrate of selected and specific CD27 bright MBCs d The interplay between CD27 dull and CD27 bright MBCs preserves B cell memory d In pregnancy, MBCs decline, but persisting CD27 dull MBCs re-expand after delivery

Additional supporting information may be found in the online version of this article at the publisher's web-site

Switched and IgM memory B cells execute different and noninterchangeable functions.We studied memory B cells in children of different ages, in peripheral blood and spleen and compared them with those of children born asplenic or unable to build germinal centers. We show that, whereas switched memory B cells are mostly generated in the germinal centers at all ages, IgM memory B cells can be distinct in three types with different developmental history. Innate IgM memory B cells, the largest pool in infants, are generated in the spleen by a germinal center-independent mechanism. With age, if the spleen is present and germinal centers are functional, innate IgM memory B cells are remodelled and accumulate somatic mutations. The third type of IgM memory B cell is a by-product of the germinal center reaction. Our data suggest that the B-cell memory developmental program is implemented during the first 5-6 years of life. Keywords:Antibodies r B cells r B cell development r Immunoglobulins r Innate immunity Additional supporting information may be found in the online version of this article at the publisher's web-site Correspondence: Dr. Rita Carsetti e-mail: rita.carsetti@opbg.net * These authors contributed equally to this work.The copyright line for this article was changed on 9th February 2018 after original online publication.C 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu 328Alaitz Aranburu et al. Eur. J. Immunol. 2017. 47: 327-344 IntroductionImmunological memory is the ability of the immune system to recognize and neutralize a previously encountered pathogen, thereby preventing re-infection and disease. Memory is acquired by the immune system through experience. Infants lack immunological memory and have a high risk of infections. The rate of infection and mortality is highest in infants and declines after the age of 5. The decline of child mortality in the last century has been one of the most significant achievements in medical history and is largely due to the introduction of vaccination and use of antibiotics. The first response to infection or vaccination generates the elements of memory: long-lived plasma cells and memory B cells [1][2][3][4]. Long-lived plasma cells continuously secrete specific antibodies (Abs) that upon re-infection will exert the first protection. Memory B cells rapidly react to the renewed challenge and produce Abs when and where these are most needed, i.e. when the pathogen again tries to invade the organism and at the site of its entry.In the adult, 50% of the B cells in the peripheral blood (PB) are memory B cells [5]. Phenotypically memory B cells can be identified by the expression of the CD27 marker. B cells that express CD27 carry somatic mutations in their immunoglobulin (Ig) genes [6]. These are permanent genetic imprints left by the mechanism of Somatic Hyper Mutation (SHM). Somatic mutations are triggered by activation events and results from the combined action of the activation-induced deaminase AID, the uracil-DNA glycosylase UNG and several DNA...

Children with Down syndrome (DS) have increased susceptibility to infections and a high frequency of leukemia and autoimmune disorders, suggesting that immunodeficiency and immune dysfunction are integral parts of the syndrome. A reduction in B-cell numbers has been reported, associated with moderate immunodeficiency and normal immunoglobulin levels. Here, we compared B-cell populations of 19 children with DS with those in healthy age-matched controls. We found that all steps of peripheral B-cell development are altered in DS, with a more severe defect during the later stages of B-cell development. Transitional and mature-naïve B-cell numbers are reduced by 50% whereas switched memory B cells represent 10–15% of the numbers in age-matched controls. Serum IgM levels were slightly reduced, but all other immunoglobulin isotypes were in the normal range. The frequency of switched memory B cells specific for vaccine antigens was significantly lower in affected children than in their equivalently vaccinated siblings. In vitro switched memory B cells of patients with DS have an increased ability to differentiate into antibody-forming cells in response to TLR9 signals. Tailored vaccination schedules increasing the number of switched memory B cells may improve protection and reduce the risk of death from infection in DS.

Down syndrome individuals should be considered a high risk group, because of their increased susceptibility to infection and reduced number of switched memory B cells. Tailored vaccination protocols are needed in order to reduce their burden of infections throughout life.

Splenectomized patients are exposed to an increased risk of septicemia caused by encapsulated bacteria. Defense against infection is ensured by preformed serum antibodies produced by long-lived plasma cells and by memory B cells that secrete immunoglobulin in response to specific antigenic stimuli. Studying a group of asplenic individuals (57 adults and 21 children) without additional immunologic defects, we found that spleen removal does not alter serum anti-pneumococcal polysaccharide (PnPS) IgG concentration, but reduces the number of PnPS-specific memory B cells, of both IgM and IgG isotypes. The number of specific memory B cells was low in splenectomized adults and children that had received the PnPS vaccine either before or after splenectomy. Seven children were given the 13-valent pneumococcal conjugated vaccine after splenectomy. In this group, the number of PnPS-specific IgG memory B cells was similar to that of eusplenic children, suggesting that pneumococcal conjugated vaccine administered after splenectomy is able to restore the pool of anti-PnPS IgG memory B cells. Our data further elucidate the crucial role of the spleen in the immunological response to infections caused by encapsulated bacteria and suggest that glycoconjugated vaccines may be the most suitable choice to generate IgG-mediated protection in these patients. Keywords: Asplenia r Memory B cells r Serum anti-PnPS r Splenectomy r Streptococcus pneumoniaeAdditional supporting information may be found in the online version of this article at the publisher's web-site Eur. J. Immunol. 2013Immunol. . 43: 2659Immunol. -2670 Introduction Asplenic patients have an increased susceptibility to bacterial infections that can evolve into severe and often lethal overwhelming postsplenectomy infection (OPSI) [1,2]. Lifetime risk of developing an OPSI is nearly 1-5% and mortality rates range between 40 and 70% [3]. Streptococcus pneumoniae is the most common pathogen causing bacteremia in splenectomized patients, followed by Haemophilus influenzae, Neisseria meningitidis, Escherichia coli, Salmonella, Pseudomonas, and Klebsiella [3,4]. The red pulp of the spleen represents a very important defense from bacteremia. In the red pulp of the spleen, the blood flows slowly in a large net of sinusoids. Macrophages, located among endothelial cells, have the function of removing and destroying particulate antigens, such as bacteria [5][6][7]. In the absence of the spleen, bacteria can therefore accumulate and replicate in the blood causing septic shock.Over the last few years, the role of the spleen in the maintenance of a pool of memory B cells involved in the protection against encapsulated bacteria has been also demonstrated. In particular, we showed that splenectomized patients lack IgM memory B cells [8]. This cell population, also known as marginal zone B cells or effector memory B cells, is produced by a T-independent mechanism [9] and inhabits the marginal zone of the spleen. IgM memory B cells generate the response to pneumococcal polysaccharide (PnP...

Pertussis is a respiratory infection caused by Bordetella pertussis that may be particularly severe and even lethal in the first months of life when infants are still too young to be vaccinated. Adults and adolescents experience mild symptoms and are the source of infection for neonates. Adoptive maternal immunity does not prevent pertussis in the neonate. We compared the specific immune response of mothers of neonates diagnosed with pertussis and mothers of control children. We show that women have pre-existing pertussis-specific antibodies and memory B cells and react against the infection with a recall response increasing the levels specific serum IgG, milk IgA, and the frequency of memory B cells of all isotypes. Thus, the maternal immune system is activated in response to pertussis and effectively prevents the disease indicating that the low levels of pre-formed serum antibodies are insufficient for protection. For this reason, memory B cells play a major role in the adult defense. The results of this study suggest that new strategies for vaccine design should aim at increasing long-lived plasma cells and their antibodies.

Children with Down Syndrome (DS) suffer from immune deficiency with a severe reduction in switched memory B cells (MBCs) and poor response to vaccination. Chromosome 21 (HSA21) encodes two microRNAs (miRs), miR-125b, and miR-155, that regulate B-cell responses. We studied B- and T- cell subpopulations in tonsils of DS and age-matched healthy donors (HD) and found that the germinal center (GC) reaction was impaired in DS. GC size, numbers of GC B cells and Follicular Helper T cells (TFH) expressing BCL6 cells were severely reduced. The expression of miR-155 and miR-125b was increased in tonsillar memory B cells and miR-125b was also higher than expected in plasma cells (PCs). Activation-induced cytidine deaminase (AID) protein, a miR-155 target, was significantly reduced in MBCs of DS patients. Increased expression of miR-155 was also observed in vitro. MiR-155 was significantly overexpressed in PBMCs activated with CpG, whereas miR-125b was constitutively higher than normal. The increase of miR-155 and its functional consequences were blocked by antagomiRs in vitro. Our data show that the expression of HSA21-encoded miR-155 and miR-125b is altered in B cells of DS individuals both in vivo and in vitro. Because of HSA21-encoded miRs may play a role also in DS-associated dementia and leukemia, our study suggests that antagomiRs may represent pharmacological tools useful for the treatment of DS.