A population of CD19highCD45R-/lowCD21low B lymphocytes poised for spontaneous secretion of IgG and IgA antibodies

Dysfunction of B Cell Leading to Failure of Immunoglobulin Response Is Ameliorated by Dietary Silk Peptide in 14-Month-Old C57BL/6 Mice

Anti-aging research suggests that immunosenescent cells can play deleterious roles in the immune system. Here, young (2 months old) and old (14 months old) C57BL/6 mice received a daily oral dose (100 or 750 mg/kg/day) of acid-hydrolyzed silk peptide (SP) for a period of 5 weeks. Mouse spleen, lymph node, and serum were analyzed to determine the immune homeostasis of SP by flow cytometry, Western blotting, ELISA, and qRT-PCR. The results suggest that SP ameliorates age-related dysfunction of T and B cells. Amelioration of B cell dysfunction improved the immunoglobulin response in aged mice. Taken together, the results suggest that SP restores immune homeostasis with respect to immunosenescent cells.

Altered marginal zone and innate-like B cells in aged senescence-accelerated SAMP8 mice with defective IgG1 responses

Aging has a strong impact on the activity of the immune system, enhancing susceptibility to pathogens and provoking a predominant pre-inflammatory status, whereas dampening responses to vaccines in humans and mice. Here, we demonstrate a loss of marginal zone B lymphocytes (MZ, CD19+CD45R+CD21++CD23lo) and a decrease of naive B cells (CD19+IgD+), whereas there is an enhancement of a CD19+CD45Rlo innate-like B cell population (B1REL) and the so-called aged B cell compartment (ABC, CD45R+CD21loCD23loCD5-CD11b-) in aged senescence-accelerated (SAMP8) mice but not in aged senescence-resistant (SAMR1) mice. These changes in aged SAMP8 mice were associated with lower IgG isotype levels, displaying low variable gene usage repertoires of the immunoglobulin heavy chain (VH) diversity, with a diminution on IgG1-memory B cells (CD11b-Gr1-CD138-IgM-IgD-CD19+CD38+IgG1+), an increase in T follicular helper (TFH, CD4+CXCR5+PD1+) cell numbers, and an altered MOMA-1 (metallophilic macrophages) band in primary follicles. LPS-mediated IgG1 responses were impaired in the B1REL and ABC cell compartments, both in vitro and in vivo. These data demonstrate the prominent changes to different B cell populations and in structural follicle organization that occur upon aging in SAMP8 mice. These novel results raise new questions regarding the importance of the cellular distribution in the B cell layers, and their effector functions needed to mount a coordinated and effective humoral response.

Reciprocal influence of B cells and tumor macro and microenvironments in the ApcMin/+ model of colorectal cancer

One of the most fascinating aspects of the immune system is its dynamism, meant as the ability to change and readapt according to the organism needs. Following an insult, we assist to the spontaneous organization of different immune cells which cooperate, locally and at distance, to build up an appropriate response. Throughout tumor progression, adaptations within the systemic tumor environment, or macroenvironment, result in the promotion of tumor growth, tumor invasion and metastasis to distal organs, but also to dramatic changes in the activity and composition of the immune system. In this work, we show the changes of the B-cell arm of the immune system following tumor progression in the Apc^Min/+ model of colorectal cancer. Tumor macroenvironment leads to an increased proportion of total and IL-10-competent B cells in draining LNs while activates a differentiation route that leads to the expansion of IgA⁺ lymphocytes in the spleen and peritoneum. Importantly, serum IgA levels were significantly higher in Apc^Min/+ than Wt mice. The peculiar involvement of IgA response in the adenomatous transformation had correlates in the gut-mucosal compartment where IgA-positive elements increased from normal mucosa to areas of low grade dysplasia while decreasing upon overt carcinomatous transformation. Altogether, our findings provide a snapshot of the tumor education of B lymphocytes in the Apc^Min/+ model of colorectal cancer. Understanding how tumor macroenvironment affects the differentiation, function and distribution of B lymphocytes is pivotal to the generation of specific therapies, targeted to switching B cells to an anti-, rather than pro-, tumoral phenotype.

Tissue adaptation: Implications for gut immunity and tolerance

Faria et al. discuss the concept that immune cells undergo specialized adaptation to tissue-specific conditions and its potential implications for tolerance and immunity.

Tissue adaptation is an intrinsic component of immune cell development, influencing both resistance to pathogens and tolerance. Chronically stimulated surfaces of the body, in particular the gut mucosa, are the major sites where immune cells traffic and reside. Their adaptation to these environments requires constant discrimination between natural stimulation coming from harmless microbiota and food, and pathogens that need to be cleared. This review will focus on the adaptation of lymphocytes to the gut mucosa, a highly specialized environment that can help us understand the plasticity of leukocytes arriving at various tissue sites and how tissue-related factors operate to shape immune cell fate and function.

PSGL-1 on Leukocytes is a Critical Component of the Host Immune Response against Invasive Pneumococcal Disease

Bacterial uptake by phagocytic cells is a vital event in the clearance of invading pathogens such as Streptococcus pneumoniae. A major role of the P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes against invasive pneumococcal disease is described in this study. Phagocytosis experiments using different serotypes demonstrated that PSGL-1 is involved in the recognition, uptake and killing of S. pneumoniae. Co-localization of several clinical isolates of S. pneumoniae with PSGL-1 was demonstrated, observing a rapid and active phagocytosis in the presence of PSGL-1. Furthermore, the pneumococcal capsular polysaccharide and the main autolysin of the bacterium ―the amidase LytA― were identified as bacterial ligands for PSGL-1. Experimental models of pneumococcal disease including invasive pneumonia and systemic infection showed that bacterial levels were markedly increased in the blood of PSGL-1^−/− mice. During pneumonia, PSGL-1 controls the severity of pneumococcal dissemination from the lung to the bloodstream. In systemic infection, a major role of PSGL-1 in host defense is to clear the bacteria in the systemic circulation controlling bacterial replication. These results confirmed the importance of this receptor in the recognition and clearance of S. pneumoniae during invasive pneumococcal disease. Histological and cellular analysis demonstrated that PSGL-1^−/− mice have increased levels of T cells migrating to the lung than the corresponding wild-type mice. In contrast, during systemic infection, PSGL-1^−/− mice had increased numbers of neutrophils and macrophages in blood, but were less effective controlling the infection process due to the lack of this functional receptor. Overall, this study demonstrates that PSGL-1 is a novel receptor for S. pneumoniae that contributes to protection against invasive pneumococcal disease.

S. pneumoniae is one of the most important and devastating human pathogens worldwide, mainly affecting young children, elderly people and immunocompromised patients. In terms of host immune defense against invasive pneumococcal isolates, professional phagocytes require receptor-mediated recognition of certain ligands on the bacterial surface for the uptake and clearance of the microorganism. In this study, we demonstrate that the P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes is involved in the phagocytosis process of S. pneumoniae by targeting the capsule and the surface protein LytA as pathogen-associated molecular patterns. To explore this process in more detail, we have used wild-type mice and mice deficient in PSGL-1 demonstrating that lack of PSGL-1 is detrimental for the host by increasing the susceptibility to the infection and the severity of the pneumococcal invasive disease. Overall, these data show the importance of PSGL-1 on leukocytes in host defense against S. pneumoniae and confirm that PSGL-1 plays a critical protective role against invasive bacterial disease.

Protection against collagen-induced arthritis in mice afforded by the parasitic worm product, ES-62, is associated with restoration of the levels of interleukin-10-producing B cells and reduced plasma cell infiltration of the joints

We have previously reported that ES-62, a molecule secreted by the parasitic filarial nematode Acanthocheilonema viteae, protects mice from developing collagen-induced arthritis (CIA). Together with increasing evidence that worm infection may protect against autoimmune conditions, this raises the possibility that ES-62 may have therapeutic potential in rheumatoid arthritis and hence, it is important to fully understand its mechanism of action. To this end, we have established to date that ES-62 protection in CIA is associated with suppressed T helper type 1 (Th1)/Th17 responses, reduced collagen-specific IgG2a antibodies and increased interleukin-10 (IL-10) production by splenocytes. IL-10-producing regulatory B cells have been proposed to suppress pathogenic Th1/Th17 responses in CIA: interestingly therefore, although the levels of IL-10-producing B cells were decreased in the spleens of mice with CIA, ES-62 was found to restore these to the levels found in naive mice. In addition, exposure to ES-62 decreased effector B-cell, particularly plasma cell, infiltration of the joints, and such infiltrating B cells showed dramatically reduced levels of Toll-like receptor 4 and the activation markers, CD80 and CD86. Collectively, this induction of hyporesponsiveness of effector B-cell responses, in the context of the resetting of the levels of IL-10-producing B cells, is suggestive of a modulation of the balance between effector and regulatory B-cell responses that may contribute to ES-62-mediated suppression of CIA-associated inflammation and inhibition of production of pathogenic collagen-specific IgG2a antibodies.

Postnatal and adult immunoglobulin repertoires of innate-like CD19(+)CD45R(lo) B Cells

The diversity in antibody repertoire relies on different B cell populations working efficiently to fulfil distinct specific functions. We recently described an innate-like CD19(+)CD45R(-/lo) (19(+)45R(lo)) cell population in postnatal unstimulated adult mice, a heterogeneous population containing cells expressing immunoglobulin M (IgM) and others behaving as differentiated mature B lymphocytes (intracytoplasmic IgG1, AID(+), Blimp-1(+)RAG2(-)). In the present study, we characterized the Ig repertoire expressed by splenic 19(+)45R(lo) cells, assuming that they would bear a restricted repertoire biased for germline rearrangements and low mutation rates similar to other innate-like cells. Sequences from 19(+)45R(lo) cells displayed a variety of V, D and J regions, and the analysis of the CDR-H3 region revealed an intermediate overall CDR-H3 length and moderate hydrophobicity. Both IgM and switched sequences of PD15 19(+)45R(lo) cells had shorter CDR-H3 region and fewer non-template N nucleotides than adult sequences, as expected for profiles that correspond to an immature phenotype. Regarding the mutation rate in the VH regions, IgG1 sequences already carried a high rate of replacement mutations at PD15, which increased further in the sequences obtained from adult mice. Moreover, statistical models suggest that a proportion of the switched sequences in adult 19(+)45R(lo) cells had experienced antigen selection, unlike other innate-like B cell compartments.

Dynamics of the splenic innate-like CD19⁺CD45Rlo cell population from adult mice in homeostatic and activated conditions

In the adult spleen, CD19⁺CD45R(-/lo) (19⁺45R(lo)) lymphocytes of embryonic origin exist as a distinct population to that of the conventional B cell lineage. These cells display a plasmablast phenotype, and they spontaneously secrete IgG1 and IgA, whereas the bone marrow population of 19⁺45R(lo) cells contains B1 progenitors. In this study, we show that 19⁺45R(lo) cells are also present in Peyer's patches and in the spleen throughout the life span of wild-type mice, beginning at postnatal day 7. Although this population is heterogeneous, the surface phenotype of most of these cells distinguishes them from follicular, transitional, marginal zone, and B1 cells. In CBA/CaHN mice, few 19⁺45R(lo) cells were detected at postnatal day 7, and none was observed in the adult spleen. Splenic 19⁺45R(lo) cells exhibited homeostatic BrdU uptake in vivo and actively transcribed cell cycle genes. When transferred to immunodeficient RAG2⁻/⁻γchain⁻/⁻ recipient mice, 19⁺45R(lo) cells survived and differentiated into IgG1- and IgA-plasma cells. Moreover, in vitro stimulation of splenic 19⁺45R(lo) cells with LPS, CpG, BAFF/IL4, and CD40/IL4 induced cell proliferation, IgG1/IgA secretion and the release of IL-10, suggesting a potential immunoregulatory role for this subset of innate-like B cells.

Nonredundant roles for B cell-derived IL-10 in immune counter-regulation

IL-10 plays a central role in restraining the vigor of inflammatory responses, but the critical cellular sources of this counter-regulatory cytokine remain speculative in many disease models. Using a novel IL-10 transcriptional reporter mouse, we found an unexpected predominance of B cells (including plasma cells) among IL-10-expressing cells in peripheral lymphoid tissues at baseline and during diverse models of in vivo immunological challenge. Use of a novel B cell-specific IL-10 knockout mouse revealed that B cell-derived IL-10 nonredundantly decreases virus-specific CD8(+) T cell responses and plasma cell expansion during murine cytomegalovirus infection and modestly restrains immune activation after challenge with foreign Abs to IgD. In contrast, no role for B cell-derived IL-10 was evident during endotoxemia; however, although B cells dominated lymphoid tissue IL-10 production in this model, myeloid cells were dominant in blood and liver. These data suggest that B cells are an underappreciated source of counter-regulatory IL-10 production in lymphoid tissues, provide a clear rationale for testing the biological role of B cell-derived IL-10 in infectious and inflammatory disease, and underscore the utility of cell type-specific knockouts for mechanistic limning of immune counter-regulation.

A role for DNA polymerase mu in the emerging DJH rearrangements of the postgastrulation mouse embryo

The molecular complexes involved in the nonhomologous end-joining process that resolves recombination-activating gene (RAG)-induced double-strand breaks and results in V(D)J gene rearrangements vary during mammalian ontogeny. In the mouse, the first immunoglobulin gene rearrangements emerge during midgestation periods, but their repertoires have not been analyzed in detail. We decided to study the postgastrulation DJ(H) joints and compare them with those present in later life. The embryo DJ(H) joints differed from those observed in perinatal life by the presence of short stretches of nontemplated (N) nucleotides. Whereas most adult N nucleotides are introduced by terminal deoxynucleotidyl transferase (TdT), the embryo N nucleotides were due to the activity of the homologous DNA polymerase mu (Polmu), which was widely expressed in the early ontogeny, as shown by analysis of Polmu(-/-) embryos. Based on its DNA-dependent polymerization ability, which TdT lacks, Polmu also filled in small sequence gaps at the coding ends and contributed to the ligation of highly processed ends, frequently found in the embryo, by pairing to internal microhomology sites. These findings show that Polmu participates in the repair of early-embryo, RAG-induced double-strand breaks and subsequently may contribute to preserve the genomic stability and cellular homeostasis of lymphohematopoietic precursors during development.