Predicting plasma cell retention and loss over a lifetime.

Plasma cells (PCs) rely on external survival cues for persistence, which limits the size of the PC pool. How, then, are new specificities incorporated into a saturated system? In this issue of Immunity, Simons and Karin put forward a mathematical framework to explain PC retention that makes testable predictions about steady-state lifespan structure, withstands tests based on accrual and displaceability, and accounts for lifespan stratification with specificity.

Interleukin-21, acting beyond the immunological synapse, independently controls T follicular helper and germinal center B cells.

Germinal centers (GCs), transient structures within B cell follicles and central to affinity maturation, require the coordinated behavior of T and B cells. IL-21, a pleiotropic T cell-derived cytokine, is key to GC biology through incompletely understood mechanisms. By genetically restricting production and receipt of IL-21 in vivo, we reveal how its independent actions on T and B cells combine to regulate the GC. IL-21 established the magnitude of the GC B cell response by promoting CD4+ T cell expansion and differentiation in a dose-dependent manner and with paracrine activity. Within GC, IL-21 specifically promoted B cell centroblast identity and, when bioavailability was high, plasma cell differentiation. Critically, these actions may occur irrespective of cognate T-B interactions, making IL-21 a general promoter of growth as distinct to a mediator of affinity-driven selection via synaptic delivery. This promiscuous activity of IL-21 explains the consequences of IL-21 deficiency on antibody-based immunity.

A Dangerous Liaison with a Conscience.

The contribution of the immunoglobulin E (IgE)-mast cell response to allergy portrays the axis as a villain with malicious intent. A new study from Starkl et al. tells a different story, highlighting a more worthwhile purpose of protecting us against bacterial toxins.

STAT6 tunes maximum T cell IL-4 production from stochastically regulated Il4 alleles.

T helper 2 (Th2) cells stochastically express from the Il4 locus but it has not been determined whether allelic expression is linked or independent. Here, we provide evidence that alleles are independently activated and inactivated. We compared Il4 locus expression in T cells from hemizygous IL-4 reporter mice in culture and in vivo following exposure to type 2 immunogens. In culture, Il4 alleles had independent, heritable expression probabilities. Modeling showed that in co-expressors, dual allele transcription occurs for only short periods, limiting per-cell mRNA variation in individual cells within a population of Th2 cells. In vivo profiles suggested that early in the immune response, IL-4 output was derived predominantly from single alleles, but co-expression became more frequent over time and were tuned by STAT6, supporting the probabilistic regulation of Il4 alleles in vivo among committed IL-4 producers. We suggest an imprinted probability of expression from individual alleles with a short transcriptional shutoff time controls the magnitude of T cell IL-4 output, but the amount produced per allele is amplified by STAT6 signaling. This form of regulation may be a relevant general mechanism governing cytokine expression.

IL-21 has a critical role in establishing germinal centers by amplifying early B cell proliferation.

The proliferation and differentiation of antigen-specific B cells, including the generation of germinal centers (GC), are prerequisites for long-lasting, antibody-mediated immune protection. Affinity for antigen determines B cell recruitment, proliferation, differentiation, and competitiveness in the response, largely through determining access to T cell help. However, how T cell-derived signals contribute to these outcomes is incompletely understood. Here, we report how the signature cytokine of follicular helper T cells, IL-21, acts as a key regulator of the initial B cell response by accelerating cell cycle progression and the rate of cycle entry, increasing their contribution to the ensuing GC. This effect occurs over a wide range of initial B cell receptor affinities and correlates with elevated AKT and S6 phosphorylation. Moreover, the resultant increased proliferation can explain the IL-21-mediated promotion of plasma cell differentiation. Collectively, our data establish that IL-21 acts from the outset of a T cell-dependent immune response to increase cell cycle progression and fuel cyclic re-entry of B cells, thereby regulating the initial GC size and early plasma cell output.

How intrinsic and extrinsic regulators of plasma cell survival might intersect for durable humoral immunity.

Plasma cells (PC) are key to protective immunity because they secrete antibodies. Surviving for periods ranging from days to decades in mammals, PC possess varying survival times that cannot be entirely stochastic or extrinsically set, as presumed half-lives vary with antigenic specificity. Here, we review the signals that impart survival potential to PC. These include signals provided during formation, and signals experienced once generated and embedded in the so-called long-lived niche. These signals all feed into survival by maintaining PC expression of MCL1, potentially synergistically with influences of other BCL2 family members. Herein, we propose that each formed PC has a capacity to respond to extrinsic cues that sets an upper maximum to its lifespan, but survival is also affected by variable availability of signals provided in BM survival niches. PC survival thus becomes a function of immunogen characteristics and niche anatomy, determined by the weighted survival benefit ascribed to each involved factor. Most factors, such as supporting cell types and secreted proteins, are predicted to influence survival times varying temporally by orders of magnitude, rather than absolute PC abundances measured at a single time, which may account for the variation in PC lifespan evident in the literature.

The origins and longevity of IgE responses as indicated by serological and cellular studies in mice and humans.

The existence of long-lived IgE antibody-secreting cells (ASC) is contentious, with the maintenance of sensitization by the continuous differentiation of short-lived IgE+ ASC a possibility. Here, we review the epidemiological profile of IgE production, and give an overview of recent discoveries made on the mechanisms regulating IgE production from mouse models. Together, these data suggest that for most individuals, in most IgE-associated diseases, IgE+ ASC are largely short-lived cells. A subpopulation of IgE+ ASC in humans is likely to survive for tens of months, although due to autonomous IgE B cell receptor (BCR) signaling and antigen-driven IgE+ ASC apoptosis, in general IgE+ ASC probably do not persist for the decades that other ASC are inferred to do. We also report on recently identified memory B cell transcriptional subtypes that are the likely source of IgE in ongoing responses, highlighting the probable importance of IL-4Rα in their regulation. We suggest the field should look at dupilumab and other drugs that prohibit IgE+ ASC production as being effective treatments for IgE-mediated aspects of disease in most individuals.

The benefit of boosters: diversity and inclusion in the COVID-19 memory response.

In a new study, a group evaluate immune responses against SARS-CoV-2 in vaccinated individuals and find evidence of durable immune memory for at least 6 months, irrespective of former infection.

CD137L and CD4 T cells limit BCL6-expressing pre-germinal center B cell expansion and BCL6-driven B cell malignancy.

Aberrant expression of the proto-oncogene BCL6 is a driver of tumorigenesis in diffuse large B cell lymphoma (DLBCL). Mice overexpressing BCL6 from the B cell-specific immunoglobulin heavy chain µ intron promoter (Iµ-Bcl6Tg/+ ) develop B cell lymphomas with features typical of human DLBCL. While the development of B cell lymphoma in these mice is tightly controlled by T cells, the mechanisms of this immune surveillance are poorly understood. Here we show that CD4 T cells contribute to the control of lymphoproliferative disease in lymphoma-prone Iµ-Bcl6Tg/+ mice. We reveal that this CD4 T cell immuno-surveillance requires signaling by the co-stimulatory molecule CD137 ligand (CD137L; also known as 4-1BBL), which may promote the transition of pre-malignant B cells with an activated phenotype into the germinal center stage via reverse signaling, preventing their hazardous accumulation. Thus, CD137L-mediated CD4 T cell immuno-surveillance adds another layer of protection against B cell malignancy to that provided by CD8 T cell cytotoxicity.

A systemic immune challenge to model hospital-acquired infections independently regulates immune responses after pediatric traumatic brain injury.

BACKGROUND: Traumatic brain injury (TBI) is a major cause of disability in young children, yet the factors contributing to poor outcomes in this population are not well understood. TBI patients are highly susceptible to nosocomial infections, which are mostly acquired within the first week of hospitalization, and such infections may modify TBI pathobiology and recovery. In this study, we hypothesized that a peripheral immune challenge such as lipopolysaccharide (LPS)-mimicking a hospital-acquired infection-would worsen outcomes after experimental pediatric TBI, by perpetuating the inflammatory immune response. METHODS: Three-week-old male mice received either a moderate controlled cortical impact or sham surgery, followed by a single LPS dose (1 mg/kg i.p.) or vehicle (0.9% saline) at 4 days post-surgery, then analysis at 5 or 8 days post-injury (i.e., 1 or 4 days post-LPS). RESULTS: LPS-treated mice exhibited a time-dependent reduction in general activity and social investigation, and increased anxiety, alongside substantial body weight loss, indicating transient sickness behaviors. Spleen-to-body weight ratios were also increased in LPS-treated mice, indicative of persistent activation of adaptive immunity at 4 days post-LPS. TBI + LPS mice showed an impaired trajectory of weight gain post-LPS, reflecting a synergistic effect of TBI and the LPS-induced immune challenge. Flow cytometry analysis demonstrated innate immune cell activation in blood, brain, and spleen post-LPS; however, this was not potentiated by TBI. Cytokine protein levels in serum, and gene expression levels in the brain, were altered in response to LPS but not TBI across the time course. Immunofluorescence analysis of brain sections revealed increased glia reactivity due to injury, but no additive effect of LPS was observed. CONCLUSIONS: Together, we found that a transient, infection-like systemic challenge had widespread effects on the brain and immune system, but these were not synergistic with prior TBI in pediatric mice. These findings provide novel insight into the potential influence of a secondary immune challenge to the injured pediatric brain, with future studies needed to elucidate the chronic effects of this two-hit insult.

BAFF, IL-4 and IL-21 separably program germinal center-like phenotype acquisition, BCL6 expression, proliferation and survival of CD40L-activated B cells in vitro.

A B cell culture system using BAFF, IL-4 and IL-21 was recently developed that generates B cells with phenotypic and functional characteristics of in vivo-generated germinal center (GC) B cells. Here, we observe discrete influences of each exogenous signal on the expansion and differentiation of a CD40L-activated B cell pool. IL-4 was expressly necessary, but neither BAFF nor IL-21 was required for B cell acquisition of the GC B cell phenotypes of peanut agglutinin binding and loss of CD38 and IgD expression. Both IL-4 and IL-21 enhanced cell cycle entry upon initial activation dose-dependently, and did so additively. Importantly, while both cytokines acted in concert to increase overall BCL6 expression amounts, IL-21 exposure uniquely caused a small proportion of cells to attain a higher level of BCL6 expression, reminiscent of in vivo GC B cells. In contrast, BAFF supported survival of a fraction of memory-like B cells in extended cultures after removal of surrogate T cell-help signals. Thus, by separably programming proliferation, survival and GC phenotype acquisition, IL-4, BAFF and IL-21 drive distinct components of activated B cell fate.

Infections after a traumatic brain injury: The complex interplay between the immune and neurological systems.

Traumatic brain injury (TBI) is a serious global health issue, being the leading cause of death and disability for individuals under the age of 45, and one of the largest causes of global neurological disability. In addition to the brain injury itself, it is increasingly appreciated that a TBI may also alter the systemic immune response in a way that renders TBI patients more vulnerable to infections in the acute post-injury period. Such infections pose an additional challenge to the patient, increasing rates of mortality and morbidity, and worsening neurological outcomes. Hospitalization, surgical interventions, and a state of immunosuppression induced by injury to the central nervous system (CNS), may all contribute to the high rate of infections seen in the population with TBI. Ongoing research to better understand the immunomodulators that underlie TBI-induced immunosuppression may aid in the development of effective therapeutic strategies to improve the recovery trajectory for patients. This review first describes the clinical scenario, posing the question of whether TBI patients are more susceptible to infections such as pneumonia, and if so, why? We then consider how cross-talk between the injured brain and the systemic immune system occurs, and further, how the additional immune challenge of an acquired infection can contribute to ongoing neuroinflammation and neurodegeneration after a TBI. Experimental models combining TBI with infection are discussed, as well as current treatment options available for this double-barreled insult. The aims of this review are to summarize current understanding of the bidirectional relationship between the CNS and the immune system when faced with a mechanical trauma combined with a concomitant infection, and to highlight key outstanding questions that remain in the field.

IL-4 Haploinsufficiency Specifically Impairs IgE Responses against Allergens in Mice.

Polymorphisms in genes involved in IL-4 responses segregate with allergic disease risk and correlate with IgE levels in humans, and IL-4 promotes IgE and IgG1 Ab production against allergens in mice. We report that mice with only one intact Il4 gene copy are significantly impaired in their ability to make specific IgE responses against allergens, whereas IgG1 responses to allergens remain unaffected. Il4-hemizygosity also resulted in a modest but detectable drop in IL-4 production by CD4+ T cells isolated from lymph nodes and prevented IgE-dependent oral allergen-induced diarrhea. We conclude that a state of haploinsufficiency for the Il4 gene locus is specifically relevant for IL-4-dependent IgE responses to allergens with the amount of IL-4 produced in the hemizygous condition falling close to the threshold required for switching to IgE production. These results may be relevant for how polymorphisms in genes affecting IL-4 responses influence the risk of IgE-mediated allergic disease in humans.

Ki67 deficiency impedes chromatin accessibility and BCR gene rearrangement.

The proliferation marker Ki67 has been attributed critical functions in maintaining mitotic chromosome morphology and heterochromatin organization during the cell cycle, indicating a potential role in developmental processes requiring rigid cell-cycle control. Here, we discovered that despite normal fecundity and organogenesis, germline deficiency in Ki67 resulted in substantial defects specifically in peripheral B and T lymphocytes. This was not due to impaired cell proliferation but rather to early lymphopoiesis at specific stages where antigen-receptor gene rearrangements occurred. We identified that Ki67 was required for normal global chromatin accessibility involving regulatory regions of genes critical for checkpoint stages in B cell lymphopoiesis. In line with this, mRNA expression of Rag1 was diminished and gene rearrangement was less efficient in the absence of Ki67. Transgenes encoding productively rearranged immunoglobulin heavy and light chains complemented Ki67 deficiency, completely rescuing early B cell development. Collectively, these results identify a unique contribution from Ki67 to somatic antigen-receptor gene rearrangement during lymphopoiesis.

B cell receptor ligation induces IgE plasma cell elimination.

The proper regulation of IgE production safeguards against allergic disease, highlighting the importance of mechanisms that restrict IgE plasma cell (PC) survival. IgE PCs have unusually high surface B cell receptor (BCR) expression, yet the functional consequences of ligating this receptor are unknown. Here, we found that BCR ligation induced BCR signaling in IgE PCs followed by their elimination. In cell culture, exposure of IgE PCs to cognate antigen or anti-BCR antibodies induced apoptosis. IgE PC depletion correlated with the affinity, avidity, amount, and duration of antigen exposure and required the BCR signalosome components Syk, BLNK, and PLCγ2. In mice with a PC-specific impairment of BCR signaling, the abundance of IgE PCs was selectively increased. Conversely, BCR ligation by injection of cognate antigen or anti-IgE depleted IgE PCs. These findings establish a mechanism for the elimination of IgE PCs through BCR ligation. This has important implications for allergen tolerance and immunotherapy as well as anti-IgE monoclonal antibody treatments.

24 h Hydration profile of collegiate soccer players training once versus twice per day in the heat.

This study examined 24-h hydration parameters among collegiate, male soccer players (n = 17) during twice (X2) and once (X1) per day practice schedules in the heat. Urine specific gravity (USG) and body mass were measured before morning practices, afternoon practice (X2)/team meeting (X1), and the next morning practices. Fluid intake, sweat losses, and urinary losses were assessed during each 24-h window. Pre-practice body mass or USG did not differ among the timepoints. Sweat losses differed among all practices (p < 0.05) and averaged approximately 2.181±0.693 (X2AM) 1.710±0.474 (X2PM), and 3.361±0.956 L (X1AM), but players averaged replacing >50% of sweat losses with fluid intake every practice. Fluid intake during and between practices from practice 1 to the afternoon practice for X2 resulted in a positive fluid balance for X2 (+0.446±0.916 L). However, higher sweat loss during the initial morning practice and lower relative fluid intake prior to the afternoon team meeting the following morning resulted in a negative fluid balance (-0.304±0.675 L; p < 0.05: Cohen's d = 0.94) over the same time period for X1. By the start of the next morning practice sessions, both X1 (+0.664±1.051 L) and X2 (+0.446±0.916 L) were in positive fluid balance, respectively. Ample fluid consumption opportunities, scaled down practice intensities during X2, and potentially intentional greater relative fluid intake during X2 training resulted in no difference in fluid shift versus an X1 schedule before the start of practices. The majority of players maintained fluid balance drinking ad libitum regardless of practice schedule.

The concerted change in the distribution of cell cycle phases and zone composition in germinal centers is regulated by IL-21.

Humoral immune responses require germinal centres (GC) for antibody affinity maturation. Within GC, B cell proliferation and mutation are segregated from affinity-based positive selection in the dark zone (DZ) and light zone (LZ) substructures, respectively. While IL-21 is known to be important in affinity maturation and GC maintenance, here we show it is required for both establishing normal zone representation and preventing the accumulation of cells in the G1 cell cycle stage in the GC LZ. Cell cycle progression of DZ B cells is unaffected by IL-21 availability, as is the zone phenotype of the most highly proliferative GC B cells. Collectively, this study characterises the development of GC zones as a function of time and B cell proliferation and identifies IL-21 as an important regulator of these processes. These data help explain the requirement for IL-21 in normal antibody affinity maturation.

Regulation of B cell fate by chronic activity of the IgE B cell receptor.

IgE can trigger potent allergic responses, yet the mechanisms regulating IgE production are poorly understood. Here we reveal that IgE+ B cells are constrained by chronic activity of the IgE B cell receptor (BCR). In the absence of cognate antigen, the IgE BCR promoted terminal differentiation of B cells into plasma cells (PCs) under cell culture conditions mimicking T cell help. This antigen-independent PC differentiation involved multiple IgE domains and Syk, CD19, BLNK, Btk, and IRF4. Disruption of BCR signaling in mice led to consistently exaggerated IgE+ germinal center (GC) B cell but variably increased PC responses. We were unable to confirm reports that the IgE BCR directly promoted intrinsic apoptosis. Instead, IgE+ GC B cells exhibited poor antigen presentation and prolonged cell cycles, suggesting reduced competition for T cell help. We propose that chronic BCR activity and access to T cell help play critical roles in regulating IgE responses.

Regulatory constraints in the generation and differentiation of IgE-expressing B cells.

B cells expressing antibodies of the immunoglobulin E (IgE) isotype are rare, yet are heavily implicated in the pathogenesis of allergies and asthma. This review discusses recent methodological advances that permit sensitive probing of IgE-expressing (IgE(+)) B cells in vivo and have accordingly clarified the basic behavior and fate of IgE(+) B cells during immune responses in mouse models. IgE antibody secreting plasma cells can arise from extrafollicular foci, germinal centers, and memory B cells. However, compared to B cells expressing other isotypes, IgE(+) B cells are susceptible to multiple additional regulatory constraints that restrict the size of the IgE(+) B cell pool at each stage, coordinately limiting the overall magnitude, affinity, and duration of the IgE antibody response.