Functional and epigenetic studies reveal multistep differentiation and plasticity of in vitro-generated and in vivo-derived follicular T helper cells.

Follicular T helper (Tfh) cells provide critical help to B cells for germinal center (GC) formation. Mutations affecting SLAM-associated protein (SAP) prevent GC formation because of defective T cell-B cell interactions, yet effects on Tfh cell differentiation remain unclear. We describe the in vitro differentiation of functionally competent "Tfh-like" cells that expressed interleukin-21, Tfh cell markers, and Bcl6 and rescued GC formation in SAP-deficient hosts better than other T helper (Th) cells. SAP-deficient Tfh-like cells appeared virtually indistinguishable from wild-type, yet failed to support GCs in vivo. Interestingly, both Tfh-like and in vivo-derived Tfh cells could produce effector cytokines in response to polarizing conditions. Moreover, Th1, Th2, and Th17 cells could be reprogrammed to obtain Tfh cell characteristics. ChIP-Seq analyses revealed positive epigenetic markings on Tbx21, Gata3, and Rorc in Tfh-like and ex vivo Tfh cells and on Bcl6 in non-Tfh cells, supporting the concept of plasticity between Tfh and other Th cell populations.

Early Th1 cell differentiation is marked by a Tfh cell-like transition.

Follicular helper T (Tfh) cells comprise an important subset of helper T cells; however, their relationship with other helper lineages is incompletely understood. Herein, we showed interleukin-12 acting via the transcription factor STAT4 induced both Il21 and Bcl6 genes, generating cells with features of both Tfh and Th1 cells. However, STAT4 also induced the transcription factor T-bet. With ChIP-seq, we defined the genome-wide targets of T-bet and found that it repressed Bcl6 and other markers of Tfh cells, thereby attenuating the nascent Tfh cell-like phenotype in the late phase of Th1 cell specification. Tfh-like cells were rapidly generated after Toxoplasma gondii infection in mice, but T-bet constrained Tfh cell expansion and consequent germinal center formation and antibody production. Our data argue that Tfh and Th1 cells share a transitional stage through the signal mediated by STAT4, which promotes both phenotypes. However, T-bet represses Tfh cell functionalities, promoting full Th1 cell differentiation.

Optimal germinal center responses require a multistage T cell:B cell adhesion process involving integrins, SLAM-associated protein, and CD84.

CD4(+) T cells deficient in signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) exhibit a selective impairment in adhesion to antigen-presenting B cells but not dendritic cells (DCs), resulting in defective germinal center formation. However, the nature of this selective adhesion defect remained unclear. We found that whereas T cell:DC interactions were primarily integrin dependent, T cell:B cell interactions had both an early integrin-dependent phase and a sustained phase that also required SAP. We further found that the SLAM family member CD84 was required for prolonged T cell:B cell contact, optimal T follicular helper function, and germinal center formation in vivo. Moreover, both CD84 and another SLAM member, Ly108, mediated T cell adhesion and participated in stable T cell:B cell interactions in vitro. Our results reveal insight into the dynamic regulation of T cell:B cell interactions and identify SLAM family members as critical components of sustained T cell:B cell adhesion required for productive humoral immunity.

T follicular helper cell diversity and plasticity.

CD4(+) T helper (Th) cells play an instrumental role in orchestrating adaptive immune responses to invading pathogens through their ability to differentiate into specialized effector subsets. Part of this customized response requires the development of T follicular helper (Tfh) cells, which provide help to B cells for the generation of germinal centers (GCs) and long-term protective humoral responses. Although initially viewed as terminally differentiated, we now recognize that Th cell subsets, including Tfh cells, display substantial flexibility and overlap in their characteristics. In this review, we highlight advances in our understanding of Tfh cell development, cytokine production, and the potential plasticity that allows Tfh cells to possess characteristics of other effector Th cell populations.

Type I IFN induces binding of STAT1 to Bcl6: divergent roles of STAT family transcription factors in the T follicular helper cell genetic program.

CD4(+) T follicular helper cells (TFH) are critical for the formation and function of B cell responses to infection or immunization, but also play an important role in autoimmunity. The factors that contribute to the differentiation of this helper cell subset are incompletely understood, although several cytokines including IL-6, IL-21, and IL-12 can promote TFH cell formation. Yet, none of these factors, nor their downstream cognate STATs, have emerged as nonredundant, essential drivers of TFH cells. This suggests a model in which multiple factors can contribute to the phenotypic characteristics of TFH cells. Because type I IFNs are often generated in immune responses, we set out to investigate whether these factors are relevant to TFH cell differentiation. Type I IFNs promote Th1 responses, thus one possibility was these factors antagonized TFH-expressed genes. However, we show that type I IFNs (IFN-α/ß) induced B cell lymphoma 6 (Bcl6) expression, the master regulator transcription factor for TFH cells, and CXCR5 and programmed cell death-1 (encoded by Pdcd1), key surface molecules expressed by TFH cells. In contrast, type I IFNs failed to induce IL-21, the signature cytokine for TFH cells. The induction of Bcl6 was regulated directly by STAT1, which bound to the Bcl6, Cxcr5, and Pdcd1 loci. These data suggest that type I IFNs (IFN-α/ß) and STAT1 can contribute to some features of TFH cells but are inadequate in inducing complete programming of this subset.

To B or not to B.

Recent data from mouse models suggest that some phenotypes of X-linked lymphoproliferative disease (XLP) result from impaired T:B-cell interactions.Hislop and colleagues now provide evidence that this may contribute to abnormal responses to Epstein-Barr virus (EBV) in XLP.

U.S.-Japan cooperative medical sciences program: 22nd International Conference on Emerging Infectious Diseases in the Pacific Rim.

This review summarizes the presentations given at the 22nd International conference on Emerging Infectious Diseases in the Pacific Rim. The purpose of this annual meeting is to foster international collaborations and address important public health issues in the Asia-Pacific region. This meeting was held in Bangkok in February 2020 and focused on emerging virus infections. Unexpectedly, the SARS-CoV-2 pandemic was in the initial stages leading to a special session on COVID-19 in addition to talks on dengue, influenza, hepatitis, AIDS, Zika, chikungunya, rabies, cervical cancer and nasopharyngeal carcinoma.

20th International Conference on Emerging Infectious Diseases in the Pacific Rim Organized by the United States-Japan Cooperative Medical Sciences Program (USJCMSP).

The 20th International Conference on Emerging Infectious Diseases in the Pacific Rim to3ok place in Shenzhen, China on January 8⁻9, 2018 followed by meetings of the acquired immunodeficiency syndrome (AIDS)/immunology, acute respiratory infections, cancer, hepatitis, and viral diseases panels on January 10⁻11. The conference was organized as part of the United States-Japan Cooperative Medical Sciences Program (USJCMSP) by the Japan Agency for Medical Research and Development (AMED) and the U.S. National Institutes of Health (NIH) and was locally hosted by the Shenzhen Third People's Hospital and the Chinese Academy of Sciences (CAS) Institute of Microbiology. The conference provides the basis for networking and fostering of collaboration opportunities between researchers in Southeast Asia and the United States based on the scientific and interactive platform of the USJCMSP and takes place in the region on an annual basis. This report summarizes the discussions and conclusions from the conference.

Waning Immunity and Microbial Vaccines-Workshop of the National Institute of Allergy and Infectious Diseases.

Since the middle of the 20th century, vaccines have made a significant public health impact by controlling infectious diseases globally. Although long-term protection has been achieved with some vaccines, immunity wanes over time with others, resulting in outbreaks or epidemics of infectious diseases. Long-term protection against infectious agents that have a complex life cycle and antigenic variation remains a key challenge. Novel strategies to characterize the short- and long-term immune responses to vaccines and to induce immune responses that mimic natural infection have recently emerged. New technologies and approaches in vaccinology, such as adjuvants, delivery systems, and antigen formulations, have the potential to elicit more durable protection and fewer adverse reactions; together with in vitro systems, these technologies have the capacity to model and accelerate vaccine development. The National Institute of Allergy and Infectious Diseases (NIAID) held a workshop on 19 September 2016 that focused on waning immunity to selected vaccines (for Bordetella pertussis, Salmonella enterica serovar Typhi, Neisseria meningitidis, influenza, mumps, and malaria), with an emphasis on identifying knowledge gaps, future research needs, and how this information can inform development of more effective vaccines for infectious diseases.

Maintenance of the CD40-related immunodeficient response in hyper-IgM B cells immortalized with a LMP1-regulated mini-EBV.

Our previous investigation of a patient (pt1) with non-X-linked hyper-immunoglobulin M syndrome revealed a CD40-mediated defect in B cell activation that resulted in low CD23 expression and absence of germ-line transcription and class-switch recombination. These deficiencies were complemented in vitro by a high threshold of sustained signaling through CD40. To further analyze the signaling defect in pt1 B cells, two types of Epstein-Barr virus lymphoblastoid cell lines (LCLs) were generated that either constitutively expressed the viral transforming protein latent membrane protein-1 (LMP1; pt1-LCL) or expressed it under the control of a tet-inducible promoter (pt1-LCL(tet)). Because LMP1 signals through the CD40 pathway, the pt1-LCL and pt1-LCL(tet) lines allow comparison of downstream functions in response to either constitutive LMP1 signals or regulated LMP1 and CD40 signals. Immortalized pt1-LCLs were initially CD23(lo)/CD38(hi) and reverted to a CD23(hi)/CD38(lo) phenotype upon extended growth in culture, suggesting that the CD40 defect was reversed by selection and/or constitutive expression of LMP1. In contrast, pt1-LCL(tet) cells retained the CD23(lo)/CD38(hi) phenotype after extended periods of culture and failed to up-regulate CD23 in response to CD40 signals. Analysis of pt1-LCL(tet) cells in response to the CD40 signals in the presence or absence of LMP1 revealed that mitogenic activation resulted only from LMP1 and not CD40, indicating a difference in the response of pt1 B cells to these two distinct signals. Together, these data demonstrate that the pt1-LCL(tet) cells maintain the CD40-related defect and provide a unique approach to study the independent effects of LMP1- and CD40-directed signals.

Protection of innate immunity by C5aR antagonist in septic mice.

Innate immune functions are known to be compromised during sepsis, often with lethal consequences. There is also evidence in rats that sepsis is associated with excessive complement activation and generation of the potent anaphylatoxin C5a. In the presence of a cyclic peptide antagonist (C5aRa) to the C5a receptor (C5aR), the binding of murine 125I-C5a to murine neutrophils was reduced, the in vitro chemotactic responses of mouse neutrophils to mouse C5a were markedly diminished, the acquired defect in hydrogen peroxide (H2O2) production of C5a-exposed neutrophils was reversed, and the lung permeability index (extravascular leakage of albumin) in mice after intrapulmonary deposition of IgG immune complexes was markedly diminished. Mice that developed sepsis after cecal ligation/puncture (CLP) and were treated with C5aRa had greatly improved survival rates. These data suggest that C5aRa interferes with neutrophil responses to C5a, preventing C5a-induced compromise of innate immunity during sepsis, with greatly improved survival rates after CLP.

c-Rel plays a key role in deficient activation of B cells from a non-X-linked hyper-IgM patient.

Our previous results demonstrated that B cells from a patient (pt1) with non-X-linked hyper-IgM syndrome (HIGM) possess an atypical CD23(lo) phenotype that is unaffected by CD40-mediated activation. To investigate the molecular mechanism underlying defective CD23 expression in pt1 B cells, we used lymphoblastoid cell lines that express LMP1 under the control of a tetracycline-inducible promoter (LCL(tet)). Our analysis revealed that the CD23(lo) phenotype in the pt1-LCL(tet) cells is a direct consequence of diminished CD23 transcription. We demonstrate a marked decrease in c-Rel-containing complexes that bind to the proximal CD23a/b promoters in pt1-LCL(tet) extracts, resulting from an overall lower expression of c-Rel in pt1-LCL(tet) cells. Analysis of c-Rel mRNA revealed relatively equal amounts in pt1-LCL(tet) and control LCL(tet) cells, indicating that diminished c-Rel protein expression is unrelated to decreased transcription. Finally, a critical role for c-Rel in CD23 regulation was demonstrated by effectively altering c-Rel expression that resulted in the direct modulation of CD23 surface expression. Collectively, these findings demonstrate that low levels of c-Rel are the underlying cause of aberrant CD23 expression in pt1 B cells and are likely to play a critical role in the pathophysiology of this form of HIGM.

Generation of C5a by phagocytic cells.

The complement activation product, C5a, is a powerful phlogistic factor. Using antibodies to detect human or rat C5a, incubation at pH 7.4 of human blood neutrophils or rat alveolar macrophages (AMs) with C5 in the presence of phorbol 12-myristate 13-acetate (PMA) led to generation of C5a. Rat AMs activated with lipopolysaccharide also generated C5a from C5. With activated neutrophils, extensive cleavage of C5 occurred, whereas activated macrophages had much more selective proteolytic activity for C5. Peripheral blood human or rat mononuclear cells and rat alveolar epithelial cells when stimulated with phorbol ester all failed to demonstrate an ability to cleave C5, suggesting a specificity of C5 cleavage by phagocytic cells. With rat AMs, C5a generation was time-dependent and was blocked if AMs were pretreated with inhibitors of transcription or protein synthesis (actinomycin D or cycloheximide). Similar treatment of activated human polymorphonuclear leukocytes only partially reduced C5a generation after addition of C5. C5a generated by activated AMs was biologically (chemotactically) active. This generation was sensitive to serine protease inhibitors but not to other classes of inhibitors. These data indicate that phagocytic cells, especially lung macrophages, can generate C5a from C5. In the context of the lung, this may represent an important C5a-generating pathway that is independent of the plasma complement system.

Complement-induced impairment of innate immunity during sepsis.

This study defines the molecular basis for defects in innate immunity involving neutrophils during cecal ligation/puncture (CLP)-induced sepsis in rats. Blood neutrophils from CLP rats demonstrated defective phagocytosis and defective assembly of NADPH oxidase, the latter being due to the inability of p47(phox) to translocate from the cytosol to the cell membrane of neutrophils after cell stimulation by phorbol ester (PMA). The appearance of these defects was prevented by in vivo blockade of C5a in CLP rats. In vitro exposure of neutrophils to C5a led to reduced surface expression of C5aR and defective assembly of NADPH oxidase, as defined by failure in phosphorylation of p47(phox) and its translocation to the cell membrane, together with failure in phosphorylation of p42/p44 mitogen-activated protein kinases. These data identify a molecular basis for defective innate immunity involving neutrophils during sepsis.

Structure-function relationships of human C5a and C5aR.

Using peptides that represent linear regions of the powerful complement activation product, C5a, or loops that connect the four alpha helices of C5a, we have defined the ability of these peptides to reduce binding of (125)I-C5a to human neutrophils, inhibit chemotactic responses of neutrophils to C5a, and reduce H(2)O(2) production in neutrophils stimulated with PMA. The data have defined likely sites of interaction of C5a with C5aR. The peptides had no functional activity per se on neutrophils and did not interfere with neutrophil responses to the unrelated chemotactic peptide, N-formyl-Met-Leu-Phe. Although previous data have suggested that there are two separate sites on C5a reactive with C5aR, the current data suggest that C5a interacts with C5aR in a manner that engages three discontinuous regions of C5a.

Expression and function of C5a receptor in mouse microvascular endothelial cells.

The complement-derived anaphylatoxin, C5a, is a potent phlogistic molecule that mediates its effects by binding to C5a receptor (C5aR; CD88). We now demonstrate specific binding of radiolabeled recombinant mouse C5a to mouse dermal microvascular endothelial cells (MDMEC) with a K(d50) of 3.6 nM and to approximately 15,000-20,000 receptors/cell. Recombinant mC5a competed effectively with binding of [(125)I]rmC5a to MDMEC. Enhanced binding of C5a occurred, as well as increased mRNA for C5aR, after in vitro exposure of MDMEC to LPS, IFN-gamma, or IL-6 in a time- and dose-dependent manner. By confocal microscopy, C5aR could be detected on surfaces of MDMEC using anti-C5aR Ab. In vitro expression of macrophage inflammatory protein-2 (MIP-2) and monocyte chemoattractant protein-1 (MCP-1) by MDMEC was also measured. Exposure of MDMEC to C5a or IL-6 did not result in changes in MIP-2 or MCP-1 production, but initial exposure of MDMEC to IL-6, followed by exposure to C5a, resulted in significantly enhanced production of MIP-2 and MCP-1 (but not TNF-alpha and MIP-1alpha). Although LPS or IFN-gamma alone induced some release of MCP-1 and MIP-2, pre-exposure of these monolayers to LPS or IFN-gamma, followed by addition of C5a, resulted in synergistic production of MIP-2 and MCP-1. Following i.v. infusion of LPS into mice, up-regulation of C5aR occurred in the capillary endothelium of mouse lung, as determined by immunostaining. These results support the hypothesis that C5aR expression on MDMEC and on the microvascular endothelium of lung can be up-regulated, suggesting that C5a in the co-presence of additional agonists may mediate pro-inflammatory effects of endothelial cells.