Bacteria colonize the intestine shortly after birth and thereafter exert several beneficial functions, including induction of protective immunoglobulin A (IgA) antibodies. The distal intestine contains IgA(2), which is more resistant to bacterial proteases than is IgA(1). The mechanism by which B cells switch from IgM to IgA(2) remains unknown. We found that human intestinal epithelial cells (IECs) triggered IgA(2) class switching in B cells, including IgA(1)-expressing B cells arriving from mucosal follicles, through a CD4(+) T cell-independent pathway involving a proliferation-inducing ligand (APRIL). IECs released APRIL after sensing bacteria through Toll-like receptors (TLRs) and further increased APRIL production by activating dendritic cells via thymic stromal lymphopoietin. Our data indicate that bacteria elicit IgA(2) class switching by linking lamina propria B cells with IECs through a TLR-inducible signaling program requiring APRIL. Thus, mucosal vaccines should activate IECs to induce more effective IgA(2) responses.

Immunoglobulin D (IgD) is an enigmatic antibody isotype that mature B cells co-express with IgM through alternative RNA splicing. We found active T cell-dependent and T cell-independent IgM-to-IgD class switching in human upper respiratory mucosa B cells. This process required activation-induced cytidine deaminase and generated local and circulating IgD-producing plasmablasts reactive to respiratory bacteria. Circulating IgD bound to basophils through a calcium-mobilizing receptor that induced antimicrobial, opsonizing, inflammatory and B cell-stimulating factors including cathelicidin, interleukin-1, interleukin-4 and B cell-activating factor BAFF upon IgD cross-linking. By showing dysregulation of IgD class-switched B cells and IgD-armed basophils in autoinflammatory syndromes with periodic fever, our data indicate that IgD orchestrates an ancestral surveillance system at the interface between immunity and inflammation.

Epithelial cells (ECs) transport class-switched immunoglobulin G (IgG) and IgA antibodies across mucous membranes. Whether ECs initiate class switching remains unknown. Here we found that ECs lining tonsillar crypts formed pockets populated by B cells expressing activation-induced cytidine deaminase (AID), an enzyme associated with ongoing class switching. ECs released B cell-activating AID-inducing factors after sensing microbial products through Toll-like receptors. The resulting class switching was amplified by thymic stromal lymphopoietin, an epithelial interleukin 7-like cytokine that enhanced the B cell 'licensing' function of dendritic cells, and was restrained by secretory leukocyte protease inhibitor, an epithelial homeostatic protein that inhibited AID induction in B cells. Thus, ECs may function as mucosal 'guardians' orchestrating frontline IgG and IgA class switching through a Toll-like receptor-inducible signaling program regulated by secretory leukocyte protease inhibitor.NOTE: In the version of this article initially published online, the middle label above Figure 6c is incorrect. The correct label should be 'BAFF'. The error has been corrected for all versions of the article.

Contact-dependent communication between immune cells generates protection, but also facilitates viral spread. We found that macrophages formed long-range actin-propelled conduits in response to negative factor (Nef), a human immunodeficiency virus type-1 (HIV-1) protein with immunosuppressive functions. Conduits attenuated immunoglobulin G2 (IgG2) and IgA class switching in systemic and intestinal lymphoid follicles by shuttling Nef from infected macrophages to B cells through a guanine exchange factor-dependent pathway involving the amino-terminal anchor, central core and carboxy-terminal flexible loop of Nef. By showing stronger virus-specific IgG2 and IgA responses in patients harboring Nef-deficient virions, our data suggest that HIV-1 exploits intercellular highways as a “Trojan horse” to deliver Nef to B cells and evade humoral immunity systemically and at mucosal sites of entry.

Meiosis is a specialized cell division that halves the genome complement, producing haploid gametes/spores from diploid cells. Proper separation of homologous chromosomes at the first meiotic division requires the production of physical connections (chiasmata) between homologs through recombinational exchange of chromosome arms after sister-chromatid cohesion is established but before chromosome segregation takes place. The events of meiotic prophase must thus occur in a strictly temporal order, but the molecular controls coordinating these events have not been well elucidated. Here, we demonstrate that the budding yeast cyclin-dependent kinase Cdc28 directly regulates the formation of the DNA double-strand breaks that initiate recombination by phosphorylating the Mer2/Rec107 protein and thereby modulating interactions of Mer2 with other proteins required for break formation. We propose that this function of Cdc28 helps to coordinate the events of meiotic prophase with each other and with progression through prophase.

IntroductionClassical Hodgkin lymphoma (HL) is a lymphoid neoplasm that stems from the clonal expansion of mononuclear Hodgkin cells and multinuclear Reed-Sternberg cells expressing the CD30 antigen. 1 Malignant Hodgkin and Reed-Sternberg (HRS) cells usually constitute less than 10% of the neoplastic mass. 2 The remaining tissue is composed of a reactive cellular infiltrate. Although rare cases with T-cell genotype have been described, 3,4 the vast majority of classical HL tumors is thought to originate from transformed germinal center (GC) B cells, because their HRS component harbors a monoclonal immunoglobulin (Ig) gene rearrangement and somatically mutated Ig V region genes. 1,[5][6][7] Despite their GC B-cell origin, HRS cells lack many molecules usually expressed by B cells and are incapable of producing functional Igs. [7][8][9][10][11][12] While nonmalignant B cells that have lost their capacity to express Igs rapidly undergo apoptosis, 13 malignant HRS cells survive. This abnormal survival is thought to be due to dysregulated activation of nuclear factor B (NF-B), 14-18 a transcription factor essential for the development of both normal and neoplastic B cells. 19,20 In classical HL, the reactive infiltrate is composed of nonmalignant T cells, B cells, plasma cells, and myeloid cells, including macrophages and granulocytes. 2,21 These cells are thought to enhance HRS cell growth through cytokines and tumor necrosis factor (TNF) family members, such as CD30 ligand (CD30L), receptor activator of NF-B ligand (RANKL), and CD40 ligand (CD40L). 15,[22][23][24][25][26][27][28][29] Recent studies show that myeloid cells express B-cell-activating factor of the TNF family (BAFF, also known as BLyS) and its homolog APRIL, a proliferation-inducing ligand, [30][31][32][33] 2 molecules essential for the survival, proliferation, and differentiation of B cells and plasma cells. 34,35 BAFF activates B cells and plasma cells by binding to transmembrane activator and calcium modulator and cyclophylin ligand interactor (TACI), B-cell maturation antigen (BCMA), and BAFF receptor (BAFF-R) receptors. APRIL activates B cells and plasma cells by binding to TACI and BCMA, but not BAFF-R. 36 By recruiting TNF receptor-associated factor (TRAF) adaptor molecules, TACI, BCMA, and BAFF-R activate an IB kinase (IKK) complex that in turn elicits phosphorylation-dependent degradation of inhibitor of NF-B (IB), which retains p50, c-Rel, and p65 NF-B proteins in a cytoplasmic inactive form. [37][38][39] IB degradation causes NF-B nuclear translocation and transcriptional activation of NF-B-responsive genes involved in B-cell survival, proliferation, and maturation. 31,[39][40][41] Of note, recent studies show that APRIL signaling via TACI and BCMA receptors is reinforced by heparan sulfate proteoglycans (HSPGs) anchored on the cell membrane or associated with the extracellular matrix. 35,42,43 The role of TACI, BCMA, BAFF-R, and HSPGs in HL remains unknown.BAFF and APRIL are implicated in B-cell neoplasias, 44-52 including non-Hodgkin lymph...

Most of what we know about proteins reflects their native folded structure. Much less is understood about the structure of unfolded proteins, which tends to be referred to as "random coil", lacking extended alpha-helix or beta-strand structure. Recent work suggests that unfolded proteins might adopt significant population of PII structure, an extended left-handed helix found in collagen and proline-rich peptides. A series of short peptides AcGGXGGNH2 has been adopted as a model for studying unfolded protein structure because of the minimal steric effect imposed by flanking glycines. Peptide AcGGAGGNH2 makes possible a host-guest conformation analysis of the middle residue alanine. NMR experiments reveal that the Phi and Psi dihedral angles of the central alanine are -73 degrees and 125 degrees , respectively, placing the alanine in the PII region of the Ramachandran plot. Circular dichroism shows a typical PII spectrum with a strong negative absorbance at 190 nm. Temperature experiments show the alanine structure shifts to increasing beta-strand at high temperature. Because the alanine side chain most closely represents unsubstituted peptide backbone, these results have significant implications for the conformational entropy of unfolded polypeptide chains.

Class switch DNA recombination (CSR) from IgM to IgG and IgA is crucial for antiviral immunity. Follicular B cells undergo CSR upon engagement of CD40 by CD40 ligand on CD4+ T cells. This T cell-dependent pathway requires 5–7 days, which is too much of a delay to block quickly replicating pathogens. To compensate for this limitation, extrafollicular B cells rapidly undergo CSR through a T cell-independent pathway that involves innate Ag receptors of the TLR family. We found that a subset of upper respiratory mucosa B cells expressed TLR3 and responded to viral dsRNA, a cognate TLR3 ligand. In the presence of dsRNA, mucosal B cells activated NF-κB, a transcription factor critical for CSR. Activation of NF-κB required TRIF (Toll/IL-1R domain-containing protein inducing IFN-β), a canonical TLR3 adapter protein, and caused germline transcription of downstream CH genes as well as expression of AID (activation-induced cytidine deaminase), a DNA-editing enzyme essential for CSR. Subsequent IgG and IgA production was enhanced by BAFF (B cell-activating factor of the TNF family), an innate mediator released by TLR3-expressing mucosal dendritic cells. Indeed, these innate immune cells triggered IgG and IgA responses upon exposure to dsRNA. By showing active TLR3 signaling and ongoing CSR in upper respiratory mucosa B cells from patients with CD40 signaling defects, our findings indicate that viral dsRNA may initiate frontline IgG and IgA responses through an innate TLR3-dependent pathway involving BAFF.