Heterologous prime-boost vaccination drives early maturation of HIV broadly neutralizing antibody precursors in humanized mice

A protective HIV vaccine will likely need to induce broadly neutralizing antibodies (bnAbs). Vaccination with the germline-targeting immunogen eOD-GT8 60mer adjuvanted with AS01B was found to induce VRC01-class bnAb precursors in 97% of vaccine recipients in the IAVI G001 phase 1 clinical trial; however, heterologous boost immunizations with antigens more similar to the native glycoprotein will be required to induce bnAbs. Therefore, we designed core-g28v2 60mer, a nanoparticle immunogen to be used as a first boost after eOD-GT8 60mer priming. We found, using a humanized mouse model approximating human conditions of VRC01-class precursor B cell diversity, affinity, and frequency, that both protein- and mRNA-based heterologous prime-boost regimens induced VRC01-class antibodies that gained key mutations and bound to near-native HIV envelope trimers lacking the N276 glycan. We further showed that VRC01-class antibodies induced by mRNA-based regimens could neutralize pseudoviruses lacking the N276 glycan. These results demonstrated that heterologous boosting can drive maturation toward VRC01-class bnAb development and supported the initiation of the IAVI G002 phase 1 trial testing mRNA-encoded nanoparticle prime-boost regimens.

Vaccine priming of rare HIV broadly neutralizing antibody precursors in nonhuman primates

Germline-targeting immunogens hold promise for initiating the induction of broadly neutralizing antibodies (bnAbs) to HIV and other pathogens. However, antibody-antigen recognition is typically dominated by heavy chain complementarity determining region 3 (HCDR3) interactions, and vaccine priming of HCDR3-dominant bnAbs by germline-targeting immunogens has not been demonstrated in humans or outbred animals. In this work, immunization with N332-GT5, an HIV envelope trimer designed to target precursors of the HCDR3-dominant bnAb BG18, primed bnAb-precursor B cells in eight of eight rhesus macaques to substantial frequencies and with diverse lineages in germinal center and memory B cells. We confirmed bnAb-mimicking, HCDR3-dominant, trimer-binding interactions with cryo-electron microscopy. Our results demonstrate proof of principle for HCDR3-dominant bnAb-precursor priming in outbred animals and suggest that N332-GT5 holds promise for the induction of similar responses in humans.

Human immunoglobulin gene allelic variation impacts germline-targeting vaccine priming

Vaccine priming immunogens that activate germline precursors for broadly neutralizing antibodies (bnAbs) have promise for development of precision vaccines against major human pathogens. In a clinical trial of the eOD-GT8 60mer germline-targeting immunogen, higher frequencies of vaccine-induced VRC01-class bnAb-precursor B cells were observed in the high dose compared to the low dose group. Through immunoglobulin heavy chain variable (IGHV) genotyping, statistical modeling, quantification of IGHV1-2 allele usage and B cell frequencies in the naive repertoire for each trial participant, and antibody affinity analyses, we found that the difference between dose groups in VRC01-class response frequency was best explained by IGHV1-2 genotype rather than dose and was most likely due to differences in IGHV1-2 B cell frequencies for different genotypes. The results demonstrate the need to define population-level immunoglobulin allelic variations when designing germline-targeting immunogens and evaluating them in clinical trials.

Human immunoglobulin gene allelic variation impacts germline-targeting vaccine priming

Vaccine priming immunogens that activate germline precursors for broadly neutralizing antibodies (bnAbs) have promise for development of precision vaccines against major human pathogens. In a clinical trial of the eOD-GT8 60mer germline-targeting immunogen, higher frequencies of vaccine-induced VRC01-class bnAb-precursor B cells were observed in the high dose compared to the low dose group. Through immunoglobulin heavy chain variable (IGHV) genotyping, statistical modeling, quantification of IGHV1-2 allele usage and B cell frequencies in the naive repertoire for each trial participant, and antibody affinity analyses, we found that the difference between dose groups in VRC01-class response frequency was best explained by IGHV1-2 genotype rather than dose and was most likely due to differences in IGHV1-2 B cell frequencies for different genotypes. The results demonstrate the need to define population-level immunoglobulin allelic variations when designing germline-targeting immunogens and evaluating them in clinical trials.

One-Sentence Summary

Human genetic variation can modulate the strength of vaccine-induced broadly neutralizing antibody precursor B cell responses.

Vaccination induces HIV broadly neutralizing antibody precursors in humans

Broadly neutralizing antibodies (bnAbs) can protect against HIV infection but have not been induced by human vaccination. A key barrier to bnAb induction is vaccine priming of rare bnAb-precursor B cells. In a randomized, double-blind, placebo-controlled phase 1 clinical trial, the HIV vaccine-priming candidate eOD-GT8 60mer adjuvanted with AS01B had a favorable safety profile and induced VRC01-class bnAb precursors in 97% of vaccine recipients with median frequencies reaching 0.1% among immunoglobulin G B cells in blood. bnAb precursors shared properties with bnAbs and gained somatic hypermutation and affinity with the boost. The results establish clinical proof of concept for germline-targeting vaccine priming, support development of boosting regimens to induce bnAbs, and encourage application of the germline-targeting strategy to other targets in HIV and other pathogens.

Long-primed germinal centres with enduring affinity maturation and clonal migration

Germinal centres are the engines of antibody evolution. Here, using human immunodeficiency virus (HIV) Env protein immunogen priming in rhesus monkeys followed by a long period without further immunization, we demonstrate germinal centre B (BGC) cells that last for at least 6 months. A 186-fold increase in BGC cells was present by week 10 compared with conventional immunization. Single-cell transcriptional profiling showed that both light- and dark-zone germinal centre states were sustained. Antibody somatic hypermutation of BGC cells continued to accumulate throughout the 29-week priming period, with evidence of selective pressure. Env-binding BGC cells were still 49-fold above baseline at 29 weeks, which suggests that they could remain active for even longer periods of time. High titres of HIV-neutralizing antibodies were generated after a single booster immunization. Fully glycosylated HIV trimer protein is a complex antigen, posing considerable immunodominance challenges for B cells1,2. Memory B cells generated under these long priming conditions had higher levels of antibody somatic hypermutation, and both memory B cells and antibodies were more likely to recognize non-immunodominant epitopes. Numerous BGC cell lineage phylogenies spanning more than the 6-month germinal centre period were identified, demonstrating continuous germinal centre activity and selection for at least 191 days with no further antigen exposure. A long-prime, slow-delivery (12 days) immunization approach holds promise for difficult vaccine targets and suggests that patience can have great value for tuning of germinal centres to maximize antibody responses.

Highly mutated antibodies capable of neutralizing N276 glycan-deficient HIV after a single immunization with an Env trimer

Elicitation of HIV broadly neutralizing antibodies (bnAbs) is challenging because unmutated bnAb precursors are rare and seldom bind HIV envelope glycoprotein (Env) trimers. One strategy to initiate bnAb responses is to use germline-targeting (GT) immunogens with high affinity to bnAb-class precursor B cells and then shepherd affinity maturation with booster immunogens that successively look more like native Env. In a mouse model where the frequency of VRC01-precursor (VRC01gHL) B cells mimics that of humans, we show that following a GT HIV Env trimer protein prime, VRC01-class B cells in the germinal center (GC) acquire high-affinity VRC01-class B cell somatic hypermutations (SHMs). Many GC-derived VRC01gHL antibodies robustly bind N276 glycan-deficient Env trimers and neutralize several N276 glycan-deficient tier 2 HIV strains. These results are encouraging for GT Env trimer vaccine designs and demonstrate accumulation of substantial SHMs, including deletions, uncommon point mutations, and functional bnAb features, after a single immunization.

Membrane cofactor protein is a receptor for adenoviruses associated with epidemic keratoconjunctivitis

Subgroup D adenovirus (Ad) types 8, 19, and 37 (Ad8, -19, and -37, respectively) are causative agents of epidemic keratoconjunctivitis and genital tract infections. Previous studies showed that Ad37 binds to a 50-kDa membrane glycoprotein expressed on human ocular (conjunctival) cells. To identify and characterize the role of the 50-kDa glycoprotein in Ad37 infection, we partially purified this molecule from solubilized Chang C conjunctival cell membranes by using lentil lectin chromatography and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Liquid chromatography coupled to nano-electrospray ionization-tandem mass spectrometry was subsequently used to identify four Ad37 receptor candidates: CD46, CD87, CD98, and CD147. Immunodepletion analyses demonstrated that the 50-kDa protein is identical to CD46 (also known as membrane cofactor protein). The Ad37, but not Ad5, fiber knob bound to the extracellular domain of CD46, demonstrating a direct interaction of an Ad37 capsid protein with CD46. An antibody specific for the N-terminal 19 amino acids of CD46 also blocked Ad37 infection of human cervical carcinoma and conjunctival cells, indicating a requirement for CD46 in infection. Finally, expression of a 50-kDa isoform of human CD46 in a CD46-null cell line increased cell binding by wild-type Ad37 and gene delivery by an Ad vector pseudotyped with the Ad37 fiber, but not by a vector bearing the Ad5 fiber. Together, these studies demonstrate that CD46 serves as an attachment receptor for Ad37 and shed further light on the cell entry pathway of subgroup D Ads.

Flexibility of the adenovirus fiber is required for efficient receptor interaction

The adenovirus (Ad) fiber protein mediates Ad binding to the coxsackievirus and Ad receptor (CAR) and is thus a major determinant of viral tropism. The fiber contains three domains: an N-terminal tail that anchors the fiber to the viral capsid, a central shaft region of variable length and flexibility, and a C-terminal knob domain that binds to cell receptors. Ad type 37 (Ad37), a subgroup D virus associated with severe ocular infections, is unable to use CAR efficiently to infect host cells, despite containing a CAR binding site in its fiber knob. We hypothesized that the relatively short, inflexible Ad37 fiber protein restricts interactions with CAR at the cell surface. To test this hypothesis, we analyzed the infectivity and binding of recombinant Ad particles containing modified Ad37 or Ad5 fiber proteins. Ad5 particles equipped with a truncated Ad5 fiber or with a chimeric fiber protein comprised of the Ad5 knob fused to the short, rigid Ad37 shaft domain had significantly reduced infectivity and attachment. In contrast, placing the Ad37 knob onto the long, flexible Ad5 shaft allowed CAR-dependent virus infection and cell attachment, demonstrating the importance of the shaft domain in receptor usage. Increasing fiber rigidity by substituting the predicted flexibility modules in the Ad5 shaft with the corresponding regions of the rigid Ad37 fiber dramatically reduced both virus infection and cell attachment. Cryoelectron microscopy (cryo-EM) single-particle analysis demonstrated the increased rigidity of this chimeric fiber. These studies demonstrate that both length and flexibility of the fiber shaft regulate CAR interaction and provide a molecular explanation for the use of alternative receptors by subgroup D Ad with ocular tropism. We present a molecular model for Ad-CAR interactions at the cell surface that explains the significance of fiber flexibility in cell attachment.

Ligand-dependent interactions of coactivators steroid receptor coactivator-1 and peroxisome proliferator-activated receptor binding protein with nuclear hormone receptors can be imaged in live cells and are required for transcription

Members of the nuclear receptor superfamily are thought to activate transcription by recruitment of one or more recently identified coactivator complexes. Here we demonstrate that both peroxisome proliferator-activated receptor binding protein (PBP) and steroid receptor coactivator-1 (SRC-1) are required for ligand-dependent transcription of transiently transfected and chromosomally integrated reporter genes by the estrogen receptor (ER) and retinoic acid receptor (RAR). To examine ligand-dependent interactions between nuclear receptors and specific coactivators in living cells, these proteins were tagged with cyan (CFP) and yellow (YFP) mutants of the green fluorescent protein. Fluorescence resonance energy transfer (FRET) from the CFP to the YFP indicated interaction between the receptor and coactivator. CFP fusions to RAR or its ligand-binding domain exhibited rapid ligand-dependent FRET to YFP-tagged nuclear receptor interaction domains of the coactivators SRC-1 and PBP. The ER-ligand-binding domain, unlike RAR, also exhibited some basal interaction with coactivators in unstimulated cells that was abolished by the receptor antagonists tamoxifen or ICI182,780. Inhibition of FRET by tamoxifen but not ICI182,780 could be reversed by estradiol, whereas estradiol-enhanced FRET could not be inhibited by either antagonist, indicating that ligand effects can show varying degrees of hysteresis. These findings suggest that ligand-dependent transcriptional activities of the RAR and ER require concurrent or sequential recruitment of SRC-1 and PBP-containing coactivator complexes.

A role for RNA helicase A in post-transcriptional regulation of HIV type 1

Retroviruses must bypass the tight coupling of splicing and nuclear export of mRNA in their replication cycle because unspliced genomic RNA and incompletely spliced mRNA must be exported to the cytoplasm for packaging or translation. This process is mediated by a cis-acting constitutive transport element (CTE) for simple retroviruses and by the trans-acting viral protein Rev in concert with its response element (RRE) for complex retroviruses (e.g., HIV). Recently, we identified RNA helicase A (RHA) as a potential cellular cofactor for CTE. Here, we report that RHA also plays a role in Rev/RRE-mediated gene expression and HIV replication. RHA binds weakly to HIV-1 RRE independently of Rev. Overexpression of RHA, but not of an RHA mutant lacking helicase activity, increased both Rev/RRE- and CTE-dependent gene expression and the levels of unspliced HIV mRNA. Microinjection of antibodies to RHA into nuclei dramatically inhibited both CTE- and Rev-dependent gene expression in human cells. Exogenous RHA cDNA, but not the mutant RHA, rescued this inhibition. We propose that RHA is required to release both CTE- and RRE-containing mRNA from spliceosomes before completion of splicing, thus freeing them for nuclear export.

Determinants of coactivator LXXLL motif specificity in nuclear receptor transcriptional activation

Ligand-dependent activation of gene transcription by nuclear receptors is dependent on the recruitment of coactivators, including a family of related NCoA/SRC factors, via a region containing three helical domains sharing an LXXLL core consensus sequence, referred to as LXDs. In this manuscript, we report receptor-specific differential utilization of LXXLL-containing motifs of the NCoA-1/SRC-1 coactivator. Whereas a single LXD is sufficient for activation by the estrogen receptor, different combinations of two, appropriately spaced, LXDs are required for actions of the thyroid hormone, retinoic acid, peroxisome proliferator-activated, or progesterone receptors. The specificity of LXD usage in the cell appears to be dictated, at least in part, by specific amino acids carboxy-terminal to the core LXXLL motif that may make differential contacts with helices 1 and 3 (or 3') in receptor ligand-binding domains. Intriguingly, distinct carboxy-terminal amino acids are required for PPARgamma activation in response to different ligands. Related LXXLL-containing motifs in NCoA-1/SRC-1 are also required for a functional interaction with CBP, potentially interacting with a hydrophobic binding pocket. Together, these data suggest that the LXXLL-containing motifs have evolved to serve overlapping roles that are likely to permit both receptor-specific and ligand-specific assembly of a coactivator complex, and that these recognition motifs underlie the recruitment of coactivator complexes required for nuclear receptor function.

Signal-specific co-activator domain requirements for Pit-1 activation

POU-domain proteins, such as the pituitary-specific factor Pit-1, are members of the homeodomain family of proteins which are important in development and homeostasis, acting constitutively or in response to signal-transduction pathways to either repress or activate the expression of specific genes. Here we show that whereas homeodomain-containing repressors such as Rpx2 seem to recruit only a co-repressor complex, the activity of Pit-1 is determined by a regulated balance between a co-repressor complex that contains N-CoR/SMRT, mSin3A/B and histone deacetylases, and a co-activator complex that includes the CREB-binding protein (CBP) and p/CAF. Activation of Pit-1 by cyclic AMP or growth factors depends on distinct amino- and carboxy-terminal domains of CBP, respectively. Furthermore, the histone acetyltransferase functions of CBP or p/CAF are required for Pit-1 function that is stimulated by cyclic AMP or growth factors, respectively. These data show that there is a switch in specific requirements for histone acetyltransferases and CBP domains in mediating the effects of different signal-transduction pathways on specific DNA-bound transcription factors.

SAP30, a component of the mSin3 corepressor complex involved in N-CoR-mediated repression by specific transcription factors

The transcriptional corepressor mSin3 is found in a large multiprotein complex containing the histone deacetylases HDAC1 and HDAC2, in addition to at least five tightly associated polypeptides. We have cloned and characterized a novel component of the mSin3 complex, SAP30, SAP30 binds to mSin3 and is capable of mediating transcriptional repression via histone deacetylases. SAP30 also binds the N-CoR corepressor and is required for N-CoR-mediated repression by antagonist-bound estrogen receptor and the homeodomain protein Rpx, as well as N-CoR suppression of transactivation by the POU domain protein Pit-1. However, SAP30 is not required for N-CoR-mediated repression by unliganded retinoic acid receptor or thyroid hormone receptor, suggesting that SAP30 is involved in the functional recruitment of the mSin3-histone deacetylase complex to a specific subset of N-CoR corepressor complexes.

Diverse signaling pathways modulate nuclear receptor recruitment of N-CoR and SMRT complexes

Several lines of evidence indicate that the nuclear receptor corepressor (N-CoR) complex imposes ligand dependence on transcriptional activation by the retinoic acid receptor and mediates the inhibitory effects of estrogen receptor antagonists, such as tamoxifen, suppressing a constitutive N-terminal, Creb-binding protein/coactivator complex-dependent activation domain. Functional interactions between specific receptors and N-CoR or SMRT corepressor complexes are regulated, positively or negatively, by diverse signal transduction pathways. Decreased levels of N-CoR correlate with the acquisition of tamoxifen resistance in a mouse model system for human breast cancer. Our data suggest that N-CoR- and SMRT-containing complexes act as rate-limiting components in the actions of specific nuclear receptors, and that their actions are regulated by multiple signal transduction pathways.

Transcription factor-specific requirements for coactivators and their acetyltransferase functions

Different classes of mammalian transcription factors-nuclear receptors, cyclic adenosine 3',5'-monophosphate-regulated enhancer binding protein (CREB), and signal transducer and activator of transcription-1 (STAT-1)-functionally require distinct components of the coactivator complex, including CREB-binding protein (CBP/p300), nuclear receptor coactivators (NCoAs), and p300/CBP-associated factor (p/CAF), based on their platform or assembly properties. Retinoic acid receptor, CREB, and STAT-1 also require different histone acetyltransferase (HAT) activities to activate transcription. Thus, transcription factor-specific differences in configuration and content of the coactivator complex dictate requirements for specific acetyltransferase activities, providing an explanation, at least in part, for the presence of multiple HAT components of the complex.

A complex containing N-CoR, mSin3 and histone deacetylase mediates transcriptional repression

Transcriptional repression by nuclear receptors has been correlated to binding of the putative co-repressor, N-CoR. A complex has been identified that contains N-CoR, the Mad presumptive co-repressor mSin3, and the histone deacetylase mRPD3, and which is required for both nuclear receptor- and Mad-dependent repression, but not for repression by transcription factors of the ets-domain family. These data predict that the ligand-induced switch of heterodimeric nuclear receptors from repressor to activator functions involves the exchange of complexes containing histone deacetylases with those that have histone acetylase activity.

The therapeutic value of cranberries in treating and preventing urinary tract infections

Studies bearing on the therapeutic value of cranberries in the prevention and/or treatment of urinary tract infections are reviewed. Research results are grouped to address the following questions: 1) Does cranberry juice ingestion acidify urine? 2) Does cranberry juice inhibit the adherence of bacteria to bladder cells? 3) What research evidence exists to support that cranberry juice is an effective intervention for urinary tract infections? And 4) Does cranberry ingestion enhance antibiotic effectiveness?

Nuclear integration of JAK/STAT and Ras/AP-1 signaling by CBP and p300

We report that interferon gamma (IFN-gamma) inhibits transcription of the macrophage scavenger receptor gene by antagonizing the Ras-dependent activities of AP-1 and cooperating ets domain transcription factors, apparently as a result of competition between AP-1/ets factors and activated STAT1 for limiting amounts of CBP and p300. Consistent with this model, STAT1 alpha interacts directly with CBP in cells, and microinjection of anti-CBP and anti-p300 antibodies blocks transcriptional responses to IFN-gamma. Cells lacking STAT1 fail to inhibit AP-1/ets activity, and overexpression of CBP both potentiates IFN-gamma-dependent transcription and relieves AP-1/ets repression. Thus, CBP and p300 integrate both positive and negative effects of IFN-gamma on gene expression by serving as essential coactivators of STAT1 alpha, modulating gene-specific responses to simultaneous activation of two or more signal transduction pathways.

Inhibition of Raf-1 signaling by a monoclonal antibody, which interferes with Raf-1 activation and with Mek substrate binding

Raf-1 is a serine/threonine specific kinase that integrates signaling by a large number of mitogens to elicit a transcriptional response in the nucleus. Activated Raf-1 phosphorylates and activates MAPK/ERK kinase Mek), thus initiating the Mek--> MAP kinase cascade, which ultimately results in the phosphorylation and activation of transcription factors by MAP kinase. Here we have characterized the mechanism by which monoclonal antibody URP26K, which binds to an epitope in the Raf-1 kinase domain, inhibits intracellular signal transduction. This antibody preferentially immunoprecipitated the underphosphorylated, non-activated form of Raf-1 from quiescent cells. Baculovirus-expressed Raf-1 immunoprecipitated with URP26K was largely refractory to phosphorylation and activation mediated by protein kinase C (PKC)alpha or the tyrosine kinase Lck. In addition, URP26K reduced the binding of Raf-1 to its substrate Mek in vitro, but did not disturb the association of Raf-1 with Ras. Microinjection of URP26K into Rat-1 cells blocked DNA synthesis initiated by serum, insulin and various purified growth factors, but it did not block DNA synthesis initiated by v-ras. Microinjected URP26K also impaired the expression of stably transfected beta-galactosidase reporter genes regulated by minimal promoter elements. These results demonstrate, (i) that the URP26K monoclonal antibody inhibits Raf-1 by preventing activating Raf-1 phosphorylation and/or association with its substrate Mek, (ii) that inhibition of Raf-1 by URP26K does not interfere with Ras-induced DNA synthesis. In contrast to dominant negative Raf-1 mutants, which also block Ras signaling by binding to the Ras effector domain, antibody mediated Raf-1 inhibition thus reveals a branchpoint of mitogenic signaling at the level of Ras.