POS0403 DYNAMIC CHANGES IN AUTOREACTIVE MEMORY B CELLS IN RHEUMATOID ARTHRITIS: KEY TO UNDERSTANDING IMMUNOLOGICAL DISEASE ACTIVITY

Background

Rheumatoid arthritis (RA) patients with autoantibodies against citrullinated antigens (ACPA) are characterized by poor chances to achieve sustained DMARD free remission. We have previously observed that autoreactive ACPA-expressing memory B cells (MBC) display an activated phenotype and frequently maintain this state of activation even in patients in clinical remission in whom joint inflammation is absent through treatment. Conceptually, these observations could reflect ongoing immunological activity. In fact, they indicate that most current therapeutic interventions do not induce immunological remission and could explain why disease frequently flares when treatment is stopped. ACPA-expressing MBC display the phenotype of recent germinal centre emigrants, actively proliferating (Ki-67 positive), expressing CD95 and co-stimulatory markers CD80 and CD86, and the signature cytokine IL-8.[1] Notably, in autoantibody-positive individuals with arthralgia that did not progress to inflammatory arthritis within 2 years, the autoreactive B cells were readily detectable in the circulation and proliferated but lacked, in most cases, the upregulation of CD80 seen at the onset of disease. Now, we studied these autoreactive B cells longitudinally in the ‘at-risk’ phase of clinically suspect arthralgia (CSA) and in a population of patients in sustained (>1 year) drug-free remission (SDFR).

Objectives

To define dynamic changes of the ACPA-expressing B cell compartment throughout clinical disease stages with the aim to evaluate this response as a possibly predictive biomarker of immunological disease activity.

Methods

ACPA-expressing MBC were identified by flow cytometry in peripheral blood of RA patients with chronic disease or at disease-onset (treatment-naïve), in ACPA-positive CSA patients ‘at-risk’ for RA, and in patients in SDFR. B cells were characterized by a combination of markers to classify B cell subpopulations and activation- and germinal centre related markers. Tetanus-toxoid-specific B cells were analysed in the same individuals as antigen-specific comparators.

Results

In patients in SDFR, significantly less autoreactive B cells expressed CD80 compared to active disease, while they persistently showed signs of proliferation, similar to the phenotype observed in CSA patients that did not progress to RA. ACPA-specific plasmablasts (defined as CD20-CD27hi) were practically absent in SDFR and CSA non-progressors, while they were readily detectable in patients with RA at disease onset, during treatment, and in CSA-individuals that later progressed to RA. Notably, two arthralgia patients that developed RA within one year showed high CD80 expression in the autoreactive MBC compartment already in the ‘at-risk’ phase, whereas patients that so far did not progress to RA during the time of follow-up, showed less CD80-positive autoreactive MBC.

Conclusion

The phenotype of autoreactive B cells is dynamic in different disease phases of RA. By studying extremes of clinical phenotypes (‘at-risk’ phase, disease onset, SDFR), we found evidence for dynamic expression of CD80 by ACPA-expressing MBC in relation to clinical disease stage. Intriguingly, CD80-positive B cells, but not CD80-negative B cells, have been shown to be able to differentiate into antibody secreting cells in mice.[2] Our finding that plasmablasts were practically absent in ACPA-positive arthralgia patients that did not progress to RA and in SDFR would fit with this notion and suggests that in SDFR autoreactive MBC lack, although actively proliferating, triggers that induce CD80 upregulation, differentiation to plasmablasts and, possibly, their active participation in processes causing joint inflammation. Based on this observation, it is possible that induction of this CD80lo phenotype in autoreactive MBC may be an important step towards achieving immunological remission, a conceptual proxy for cure.

References

[1]H. Kristyanto et al., Sci Transl Med12, (2020).

[2]G. V. Zuccarino-Catania et al., Nat Immunol15, 631-637 (2014).

Disclosure of Interests

None declared.

AB0022 AUTOREACTIVE B CELLS IN RHEUMATOID ARTHRITIS DISPLAY AN ACTIVATED PHENOTYPE OF RECENT ANTIGEN EXPOSURE

Background:

Rheumatoid arthritis, in particular ACPA+ RA, is characterized by frequent disease flares and poor chances to achieve DMARD-free sustained remission. Recently, we have shown that ACPA-expressing memory B cells (MBC) remain in a persistently activated state throughout disease, even in patients in DMARD-induced clinical remission.(1) The reasons why the ACPA B cell response is continuously activated are unknown, as well as why the response does not revert to a resting, ‘quiescent’ state. We hypothesized that continuous antigen exposure in germinal centres drives ACPA B cell activation, leading to a ‘recent germinal centre emigrant’ phenotype of these cells in the circulation.

Objectives:

To understand whether the activated phenotype of ACPA-expressing B cells could be induced by recent antigen exposure, to thereby discern the processes of immune activation that remain active in patients even in clinical remission and to argue whether these processes could be targets for therapeutic intervention.

Methods:

ACPA-expressing B cells were identified in peripheral blood of RA patients by flow cytometry during different stages of disease and characterized by a panel of activation- and germinal centre related markers (CD80, CD86, CD32, CD95, Ki-67). In addition, three healthy donors received a TT booster vaccination. TT-specific MBC were identified in blood at different timepoints (before vaccination and up to 22 weeks after vaccination) and analysed phenotypically over time.

Results:

The majority of ACPA-expressing B cells strongly expressed CD95 and the co-stimulatory marker CD80. A part was also positive for the proliferation marker Ki-67 (on average 30%), and most cells downregulated the inhibitory marker CD32. TT-specific MBC adopted a comparable phenotype after booster vaccination, but most markers returned to the pre-vaccination expression level gradually over time. These effects were antigen-dependent because the phenotype of TT-negative B cells remained unchanged. The phenotypic composition of the proliferating ACPA-positive B cell pool most closely corresponded to a stimulation history of 1-2 weeks after antigen exposure. Notably, none of the Ki-67 negative ACPA-specific MBC showed phenotypic quiescence, indicating either a short life-time (in circulation) after antigen encounter or persistent additional factors of activation.

Figure 1.

Ki-67 expression on ACPA-specific MBC in RA (A) and on TT-specific MBC in 3 healthy donors before and after booster vaccination (B).

Conclusion:

ACPA-expressing MBC phenotypically resemble TT-specific MBC after recent (1-2 weeks) booster vaccination, reflecting the phenotype of recent germinal centre emigrants, and remain activated, whereas TT-specific MBC lose this marker profile over time. These observations suggest that ACPA-expressing MBC either home to tissue or survive shortly in the circulation, or that additional factors drive or program these cells to persistent activation. Transcriptomic profiling and analysis of the homing marker profile may help to answer these questions. Furthermore, it will be important to understand the association of persistent activation of ACPA-expressing B cells in clinical remission and the risk for disease flares upon treatment discontinuation.

References:

[1]Kristyanto H, Blomberg NJ, Slot LM, van der Voort EIH, Kerkman PF, Bakker A, et al. Persistently activated, proliferative memory autoreactive B cells promote inflammation in rheumatoid arthritis. Sci Transl Med. 2020;12(570).

Disclosure of Interests:

Nienke Blomberg: None declared, Hendy Kristyanto: None declared, Thomas Huizinga Grant/research support from: Gilead, Rene Toes: None declared, Hans Ulrich Scherer Grant/research support from: Pfizer, Lilly, Sanofi, BMS

Towards a European health research and innovation cloud (HRIC)

The European Union (EU) initiative on the Digital Transformation of Health and Care (Digicare) aims to provide the conditions necessary for building a secure, flexible, and decentralized digital health infrastructure. Creating a European Health Research and Innovation Cloud (HRIC) within this environment should enable data sharing and analysis for health research across the EU, in compliance with data protection legislation while preserving the full trust of the participants. Such a HRIC should learn from and build on existing data infrastructures, integrate best practices, and focus on the concrete needs of the community in terms of technologies, governance, management, regulation, and ethics requirements. Here, we describe the vision and expected benefits of digital data sharing in health research activities and present a roadmap that fosters the opportunities while answering the challenges of implementing a HRIC. For this, we put forward five specific recommendations and action points to ensure that a European HRIC: i) is built on established standards and guidelines, providing cloud technologies through an open and decentralized infrastructure; ii) is developed and certified to the highest standards of interoperability and data security that can be trusted by all stakeholders; iii) is supported by a robust ethical and legal framework that is compliant with the EU General Data Protection Regulation (GDPR); iv) establishes a proper environment for the training of new generations of data and medical scientists; and v) stimulates research and innovation in transnational collaborations through public and private initiatives and partnerships funded by the EU through Horizon 2020 and Horizon Europe.

[A Randomized Controlled Trial on the Effect of Earmuffs on Intermittent Hypoxia and Bradycardia in Preterm Infants]

BACKGROUND

Irregular breathing causing hypoxia and bradycardia is a common problem of preterm infants but its pathophysiology is incompletely understood. Agitation provoked by environmental noise may play a role. We wanted to know if earmuffs can at least halve the rate of intermittent hypoxia in premies.

PATIENTS AND METHODS

In this randomized controlled trial 31 infants (14 male; median [min.-max.] birth weight and gestational age: 1 323 g [560-1 990] and 30(1/7) weeks [25(5/7)-33(0/7)]) had the effect of earmuffs on the frequency of pulse oximeter desaturations (SpO(2) <80%) and bradycardia events (<80 beats per minute) tested, documented via a standard home monitor. Infants were measured 2 h each with or without earmuffs; the sequence of intervention was randomised. Measurement conditions were kept constant while a noise meter recorded sound pressure levels at a 1 Hz sampling rate.

RESULTS

Median sound pressure level was 46.8 dB(A). In a pre-study, ear muffs yielded a sound reduction by 7.2 dB. 19 infants had a total of 474 desaturations. The median (25.-75. percentile) number of desaturations was 1 (1-10.5) without, and 1 (1-10) with earmuffs. The amount of infants with at least one desaturation was equal in both treatment protocols. Only 7 bradycardias occurred.

CONCLUSION

The earmuffs had, in a rather quiet environment, no effect on intermittent hypoxia in these infants.

Structure of the PPARalpha and -gamma ligand binding domain in complex with AZ 242; ligand selectivity and agonist activation in the PPAR family

BACKGROUND

The peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors belonging to the nuclear receptor family. The roles of PPARalpha in fatty acid oxidation and PPARgamma in adipocyte differentiation and lipid storage have been characterized extensively. PPARs are activated by fatty acids and eicosanoids and are also targets for antidyslipidemic drugs, but the molecular interactions governing ligand selectivity for specific subtypes are unclear due to the lack of a PPARalpha ligand binding domain structure.

RESULTS

We have solved the crystal structure of the PPARalpha ligand binding domain (LBD) in complex with the combined PPARalpha and -gamma agonist AZ 242, a novel dihydro cinnamate derivative that is structurally different from thiazolidinediones. In addition, we present the crystal structure of the PPARgamma_LBD/AZ 242 complex and provide a rationale for ligand selectivity toward the PPARalpha and -gamma subtypes. Heteronuclear NMR data on PPARalpha in both the apo form and in complex with AZ 242 shows an overall stabilization of the LBD upon agonist binding. A comparison of the novel PPARalpha/AZ 242 complex with the PPARgamma/AZ 242 complex and previously solved PPARgamma structures reveals a conserved hydrogen bonding network between agonists and the AF2 helix.

CONCLUSIONS

The complex of PPARalpha and PPARgamma with the dual specificity agonist AZ 242 highlights the conserved interactions required for receptor activation. Together with the NMR data, this suggests a general model for ligand activation in the PPAR family. A comparison of the ligand binding sites reveals a molecular explanation for subtype selectivity and provides a basis for rational drug design.

Structure of a PH domain from the C. elegans muscle protein UNC-89 suggests a novel function

BACKGROUND

Pleckstrin homology (PH) domains constitute a structurally conserved family present in many signaling and regulatory proteins. PH domains have been shown to bind to phospholipids, and many function in membrane targeting. They generally have a strong electrostatic polarization and interact with negatively charged phospholipids via the positive pole. On the basis of electrostatic modeling, however, we have previously identified a class of PH domains with a predominantly negative charge and predicted that these domains recognize other targets. Here, we report the first experimental structure of such a PH domain.

RESULTS

The structure of the PH domain from Caenorhabditis elegans muscle protein UNC-89 has been determined by heteronuclear NMR. The domain adopts the classic PH fold, but has an unusual closed conformation of the "inositol binding loops. This creates a small opening to a deep hydrophobic pocket lined with negative charges on one side, and provides a molecular explanation for the lack of association with inositol-1,4,5-triphosphate. As predicted, the PH domain of UNC-89 has a strongly negative overall electrostatic potential. Modeling the Dbl homology (DH)-linked PH domains from the C. elegans genome shows that a large proportion of these modules are negatively charged.

CONCLUSIONS

We present the first structure of a PH domain with a strong negative overall electrostatic potential. The presence of a deep pocket lined with negative charges suggests that the domain binds to ligands other than acidic phospholipids. The abundance of this class of PH domain in the C. elegans genome suggests a prominent role in mediating protein-protein interactions.

A new approach for applying residual dipolar couplings as restraints in structure elucidation

Residual dipolar couplings are useful global structural restraints. The dipolar couplings define the orientation of a vector with respect to the alignment tensor. Although the size of the alignment tensor can be derived from the distribution of the experimental dipolar couplings, its orientation with respect to the coordinate system of the molecule is unknown at the beginning of structure determination. This causes convergence problems in the simulated annealing process. We therefore propose a protocol that translates dipolar couplings into intervector projection angles, which are independent of the orientation of the alignment tensor with respect to the molecule. These restraints can be used during the whole simulated annealing protocol.

Classification of protein sequences by homology modeling and quantitative analysis of electrostatic similarity

Protein electrostatics plays a key role in ligand binding and protein-protein interactions. Therefore, similarities or dissimilarities in electrostatic potentials can be used as indicators of similarities or dissimilarities in protein function. We here describe a method to compare the electrostatic properties within protein families objectively and quantitatively. Three-dimensional structures are built from database sequences by comparative modeling. Molecular potentials are then computed for these with a continuum solvation model by finite difference solution of the Poisson-Boltzmann equation or analytically as a multipole expansion that permits rapid comparison of very large datasets. This approach is applied to 104 members of the Pleckstrin homology (PH) domain family. The deviation of the potentials of the homology models from those of the corresponding experimental structures is comparable to the variation of the potential in an ensemble of structures from nuclear magnetic resonance data or between snapshots from a molecular dynamics simulation. For this dataset, the results for analysis of the full electrostatic potential and the analysis using only monopole and dipole terms are very similar. The electrostatic properties of the PH domains are generally conserved despite the extreme sequence divergence in this family. Notable exceptions from this conservation are seen for PH domains linked to a Db1 homology (DH) domain and in proteins with internal PH domain repeats.

The PH superfold: a structural scaffold for multiple functions

Pleckstrin homology (PH) domains form a structurally conserved family that is associated with many regulatory pathways within the cell. Domains with a nearly identical fold are found in other families that share no sequence similarity, suggesting the existence of a stable PH superfold. The PH domains generally function as regulated membrane-binding modules that bind to inositol lipids and respond to upstream signals by targeting the host proteins to the correct cellular sites. The other domains with a similar fold, such as the phosphotyrosine binding domains, recognize protein ligands.

Functional diversity of PH domains: an exhaustive modelling study

BACKGROUND

Pleckstrin homology (PH) domains are found in many proteins involved in signal transduction or cytoskeletal organization. The general function for the domain is still unclear; phospholipid binding of some PH domains and a strong electrostatic polarization in the experimental structures suggest a role in localization on membranes. We have analyzed the electrostatic properties and the spatial amino acid distribution from homology models of the entire PH domain family.

RESULTS

Despite the sequence divergence, the quality of the models is sufficient for our study. Most PH domains have an electrostatic polarization similar to the experimental structures. but roughly half of the PH domains linked to a Dbl homology domain have very different electrostatic properties. We also found a striking electrostatic complementarity in some internal PH domain repeats. The analysis of the spatial distribution of amino acids identified residues in the phospholipid-binding site of the spectrin and dynamin PH domains as specific for these domains.

CONCLUSIONS

The mostly conserved electrostatic polarization supports a general function in binding to phospholipid membranes. But the presence of PH domains with opposite polarity suggests that ligands and functions have diverged during evolution. We also demonstrate homology modelling as a general sequence analysis tool that can yield significantly more information than conventional analysis.