Platelet LGALS3BP as a Mediator of Myeloid Inflammation in Systemic Lupus Erythematosus

OBJECTIVE

Platelets are mediators of inflammation with immune effector cell properties and have been implicated in the pathogenesis of systemic lupus erythematosus (SLE). This study investigated the role of platelet-associated lectin, galactoside-binding, soluble 3 binding protein (LGALS3BP) as a mediator of inflammation in SLE and as a potential biomarker associated with clinical phenotypes.

METHODS

We performed RNA sequencing on platelets from patients with SLE (n = 54) and on platelets from age-, sex-, and race/ethnicity-matched healthy controls (n = 18) and measured LGALS3BP levels in platelet releasate and in circulating serum. We investigated the association between LGALS3BP levels and the prevalence, disease severity, and clinical phenotypes of SLE and studied platelet-mediated effects on myeloid inflammation.

RESULTS

Platelets from patients with SLE exhibited increased expression of LGALS3BP (fold change 4.0, adjusted P = 6.02 × 10-11 ). Platelet-released LGALS3BP levels were highly correlated with circulating LGALS3BP (R = 0.69, P < 0.0001), and circulating LGALS3BP levels were correlated with the severity of disease according to the SLE Disease Activity Index (r = 0.32, P = 0.0006). Specifically, circulating LGALS3BP levels were higher in SLE patients with lupus nephritis than in patients with inactive disease (4.0 μg/ml versus 2.3 μg/ml; P < 0.001). Interferon-α induced LGALS3BP transcription and translation in a megakaryoblastic cell line (MEG-01) in a dose-dependent manner. Recombinant LGALS3BP and platelet releasates from SLE patients enhanced proinflammatory cytokine production by macrophages.

CONCLUSIONS

Our results support that platelets act as potent effector cells that contribute to the pathogenesis of SLE by secreting proinflammatory LGALS3BP, which also represents a novel biomarker of SLE clinical activity.

Modeling of clinical phenotypes in systemic lupus erythematosus based on the platelet transcriptome and FCGR2a genotype

BACKGROUND

The clinical heterogeneity of SLE with its complex pathogenesis remains challenging as we strive to provide optimal management. The contribution of platelets to endovascular homeostasis, inflammation and immune regulation highlights their potential importance in SLE. Prior work from our group showed that the Fcγ receptor type IIa (FcγRIIa)-R/H131 biallelic polymorphism is associated with increased platelet activity and cardiovascular risk in SLE. The study was initiated to investigate the platelet transcriptome in patients with SLE and evaluate its association across FcγRIIa genotypes and distinct clinical features.

METHODS

Fifty-one patients fulfilling established criteria for SLE (mean age = 41.1 ± 12.3, 100% female, 45% Hispanic, 24% black, 22% Asian, 51% white, mean SLEDAI = 4.4 ± 4.2 at baseline) were enrolled and compared with 18 demographically matched control samples. The FCGR2a receptor was genotyped for each sample, and RNA-seq was performed on isolated, leukocyte-depleted platelets. Transcriptomic data were used to create a modular landscape to explore the differences between SLE patients and controls and various clinical parameters in the context of FCGR2a genotypes.

RESULTS

There were 2290 differentially expressed genes enriched for pathways involved in interferon signaling, immune activation, and coagulation when comparing SLE samples vs controls. When analyzing patients with proteinuria, modules associated with oxidative phosphorylation and platelet activity were unexpectedly decreased. Furthermore, genes that were increased in SLE and in patients with proteinuria were enriched for immune effector processes, while genes increased in SLE but decreased in proteinuria were enriched for coagulation and cell adhesion. A low-binding FCG2Ra allele (R131) was associated with decreases in FCR activation, which further correlated with increases in platelet and immune activation pathways. Finally, we were able to create a transcriptomic signature of clinically active disease that performed significantly well in discerning SLE patients with active clinical disease form those with inactive clinical disease.

CONCLUSIONS

In aggregate, these data demonstrate the platelet transcriptome provides insight into lupus pathogenesis and disease activity, and shows potential use as means of assessing this complex disease using a liquid biopsy.

P2Y12 Inhibition Suppresses Proinflammatory Platelet-Monocyte Interactions

BACKGROUND

 Monocyte-platelet aggregates (MPAs) represent the crossroads between thrombosis and inflammation, and targeting this axis may suppress thromboinflammation. While antiplatelet therapy (APT) reduces platelet-platelet aggregation and thrombosis, its effects on MPA and platelet effector properties on monocytes are uncertain.

OBJECTIVES

 To analyze the effect of platelets on monocyte activation and APT on MPA and platelet-induced monocyte activation.

METHODS

 Agonist-stimulated whole blood was incubated in the presence of P-selectin, PSGL1, PAR1, P2Y12, GP IIb/IIIa, and COX-1 inhibitors and assessed for platelet and monocyte activity via flow cytometry. RNA-Seq of monocytes incubated with platelets was used to identify platelet-induced monocyte transcripts and was validated by RT-qPCR in monocyte-PR co-incubation ± APT.

RESULTS

 Consistent with a proinflammatory platelet effector role, MPAs were increased in patients with COVID-19. RNA-Seq revealed a thromboinflammatory monocyte transcriptome upon incubation with platelets. Monocytes aggregated to platelets expressed higher CD40 and tissue factor than monocytes without platelets (p < 0.05 for each). Inhibition with P-selectin (85% reduction) and PSGL1 (87% reduction) led to a robust decrease in MPA. P2Y12 and PAR1 inhibition lowered MPA formation (30 and 21% reduction, p < 0.05, respectively) and decreased monocyte CD40 and TF expression, while GP IIb/IIIa and COX1 inhibition had no effect. Pretreatment of platelets with P2Y12 inhibitors reduced the expression of platelet-mediated monocyte transcription of proinflammatory SOCS3 and OSM. CONCLUSIONS:  Platelets skew monocytes toward a proinflammatory phenotype. Among traditional APTs, P2Y12 inhibition attenuates platelet-induced monocyte activation.

Diversity of Functionally Distinct Clonal Sets of Human Conventional Memory B Cells That Bind Staphylococcal Protein A

Staphylococcus aureus, a common cause of serious and often fatal infections, is well-armed with secreted factors that disarm host immune defenses. Highly expressed in vivo during infection, Staphylococcal protein A (SpA) is reported to also contribute to nasal colonization that can be a prelude to invasive infection. Co-evolution with the host immune system has provided SpA with an Fc-antibody binding site, and a Fab-binding site responsible for non-immune superantigen interactions via germline-encoded surfaces expressed on many human BCRs. We wondered whether the recurrent exposures to S. aureus commonly experienced by adults, result in the accumulation of memory B-cell responses to other determinants on SpA. We therefore isolated SpA-specific class-switched memory B cells, and characterized their encoding VH : VL antibody genes. In SpA-reactive memory B cells, we confirmed a striking bias in usage for VH genes, which retain the surface that mediates the SpA-superantigen interaction. We postulate these interactions reflect co-evolution of the host immune system and SpA, which during infection results in immune recruitment of an extraordinarily high prevalence of B cells in the repertoire that subverts the augmentation of protective defenses. Herein, we provide the first evidence that human memory responses are supplemented by B-cell clones, and circulating-antibodies, that bind to SpA determinants independent of the non-immune Fc- and Fab-binding sites. In parallel, we demonstrate that healthy individuals, and patients recovering from S. aureus infection, both have circulating antibodies with these conventional binding specificities. These findings rationalize the potential utility of incorporating specially engineered SpA proteins into a protective vaccine.

Hydroxychloroquine is associated with lower platelet activity and improved vascular health in systemic lupus erythematosus

OBJECTIVE

Hydroxychloroquine (HCQ) is a mainstay of therapy in the treatment of SLE. The effect of HCQ on platelets and vascular health is uncertain. We investigated the relationship between HCQ use and dose with platelet activity, platelet transcriptomics and vascular health in patients with SLE.

METHODS

Platelet aggregation, platelet mRNA expression and vascular health (sublingual capillary perfused boundary region (PBR), red blood cell filling (RBCF) and brachial artery reactivity testing) were analysed by HCQ use and dose.

RESULTS

Among 132 subjects with SLE (age: 39.7±12.9 years, 97% female), 108 were on HCQ. SLE disease activity was similar between subjects on and off HCQ. Platelet aggregation in response to multiple agonists was significantly lower in patients on HCQ. There were inverse relationships between HCQ dose and gene expression pathways of platelet activity. Gene expression of P-selectin (SELP) was inversely correlated with HCQ dose (r=-0.41, p=0.003), which was validated at the protein level. Subjects on HCQ had improved vascular function correlating with HCQ dose as measured by lower PBR (r=-0.52, p=0.007), higher RBCF (r=0.55, p=0.004) and greater brachial artery reactivity (r=0.43, p=0.056).

CONCLUSION

HCQ use was associated with decreased platelet activation and activation-related transcripts and improved vascular health in SLE.

Platelet-conditioned media induces an anti-inflammatory macrophage phenotype through EP4

BACKGROUND

Platelets are increasingly recognized as immune cells. As such, they are commonly seen to induce and perpetuate inflammation; however, anti-inflammatory activities are increasingly attributed to them. Atherosclerosis is a chronic inflammatory condition. Similar to other inflammatory conditions, the resolution of atherosclerosis requires a shift in macrophages to an M2 phenotype, enhancing their efferocytosis and cholesterol efflux capabilities.

OBJECTIVES

To assess the effect of platelets on macrophage phenotype.

METHODS

In several in vitro models employing murine (RAW264.7 and bone marrow-derived macrophages) and human (THP-1 and monocyte-derived macrophages) cells, we exposed macrophages to media in which non-agonized human platelets were cultured for 60 minutes (platelet-conditioned media [PCM]) and assessed the impact on macrophage phenotype and function.

RESULTS

Across models, we demonstrated that PCM from healthy humans induced a pro-resolving phenotype in macrophages. This was independent of signal transducer and activator of transcription 6 (STAT6), the prototypical pathway for M2 macrophage polarization. Stimulation of the EP4 receptor on macrophages by prostaglandin E2 present in PCM, is at least partially responsible for altered gene expression and associated function of the macrophages-specifically reduced peroxynitrite production, increased efferocytosis and cholesterol efflux capacity, and increased production of pro-resolving lipid mediators (ie, 15R-LXA4 ).

CONCLUSIONS

Platelet-conditioned media induces an anti-inflammatory, pro-resolving phenotype in macrophages. Our findings suggest that therapies targeting hemostatic properties of platelets, while not influencing pro-resolving, immune-related activities, could be beneficial for the treatment of atherothrombotic disease.

Human low-affinity IgG receptor FcγRIIA polymorphism H131R associates with subclinical atherosclerosis and increased platelet activity in systemic lupus erythematosus

Essentials Systemic lupus erythematosus (SLE) patients are at increased risk for premature CVD. Platelet activity, vascular dysfunction and carotid artery plaque are associated with FcγRIIA genotype in SLE. FcγRIIA genotype was not associated with platelet activity or carotid plaque in healthy controls. FcγRIIA represents a link that connects platelet activity, vascular health and CVD in SLE. SUMMARY: Background Systemic lupus erythematosus (SLE) is a complex autoimmune disease associated with an elevated risk of premature cardiovascular disease. Platelets express receptors contributing to inflammation and immunity, including FcγRIIA, the low affinity receptor of the Fc portion of IgG antibodies. The variation at a single amino acid substitution, H131R, in the extracellular binding domain alters the affinity for IgG, which may account for individual variation in platelet activity and platelet-mediated disease. Objectives This study was performed to investigate the association between FcγRIIA genotype, preclinical atherosclerosis, platelet reactivity and vascular health. Methods FcγRIIA was genotyped in 80 SLE patients and 30 healthy controls. Carotid ultrasound plaque, soluble E-selectin and platelet aggregability were evaluated in SLE and matched controls. Results Carotid plaque was significantly more prevalent in SLE patients carrying a variant allele compared to those with a homozygous ancestral allele (58% vs. 25%, P = 0.04). In contrast, prevalent carotid plaque was not associated with genotype in controls. Consistently, SLE variant FcγRIIA carriers vs. ancestral allele carriers had a significant increase in the levels of soluble E-selectin, which was not observed in controls. Monocyte and leukocyte-platelet aggregation and platelet aggregation in response to submaximal agonist stimulation were significantly elevated in SLE patients with the variant vs. ancestral genotype. Conclusions Carotid ultrasound plaque, soluble E-selectin levels and platelet activity were more frequently prevalent in SLE patients carrying variant FcγRIIA. The interplay between FcγRIIA-mediated platelet activation and endothelial cells might represent a mechanism underlying the pathogenesis of cardiovascular disease in SLE patients.

Unbiased RACE-Based Massive Parallel Surveys of Human IgA Antibody Repertoires

For investigations of human B-cell receptor (BCR) repertoires, we have developed a protocol for large-scale surveys of human antibody heavy chain (VH) rearrangements. Here we study IgA repertoires, as more IgA antibodies are synthesized in the human body on a daily level than all other isotypes combined. In fact, IgA is secreted at all mucosal surfaces, and it is also secreted in the perspiration that coats our cutaneous surfaces. In these studies we can characterize the IgA clonal diversity of B-cell populations obtained from any donor. To recover representative repertoire libraries, we make our libraries from antibody gene transcript templates (i.e., cDNA), as these are closer reflections of the immune repertoire expressed at the antibody protein level. To avoid biases potentially introduced by upstream oligonucleotide primers that hybridize to variable region framework regions, our approach also uses rapid amplification of cDNA ends (RACE) of antibody transcripts. For exploration of human IgA responses, we have designed a duplexing antisense constant region primer that efficiently amplifies, side-by-side, heavy chain transcripts of both the IgA1 and IgA2 subclasses. By these methods we have begun to define the molecular differences in the IgA1 and IgA2 responses occurring simultaneously in different donors. These methods will be used to investigate the effects of microbial virulence factors on host defenses, during autoimmune responses, and in B-cell malignancies.