Recombinant Human V-set and transmembrane domain-containing protein 1 (VSTM1)

What is VSTM1 and what are its key structural characteristics?

VSTM1 (V-set and transmembrane domain-containing protein 1) is a single-pass membrane protein encoded by the VSTM1 gene in humans. It contains one Ig-like V-type domain and functions as an inhibitory immune receptor involved in the regulation of phagocytes . The protein structure includes extracellular, transmembrane, and intracellular domains, with the latter containing immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that serve as docking sites for SHP1 and SHP2 phosphatases when phosphorylated . Recombinant human VSTM1 typically spans amino acids Tyr17-Thr135, with a molecular mass of 14.6 kDa, though it appears as 25-35 kDa in SDS-PAGE analysis due to post-translational modifications .

Which cell types express VSTM1 and what is its distribution pattern?

VSTM1 is predominantly expressed on human myeloid cells, including neutrophils, eosinophils, and monocytes, but not on lymphoid cells . The membrane-bound form, also known as Signal inhibitory receptor on leukocytes-1 (SIRL-1), serves as an immune inhibitory receptor . The expression pattern is consistent with its role in regulating innate immune responses, particularly in neutrophil function. This selective expression on myeloid lineage cells suggests specialized functions in innate immunity rather than adaptive immune responses.

What are the known isoforms of VSTM1 and how do they differ?

There are at least two recognized isoforms of VSTM1:

• SIRL-1 (membrane-bound form): Contains transmembrane and cytoplasmic domains with ITIMs that mediate inhibitory signaling .

• VSTM1-v2 (soluble isoform): Lacks the exon encoding the transmembrane domain of SIRL-1 and is therefore predicted to be secreted . Additionally, activated neutrophils can shed the ectodomain of SIRL-1, releasing soluble SIRL-1 (sSIRL-1) .

The functional differences between these isoforms remain under investigation, with contradictory findings regarding VSTM1-v2's role in Th17 cell differentiation .

How is VSTM1 involved in immune regulation?

VSTM1 functions as an immune inhibitory receptor that helps maintain immune system balance . Upon ligation and subsequent phosphorylation, VSTM1 recruits SHP1 and SHP2 phosphatases that negatively regulate cellular activation . Specifically, VSTM1 restricts essential immune responses such as Fc receptor-induced reactive oxygen species (ROS) production and increased cellular respiration . Research has demonstrated that VSTM1 deficiency results in both increased ROS levels and decreased cell viability, whereas VSTM1 activation via agonist antibodies preserves neutrophil viability by suppressing ROS production .

What is the current understanding of VSTM1-v2's role in Th17 cell differentiation, and how do recent findings challenge earlier research?

The role of VSTM1-v2 in Th17 cell differentiation has become controversial. While earlier research by Guo et al. suggested that VSTM1-v2 functions as a Th17 polarizing cytokine capable of inducing Th17 cell differentiation and activation, recent studies have directly challenged these findings .

Recent experiments investigated VSTM1-v2's effect on both dendritic cell (DC)-driven and cytokine-driven Th17 cell development. Results showed that:

• Neutrophils induced DC-driven Th17 cell differentiation, but this was not enhanced by VSTM1-v2 .

• VSTM1-v2 had no effect on cytokine-driven Th17 cell development .

• No binding of VSTM1-v2 to lymphocytes, monocytes, or granulocytes was observed .

How does VSTM1 signaling affect neutrophil function and viability in inflammatory conditions?

VSTM1 signaling significantly influences neutrophil function and viability, particularly in inflammatory settings. Research indicates that:

• VSTM1 deficiency leads to robustly increased ROS levels and decreased cell viability in neutrophils .

• Activation of VSTM1 using agonist antibodies preserves neutrophil viability by suppressing ROS production .

• SIRL-1 ligation inhibits Th17 cell development, potentially by suppressing neutrophil degranulation and release of neutrophil elastase .

• The redox status of peripheral blood neutrophils in SLE patients positively correlates with disease activity and neutropenia .

These findings suggest that VSTM1 plays a protective role for neutrophils in inflammatory conditions by modulating ROS production and preventing excessive neutrophil activation and subsequent cell death. This mechanism appears specific to neutrophil subsets rather than affecting other immune cells such as monocytes, B cells, or T cells .

What is known about potential ligands for VSTM1, and what are the challenges in identifying them?

The natural ligand(s) of VSTM1 remain largely unknown, presenting a significant knowledge gap in understanding its biological function . Recent research has explored several potential candidates:

• S100 proteins and LL-37 have been proposed as potential ligands for SIRL-1, though direct binding could not be detected, possibly due to low affinity interactions or the requirement for additional binding partners .

• Xu et al. investigated whether oxidized galectin-1 (Gal-1) in SLE patients might bind to VSTM1 but found that it does not, suggesting other molecules must serve as the physiological ligand(s) .

• In vitro pull-down assays have been employed to search for potential VSTM1-binding partners .

The challenges in identifying VSTM1 ligands include:

• Potentially low-affinity interactions that are difficult to detect with conventional binding assays

• The possible requirement for co-receptors or additional binding partners

• The dynamic nature of ligand-receptor interactions in different inflammatory contexts

• Technical limitations in recreating physiological conditions in vitro

What is the relationship between VSTM1 expression and autoimmune diseases?

There is emerging evidence connecting VSTM1 expression with autoimmune conditions:

• VSTM1-v2 mRNA expression has been reported to be increased in peripheral blood mononuclear cells (PBMCs) of rheumatoid arthritis patients compared to controls .

• A positive correlation between VSTM1-v2 mRNA and IL-17 mRNA expression has been observed in these patients .

• In systemic lupus erythematosus (SLE), research has examined the relationship between VSTM1 and neutrophil ROS production, finding that the redox status of peripheral blood neutrophils positively correlates with disease activity and neutropenia .

• Soluble SIRL-1 (sSIRL-1) concentrations were found to be increased in COVID-19 and RSV bronchiolitis patients, though this may represent a combination of shed SIRL-1 ectodomain and VSTM1-v2 .

While these associations suggest potential roles in disease pathogenesis, the exact mechanisms underlying these relationships require further investigation.

What are the optimal methods for producing and purifying recombinant VSTM1 protein?

Based on established protocols, the following approach is recommended for producing and purifying recombinant VSTM1:

• Expression System: Mammalian expression systems are preferred for producing human VSTM1 to ensure proper post-translational modifications. Human cell lines have been successfully used to express the protein encompassing amino acids Tyr17-Thr135 .

• Fusion Tags: A C-terminal 6xHis tag facilitates purification while minimizing interference with protein function .

• Purification Process:

  • Initial capture using nickel affinity chromatography

  • Secondary purification via size exclusion chromatography

  • Quality assessment by SDS-PAGE (>95% purity standard)

• Endotoxin Removal: Critical for functional studies, endotoxins should be removed using Triton X-114 and SM-2 beads, with validation using the PyroGene™ Recombinant Factor C Assay to confirm levels <1 EU/mL .

• Storage Considerations: Lyophilized protein can be stored at -20 to -80°C for up to 12 months. Once reconstituted, the solution remains stable at 4-8°C for 2-7 days, with aliquots stable at <-20°C for 3 months .

How can researchers effectively design experiments to study VSTM1's role in Th17 cell differentiation?

When designing experiments to investigate VSTM1's role in Th17 cell differentiation, consider the following methodological approach based on recent research:

• Co-culture Systems:

  • For DC-driven Th17 differentiation: Use naive CD4+ T cells, Candida albicans-activated monocyte-derived dendritic cells (moDCs), and neutrophils .

  • For memory T cell activation: Co-culture memory CD4+ T cells with C. albicans-activated moDCs .

• Cytokine-Driven System:

  • Activate total CD4+ T cells with anti-CD3, anti-CD28, and a Th17 polarizing cytokine mix .

• Critical Controls:

  • Include conditions with and without neutrophils to demonstrate their requirement

  • Use appropriate recombinant protein controls (e.g., sLAIR-1 ecto, which has 30% sequence identity with VSTM1-v2)

  • Include multiple time points for analysis (both early [72h] and late [10-12 days])

• Assessment Methods:

  • Flow cytometry to quantify IL-17+ T cells

  • ELISA to measure cytokine production

  • Binding assays using fluorescently labeled anti-His to detect protein interactions

• Validation Approaches:

  • Test binding of VSTM1-v2 to various leukocyte populations in erythrocyte-lysed whole blood to rule out competition effects

  • Perform parallel experiments using both DC-driven and cytokine-driven systems to comprehensively assess effects

Sample experimental design table:

What techniques are most effective for detecting endogenous VSTM1-v2 expression?

Detection of endogenous VSTM1-v2 presents several challenges, as it has primarily been identified at the mRNA level rather than as a protein . Based on research findings, the following approaches are recommended:

• RNA-based Detection:

  • RT-PCR with isoform-specific primers to distinguish VSTM1-v2 from SIRL-1

  • RNA-sequencing for comprehensive isoform analysis

  • qPCR for quantitative assessment of expression levels in different cell types

• Protein-based Detection:

  • Development of isoform-specific antibodies that can distinguish VSTM1-v2 from shed SIRL-1 ectodomain

  • ELISA systems capable of detecting soluble VSTM1 forms (existing ELISAs can recognize both shed SIRL-1 and VSTM1-v2)

  • Western blotting with appropriate controls to distinguish between isoforms

• Cell-specific Expression Analysis:

  • Single-cell RNA sequencing to identify specific cellular sources

  • Flow cytometry after cell permeabilization using isoform-specific antibodies

  • Immunohistochemistry on tissue sections to determine anatomical distribution

• Validation Strategies:

  • Use recombinant VSTM1-v2 as a positive control

  • Include samples from various inflammatory conditions (e.g., rheumatoid arthritis) where VSTM1-v2 mRNA has been reported to be elevated

  • Confirm specificity using VSTM1 knockout models or knockdown approaches

How can researchers investigate VSTM1's effects on neutrophil ROS production and viability?

To investigate VSTM1's effects on neutrophil ROS production and viability, researchers should consider the following methodological approaches:

Sample experimental workflow:

How should researchers reconcile contradictory findings regarding VSTM1-v2's role in Th17 cell differentiation?

The contradictory findings between studies by Guo et al. and more recent research regarding VSTM1-v2's role in Th17 cell differentiation require careful consideration of several factors:

• Experimental Timelines:

  • The original study measured IL-17 release after 72 hours of stimulation

  • More recent research evaluated IL-17 expression after 10-12 days of culture

  • Consider that earlier timepoints might reflect activation of pre-existing Th17 cells rather than de novo differentiation

• Readout Differences:

  • Examine whether studies measured percentage of IL-17+ cells versus total IL-17 production

  • Different metrics may capture distinct biological phenomena

• Protein Quality and Verification:

  • Ensure molecular weight verification (similar molecular weight on SDS-PAGE as reported by Guo et al.)

  • Confirm endotoxin removal, as contamination could affect results

  • Verify biological activity using established positive controls

• Experimental Design Variations:

  • Different compositions of Th17 polarizing cytokine mixes

  • Variations in T cell purification methods

  • Presence of additional cell types that might influence the outcome

• Reproducibility Analysis:

  • Implement systematic replication of key experimental conditions

  • Consider multi-laboratory validation studies

  • Evaluate statistical power and sample sizes

When reconciling these contradictions, researchers should design comprehensive experiments that systematically address these variables, potentially including conditions that replicate both published protocols side-by-side.

What are the implications of VSTM1's role in neutrophil function for inflammatory and autoimmune disease research?

The role of VSTM1 in regulating neutrophil function has significant implications for inflammatory and autoimmune disease research:

• Neutrophil Survival and Inflammation Resolution:

  • VSTM1 activation preserves neutrophil viability by suppressing ROS production

  • This suggests VSTM1 agonists could potentially limit neutrophil-mediated tissue damage in acute inflammation

  • Conversely, VSTM1 deficiency might contribute to enhanced neutrophil death and subsequent inflammatory responses

• Autoimmune Disease Pathogenesis:

  • The positive correlation between redox status of peripheral blood neutrophils in SLE patients with disease activity and neutropenia suggests VSTM1 dysfunction may contribute to disease mechanisms

  • Elevated VSTM1-v2 mRNA in rheumatoid arthritis PBMCs indicates potential involvement in chronic inflammatory conditions

• Therapeutic Targeting Potential:

  • VSTM1 represents a myeloid-specific target for modulating neutrophil function

  • VSTM1 agonists could potentially limit neutrophil-mediated tissue damage

  • Inhibitors of VSTM1-v2 might be explored if its role in promoting inflammation is confirmed

• Biomarker Development:

  • Soluble SIRL-1/VSTM1-v2 levels might serve as disease activity markers

  • Increased concentrations in COVID-19 and RSV bronchiolitis patients suggest relevance beyond autoimmunity

This understanding invites development of therapeutic strategies that modulate VSTM1 function to control neutrophil activity in inflammatory diseases.

How can researchers determine whether VSTM1-v2 protein is expressed endogenously in biological samples?

Determining whether VSTM1-v2 protein is expressed endogenously requires a multi-faceted approach:

• Transcriptional Evidence:

  • Confirm presence of VSTM1-v2 mRNA using isoform-specific primers

  • Quantify expression levels relative to housekeeping genes

  • Compare with membrane-bound VSTM1/SIRL-1 transcript levels

• Protein Detection Strategy:

  • Develop isoform-specific antibodies that can distinguish VSTM1-v2 from shed SIRL-1 ectodomain

  • Current ELISAs can recognize both recombinant VSTM1-v2 and shed SIRL-1, making it difficult to differentiate between them

  • Consider immunoprecipitation followed by mass spectrometry to confirm protein identity

• Biological Samples to Analyze:

  • Serum/plasma from healthy individuals and disease states (particularly rheumatoid arthritis where mRNA has been reported elevated)

  • Cell culture supernatants from myeloid cells

  • Cell lysates to detect intracellular protein prior to secretion

• Controls and Validation:

  • Use recombinant VSTM1-v2 as a positive control

  • Include samples from VSTM1 knockout models as negative controls

  • Perform parallel analysis of shed SIRL-1 ectodomain to establish differential detection methods

As noted in the research, VSTM1-v2 has thus far been identified primarily at the mRNA level, with protein expression remaining to be conclusively demonstrated . Future research requires VSTM1-v2-specific antibodies to enable investigation of endogenous protein expression.

What are the most promising future research directions for VSTM1 in immune regulation?

Several promising research directions emerge from current understanding of VSTM1 in immune regulation:

• Ligand Identification: Determining the natural ligand(s) for VSTM1 remains a critical priority. Pull-down assays, receptor-ligand interaction screens, and proteomic approaches may help identify these binding partners .

• Isoform-Specific Function: Developing tools to distinguish between membrane-bound SIRL-1 and soluble VSTM1-v2 will enable more precise understanding of their respective roles. This includes generating isoform-specific antibodies and establishing knockout models that selectively target each form .

• Therapeutic Targeting: Exploring VSTM1-targeting strategies for inflammatory conditions where neutrophil dysregulation plays a role. This includes development of agonistic antibodies to suppress neutrophil ROS production and enhance viability in inflammatory settings .

• Disease Associations: Further investigating the relationship between VSTM1 expression/function and autoimmune diseases, particularly rheumatoid arthritis and SLE where preliminary associations have been observed .

• Signaling Mechanisms: Elucidating the full signaling pathway downstream of VSTM1, beyond SHP1/SHP2 recruitment, to identify potential intervention points for therapeutic development .

The contradictory findings regarding VSTM1-v2's role in Th17 cell differentiation also warrant systematic investigation to resolve these discrepancies and establish a definitive understanding of its immunomodulatory functions .

What are the key technical challenges that must be overcome to advance VSTM1 research?

Several technical challenges currently limit progress in VSTM1 research:

• Isoform Discrimination: Current detection methods cannot reliably distinguish between shed SIRL-1 ectodomain and VSTM1-v2 . Development of isoform-specific antibodies and assays is essential for accurately studying each form's distinct functions.

• Ligand Identification: The natural ligand(s) for VSTM1 remain unknown . Low-affinity interactions or requirements for additional binding partners complicate their identification using conventional methods.

• Protein Expression Verification: While VSTM1-v2 has been detected at the mRNA level, confirmation of endogenous protein expression has not been conclusively demonstrated . This requires development of sensitive and specific detection methods.

• Functional Assay Standardization: The contradictory findings regarding VSTM1-v2's role in Th17 cell differentiation highlight the need for standardized functional assays . Variations in experimental timeline, readouts, and conditions must be systematically addressed.

• Physiological Relevance: Translating in vitro findings to in vivo significance requires appropriate animal models, which is complicated by potential species differences in VSTM1 function and expression.