Recombinant Bovine Cell surface glycoprotein CD200 receptor 1 (CD200R1)

What is the basic structure of bovine CD200R1?

Bovine CD200R1 is a type-1 cell membrane glycoprotein belonging to the immunoglobulin supergene family. The recombinant form typically encompasses amino acids 27-355, with the extracellular domain composed of one Ig-like V-type domain and one Ig-like C2-type domain . The full sequence of the extracellular portion is: AGMEGTKTSNNSMQQLDNGNHSSVSTTSSTERKQSTVTLYAEVKTSLSVLVDTKAVLTCPPVLWPSVLVVTWEIVLRDKPPCFGAYRRDTNQTTRGNCTDKRITWASRPDENPALQVDPVAITHDGNYTCQIVTSDGNFHHEYHLQVLVPPEVTLIQTEKGTAVCKAAAGKPAAQISWTPEGDCDTEQGPYWGDGTVTVQSTCRWGSRHVLNVSCSVSHLAGNKSLSIQLSQGAEIPAHLKNLYITAPIFIILIVVGSIWLLKISGCRKCKLKKTEHTPVVQEDEMEPYASYTEKNNPLYDITNRVKTSQVLQSEVDGMNLHTIYVPRV .

How does CD200R1 signaling function at the molecular level?

CD200R1 signaling involves a unique mechanism distinct from conventional inhibitory receptors. Unlike most immune inhibitory receptors, CD200R1 does not possess a conventional immunoreceptor tyrosine-based inhibitory motif (ITIM) . Upon CD200/CD200R1 interaction, phosphorylation occurs at conserved tyrosine residues in the cytoplasmic tail. Particularly, Y297 situated within an NPxY signaling motif is crucial . This phosphorylation recruits the inhibitory adaptor proteins Dok1 and Dok2, which become phosphorylated and subsequently inhibit Ras/MAPK activation . The Dok2/RasGAP complex inhibits Ras activation, disrupting signaling through the Ras/MAPK pathway and suppressing cytokine production, particularly in myeloid cells .

What functional assays can verify the biological activity of recombinant bovine CD200R1?

The biological activity of recombinant bovine CD200R1 is primarily determined by its binding capabilities in functional ELISA . Researchers should develop binding assays to measure the interaction between the recombinant CD200R1 and its ligand CD200. Functional validation can include:

• Binding affinity measurements using surface plasmon resonance

• Verification of inhibitory signaling in myeloid cells using phosphorylation assays for Dok1/Dok2

• Functional inhibition assays measuring the suppression of pro-inflammatory cytokine production

• Western blot analysis to confirm proper folding and expression

A comparative approach with human CD200R1 can be valuable, as seen in cynomolgus monkey CD200R1 studies where binding to human CD200 Fc Chimera was measured, showing an ED50 of 0.02-0.12 μg/mL .

What expression systems are optimal for producing recombinant bovine CD200R1?

For more native-like protein with proper glycosylation, mammalian expression systems should be considered:

• HEK293 cells

• CHO cells

• Insect cells (baculovirus expression system)

Each system offers different advantages in terms of yield, glycosylation patterns, and experimental applications .

How can I detect native CD200R1 protein in bovine tissue samples?

Detection of native CD200R1 in tissue samples presents challenges due to its relatively low expression levels, particularly in the central nervous system. Based on methodologies applied to other species, researchers should consider:

• Immunohistochemistry: Use antibodies specific to the extracellular domain, noting that commercially available antibodies typically detect only the long transmembrane protein isoforms .

• Flow cytometry: Particularly effective for detecting CD200R1 on myeloid cells including macrophages, mast cells, basophils, and dendritic cells .

• Western blotting: Should show bands at approximately 60-67 kDa under reducing and non-reducing conditions .

• Isolation of specific cell populations: Given the low percentage of CD200R1-expressing cells in tissues, enrichment techniques may be necessary before analysis .

What are the known variants of CD200R1 and how do they differ functionally?

Studies in human systems have identified four CD200R1 mRNA variants resulting from alternative splicing, with potentially different functional implications:

Table 1: Key Characteristics of CD200R1 Variants

Research indicates differential expression of these variants across cell types. For instance, in human studies, microglia-like cells show similar V1 and V4 expression to monocytes but significantly lower V2 and V3 expression . While bovine-specific variants are less characterized, researchers should anticipate similar diversity when studying bovine CD200R1.

How does CD200R1 expression change in response to inflammatory stimuli?

CD200R1 expression demonstrates stimulus-specific regulation that varies by cell type and variant. In microglia-like cells, studies have shown:

• Pro-inflammatory stimuli (LPS): Significant decrease in V3 and V4 CD200R1 mRNA variants after 24 hours of treatment .

• Anti-inflammatory stimuli (IL-4): Significant increase in V1 CD200R1 mRNA expression after 24 hours .

This differential regulation highlights the complex role CD200R1 plays in inflammatory responses. Researchers studying bovine CD200R1 should design experiments that account for these stimulus-specific changes when evaluating receptor function in inflammatory contexts.

What is currently known about soluble forms of CD200R1?

Short CD200R1 mRNA variants (V2 and V3) appear to encode soluble forms of the receptor that may serve as decoy receptors . While their exact physiological function remains under investigation, hypotheses include:

• Potential inhibition of the CD200-CD200R1 system by preventing binding between membrane-bound forms of the ligand and receptor .

• Possible competition with membrane-bound receptors for available CD200 ligand .

Increased expression of these soluble forms has been observed in certain pathological conditions such as Parkinson's disease, suggesting a role in disease pathophysiology . Future research should focus on isolating and characterizing these soluble forms from biological samples to better understand their functional significance.

How is the CD200/CD200R1 axis implicated in neurological disorders?

The CD200/CD200R1 pathway plays a critical role in maintaining immune homeostasis in the central nervous system. Alterations in this pathway have been implicated in several neurological disorders:

• Parkinson's Disease (PD):

  • Increased expression of CD200R1 (mRNA variants and protein isoforms) in brain tissue of PD patients, particularly in the hippocampus .

  • Increased CD200 expression (CD200tr mRNA) in brain tissue of PD patients .

  • Elevated expression of CD200full and CD200tr mRNAs in iPSCs-derived dopaminergic neurons from PD patients .

  • These changes may affect microglial function and represent potential therapeutic targets .

• Multiple Sclerosis and Alzheimer's Disease:

  • Decreased expression of CD200 and/or CD200R1 reported in brain tissues from patients with these conditions .

  • This suggests that the inhibitory mechanism provided by CD200/CD200R1 signaling may be overwhelmed in these disorders .

Understanding these alterations provides insights into neuroinflammatory mechanisms and potential therapeutic approaches for neurological disorders.

What role does CD200R1 play in autoimmune conditions like rheumatoid arthritis?

CD200R1 serves as a critical regulator of immune responses in autoimmune conditions, with particularly well-documented roles in rheumatoid arthritis (RA):

• Expression patterns in RA:

  • The proportion of CD200+ cells and CD200R1+ cells in peripheral blood mononuclear cells, CD14+ cells, and CD4+ T cells is significantly reduced in RA patients compared to healthy controls .

  • Paradoxically, CD200+ cells are elevated in synovium from RA patients .

• Functional effects in RA models:

  • CD200R1-deficient mice show significantly greater clinical severity of arthritis than wild-type controls, confirming the physiological role of CD200R1 in limiting arthritis severity .

  • CD200Fc (an agonist of CD200R1) treatment in vitro:

    • Partially inhibits CD4+ T cell proliferation

    • Promotes CD4+ T cell apoptosis

    • Reduces Th17 cell differentiation

    • Down-regulates CCR6-mediated Th17 chemotaxis in cells from RA patients

  • Engagement of CD200R1 on CD14+ cells reduces osteoclastogenesis and inhibits Th17 differentiation .

• Therapeutic implications:

  • Increased peripheral CD200/CD200R1 expression correlates with decreased disease activity after treatment with infliximab and MTX .

  • This suggests the CD200/CD200R1 axis may serve as both a biomarker and therapeutic target in RA.

How does CD200R1 contribute to peripheral nerve injury healing?

CD200R1 plays a significant role in modulating inflammatory responses during peripheral nerve injury and regeneration:

• Expression changes after injury:

  • Upregulation of CD200R1 mRNA occurs after sciatic nerve crush injury in mice.

  • Conversely, CD200 is downregulated acutely after nerve injury .

• Functional contributions:

  • CD200R1 blockade using specific antibodies significantly reduces acute entrance of both neutrophils and monocytes from blood after nerve injury.

  • This intervention impairs spontaneous functional recovery, suggesting CD200R1 has a crucial role in mounting a successful acute inflammatory reaction necessary for effective regeneration .

• Proposed mechanism:

  • CD200R1 appears to regulate the quality and timing of inflammatory responses following nerve injury.

  • Proper CD200R1 function may be necessary for appropriate inflammatory cell recruitment and subsequent resolution phases required for successful nerve regeneration .

These findings challenge simplistic views of inflammation as uniformly detrimental in injury contexts, highlighting the nuanced role of immune regulation in repair processes.

What approaches are being investigated to therapeutically target the CD200/CD200R1 pathway?

Several therapeutic strategies targeting the CD200/CD200R1 pathway are under investigation:

• CD200R1 agonists:

  • CD200Fc fusion proteins have demonstrated neuroprotective effects in experimental models of neurological disorders .

  • These agonists aim to enhance the inhibitory function of CD200R1, potentially controlling microglial activation and associated neurotoxicity .

• CD200R1 antagonists:

  • 23ME-00610, a humanized IgG1 investigational antibody that binds human CD200R1 with high affinity, is being developed as a potential first-in-class therapy .

  • This approach aims to block the inhibitory function of CD200R1, potentially enhancing anti-tumor immune responses .

• CD200 Activation Receptor-Ligand (CD200AR-L):

  • A novel immunotherapy targeting multiple checkpoints with a single peptide inhibitory ligand.

  • This approach activates the immune system by downregulating CD200-inhibitory receptor, PD-1/PD-L1, and CTLA-4 .

  • Clinical testing is ongoing for recurrent glioblastoma (NCT04642937) .

• Engineered surrogate antibodies:

  • Development of species-specific variants, such as 23ME-00611, which was engineered for binding to cynomolgus monkey CD200R1 to enable preclinical toxicology studies .

What is the current status of clinical trials targeting CD200R1?

While most CD200R1-targeted therapies remain in preclinical development, early clinical trials are underway:

• CD200AR-L for Glioblastoma (NCT04642937):

  • First-in-human, dose-escalation phase 1 clinical trial utilizing a 3+3 design.

  • Treatment involves CD200AR-L administered by intradermal injection after topical imiquimod, combined with an allogeneic vaccine.

  • Preliminary results from 6 patients (ages 37-65) showed one dose-limiting toxicity of grade-III encephalopathy and non-dose-limiting grade-III toxicities including lymphopenia and immunotherapy-related intracranial edema.

  • The estimated completion date is November 2023 .

• 23ME-00610:

  • A humanized IgG1 investigational antibody that binds human CD200R1 with high affinity.

  • Crystal structure of 23ME-00610 Fab in complex with human CD200R1 has been solved, showing it blocks CD200-CD200R1 interaction through steric hindrance.

  • The therapy is advancing in clinical studies to understand the human CD200R1 immune checkpoint as a target in immuno-oncology .

Researchers should monitor clinical trial registries for updates on these and emerging CD200R1-targeted therapies.

What challenges exist in developing CD200R1-targeted therapies?

Several significant challenges must be addressed in developing effective CD200R1-targeted therapeutics:

• Species-specific differences:

  • Significant sequence variation exists between human and animal CD200R1 orthologs. For example, within the extracellular domain, cynomolgus CD200R1 shares 91% amino acid sequence identity with human, but only 54% and 57% with mouse and rat CD200R1, respectively .

  • This complicates preclinical testing, often requiring development of surrogate antibodies with species-specific binding properties .

• Variable expression patterns:

  • CD200R1 expression varies significantly between different cell types and can be regulated differently in response to various stimuli .

  • This heterogeneity complicates targeting strategies and may lead to variable treatment responses.

• Isoform complexity:

  • The existence of multiple CD200R1 isoforms with potentially different functions (membrane-bound versus soluble) creates challenges in developing precisely targeted therapies .

  • Current antibodies typically detect only the long transmembrane protein isoforms, potentially missing important biology related to soluble forms .

• Balancing immune activation vs. suppression:

  • The optimal therapeutic approach may differ by disease context. In cancer, blocking CD200R1 inhibitory signals may enhance anti-tumor immunity, while in autoimmune or inflammatory conditions, enhancing CD200R1 signaling might be beneficial .

  • This dual nature requires careful consideration of therapeutic design and patient selection.

How might single-cell technologies advance our understanding of CD200R1 biology?

Single-cell technologies offer unprecedented opportunities to elucidate CD200R1 biology across diverse cell populations:

• Single-cell RNA sequencing (scRNA-seq):

  • Can resolve the heterogeneity in CD200R1 variant expression across individual cells within tissues.

  • May uncover previously unrecognized cell populations with unique CD200R1 expression patterns.

  • Could identify co-expression patterns with other immunoregulatory molecules, providing insights into functional networks.

• CyTOF (mass cytometry):

  • Allows simultaneous detection of CD200R1 protein expression alongside dozens of other cellular markers.

  • Can elucidate how CD200R1 expression correlates with cell activation states, cytokine production, and other functional parameters at the single-cell level.

• Spatial transcriptomics:

  • Offers the ability to map CD200R1 variant expression within intact tissue contexts.

  • Particularly valuable for understanding CD200R1 biology in complex environments like the brain or inflammatory lesions.

What structural determinants govern the interaction between bovine CD200 and CD200R1?

Understanding the structural basis of bovine CD200-CD200R1 interaction remains an important research frontier:

• Key interaction domains:

  • Association of CD200 with CD200R1 primarily occurs between their respective N-terminal Ig-like domains .

  • Crystal structures of human CD200-CD200R1 complexes suggest specific binding interfaces that may be conserved in bovine proteins.

• Research approaches:

  • X-ray crystallography or cryo-EM studies of bovine CD200-CD200R1 complexes.

  • Mutagenesis studies targeting predicted interaction residues to validate their functional importance.

  • Molecular dynamics simulations to explore the energetics and conformational changes involved in binding.

• Comparative analysis:

  • Studies have shown that cynomolgus monkey CD200R1 can bind human CD200 Fc Chimera with an ED50 of 0.02-0.12 μg/mL .

  • Similar cross-species binding studies with bovine CD200R1 would provide insights into structural conservation and species specificity.

How do viral homologs of CD200 interact with bovine CD200R1?

Viral exploitation of the CD200-CD200R1 pathway presents a fascinating area for investigation:

• Known viral CD200 homologs:

  • Viral CD200 (vCD200) homologs are encoded by Kaposi's sarcoma-associated herpesvirus (KSHV) and rhesus macaque rhadinovirus (RRV) .

  • RRV vCD200 has been shown to induce functional signals through rhesus macaque CD200R .

  • RRV can express both membrane-associated and secreted forms of vCD200 (vCD200-Sec) .

• Functional distinctions:

  • Membrane-expressed RRV vCD200 can induce signal transduction via rhesus macaque CD200R.

  • The secreted form of vCD200 appears to be nonfunctional in the same context .

  • KSHV vCD200 does not efficiently induce signaling via rhesus macaque CD200R .

• Research directions for bovine systems:

  • Investigate whether bovine herpesviruses encode CD200 homologs.

  • Determine if known viral CD200 homologs can bind and signal through bovine CD200R1.

  • Explore whether viral exploitation of this pathway contributes to immune evasion in bovine viral diseases.

Understanding these viral mimicry strategies may provide insights into both viral pathogenesis and the fundamental biology of the CD200-CD200R1 pathway.

What are optimal methods for studying CD200R1 signaling pathways?

Investigating CD200R1 signaling requires specialized approaches to capture its unique signaling mechanisms:

• Phosphorylation assays:

  • Focus on detecting phosphorylation of key tyrosine residues in the cytoplasmic tail.

  • Assess recruitment and phosphorylation of Dok1 and Dok2 adaptor proteins.

  • Monitor inhibition of Ras/MAPK pathway activation .

• Co-immunoprecipitation studies:

  • Examine the formation of signaling complexes involving CD200R1, Dok proteins, and RasGAP.

  • Investigate potential species-specific differences in complex formation .

• Functional readouts:

  • Measure inhibition of pro-inflammatory cytokine production (TNF-α, IL-2, IFN-γ).

  • Assess suppression of MAPK and STAT3 signaling pathways .

  • Evaluate effects on myeloid cell activation markers.

• Genetic approaches:

  • CRISPR/Cas9 editing to introduce specific mutations in tyrosine residues.

  • Generation of variant-specific knockouts to assess individual contributions of CD200R1 isoforms.

What cell models are most appropriate for investigating bovine CD200R1 function?

Selection of appropriate cell models is critical for meaningful studies of bovine CD200R1 function:

• Primary bovine cells:

  • Peripheral blood mononuclear cells (PBMCs) - provide a mixed population including monocytes and lymphocytes.

  • Bovine macrophages - either derived from blood monocytes or isolated from tissues.

  • Bovine microglial cells - relevant for CNS-related studies.

• Cell line options:

  • Bovine macrophage cell lines (where available).

  • Consideration of heterologous expression systems where bovine CD200R1 is expressed in well-characterized human or murine cell lines.

• Co-culture systems:

  • To study CD200-CD200R1 interactions, co-culture systems can be established using:

    • B cells or other cells expressing CD200 co-cultured with myeloid cells expressing CD200R1.

    • This approach mimics interactions predicted to occur in vivo .

• iPSC-derived models:

  • For neurological applications, bovine iPSC-derived neurons and glial cells could provide valuable insights, similar to approaches used in human studies .

What are the most reliable approaches for functional validation of CD200R1 variants?

Functional validation of CD200R1 variants requires comprehensive approaches:

• Binding assays:

  • Surface plasmon resonance to measure binding kinetics between different CD200R1 variants and CD200.

  • Flow cytometry-based binding assays using differentially labeled CD200 to detect binding to cells expressing distinct CD200R1 variants.

• Signaling assays:

  • Phospho-flow cytometry to detect activation of signaling pathways in cells expressing different variants.

  • Western blotting to assess phosphorylation of key signaling molecules downstream of each variant.

• Cellular function assays:

  • Effects on cytokine production (measure by ELISA or multiplex assays).

  • Impact on phagocytosis (particularly for myeloid cells).

  • Influence on cell migration and chemotaxis.

  • Modulation of T cell differentiation (particularly Th17 vs. Treg balance) .

• Blocking studies:

  • Use of variant-specific antibodies or engineered blocking proteins.

  • CRISPR-based knockout/knockin approaches to specifically manipulate individual variants.

These approaches should be systematically applied to compare membrane-bound versus soluble variants, helping to resolve their distinct functional roles.