Recombinant Macaca mulatta (Rhesus macaque) C5a anaphylatoxin chemotactic receptor (C5AR1)

What is the structure and function of Macaca mulatta C5AR1 protein?

C5AR1 (Complement component 5a receptor 1), also known as CD88, is a seven-transmembrane G protein-coupled receptor with a molecular weight of approximately 39.1 kDa. It functions primarily as a receptor for the chemotactic and inflammatory peptide anaphylatoxin C5a. The receptor interacts with its ligand through at least two sites: a high-affinity site on the extracellular N-terminus and a second site in the transmembrane region that activates downstream signaling events. C5AR1 activation stimulates chemotaxis, granule enzyme release, intracellular calcium release, and superoxide anion production . In Macaca mulatta, C5AR1 shares significant structural homology with human C5AR1, making macaques valuable models for studying complement-related immune functions .

How can researchers validate the functional activity of recombinant Macaca mulatta C5AR1?

Functional validation of recombinant Macaca mulatta C5AR1 can be performed through several methodological approaches:

• Binding assays: Measure the binding ability using functional ELISA. Immobilized human C5AR1 can bind Anti-C5AR1 recombinant antibody with an EC₅₀ of 1.239-3.760 ng/mL .

• Receptor-ligand interaction studies: Assess the interaction between recombinant C5AR1 and C5a using techniques such as surface plasmon resonance or fluorescence-based binding assays.

• Cellular activation assays: Determine if the recombinant protein can initiate signaling cascades when exposed to C5a by measuring:

  • Calcium mobilization

  • ERK phosphorylation

  • Chemotaxis of relevant immune cells

• Cross-reactivity validation: Confirm species-specific binding properties using antibodies that recognize both human and rhesus macaque C5AR1, such as clone P12/1 .

Expression and Purification Methodologies

Native C5a contains a large N-linked glycosylation site at Asn 64, which accounts for up to 25% of its molecular weight. Research comparing recombinant E. coli-generated (non-glycosylated) C5a to purified plasma-derived (glycosylated) C5a showed that:

These findings suggest that researchers should pair studies using recombinant C5a with C5aR1 inhibitors or purified/synthetic C5a to confirm relevant findings, particularly in functional/cytokine assays conducted in primary immune cells .

How can recombinant Macaca mulatta C5AR1 be used to study neuroinflammation in primate models?

Recombinant Macaca mulatta C5AR1 can be leveraged for neuroinflammation studies through several approaches:

• Cell line development: Researchers have established IFN-responsive cell lines from rhesus macaque brain tissue that can be used to study neuroinflammatory mechanisms. These cell lines, derived from trigeminal ganglia, cortex, and hippocampus, were immortalized using lentiviral transduction and can serve as platforms for studying C5AR1 signaling in a neural context .

• Alzheimer's disease models: C5aR1 antagonism has been shown to suppress inflammatory glial responses and protect cognitive function in mouse models of Alzheimer's disease. Similar studies in macaque models could provide more translatable insights into human disease processes. C5aR1 inhibition prevented cognitive loss, limited detrimental glial polarization while permitting neuroprotective responses .

• Single-cell RNA-seq analysis: This approach can identify neurotoxic disease-associated microglia clusters that are C5aR1-dependent, while revealing microglial genes associated with synapse organization, transmission, and learning that are overrepresented following C5aR1 antagonist treatment .

• Ex vivo tissue models: Using primary cultures from macaque brain tissue to study C5AR1-mediated inflammatory responses allows for assessment of species-specific signaling pathways in a more physiologically relevant context .

What are the differences in C5a binding affinity between Macaca mulatta C5AR1 and human C5AR1?

The binding affinity differences between Macaca mulatta and human C5AR1 have important implications for translational research:

• Species-specific recognition: Structural studies have revealed that some C5a antagonists, such as the Spiegelmer NOX-D20, show specificity for mouse and human C5a against macaque and rat C5a. This suggests structural differences in the binding epitopes between species .

• Antibody cross-reactivity: The monoclonal antibody clone P12/1 recognizes both human and rhesus macaque C5AR1, indicating conservation of certain epitopes. This antibody recognizes the N-terminal domain of C5AR1 (Met1-Asn31) and has been shown to inhibit C5a binding to its receptor at a concentration of 3.3 µM .

• Functional responses: When binding studies were performed with C5a, both species' receptors showed activation of similar downstream pathways, but potentially with different magnitudes of response or kinetics .

These differences must be considered when extrapolating experimental results between species or when developing therapeutic agents targeting the C5a-C5AR1 axis.

How is C5AR1 implicated in cancer immunology, and how can recombinant proteins advance this research?

C5AR1 plays significant roles in cancer immunology, particularly in shaping the tumor microenvironment:

• Expression patterns: C5AR1 is overexpressed in tumor-infiltrating dendritic cells compared to DCs from blood or spleen. This expression is restricted to type 2 conventional DCs and monocyte-derived DCs, which display a tolerogenic phenotype capable of inhibiting T-cell activation and promoting tumor growth .

• Migration control: C5aR1 engagement in DCs drives their migration from tumors to tumor-draining lymph nodes, where C5a levels are higher .

• Immunotherapy enhancement: In three syngeneic tumor models, C5aR1 inhibition significantly enhanced the efficacy of poly I:C (a Toll-like receptor 3 agonist) in combination with PD-1/PD-L1 blockade. This antitumor activity relied on type 1 conventional DCs and antigen-specific CD8+ T cells .

• Ovarian cancer applications: C5aR1 expression independently predicted dismal prognosis in high-grade serous ovarian cancer and was linked to the immunoevasive subtype characterized by increased infiltration of pro-tumor cells (Treg cells, M2-polarized macrophages, and neutrophils) and impaired CD8+ T cell functions .

Recombinant C5AR1 proteins can advance this research by facilitating binding studies with potential antagonists, serving as immunogens for antibody development, and enabling structure-function analyses to design more effective therapeutic agents.

What methodological approaches can be used to study C5AR1 antagonism in infectious disease models using Macaca mulatta?

Several methodological approaches can be employed to study C5AR1 antagonism in infectious disease models using Macaca mulatta:

• Vaginal SHIV challenge models: C5A peptide has been tested as a microbicide for HIV prevention. In macaque studies, C5A protected 8 of 9 animals from SHIV-162P3 vaginal challenge, compared to only 2 of 9 protected in the placebo group .

• COVID-19 research applications: C5a levels are elevated in COVID-19 patients and correlate with disease severity. In human samples, C5a was detected in the bronchoalveolar lavage fluid of patients with COVID-19 who had ARDS, and large numbers of neutrophils and monocytes expressing C5AR1 were found in these samples . Similar studies could be conducted in macaque models.

• Ex vivo tissue cultures: Researchers can use Macaca mulatta tissue explants to test the effects of C5AR1 antagonists on viral infection. This approach has been used to demonstrate that C5A blocks herpes simplex virus infection of epithelial cells both in vitro and ex vivo by disrupting viral membrane integrity .

• Cell-based assays: The development of rhesus macaque astrocyte cell lines supporting viral infections provides a valuable tool for testing C5AR1 antagonists against neurotropic viruses. These cell lines were permissive to infection with Zika virus and Papiine alphaherpesvirus 2, making them useful for analyzing neurotropic viral infections in rhesus macaque models .

What are the key considerations when designing antibodies against Macaca mulatta C5AR1?

When designing antibodies against Macaca mulatta C5AR1, researchers should consider several factors:

• Epitope selection: The N-terminal domain of C5AR1 (Met1-Asn31) has been successfully targeted by antibodies like clone P12/1 that cross-react between human and rhesus macaque C5AR1 .

• Functional interference: Antibodies that bind to the N-terminal domain can inhibit C5a binding to its receptor. For example, antibody S5/1 inhibits C5a binding to its receptor at a concentration of 3.3 µM .

• Application-specific considerations:

  • For flow cytometry: Antibodies should be tested at dilutions of 1:50 - 1:100, using approximately 10µl of the suggested working dilution to label 5 x 10^5 cells in 100µl .

  • For immunohistochemistry: Tissue sections can be fixed in 2% paraformaldehyde and pretreated with 5% normal serum before applying antibodies .

  • For Western blotting: When using smooth muscle cell extracts separated by SDS-PAGE, the expected band size with anti-C5AR1 antibodies is approximately 42kDa .

• Validation methods: Confirm antibody specificity through techniques like immunoprecipitation followed by mass spectrometry, knockout/knockdown controls, and cross-reactivity testing between species .

How can researchers address the off-target activities observed with recombinant C5a in their experimental designs?

Research has shown that recombinant C5a can exhibit unexpected off-target activities in experimental systems. To address these issues, researchers should implement the following strategies:

• Compare multiple C5a forms: Multiple commercial sources of recombinant C5a, but not plasma-purified or synthetic C5a versions, induced human monocyte-derived macrophages to produce IL-6 and IL-10 in a C5a receptor-independent manner .

• Include proper controls:

  • Use C5aR1 inhibitors alongside recombinant C5a to confirm receptor-specific effects

  • Compare results with purified/synthetic C5a

  • Include receptor knockout/knockdown controls

• Signaling pathway verification: The off-target effects of recombinant C5a were found to be driven through Syk and NF-κB signaling, apparently not due to endotoxin contamination. Researchers should verify which signaling pathways are activated in their experimental systems .

• Alternative approaches: Consider using novel recombinant C5a derivatives, such as C5a-N19 (containing a 19-residue amino-terminal extension peptide), which has been characterized as biologically indistinguishable from unmodified recombinant anaphylatoxin for certain activities but offers advantages for detection and isolation of receptor complexes .

How might single-cell RNA sequencing advance our understanding of C5AR1 biology in Macaca mulatta models?

Single-cell RNA sequencing (scRNA-seq) presents transformative opportunities for C5AR1 research in Macaca mulatta models:

• Cell-type specific expression patterns: scRNA-seq analysis has revealed that C5AR1 is predominantly expressed in specific cell types such as tumor-associated macrophages with an immunosuppressive-related expression signature . Similar approaches in macaque models could identify tissue-specific and disease-specific expression patterns.

• Therapy response prediction: In studies of C5aR1 antagonism in Alzheimer's disease models, scRNA-seq identified neurotoxic disease-associated microglia clusters that are C5aR1-dependent, while revealing microglial genes associated with synapse organization that were overrepresented following treatment .

• Intercellular communication networks: Analysis of scRNA-seq data can predict signaling pathways between brain cell types associated with cell growth and repair versus inflammatory pathways, revealing how C5aR1 signaling influences cellular communication networks .

• Species-specific immune signatures: Comparative scRNA-seq studies between human and macaque samples could identify conserved and divergent aspects of C5AR1 signaling, improving the translational relevance of macaque models for human disease .

• Temporal dynamics: Time-course scRNA-seq experiments during disease progression or treatment could reveal dynamic changes in C5AR1 expression and associated signaling networks, providing insights into optimal therapeutic timing .

What novel therapeutic approaches targeting the C5a-C5AR1 axis are being developed using Macaca mulatta models?

Several innovative therapeutic approaches targeting the C5a-C5AR1 axis are being developed and tested in preclinical models, including Macaca mulatta:

• Small molecule antagonists:

  • PMX53 and PMX205 have shown efficacy in antagonizing C5aR1 signaling in various disease models

  • PMX53 significantly reduced tumor growth and modulated immunosuppressive mechanisms when combined with aPD-1 therapy

  • PMX205 improved outcomes in Alzheimer's disease models, preventing deficits in short-term memory

• Aptamer-based therapeutics:

  • NOX-D20, an L-oligonucleotide aptamer (Spiegelmer), efficiently targets C5a

  • Crystal structures of NOX-D20 bound to mouse C5a reveal a complex 3D architecture that wraps around C5a, including an intramolecular G-quadruplex stabilized by a central Ca²⁺ ion

  • These structures provide insights into improving affinity through nucleotide modifications and inhibition of the C5a:C5aR interaction

• Peptide-based approaches:

  • C5A, a peptide matching amino acids 3-20 of HCV nonstructural protein 5A, has shown protection against vaginal SHIV transmission in macaques

  • 7 of 9 placebo-treated animals were infected, whereas 8 of 9 C5A-treated animals remained uninfected

  • Mechanism involves disruption of viral membranes without affecting cellular membranes

• Combination immunotherapies:

  • C5aR1 inhibition significantly enhanced the efficacy of poly I:C in combination with PD-1/PD-L1 blockade in cancer models

  • The combination of C5aR1 and PD-L1 targeting was associated with specific molecular characteristics and matched clinical response annotations in ovarian cancer