CD300C Human

What is CD300C and how does it relate to other CD300 family members?

CD300C (also known as LMIR2, CMRF-35, or CMRF35-A antigen) is a type I transmembrane glycoprotein belonging to the CD300 family of paired immune receptors within the immunoglobulin (Ig) superfamily . It functions as an activating receptor, in contrast to its inhibitory counterpart CD300A .

CD300C shares over 80% homology with CD300A in their extracellular immunoglobulin domain but differs significantly in its cytoplasmic structure . Unlike CD300A, which contains inhibitory ITIMs (Immunoreceptor Tyrosine-based Inhibitory Motifs), CD300C has a short cytoplasmic tail and a charged glutamic acid in its transmembrane domain that facilitates association with adapter molecules like FcRγ and DAP12 .

What is the molecular structure of CD300C?

Human CD300C is synthesized as a 224 amino acid precursor with the following structure:

• 20 amino acid signal sequence

• 163 amino acid extracellular domain (ECD) containing an Ig-like V-type domain (aa 22-128)

• Two N-linked glycosylation sites (aa 90 and 99)

• 21 amino acid transmembrane region with a charged glutamic acid residue

• 20 amino acid cytoplasmic tail

The membrane-proximal region (aa 128-183) has a distinctive composition with high proportions of proline (18%), serine (20%), and threonine (13%) .

Which human immune cells express CD300C?

CD300C shows distinctive expression patterns across immune cell populations:

Studies using discriminatory antibodies have confirmed that freshly isolated blood monocytes are the primary peripheral leukocytes expressing CD300C on their surface .

How is CD300C expression regulated?

CD300C expression is dynamically regulated by several mechanisms:

• Cytokine regulation: IL-2 and IL-15 significantly induce CD300C expression specifically on CD56bright NK cells . This upregulation requires STAT5 signaling and is inhibited by IL-4 .

• T cell interaction: IL-2 secreted from activated CD4+ T cells specifically induces CD300C expression on CD56bright NK cells, demonstrating potential cross-talk between adaptive and innate immunity .

• TLR-mediated regulation: TLR ligands like LPS and flagellin dynamically regulate CD300C expression on monocytes .

• Differentiation-dependent expression: CD300C is differentially expressed during monocyte differentiation into macrophages and dendritic cells .

What are the functional consequences of CD300C activation?

Cross-linking of CD300C with specific antibodies triggers multiple functional responses:

• In monocytes:

  • Calcium mobilization

  • Upregulation of costimulatory molecule CD86

  • Production of inflammatory cytokines

  • Enhanced LPS-induced inflammatory responses

• In mast cells:

  • Cytokine/chemokine production

  • Degranulation

• In CD56bright NK cells:

  • Enhanced degranulation

  • Increased chemokine and cytokine secretion

These findings establish CD300C as a potent activating receptor with a potential role in amplifying inflammatory responses .

What is the relationship between CD300C signaling and FcRγ?

Fc receptor γ (FcRγ) is critical for CD300C function through two mechanisms:

• Surface expression: FcRγ is indispensable for efficient surface expression of CD300C .

• Signal transduction: FcRγ is essential for CD300C-mediated activating functions in mast cells and monocytes .

The interaction likely occurs through the charged glutamic acid residue in CD300C's transmembrane domain, which facilitates association with FcRγ and subsequent activation of downstream signaling cascades .

What are the ligands for CD300C and how do they compare to CD300A ligands?

Research using reporter cell systems has identified several potential CD300C ligands:

Importantly, when CD300A and CD300C are co-expressed (as in most monocytes), CD300A appears to dominate PE recognition and signaling . This helps explain why PE fails to stimulate cytokine production in monocytes expressing both receptors despite CD300C's activating potential .

How can researchers distinguish between CD300A and CD300C experimentally?

Due to high sequence homology, specialized tools are required to differentiate between CD300A and CD300C:

• Specific antibodies:

  • Clone TX45: Recognizes only CD300C

  • Clone E59.126: Recognizes both CD300A and CD300C

  • Epitope differences: Antibodies recognize unique regions involving amino acid residues CD300A(F56-L57) and CD300C(L63-R64)

• Molecular techniques:

  • RT-PCR with receptor-specific primers

  • Expression constructs in 293T cells or YTS cells

• Functional assays:

  • CD300C cross-linking induces activation

  • CD300A cross-linking induces inhibition

What reporter systems can be used to study CD300C ligand interactions?

Researchers have developed several reporter systems for studying CD300C:

• Chimeric receptor reporter cells:

  • Design: Extracellular CD300C domain fused to intracellular CD3ζ domain

  • Readout: GFP expression upon ligand binding

  • Applications: Identification of phosphatidylethanolamine and apoptotic cells as ligands

• Transfection systems:

  • 293T cells transiently transfected with CD300C constructs

  • YTS stable transfectants expressing CD300C

These systems allow for controlled study of receptor-ligand interactions and downstream signaling events.

What methodologies are effective for studying CD300C function in primary cells?

Several approaches have proven effective:

• Antibody-based methods:

  • Receptor cross-linking with specific monoclonal antibodies (e.g., TX45)

  • Flow cytometry for expression analysis and calcium mobilization

  • ELISA for cytokine/chemokine production measurement

• Genetic approaches:

  • RNA isolation from primary cells (e.g., NK cells, monocytes)

  • cDNA synthesis and cloning of CD300C

  • Expression in cell lines for functional studies

• Functional assays:

  • Calcium mobilization measurements

  • CD86 upregulation analysis

  • Cytokine production quantification

  • Co-stimulation with TLR ligands like LPS

How do CD300A and CD300C function as paired receptors in immune regulation?

While substantial progress has been made, several aspects of CD300A/C paired receptor function remain to be fully elucidated:

• The molecular basis for the dominance of CD300A over CD300C in phosphatidylethanolamine recognition when co-expressed

• The potential co-localization of these receptors in lipid rafts or signaling complexes

• The exact stoichiometry and kinetics of ligand binding to each receptor

Research suggests that the balance between these inhibitory (CD300A) and activating (CD300C) paired receptors may represent an important regulatory mechanism for fine-tuning immune responses .

What is the potential role of CD300C in inflammatory diseases?

Given CD300C's role as an activating receptor that enhances inflammatory responses, it may have significant implications for inflammatory diseases:

• Potential significance:

  • Enhanced LPS-induced cytokine production suggests a role in amplifying inflammatory responses

  • Upregulation on CD56bright NK cells by IL-2/IL-15 indicates potential involvement in inflammatory conditions where these cytokines are elevated

• Future research directions:

  • Expression analysis in tissues from patients with inflammatory disorders

  • Functional studies of CD300C+ cells in inflammatory microenvironments

  • Potential as a biomarker or therapeutic target

How might NK cell-specific CD300C expression influence immune surveillance?

The selective expression of CD300C on CD56bright NK cells after IL-2/IL-15 stimulation raises intriguing questions:

• Does CD300C contribute to NK cell-mediated immunosurveillance against tumors or infections?

• How does the interplay between CD300C and other NK cell receptors shape immune responses?

• Could targeting CD300C enhance NK cell-based immunotherapies?

The enhanced degranulation and cytokine secretion observed upon CD300C engagement suggest it may play an important role in augmenting NK cell effector functions in specific immunological contexts .