Recombinant Human IL-21 Protein

What it means: This IL-21 is produced in Human cells rather than in E. coli or insect cells. The manufacturer gives the source as human cells and does not name a specific cell line.

Why it matters:

• Expression in a human host lets the human IL-21 sequence be folded and processed by human cellular machinery.
• Production in human cells avoids bacterial host-cell contaminants that can affect sensitive immune-cell and stem-cell cultures.
• Together with the animal-free, xeno-free manufacturing format, it fits fully human culture systems.

How to interpret here: An expression system of Human cells means the IL-21 was made by a human cell line. It is intended for T-cell, NK-cell and immunotherapy research with human cells.

Tip: When switching from an IL-21 made in another host, re-titrate the working concentration rather than assuming the same activity per microgram.

What it means: The protein is Tag-free: no His, Fc, GST or other affinity tag is fused to the IL-21 sequence, and no tag needs to be removed before use.

Why it matters:

• A tag adds extra amino acids that can change folding, receptor binding, stability or the apparent molecular weight of a cytokine.
• In defined immune-cell and stem-cell culture media, every added component is a variable; a tag-free protein keeps the ligand close to the native sequence.
• Tag-free proteins are also preferred when results will be compared with published work that used untagged cytokines.

How to interpret here: Tag-free here means the vial contains only the IL-21 protein. It cannot be captured or detected with anti-His, anti-GST or Protein A reagents, so plan detection with an antibody against IL-21 itself.

What it means: Purity of >95% by SDS-PAGE means that when the protein is run on a denaturing gel and stained, at least 95% of the stained protein is the IL-21 band. It is the purity specification stated by the manufacturer.

Why it matters:

• High purity limits other proteins in the vial that could act on cells or interfere with downstream readouts.
• Dosing is done by protein mass, so a pure preparation makes the amount of cytokine added more accurate and comparable between experiments.
• Purity is stated together with endotoxin (<0.5 EU/µg), so both protein and non-protein contaminants are addressed.

How to interpret here: A specification of >95% by SDS-PAGE is a minimum value. The exact result for each lot is on the Certificate of Analysis (CoA), available from BioHippo on request.

Tip: SDS-PAGE purity shows how clean the preparation is, not how active it is; check the activity data separately.

What it means: The species is Human: the amino-acid sequence is that of human IL-21. The protein is also produced in human cells, so both the sequence and the expression host are human.

Why it matters:

• Matching the species of the cytokine to the species of the cells avoids differences in receptor binding between species.
• Human sequence is the usual choice for work with human primary immune cells, cell lines and stem-cell-derived cells.
• Recording the species in protocols matters because the same cytokine is sold from several species.

How to interpret here: Human here refers to the protein sequence. When reproducing a published protocol, check that it also used Human IL-21; if it used a protein from another species, the working concentration may need adjusting.

What it means: The protein is supplied Lyophilized, that is freeze-dried to a powder, from a 0.2 µm-filtered PBS solution and without carrier protein.

Why it matters:

• Lyophilized protein is more stable during storage than a solution, which gives a long shelf life when stored correctly.
• The stock concentration is chosen at reconstitution, so it can be matched to the volumes used in a protocol.
• As supplied, store at −20°C to −80°C for up to 12 months from receipt; after reconstitution keep it under sterile conditions at 2°C to 8°C for up to 1 month or at −20°C to −80°C for up to 3 months.

How to interpret here: Lyophilized means the vial must be reconstituted before use: briefly centrifuge the vial before opening, then reconstitute in sterile PBS containing 0.1% endotoxin-free recombinant human serum albumin.

Tip: Avoid repeated freeze–thaw cycles of the reconstituted protein; a manual-defrost freezer is recommended.

What it means: The molecular weight is ~15 kDa, as stated by the manufacturer. The manufacturer describes the recombinant protein as a glycosylated monomer of 132 amino acid residues and as a 15.4 kDa protein.

Why it matters:

• Molecular weight is the first check of identity when the protein is run on a gel or analysed by other methods.
• Glycosylation can make the band appear slightly larger or broader than 15.4 kDa.
• The manufacturer gives a nominal value; lot-level data are on the Certificate of Analysis.

How to interpret here: ~15 kDa is a nominal value. Refer to the lot-specific Certificate of Analysis (CoA), available from BioHippo on request, for lot data.

Tip: Note whether a gel was run under reducing or non-reducing conditions when comparing band sizes.

What it means: Biological activity is tested in a cell proliferation assay using human N1186 T cells. The typical result is ED50 0.5–5 ng/mL, the concentration that gives half of the maximal effect.

Why it matters:

• IL-21 controls proliferation of target immune cells, so a T-cell proliferation assay reflects its normal function.
• A cell-based activity test confirms that the protein is functionally active, which purity alone cannot show.
• The assay gives a common reference point when comparing lots or comparing this protein with other preparations tested the same way.

How to interpret here: ED50 0.5–5 ng/mL describes potency in the N1186 assay. It is not a recommended working dose; concentrations for each culture should be optimized for the cells and readout used.