What are the key considerations when performing single-cell experiments using blood samples?

Answer:
Blood samples present several unique challenges that can affect both experimental performance and data interpretation.

Storage conditions can significantly alter cellular composition. Overnight storage may increase the proportion of polymorphonuclear cells (PMNs) while reducing the proportion of intact neutrophils. Therefore, if neutrophils are a primary focus of the study, prolonged storage should be avoided.

Neutrophils have a short half-life and are highly prone to lysis and apoptosis. In addition, they contain relatively low RNA content. Consequently, samples enriched in neutrophils often exhibit reduced median gene counts, typically in the range of 300–700 genes per cell. This should be interpreted as an intrinsic biological characteristic rather than a technical failure. 

In some samples, particularly under pathological conditions, low-density granulocytes (LDGs) may be present. These cells have densities similar to PBMCs and cannot be effectively removed by standard density gradient centrifugation, which may lead to reduced gene complexity.

Granulocytes are highly susceptible to lysis, which can increase ambient RNA background and reduce effective cell recovery. This issue is commonly observed in blood and other granulocyte-rich samples.

If neutrophils are not of interest, magnetic bead-based depletion of neutrophils is recommended to improve data quality and reduce background RNA contamination. 

Commonly used magnetic beads:

• Human neutrophil isolation (positive selection): CD15 MicroBeads, human, Miltenyi, Cat. No. 130-046-601. 
• Mouse neutrophil isolation (negative selection): Neutrophil Isolation Kit, mouse, Miltenyi, Cat. No. 130-097-658. 
• Mouse neutrophil isolation (positive selection): Anti-Ly-6G MicroBeads UltraPure, mouse, Miltenyi, Cat. No. 130-120-337. 

In certain disease contexts, such as sepsis, the proportion of granulocytes may be substantially increased. In such cases, density gradient separation alone may not be sufficient, and additional depletion strategies should be considered.

Nucleated red blood cells may also be present in some samples and cannot be efficiently removed. Their presence is often associated with specific pathological conditions and should be considered during downstream data interpretation.

Finally, direct red blood cell lysis without prior density gradient separation is not recommended, as it may lead to excessive residual contamination and reduced sample quality.