Targeting blood cells (hematopoietic cells) with AAV is uniquely challenging. Because blood cells are non-adherent and constantly circulating, and because hematopoietic stem cells (HSCs) are largely quiescent (resting), they are naturally resistant to AAV infection.

Most successful “blood” gene therapies actually target the Bone Marrow (the factory) or use Ex Vivo transduction (treating cells in a lab and then re-infusing them).

Quick Legend: Administration Routes

• IV (Intravenous): Systemic delivery. Faces high “washout” and liver sequestration.
• IBM (Intrabone Marrow): Direct injection into the marrow cavity (e.g., femur or iliac crest). Best for targeting the stem cell niche.
• Ex Vivo: Cells are collected via apheresis, transduced with AAV in a controlled lab environment, and then transplanted back into the patient.

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1. Natural AAV Serotypes

Serotype	Primary Target Cells	Preferred Route	Key Characteristics & Notes
AAV6	Hematopoietic Stem Cells (HSCs), T-cells	Ex Vivo	The Gold Standard for Blood. • The most efficient natural serotype for transducing human HSCs. • Widely used in CRISPR/Cas9 workflows to deliver repair templates for gene editing.
AAV1	Monocytes, Progenitor Cells	Ex Vivo	Closely related to AAV6; shows high efficiency in some myeloid lineages but generally secondary to AAV6 for stem cells.
AAV2	B-cells, Macrophages	Ex Vivo	Historically used for early blood research. While it can transduce these cells, the efficiency is significantly lower than engineered variants or AAV6.
AAV5	Monocytes / Dendritic Cells	Ex Vivo	Better suited for “Antigen Presenting Cells” (APCs) rather than the primary red/white blood cell lineages.

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2. Engineered & Synthetic Capsids

Engineered capsids for blood focus on overcoming the “attachment” problem—adding ligands that stick to receptors on moving blood cells.

Variant	Primary Target Cells	Preferred Route	Key Characteristics & Notes
AAV6.2	HSCs, T-cells	Ex Vivo	A modified AAV6 with a single mutation (F129L) that increases stability and improves the rate of gene delivery to blood progenitors.
AAV-DJ	Broad Leukocytes	Ex Vivo	A synthetic hybrid with high “in vitro” potency. It is excellent for lab-based transduction of blood-derived cell lines.
AAV-HSC (1-17)	Bone Marrow Stem Cells	IV / IBM	A specialized class of AAVs isolated from human CD34+ cells. Unlike other AAVs, these are designed specifically to “home” to the hematopoietic niche.
AAV2-Rec7	Progenitor Cells	IV	A bioengineered capsid designed to avoid liver trapping, allowing more of the vector to reach the bone marrow via systemic circulation.

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3. Tropism by Blood Lineage

If you are targeting a specific blood component, start here:

• Hematopoietic Stem Cells (HSCs): AAV6 (Ex Vivo) or AAV-HSC15 (In Vivo).
• T-Cells (CAR-T Applications): AAV6 is currently the preferred “repair template” vehicle for T-cell engineering.
• B-Cells: AAV2 or AAV6, though Lentivirus is often preferred for B-cells due to higher efficiency.
• Monocytes/Macrophages: AAV1 or AAV5.

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Critical Factor: Quiescence & Integration

The “Blood Problem” has two major hurdles:

1. The “Moving Target”: Systemic IV delivery rarely works for blood cells because the contact time between the virus and the moving cell is too short. Ex Vivo is almost always required.
2. Cell Division: AAV is typically “episomal” (it sits outside the DNA). Because blood cells divide rapidly, the AAV signal is “diluted” and lost within days.
   • The Solution: Use AAV as a carrier for CRISPR/Cas9. The AAV delivers the “patch,” and the CRISPR integrates it permanently into the blood cell’s genome so that all future generations of that cell carry the fix.

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Summary of Specialized Tissue Tropism

Tissue	Barrier	Best Capsid	Best Route
Blood Cells	Cell Motion / Quiescence	AAV6	Ex Vivo
Bone Marrow	Bony Compartment	AAV-HSC	Intrabone (IBM)
Spleen	Blood Filtration	AAV9	IV