CD34 serves as an intrinsic innate immune guardrail protecting stem cells from replicating retroviruses

It has long been recognized that hematopoietic stem and progenitor cells are remarkably resistant to retroviral infections, such as HIV-1, compared to their more differentiated progeny. While this resistance is partially attributed to known restriction factors and interferon-stimulated genes (ISGs), the underlying mechanisms remain incomplete, especially given that many viruses have evolved accessory proteins to counteract these defenses. CD34, a transmembrane glycoprotein and a well-known marker of hematopoietic stem cells, has remained largely mysterious in its functional capacity. This study investigates whether CD34 acts as a previously unidentified component of the intrinsic innate immune system in stem cells.

The researchers examined the effect of CD34 on HIV-1 replication by expressing it in various cell types, including CD4+ T cells and the Kasumi-3 hematopoietic progenitor cell line. They utilized CRISPR/Cas9 to knockout the CD34 gene and employed cytokines (GM-CSF and TNF-α) to induce the downregulation of CD34 on the cell surface. Viral infectivity was measured using reporter systems that track progeny virion activity, and viral attachment was assessed using biochemical assays to determine if CD34 physically inhibits the virus's ability to bind to target cells.

The study demonstrates that CD34 is not a simple bystander or cell marker but an active antiviral factor. CD34 does not block viral entry, genome integration, or the release of viral particles. Instead, it is incorporated onto the surfaces of budding progeny virions. Once incorporated, CD34 physically hinders the virion's ability to attach to target cells, effectively rendering it non-infectious. Notably, unlike other host restriction factors, CD34 is resistant to downregulation by HIV accessory proteins, positioning it as a robust final guardrail for stem cells. The researchers also confirmed that when CD34 is removed—either through CRISPR knockout or cytokine-induced differentiation—the resulting virions regain significant infectivity.