Researchers at the University of Southern California (USC) have developed a method for expanding and genetically engineering immune cell progenitors, potentially opening a new route toward scalable cancer immunotherapies.
The study, published inCell, demonstrates how granulocyte-monocyte progenitors (GMPs) can be expanded in laboratory conditions while retaining their ability to differentiate into functional immune cells. When engineered with chimeric antigen receptors (CARs), these progenitors demonstrated antitumor activity against both blood cancers and solid tumors in mouse models.
The research addresses several limitations associated with CAR-macrophage (CAR-M) therapies, including poor cell expansion, inefficient genetic modification and limited persistence following administration.
Unlike mature macrophages, which have little capacity to proliferate outside the body, GMPs are immature cells capable of generating macrophages and granulocytes. However, their normally limited proliferative lifespan has historically restricted their potential as a manufacturing platform.
Unlocking scalable cell expansion
The team developed a chemically defined culture formulation, termed SCF/2i, containing stem cell factor and two small molecules, GDC-0879 and SKL2001.
Under these conditions, mouse GMPs expanded for more than 50 days, increasing in number by nearly eight orders of magnitude while maintaining their progenitor identity and differentiation potential.
To optimize the approach for human cells, the researchers introduced a more potent SKL2001 analog, TN2-30, alongside additional growth factors. The resulting SCF/2i-IG formulation supported extensive expansion of human GMPs derived from cord blood, bone marrow and mobilized peripheral blood.
Importantly, whole-genome sequencing revealed no detectable increase in somatic mutation burden after 28 days of culture, although longer-term safety assessments remain necessary.
Further investigations identified myeloperoxidase (MPO) as an unexpected regulator of GMP proliferation. Inhibiting MPO promoted sustained expansion, revealing a potential mechanism for maintaining progenitor populations during manufacturing.
Engineering more effective immunotherapies
The researchers subsequently demonstrated that expanded GMPs could be efficiently modified using lentiviral gene delivery and CRISPR-Cas9, overcoming genetic engineering limitations associated with mature macrophages.
Following infusion into mice, the progenitors established themselves in hematopoietic tissues and generated functional myeloid cells. Human GMP-derived cells persisted for at least 180 days in immunodeficient mice, whereas infused mature macrophages became undetectable within seven days.
In a mouse model of CD19-positive B-cell acute lymphoblastic leukemia, six of 10 animals treated with CAR-engineered human GMPs survived beyond 80 days, compared with none of the untreated controls surviving beyond day 38. HER2-targeted CAR-GMPs also substantially suppressed tumor growth and improved survival in a mouse model of ovarian cancer.
The team additionally developed a modified CAR containing an immunoglobulin G Fc domain, designed to recruit the recipient's own immune cells. This CAR-Fc approach enhanced antitumor activity in immunocompetent mouse models of lymphoma and breast cancer, even when donor and recipient cells were immunologically mismatched.
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