Genetically-Modified Macrophages Accelerate Myelin Repair

Background

Myelin, produced by oligodendrocytes, is an essential component of the insulating sheath surrounding axons and ensures proper spread of nerve impulses¹. In Multiple Sclerosis (MS), a chronic inflammatory disease of the central nervous system (CNS), the destruction of this myelin sheath by an autoimmune attack results in neuronal damage and loss, ultimately leading to typical clinical manifestations such as visual problems, fatigue and muscle weakness². Although various immunomodulatory treatment options exist, so far it is not possible to completely stop progression or cure the disease. New strategies are focusing on enhancing myelin repair, which is critical to restore neurological function and prevent further axonal/neuronal damage¹.

In some demyelinated regions in the CNS of MS patients, a loss of myelinating oligodendrocytes is observed. Since these glial cells derive from oligodendrocyte precursor cells (OPCs), the recruitment of OPCs to the affected areas is vital for myelin repair. The migration of OPCs is among others controlled by guidance molecules such as Semaphorin 3F (Sema3F)³.

A key feature of MS lesions, defined as white matter scars, is the infiltration of inflammatory cells, among others activated monocyte-derived macrophages. These are peripheral immune cells invading the CNS which play dual roles during the disease course. While they contribute to neuroinflammatory events and promote demyelination during the early disease stage, they trigger remyelination and alleviate disease in recovery phases⁴.

Recently, macrophages were used as drug delivery system in the context of tumor treatment⁵.  The present study investigates whether myeloid lineage cells can also serve as vehicle for myelin repair-sustaining molecules under demyelinating conditions by transplanting genetically modified hematopoietic stem cells (HSCs) overexpressing Sema3F.

What was done?

The authors generated Sema3F-overexpressing HSCs by transduction with a lentiviral vector carrying the Sema3F- and GFP-transgene. Control cells were transduced with a vector only encoding for GFP. Vector functionality was confirmed in vitro among others by OPC migration assays using the supernatant of transduced HEK cells.

Subsequently, they genetically modified HSCs isolated from bone marrow of female mice by transduction with the validated lentiviral constructs. After in vitro expansion, the obtained cells were viable, retained their proliferation and differentiation potential, and secreted Sema3F.

Transplantation of these cells into male recipient mice resulted in a high reconstitution with donor cells eight weeks after retro-orbital injection. Of note, although GFP-Sema3F-transduced cells showed an increase in the generation of CD11b⁺ over CD19⁺ cells, general blood cell composition was not altered upon HSC treatment.

In the next step, they analyzed the impact of genetically engineered HSC transplantation on acute demyelination in the spinal cord. To induce focal lesions in recipient mice, they applied lysolecithin, a toxin eliciting a well characterized temporal pattern of de- and remyelination. GFP⁺ cells were detected in the lesions 7 and 10 days after lysolecithin treatment and numbers correlated with the percentage of GFP⁺ cells in the blood. Co-immunolabelling with the macrophage/microglial marker Iba1 showed that these cells were mostly of monocyte origin and further indicated that the general inflammatory response was similar in Sema3F-GFP HSC-transplanted mice and the GFP HSC control group. In the phase of maximal OPC repopulation, the number of OPCs and oligodendroglia was notably increased in Sema3F-GFP HSC-transplanted mice. Consistent with this enhanced OPC migration, significantly more newly generated oligodendrocytes were present in the lesions. Furthermore, regeneration of myelin was already observed at a very early time point of the remyelination stage.

Why I like this preprint

MS is a devastating disease affecting young individuals with a major impact on their daily life. To pave the way for a close to normal lifestyle, it is essential to develop new and safer strategies to stop the loss of neuronal function, for example by boosting remyelination. In my opinion, the concept chosen by Tepavčević et al. is very elegant and holds promising potential towards promoting myelin repair. It will be worth to investigate this approach in more depth in future studies. Importantly, besides MS, this strategy could also be used for the treatment of other demyelinating disorders.

Conclusion and future perspectives

Overall, these findings indicate that Sema3F-expressing blood-derived macrophages can attract OPCs to focal demyelinating lesions, thereby promoting oligodendrogenesis and myelin repair. However, to consolidate the efficiency and possible application as MS therapy, it will be key to validate these results in other models of demyelination such as the cuprizone model or experimental autoimmune encephalomyelitis; in particular as the complex immunological etiology of MS is not reflected in the lysolecithin model.

Questions to the authors

1. Very few GFP⁺ cells seem to be present in the brain. Did genetically modified macrophages accumulate in peripheral organs?
2. Do the authors have an explanation why SemaF3-GFP-transduced cells preferentially differentiate into CD11b⁺ compared to CD19⁺ cells?
3. The used lysolecithin model is a model of acute demyelination lacking the presence of other immune cells at the lesion site. Do you expect comparable results in chronic demyelination? Will the presence of other immune cells influence the recruitment of genetically modified HSC to the lesions and their ability to attract OPCs?
4. The increase in remyelination was already observed at a very early time point. Did you also determine myelin repair at later time points?

References

1. Lubetzki, C., Zalc, B., Williams, A., Stadelmann, C. & Stankoff, B. Remyelination in multiple sclerosis: from basic science to clinical translation. The Lancet Neurology 19, 678–688 (2020).
2. Thompson, A. J., Baranzini, S. E., Geurts, J., Hemmer, B. & Ciccarelli, O. Multiple sclerosis. The Lancet 391, 1622–1636 (2018).
3. Piaton, G. et al. Class 3 semaphorins influence oligodendrocyte precursor recruitment and remyelination in adult central nervous system. Brain 134, 1156–1167 (2011).
4. Wang, J. et al. Targeting Microglia and Macrophages: A Potential Treatment Strategy for Multiple Sclerosis. Front. Pharmacol. 10, (2019).
5. Moyes, K. W. et al. Genetically Engineered Macrophages: A Potential Platform for Cancer Immunotherapy. Human Gene Therapy 28, 200–215 (2016).

 

Tags: hematopoeitic stem cells, macrophages, multiple sclerosis, myelin

doi: https://doi.org/10.1242/prelights.25725

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