Characterization of immune cells in X-linked adrenoleukodystrophy

About 65% of all male X-linked adrenoleukodystrophy (X-ALD) patients develop a rapidly progressive, inflammatory, demyelinating cerebral form of X-ALD either in childhood or adulthood. Without treatment, the 5- year survival rate (from the occurrence of initial symptoms) is 59% with considerable variation in individual survival times. So far, all studies attempting immunosuppressive or immunomodulatory therapies have not been successful. Hematopoietic stem cell therapy (HSCT) can halt the progression and stop the inflammation when performed at an early phase of inflammatory progression. However, the molecular mechanism underlying the success of this treatment is unclear. X-ALD is caused by inherited mutations in the ABCD1 gene encoding the peroxisomal ABC transporter ABCD1. The functional loss of ABCD1 leads to accumulation of very long-chain fatty acids. It has long been assumed that all cell types expressing ABCD1 are equally affected in X-ALD. Recently, we and others have demonstrated that a homologous peroxisomal ABC transporter, ABCD2, can compensate for the lack of ABCD1 upon overexpression in vitro and in vivo. In preliminary experiments, we could demonstrate that individual immune cell types are considerably more affected than others. Thus, within the proposed project, we will investigate various purified immune cells from healthy controls and from X-ALD patients in order to identify differently affected immune cell in X-ALD. In addition, we will perform a functional characterization of purified monocytes / macrophages isolated from X-ALD patients. The results of these studies should help to substantially improve our understanding of the respective mechanisms underlying the success of HSCT in X-ALD and the failure of immunosuppressive therapies. In a separate set of experiments, we will use a mouse model for X-ALD to elucidate the amount of ABCD2 required to compensate for the lack of ABCD1. Furthermore, we will investigate the regulation of ABCD2 gene expression in purified immune cells in order to find novel mechanisms to induce the ABCD2 gene for therapeutic attempts. The results of this work should lead to novel possibilities for a pharmacological induction of ABCD2 in order to halt the cerebral inflammation in patients that could not undergo HSCT due to late diagnosis of X-ALD or to the lack of an appropriate donor.

X-linked adrenoleukodystrophy (X-ALD) is a rare, inherited disorder with a broad clinical variability. Whereas essentially all male X-ALD patients develop a slowly progressive dying-back axonopathy affecting both ascending and descending spinal cord tracts in young adults, in about 60%, a devastating, rapidly progressive form of cerebral inflammatory demyelination occur either in childhood or adulthood. Only if the cerebral inflammation is diagnosed and treated at an early stage, can the exchange of the immune cells by hematopoietic stem cell transplantation stop the inflammation and rescue the patients. Within this project, we could show that, among the different immune cell types, predominantly the macrophages are metabolically affected in X-ALD. Moreover, we demonstrated, both in vitro and in vivo, that the intrinsic defect of X-ALD macrophages impairs their ability to convert from a pro-inflammatory to an anti-inflammatory status. A direct comparison between pro-inflammatory lesions of post-mortem brain tissue from patients with multiple sclerosis or cerebral X-ALD clearly demonstrated a lack of anti-inflammatory macrophages in X-ALD. This inability to establish an anti-inflammatory milieu would explain why spreading of the inflammatory, demyelinating lesion in X-ALD cannot halt spontaneously and why it is refractory to anti-inflammatory treatments. Moreover, this might also explain the success of hematopoietic stem cell transplantation in X-ALD, in which ? among other cell types ? also the patients macrophages are exchanged by those derived from the donor stem cells. However, often the inflammatory demyelination in X-ALD patients is diagnosed at an advanced stage, too late for hematopoietic stem cell transplantation to be beneficial, leaving many patients without any curative treatment. The research results of this project identified macrophages as a main therapeutic target for stopping the devastating inflammatory demyelination in X-ALD. Moreover, we identified compounds that are able to induce a homologous gene, which is intact in X-ALD, to an extent that may be sufficient for rescue of the inherited defect in X-ALD macrophages. The application of such compounds to cerebral X-ALD patients potentially represent a novel therapeutic pharmacological approach to halt the inflammation in X-ALD.