A study published September 25 in JAMA Neurology supports the idea that the immune system and amyotrophic lateral sclerosis have a complicated relationship. Tracking different immune cells in the blood of people with ALS, researchers led by Eva Feldman at the University of Michigan in Ann Arbor reported that many, including neutrophils, monocytes, and other myeloid cells, increased in number as symptoms worsened. Conversely, in some of those cells, elevated expression of CX3CR1—a receptor that binds ligands on neurons and dampens inflammation in the CNS—correlated with slower progression. The findings add weight to the idea that ALS and the immune response are intimately linked. They also point to potential therapeutic targets and progression markers.

  • People with ALS have more myeloid cells in their blood than do controls.
  • As symptoms worsen, leukocytes and neutrophils grow, while CD4+ T cells dwindle.
  • The number of monocytes expressing the CNS-homing receptor CX3CR1 tracks with slower progression.

“While injury to motor neurons may initiate the disease process, this paper and others support the idea that ALS also triggers, in a very meaningful way, responses in immune cells outside of the CNS,” commented Stanley Appel of Houston Methodist Neurological Institute.

There is no doubt that the immune system kicks into high gear during ALS, but whether the response is helpful or hurtful is unclear. A prevalent hypothesis claims that the immune response protects motor neurons at first, but later exacerbates neurodegeneration (for review, see Hooten et al., 2015, and Murdock et al., 2015). In the spinal cord, agitated microglia and astrocytes inflame the tissue, while immune cells outside of the CNS proliferate as well (Turner et al., 2004Murdock et al., 2016May 2017 news). Whether and how those peripheral cells might affect ALS pathology remains unclear, though recent studies indicate that expression of immune genes in peripheral cells comes with faster disease progression (Mar 2017 news). Broad-spectrum immunosuppressive drugs either have been ineffective or worsened disease in ALS patients, indicating that at least part of the immune response helps.

To identify which peripheral immune cell types correlate with disease progression, first author Benjamin Murdock and colleagues took blood samples from 119 people with ALS and 35 controls, and counted cells in the different leukocyte populations. On average, people with ALS had higher total leukocyte counts than controls—5.53 million versus 4.57 million cells per mL—although there was significant overlap between the two groups. In particular, natural killer cells, neutrophils, monocytes, and other myeloid cells were all higher in people with the disease.

Next, Murdock and colleagues tracked these cell populations longitudinally in 24 ALS patients within three years of their diagnosis. Leukocyte counts varied dramatically between patients and between different visits in individual patients. Even so, during the next two years average numbers of total leukocytes, neutrophils, some monocytes, and natural killer (NK) cells notched upward, by an average of 60,000, 110,000, 20,000 and 80,000 cells per mL per year, respectively.

In addition, total leukocyte and neutrophil counts correlated with worsening scores on the ALSFRS-R scale, a measure of disease progression. In contrast, numbers of CD4+ T cells negatively correlated with decline—cell numbers shrank as symptoms worsened. A portion of these CD4 T cells may have been regulatory T cells, although the researchers did not stain the cells for regulatory T cell markers.

The researchers also found that the amount of the fractalkine receptor CX3CR1, expressed on the surface of monocytes, correlated with slower disease progression. Similarly, decreasing numbers of other myeloid cells expressing this receptor also correlated with slower decline.

The findings paint a picture of a rising systemic immune response as disease progresses, but as the authors and other commentators pointed out, the nature of the connection remains murky. “The real question is: ‘Are these peripheral immune cells contributing in a significant way to the ultimate injury of motor neurons?’” Appel asked.

Johnathan Cooper-Knock of the University of Sheffield in England wondered the same thing. “We know that changes in the peripheral immune system correlate with changes in ALS. What we do not know is whether these peripheral changes are causative and therefore a good therapeutic target, or whether they are a secondary downstream effect of changes in the CNS,” he wrote to Alzforum.

Despite the lack of a causative link, Cooper-Knock considered the fractalkine receptor findings promising. “CX3CR1 has been implicated in myeloid cell trafficking to the CNS and therefore this observation may be consistent with a protective effect of migrating peripheral monocytes,” he wrote to Alzforum. “This is an exciting observation which may provide a peripherally accessible therapeutic target.”

Murdock told Alzforum that he is focusing on NK cells as a potential therapeutic target. These cells normally ignore the body’s own cells, but ALS motor neurons downregulate major histocompatibility class I molecules that identify them to the immune system as “self” (Mar 2016 news on Song et al., 2016). In addition, he mentioned that shelved NK-targeting drugs, initially designed to treat rheumatoid arthritis, could be ripe for repurposing.

What about immune cells as markers of disease progression? Though mounting evidence suggests that systemic inflammation correlates with disease, Murdock and other researchers agreed that immune cell counts make poor biomarkers. Appel said that immune cell counts in controls overlap those in people with ALS, rendering the measurements meaningless at the individual level.

Robert Miller of California Pacific Medical Center in San Francisco called the fluctuations in immune cell populations within individual patients “disappointing,” though he considers the overall results good news. “Biomarkers of inflammation are becoming a very promising area of focus,” he wrote to Alzforum. “We are certainly not there yet in finding the best cell type or combination of cell types to study, but reports like these encourage us to soldier on.”—Jessica Shugart

Comments

  1. Murdock and colleagues have conducted a detailed study of leukocyte populations in peripheral blood samples taken from ALS patients and controls. Serial samples were collected from a number of the ALS patients, allowing the authors to track changes in leukocyte subpopulations over time. They discovered that total leukocyte counts, and counts of a number of subpopulations, including neutrophils, natural killer (NK) cells, but not CD4+ or CD8+ T-cells, are increased in ALS patients compared to controls. They went on to test whether leukocyte counts correlated with disease progression as measured by change in ALS-FRS score.

    Change over time in several of the cell populations correlated significantly with change in the ALS-FRS score. However, ALS is a relentless disease that will always progress over time, and therefore any marker that changes in a consistent manner over time might be expected to correlate with changes in the ALS-FRS score. This highlights the great unanswered question for study of the peripheral immune system in ALS—correlation or causation? We know that changes in the peripheral immune system correlate with changes in ALS: for example, work from our group has shown that gene expression in peripheral lymphocytes can be used to classify ALS patients as fast or slow progressors (Mar 2017 news on Cooper-Knock et al., 2017). What we do not know is whether these peripheral changes are causative and therefore a good therapeutic target, or whether they are a secondary downstream effect of changes in the CNS—a limitation acknowledged by the authors of this work. A recent study of CNS microglia has suggested that these cells are relatively long-lived, tend to remain in their local environment, and increase their numbers through cell division rather than recruitment from outside the CNS (Aug 2017 news on Réu et al., 2017, and Füger et al., 2017). This would suggest that the CNS immune system is relatively independent but there is much more work to be done.

    A very interesting aspect of this study was the dissection of myeloid cell subgroups using cell surface markers. The authors discovered that, although changes in CD16+ and CD16- monocytes overall did not correlate with changes in the ALS-FRS, when the analysis was limited to monocytes expressing CX3CR1 there was a significant relationship. Increased numbers of CX3CR1+ cells associated with slower disease progression. CX3CR1 has been implicated in myeloid cell trafficking to the CNS and therefore this observation may be consistent with a protective effect of migrating peripheral monocytes. This is an exciting observation which may provide a peripherally accessible therapeutic target.

    References:

    . A data-driven approach links microglia to pathology and prognosis in amyotrophic lateral sclerosis. Acta Neuropathol Commun. 2017 Mar 16;5(1):23. PubMed.

    . The Lifespan and Turnover of Microglia in the Human Brain. Cell Rep. 2017 Jul 25;20(4):779-784. PubMed.

    . Microglia turnover with aging and in an Alzheimer's model via long-term in vivo single-cell imaging. Nat Neurosci. 2017 Oct;20(10):1371-1376. Epub 2017 Aug 28 PubMed.

  2. This study highlights the importance of the interaction between the immune system and ALS disease progression. The strengths of the study are the number of patients who participated and the fact that a part of the study analyzed blood cells longitudinally.

    An important aspect is that different leukocyte populations show a difference in their number in ALS patients compared with controls. This could, therefore, be used as a marker of the disease if this happens to be specific for ALS (data would have to be compared with other related neurodegenerative diseases) and, importantly, if the results were confirmed in other ALS populations.

    Interestingly, some variations with cell population numbers correlated with the ALSFRS-R score, which hints at the potential impact of the immune system in ALS. From these data, it is not possible to conclude whether ALS disease progression impacts the immune system leading to variations in the number of subpopulations of leukocytes or if those variations impact disease progression.

    Some perspective comes from other new findings showing that the most common genetic cause of ALS (C9ORF72) has an important immune function, as determined in several studies analyzing the effect of C9ORF72 knockout in mice, and that some genes previously known for their implication in the immune system are mutated in ALS (TBK1). In addition, the results provided by this new study from Feldman and collaborators are important in regard to clinical trials currently testing the impact of compounds targeting immune cells. It would be interesting to analyze the impact of these compounds on the cell populations highlighted in this study.

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References

News Citations

  1. Inside Out, or Outside In? ALS Turns on Monocytes in Blood
  2. Can Immune Gene Expression Predict Pace of Motor Neuron Destruction?
  3. Learning About ALS by Eavesdropping on Neuron-Astrocyte Chatter

Paper Citations

  1. . Protective and Toxic Neuroinflammation in Amyotrophic Lateral Sclerosis. Neurotherapeutics. 2015 Apr;12(2):364-75. PubMed.
  2. . The dual roles of immunity in ALS: injury overrides protection. Neurobiol Dis. 2015 May;77:1-12. Epub 2015 Feb 26 PubMed.
  3. . Evidence of widespread cerebral microglial activation in amyotrophic lateral sclerosis: an [11C](R)-PK11195 positron emission tomography study. Neurobiol Dis. 2004 Apr;15(3):601-9. PubMed.
  4. . Increased ratio of circulating neutrophils to monocytes in amyotrophic lateral sclerosis. Neurol Neuroimmunol Neuroinflamm. 2016 Aug;3(4):e242. Epub 2016 Jun 1 PubMed.
  5. . Major histocompatibility complex class I molecules protect motor neurons from astrocyte-induced toxicity in amyotrophic lateral sclerosis. Nat Med. 2016 Apr;22(4):397-403. Epub 2016 Feb 29 PubMed.

Further Reading

Primary Papers

  1. . Correlation of Peripheral Immunity With Rapid Amyotrophic Lateral Sclerosis Progression. JAMA Neurol. 2017 Sep 25; PubMed.