Different T-cell subsets may have opposite effects, either protective or toxic, on the development of ALS. Two previous MR studies(39,40) reported inconsistent results regarding the causal link between white blood cell and total lymphocyte counts and the risk of ALS. Our study, to the best of our knowledge, is the first MR investigation that reveals the causal connections between T cell-related immune characteristics and the risk of ALS. The T-cell subsets characterized by the levels of CD3 molecules were highlighted as they showed consistent protective effects against ALS. In addition, the proportion of Tregs in CD4+ T cells and CCR7 expression on both naïve CD4+ T cells and naïve CD8+ T cells were potential association with the risk of ALS. However, given that these three traits were excluded from the sensitivity analysis, biological interpretations should be made with caution.
CD3 is a pan-T-cell marker expressed on all T-cells. Emerging evidence indicate that CD3+ T cells can be detected in the spinal cords of both ALS mouse models and ALS patients. (9,25,59). These cells have been observed in patients of every age examined, with the numbers increasing with disease progression(26). Notably, patients with longer survival times show higher CD3 counts than those with shorter survival times(10). However, there is a dearth of literature that provide a detailed characterization of the association between the levels of CD3 expression in different T-cell subsets and the risk of ALS.
Interestingly, a longitudinal study showed that senescent T and B cells remain relatively stable as ALS progresses, in contrast to the elevated CD3 levels observed between baseline and the third clinical visit(13). The increase in CD3 may compensate for the increase in the number of functionally impaired senescent T cells. In the present study, T-cell subsets characterized by the level of CD3 molecules exhibited a potential protective tendency against the pathogenesis of ALS. However, further research is required to confirm the potential functional mechanism that supports this effect.
Research on the pathophysiology of ALS has been primarily focused on the role of Tregs. In this study, we confirmed a potential negative association between the proportion of Tregs in CD4+ T cells and the risk of developing ALS by managing residual confounding factors and performing a reverse causality examination. This discovery aligns with observations made in earlier studies. Tregs are strongly associated with disease progression rate(21–23) and survival(14) in ALS patients and mouse models. Previous animal studies have demonstrated that expanded or activated Tregs effectively postpone the decline of motor abilities and extend the lifespan in ALS mouse models(21,23,24,26). In the early stages of ALS, Tregs effectively activate protective M2 microglia(25) and suppress neurotoxic microglia(23). Expansion of Tregs helps maintain the size of the MN soma and boosts the neurotrophic factor gene expression in the spinal cord and peripheral nerves of ALS mice(21). Treg therapy can potentially reduce peripheral oxidative stress and the increased levels of the associated circulating acute-phase proteins induced by inflammation(60).
We performed an extensive literature review and found that only one study indicated that patients with ALS show higher CCR7 expression on CD8+ T cells than healthy individuals(34) (Table 1). Interestingly, CCR7 signalling is crucial for the optimal recruitment of activated CD8+ T cells into the inflamed central nervous system(61). Our findings indicate that the positive correlation between the expression of CCR7 on both naïve CD4+ and naïve CD8+ T cells and the risk of ALS may not be coincidental.
Table 1. Evidence of the known associations between T cell subsets and ALS from previous studies
Trait
|
Evidence of disease association
|
PMID
|
CD3 T cells
|
Present in the spinal cords of ALS mouse models*
|
21596768, 35688338
|
Infiltration in the lumbar spinal cord was observed before the cervical region*
|
21176785
|
Increased as the disease progressed*
|
18809917
|
Increased from baseline to the third clinical visit
|
36323511
|
No difference between ALS and HC
|
32246039, 21596768
|
Positively associated with disease progression rate
|
36347843
|
Patients with longer survival times had higher counts
|
28742871
|
CD4 central memory cell
|
Negatively associated with the risk of death
|
35287794
|
Negatively associated with survival from onset
|
36323511
|
Treg
|
Increased/reduced in ALS compared to HC
|
36359827
|
Activated Treg was positively associated with survival
|
36347843
|
Increased/reduced as the disease progressed
|
19307024, 21651502
|
Increased during the slowly progressing phase and decreased during the rapidly progressing phase
|
22820142, 23143995
|
Negatively associated with disease progression rate
|
29507931
|
CCR7 on CD8+ T cell
|
Increased in ALS compared to HC
|
36359827
|
Abbreviations: HC, healthy control; ALS, amyotrophic lateral sclerosis
*The reported results were obtained from ALS mouse models, not patients.
This study has several limitations that should be taken into account when interpreting the findings. First, we implemented a stringent p-value threshold of P < 5×10-8 to minimize any bias from weak instrumental variables. However, this resulted in a limited number of IVs, which led to a low statistical power. Second, after the Bonferroni correction, we observed a nominally significant association between 24 subtypes and the risk of ALS, but none of these associations were deemed statistically significant. Consequently, it is imperative to investigate further and validate these potential associations using larger cohorts and GWAS. Third, given that we only included samples from individuals of European ancestry to overcome population stratification, the generalizability of our findings to other ethnic groups is limited. Fourth, while we used the PhenoScannerV2 database to identify potential secondary phenotypes of the IVs, we cannot completely rule out the possibility of pleiotropy. Lastly, we only used a GWAS of T-cell subtypes in peripheral blood. However, existing evidence suggests that the associations between T-cell phenotypes and the progression rate of ALS are stronger in samples taken from cerebrospinal fluid (CSF) rather than blood(14). Thus, future blood- and CSF-based T-cell marker studies may allow for better understanding of the immunological processes underlying ALS.