Increasing evidence suggests that immune system alterations, including inflammation and autoimmunity, are associated with both sporadic and genetic forms of FTLD [4, 10, 12, 26, 27]. Several murine model studies have indicated that C9orf72 loss-of-function leads to severely altered peripheral inflammatory marker levels (cytokines, autoantibodies and peripheral blood counts) coinciding with autoimmune disease-like phenotypes and increased mortality [1,2,3]. To our knowledge, the present study is the first one describing systemic inflammatory markers in human patients carrying the C9orf72 HRE. It should be noted that only a few studies have so far addressed the levels of peripheral inflammatory molecules in FTLD in general [28,29,30]. Elevated levels of IL-6 in sporadic FTLD and FTLD patients with GRN mutations have been observed [29, 30]. In another study, serum and CSF levels of 27 inflammatory molecules (including IL-6) were screened, but no differences in the serum in GRN mutation-carrying or sporadic FTLD patients were identified . We observed that out of the measured factors (MCP-1, RANTES, IL-10, IL-17A, IL-12p, IFN-γ, IL-8, IL-1β, hemoglobin, thrombocytes, leukocytes and hs-CRP), only hemoglobin and IL-10 differed between C9orf72 HRE carriers and non-carriers (higher levels in the carriers). These differences were observed mostly in male subjects, suggesting a possible gender-specific effect of the C9orf72 HRE. However, the reason and underlying mechanisms of the elevated levels of IL-10 and hemoglobin observed here in C9orf72 HRE-carrying males require further studies. In contrast to our data in male patients, C9orf72 homozygous knock-out mice previously showed decreased levels of hemoglobin [1, 2]. Thus, in patients harboring one healthy and one expanded allele, the C9orf72 HRE-related haploinsufficiency does not appear to alter peripheral cytokines or blood count factors in a similar manner to the total knock-out mice. In the present study, we also observed several other gender-specific differences, as the levels of IL-8, RANTES and hs-CRP were all higher in females compared to males, regardless of whether the patients carried the C9orf72 HRE or not. Although these differences might not be specifically related to FTLD, they suggest that the gender may potentially influence the immunological profiles in FTLD.
Associations between peripheral inflammatory molecules and cognitive or psychometric measures or between peripheral inflammation and longitudinal data reflecting disease progression in FTLD patients have not been so far reported [28,29,30]. Our present data provide novel evidence for the association between specific altered inflammatory markers and disease progression or clinical manifestation of FTLD. The levels of IL-10 correlated negatively and those of RANTES positively to a more rapid disease progression (measured with a decline rate in the ADCS-ADL score). Additionally, increased levels of MCP-1 correlated with a more rapid cognitive decline in FTLD patients as measured with a decline rate in MMSE. In line with these findings, higher plasma MCP-1 levels were also recently associated with a more rapid cognitive decline in Alzheimer’s disease patients . Interestingly, MCP-1 and RANTES are pro-inflammatory immune mediators that recruit immune cells to the site of inflammation [32,33,34], whereas IL-10 is a key anti-inflammatory cytokine [35, 36]. Our results suggest that high levels of IL-10 might be protective against rapid disease progression and, conversely, low levels of IL-10 may associate with a more rapid progression in FTLD patients. Elevated levels of the pro-inflammatory RANTES and MCP-1, in turn, could lead to accelerated disease progression, even though it is also possible that inflammation represents a secondary phenomenon rather than a driver in patients with a more rapid progression.
Interestingly, patients manifesting with Parkinsonism showed lower levels of IL-10, elevated levels of MCP-1, and a nonsignificant trend towards elevated levels of RANTES, suggesting that the clinical FTLD phenotype with Parkinsonism is associated with a disadvantageous pro-inflammatory profile. This idea is supported by previous studies showing increased MCP-1 and RANTES levels in patients with Parkinson’s disease (PD) [24, 37], suggesting that increased levels of these cytokines may associate with extrapyramidal neuronal system decline. Moreover, alterations in cytokine levels, abnormal glial activation, and T-cell infiltration have been observed in PD patients, further pointing to inflammation as an important contributor to PD pathogenesis . Another interesting finding in the present study was that FTLD patients with psychotic symptoms had elevated IL-10 levels. This finding is supported by previous reports showing that especially patients with first-episode schizophrenia or bipolar disorder with psychotic symptoms display elevated levels of IL-10 [39, 40]. Based on these observations, we hypothesize that inflammation associated with altered IL-10, MCP-1 and RANTES levels might at least partially underlie and predict the different clinical phenotypes or progression rates in patients under the heterogeneous FTLD spectrum. Moreover, the blood samples in this study were mainly collected at the early symptomatic phase of FTLD (at the first visit in the neurology clinic), which suggests that early pro-inflammatory cytokine changes may be disadvantageous.
The strengths of our study are that the frequency of the C9orf72 HRE carriers in our cohort is considerably high and that the association between the C9orf72 HRE carriers and inflammatory markers has not been previously studied. Our study provides novel information on the associations between inflammation and several carefully defined clinical features and disease progression. Furthermore, the Simoa used for IL-8, IL-10, IL-1β and MCP-1 analyses, provides ultrasensitive methodology that enables more reliable detection of low-grade inflammation compared to standard ELISA-based methods. The limitations include the fact that our cohort is rather limited in size for explicit conclusions and replication in other cohorts (from other geographical populations) is required to confirm our findings. As we did not include corrections for multiple comparisons, our results represent preliminary suggestions rather than explicit associations. Notably, as the observed differences related to disease progression and Parkinsonism were biologically rational (inverse effects of pro-inflammatory RANTES and MCP-1 and anti-inflammatory IL-10), the results are likely to suggest biological pathways rather than coincidental type 1 errors. Thus, validation of the present findings warrants further studies. We cannot exclude the fact that other factors (apart from age and gender), unrelated to the disease process, may have affected the results of this study. Additionally, as our study provides a cross-sectional analysis at one time point for the cytokine levels (early symptomatic phase of FTLD), further longitudinal assessments are needed in the future. Finally, we did not have the opportunity to include healthy controls in our study, and thus at this point cannot evaluate whether the observed differences are detected only inside the FTLD spectrum or whether the findings in specific subgroups (such as FTLD with Parkinsonism) would also differ from those in non-FTLD participants.
In conclusion, our study suggests that pro-inflammatory changes (indicated by elevated levels of the pro-inflammatory RANTES and MCP-1 and decreased levels of the anti-inflammatory IL-10) observed in the early symptomatic phase of FTLD are associated with distinct clinical profiles and a more rapid disease progression. Furthermore, we show that the C9orf72 HRE and gender may affect the inflammatory profile in FTLD, although the observed differences related to the C9orf72 HRE were not as prominent as in the previous studies in C9orf72 knock-out murine models [1,2,3]. Future longitudinal studies in other cohorts are warranted to confirm the suggested correlation of the inflammatory profiles and clinical manifestations in FTLD in general and in FTLD patients carrying the C9orf72 HRE.