Gut-related Immune Activation in Parkinson's Disease with Asian LRRK2 Risk Variants: Associations with Systemic Inflammation and Clinical Severity

Technical Summary: Gut-related Immune Activation in Parkinson's Disease with Asian LRRK2 Risk Variants

Problem Statement
Parkinson's disease (PD) is increasingly understood through the lens of the microbiome–gut–brain axis, where intestinal barrier dysfunction and gut inflammation may drive systemic inflammation and neurodegeneration. A key mechanism involves the translocation of lipopolysaccharide (LPS) from the gut lumen into systemic circulation, triggering innate immune signaling via the LPS-binding protein (LBP), soluble CD14 (sCD14), and the TLR4/MD2 complex. While previous studies have assessed intestinal permeability markers like calprotectin and zonulin, evidence regarding LBP and sCD14 in PD remains limited and conflicting. Furthermore, it is unclear whether host genetic factors, specifically the LRRK2 gene, modulate these gut-related immune responses. The LRRK2 p.G2385R and p.R1628P variants are common in Asian populations and are known to influence PD risk and immune regulation, yet their specific impact on gut permeability and endotoxin-related immune activation in PD patients has not been evaluated.

Methodology
This cross-sectional study recruited a cohort of 237 participants from Universiti Malaya, stratified into five groups: healthy controls (HC, n=58), idiopathic PD (iPD, n=61), PD patients carrying the LRRK2 p.G2385R variant (n=56), PD patients carrying the p.R1628P variant (n=60), and double-variant carriers (n=5). All PD patients were diagnosed by movement disorder specialists.

The study measured plasma concentrations of LBP and sCD14 as surrogate markers for gut permeability and endotoxin exposure, respectively. Additionally, a panel of systemic inflammatory markers (cytokines IL-6, TNF-α; chemokines CCL2, CCL5, CX3CL1, VCAM1) was quantified using a multiplex immunoassay system. Clinical severity was assessed using the MDS-UPDRS (Parts I–IV), the Clinical Impression of Severity Index for Parkinson's Disease (CISI-PD), Montreal Cognitive Assessment (MoCA), and constipation metrics (Bristol Stool Form Scale, ROME IV criteria, PAC-SYM).

Statistical analyses involved Spearman's rank correlation to assess associations between LBP/sCD14 and inflammatory/clinical variables, followed by generalized linear models (GLM) to adjust for covariates such as age, disease duration, and diagnosis age.

Key Results

1. Group Differences: No significant differences were observed in plasma LBP or sCD14 levels when comparing HC, iPD, and the various LRRK2 variant carrier groups (p=0.172 and p=0.326, respectively). This indicates that the presence of these specific LRRK2 risk variants does not substantially alter circulating levels of these gut-related immune markers.
2. Correlations between Markers: LBP and sCD14 levels were positively correlated across the overall cohort and within specific PD subgroups, even after adjusting for age.
3. Association with Systemic Inflammation:
   • LBP: Demonstrated robust positive correlations with pro-inflammatory cytokines IL-6 and TNF-α across the overall PD cohort and within all PD subgroups (including iPD, G2385R, and R1628P carriers).
   • sCD14: Showed more modest and inconsistent associations, correlating significantly with IL-6 and CCL5 in the overall PD cohort, but with weaker or non-significant links in specific subgroups.
4. Association with Clinical Severity:
   • LBP: Higher levels were significantly correlated with worse motor function (higher MDS-UPDRS Part III and CISI-PD motor signs), greater disability (MDS-UPDRS Part II, CISI-PD disability), and remained significant after covariate adjustment.
   • sCD14: Higher levels were associated with worse constipation severity (higher ROME IV scores) and, in unadjusted analyses, worse motor complications. However, the association with motor complications lost significance after covariate adjustment. No associations were found with cognitive function (MoCA) or stool form.

Key Contributions and Significance
This study provides the first evaluation of circulating LBP and sCD14 in PD patients stratified by the Asian-prevalent LRRK2 p.G2385R and p.R1628P risk variants. The primary contribution is the demonstration that while these specific genetic variants do not differentiate patients based on gut permeability or endotoxin exposure markers, the markers themselves retain strong clinical relevance.

The paper claims that plasma LBP serves as an informative marker of gut permeability and endotoxin exposure that correlates with a proinflammatory profile (elevated IL-6 and TNF-α) and greater motor disability in PD, independent of LRRK2 genotype status. This suggests that gut-related endotoxin-induced immune activation is a feature of PD pathophysiology that operates alongside, rather than being strictly defined by, these specific genetic risk factors.

The authors conclude that plasma LBP may represent a promising biomarker for inflammatory burden, potentially useful for patient stratification and disease prognosis, particularly in the context of future gut-targeted therapeutic trials. The study emphasizes the need for prospective longitudinal validation to confirm these cross-sectional findings and to further elucidate the role of sCD14 in gut-related immune dysregulation.